Sections
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Caption: Earth from
Apollo 17,
1972
Dec07.
Credit/Permission:
NASA,
NASA: Image #AS17-148-22742,
1972 /
Public domain.
Image link: Itself.
You can see the Baja California Peninsula (which is about 1250 km long), the Rocky Mountains, and, towards the top, Lake Superior and Lake Michigan.
Credit/Permission:
NASA,
NASA: Image #AS16-118-18873,
1972 /
Public domain.
Image link: Itself.
North is to the upper left. The Las Vegas Wash pours into the leftmost corner of Lake Mead. The Highway 95 to the north-west is clear. Interstate 15 is probably the curve that streaks from the word Las Vegas up to the right and then more or less to the top middle. The University of Nevada, Las Vegas (UNLV) is probably off the picture south of the word Las Vegas.
The city looks like a grey mold spreading over the Mojave Desert.
Credit/Permission: NASA,
NASA: ISS EarthKam,
before or circa 2004 /
Public domain.
Image link: Itself.
Earth, the one and only.
All other planets are failed Earths.
There are mountains, valleys, and continents etc. due to geological processes that we will discuss, but the range from highest to lowest points is only about 20 km:
Caption: "NASA pan photo mosaic of the Himilayas with Makalu and Mount Everest. The photo mosaic uses photos ISS008-E-13302 TO 13307 taken 2004 Jan28 taken from the International Space Station (ISS), Expedition 8 and added to The Gateway to Astronaut Photography. From The Gateway to Astronaut Photography of Earth." (Slightly edited.)
The image looks south from over the Tibetan Plateau. Makalu (8462 m, the 5th highest peak) is somewhere on left; Mount Everest (8850 m) somewhere on the right.
Credit/Permission: NASA,
2004 /
Public domain.
Image link: Wikipedia.
See Mount Everest videos below/at link:
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Additionally, the Earth
(not counting the atmosphere
for the moment)
is slightly
oblate
(Cox-240) due to the
centrifugal force.
For further explication, see the figure below.
The Earth's modulations from sphericity
are small as discussed above.
So basically the Earth is a
sphere.
In any large scale images, differences from a
sphere are unnoticeable.
Gravity has pulled it into this shape.
An ideal fluid CANNOT resist a shearing force; it can only
resist COMPRESSION. The Earth is
NOT an ideal fluid, but
under the weight of its own mass it acts approximately like a fluid in bulk.
The combination of self-gravity and
pressure force results in the
nearly spherical shape.
The solid and liquid pressure holds up the Earth's atmosphere
too.
The atmosphere
is internally held up by its own gas pressure, of course.
Chemical bond forces of the rock
and metals, the tidal forces
of the Moon and Sun,
and the centrifugal force provide only a bit of
modulation from sphericity.
A fuller explanation of why sufficiently massive dense
astro-bodies
become approximately spherical is given in the figure below.
The centrifugal force
effect on Earth is relatively small.
It is much larger for Jupiter
and Saturn
which are distinctly oblate
in some images.
See the false-color image of
Saturn showing its
oblateness.
We can determine the mass of the Earth from
the gravitation law
and Newton's 2nd law
(i.e., F=ma).
Caption: Determining the Earth's mass.
The accepted value for the
Earth's mass is
Earth mass M_⊕ = 5.9722(6)*10**24 kg
= 3.0033*10**(-6) M_☉.
Note we have used the
astronomical symbols
⊕ and ☉ to denote
Earth
and Sun
and
M_☉
is the solar mass unit.
Credit/Permission: © David Jeffery,
2003 / Own work.
Density (without qualification) is the ratio of mass to volume
of an object.
We can calculate the Earth's
mean density---which is a very, very cruel
density---see the figure below.
Caption: Determining the Earth's
mean density.
Credit/Permission: © David Jeffery,
2003 / Own work.
The mean density of 5.5148 g/cm**3
(Cox-240)
is partially set by matter under strong compression
in the deep interior.
From modeling,
the uncompressed density of the Earth is
estimated to be 4.2 g/cm**3
(Se-418).
This is the density
the Earth material would have if
it were uncompressed.
The modeling involves the ingredients we discuss in the
next section The Interior of the Earth.
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Question: What holds the Earth up from collapsing under its own
self-gravity?
The pressure force of solids (rock and metal)
and fluids (atmosphere, oceans, and internal
molten rock and metal) sustains the Earth from collapse
under its own self-gravity.
Answer 3 is right.
php require("/home/jeffery/public_html/astro/fluids/hydrostatic_equilibrium_sphere.html");?>
Question: What is the centrifugal force and how does it
affect the Earth's shape?
Answer 2 is right.
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Image link: Itself.
Image link: Itself.
The deepest drilling has only gone a few kilometers down---which is just scratching the surface on a planet with a mean radius of 6371.0 km. (see Wikipedia: Earth).
This understanding has had to be inferred from indirect evidence: density, primordial solar nebula composition, seismology, heat flow from the interior, other evidences (too numerous to mention), and modeling.
Let's detail the ingredients needed to understand the Earth's interior structure:
The Earth's mean density is 5.5148 g/cm**3 (Cox-240). But the density of the surface material (mainly silicates) is of order 2.5--3 g/cm**3 (Cox-257).
Thus there must be interior material that is denser (when uncompressed) or of compressed material. In fact, both conjectures are true: there is material denser when uncompressed, but it is also compressed to high density by weight of the overlying material.
The intrinsically denser materials that made up the newly formed hot and molten Earth sank and the intrinsically less dense materials rose all due to the buoyancy effect
This process is called chemical differentiation---which we discuss in IAL 10: Solar System Formation.
The buoyancy effect and "chemical differentiation" is illustrated in the figure below.
The Earth formed out of the
primordial solar nebula whose
composition is known from the solar photosphere composition and
primitive meteorites.
This composition is believed to have been fairly uniform throughout
the solar system before chemical differentiation and other processes
caused composition variations to arise.
We know that the volatiles are depleted
on Earth because they were never strongly condensed and/or escaped
from the early hot Earth.
But what of the elements that
stayed on Earth and
chemically differentiated?
For example, consider the most abundant dense
refractory
element
iron (Fe):
By studying the propagation of
seismic waves
from earthquakes
plus modeling
(i.e., by seismology),
the interior structure of the Earth (including
temperature,
pressure, phase of matter) can be inferred.
The phase information is particularly interesting: the
seismic wave study allows us to see where the interior
is solid and where it is liquid.
See
Seismic waves and earthquake videos below/at link:
We can measure heat flow and temperature
directly at the surface and to some depth
(Wikipedia: Geothermal gradient).
All kinds of other bits of evidence is used.
Among other things, the surface of the Earth
is a boundary condition for the interior and it gives many constaints on interior conditions.
We have already noted some constraints: e.g., crustal composition.
One has to construct computer models
of the Earth that
match the observations discussed in the subsection above.
Solving for the interior structure of the Earth
is, in fact, an inverse problem where one must
obtain the model parameters from the observational data.
But it's NOT a simple
inverse problem where
one puts the data values into formulae and calculates the model parameters.
It's one where you assume model parameters and calculate predictions and compare those
to actual observations.
Usually, an iteration is needed to bring the predictions into agreement with the observations.
Of course, the model parameters obtained are fits to the observations and are only meaningful to the
degree that the model is an adequate simplified representation of the
interior structure.
If it's NOT an adequate representation, then the model parameters may NOT mean much.
Among other conditions imposed on the Earth model,
the model must have
hydrostatic equilibrium: i.e., the pressure at every
radius must be able to hold up the mass of all above that radius
from collapse under the Earth's
self-gravity.
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EOF
These layers are shown in the figure below.
Caption: A cartoon of the interior structure of the Earth (HI-118; Ze2002-160).
My cartoon is from before or circa 2010 and is slightly dated.
An analysis from 2013, suggests that sulfur (S) is probably NOT a main component of the Earth's core (see Lidunka Vocadlo, 2013, Earth science: Core composition revealed, Nature).
It is now thought that oxygen (O) and silicon (Si) are the main components after iron (Fe) (approximately 80 % by mass) and nickel (Ni) (approximately 10 % by mass)
The liquid outer core may be about 10 % by mass O and Si and the solid inner core maybe about 3 % by mass O and Si.
Why were the lighter elements NOT excluded from the Earth's core by chemical differentiation?
Chemical differentiation isn't a perfect process. Some chemical chemical bonding may always keep some elements stuck together to some degree depending on the situation.
Credit/Permission: © David Jeffery,
2003 / Own work.
Image link: Itself.
The inner core is hotter than the outer core, but is a solid because the higher pressures near the center favor the solid phase.
Solid-state physicists can measure or calculate melting temperatures under high pressure.
The temperature and density profiles of the Earth can be calculated too.
Caption: A cartoon of the Earth's interior temperature profile from modeling (Se-431).
Credit/Permission: © David Jeffery,
2003 / Own work.
Image link: Itself.
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The crust by direct inspection is mostly
silicates:
i.e., rock composed
mainly of silicon (Si)
and oxygen (O) plus some amounts of all other
refractories and
volatiles that are locked up in the rock.
The densities are 2.5--3.5 g/cm**3
(Se-431).
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The mantle is also mainly
silicates by INFERENCE
with density varying
over the range 3.5--5.8 g/cm**3
(Se-431).
The silicates
are believed to be richer in
magnesium and iron than the
crust
(CW-52).
The solid inner core and liquid outer core are probably mainly iron, but significant nickel (Ni), oxygen (O), and silicon (Si) are also present as discussed above.
There is also a second layering classification: lithosphere and asthenosphere.
Form groups of 2 or 3---NOT more---and tackle homework 11 problems 2--10 on the Earth's structure.
Discuss each problem and come to a group answer.
Let's work for 5 or so minutes.
See solutions 11.
The winners get chocolates.
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The electromagnetic radiation (EMR) flux above the atmosphere impinging on the Earth is on average 1367.6 W/m**2 (see NASA Earth fact sheet).
Answer 1 is right, but as we saw in
IAL 8: The Sun, the
solar constant
does vary.
The solar constant is defined to be the flux at the mean orbital radius of the Earth, and so does NOT have variation due to the eccentricity of the Earth's orbit. That variation is quite large, but averages out over a year.
But once the very short-time variations (due to sunspots mainly???) have been smoothed away the solar cycle variations are only of order 0.1 %.
Of course, very long term variations exist. Over centuries and over gigayears. Recall the Sun is has probably brightened by 30 % since 4.6 Gyr ago will probably brighten by 30 % more in the next 3.5 Gyr (WB-106).
The solar constant flux is captured by the Earth's circular cross-sectional area of pi*R**2 where R is the Earth's radius (Wikipedia: Sphere).
But to get the average energy flux at the top of the atmosphere, one must spread the captured power over the whole spherical surface area of 4*π*R**2 (Wikipedia: Sphere).
So the average flux at the top of the atmosphere is
solar constant * π*R**2 1367.6 W/m**2 * π*R**2 1367.6 W/m**2 ------------------------- = ---------------------- = ------------- ≅ 340 W/m**2 , 4*π*R**2 4*π*R**2 44π*R**2 is the surface area of the Earth.
About 30 % of this average flux is reflected by ground and Earth's atmosphere: see Wikipedia: Earth's energy budget and the figure below.
So the heating of the Earth's atmosphere and the Earth's surface is done by ∼ 240 W/m**2 of absorbed flux.
About 170 W/m**2 is absorbed by the Earth surface. This is an average value, of course. There is nothing at night and daytime absorption is very variable and depends on weather, latitude, topography, and other lesser features.
Solar power is the biggest of all renewable energy sources, and so eventually, I think, it must be the main energy source of the future---but when that future will arrive is very uncertain---20 years, 100 years?
The is no build up of heat energy on Earth: if there was temperature would increase forever.
We'll discuss how Earth's temperature is set below in the section The Greenhouse Effect.
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What of other heat flows to the
Earth surface?
The mean heat flow to the surface from the hot interior of the Earth is only about 0.08 W/m**2 (see Wikipedia: Earth's energy budget).
The tidal heating is 0.0059 W/m**2 (see Wikipedia: Earth's energy budget).
Answer 2 is right.
Clearly, geothermal heating is NOT what keeps us warm.
In the oceanic basins, the heat flux is about 0.099 W/m**2 and comes from the deep interior???.
I can't find the reference for these numbers at the moment, but I must have got them from somewhere once upon a time.
Some of it is residual heat energy from formation (∼ 50 %) and some is from radioactive decay past and present of radioactive isotopes (∼ 50 %) (see Wikipedia: Earth's internal heat budget).
The combined internal heat energy drives primordial-radiogenic heat geology (see also Wikipedia: Earth's internal heat budget: Radiogenic heat: Primordial heat).
The figure below explicates.
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Why is that past heat energy from
formation and past radioactive decay still there
inside the Earth?
It just takes a very long time for heat energy to leak out from the interior of a massive object.
Caption: Cooling time scale for rocky spherical bodies in the solar system.
Credit/Permission: © David Jeffery,
2003 / Own work.
Image link: Itself.
Size matters.
The calculation ignores many complicated effects (e.g., real structure and differences in composition), but it it gives the right scaling of the effect of size on cooling.
The amount of residual/radioactive heat energy that a rocky body has is important for its geological activity since the heat flow of this energy is a major geological driver.
Note there are two aspects:
Caption: A relative size comparison of the four rocky planets Mercury, Venus, Earth, and Mars and the rocky dwarf planet Ceres (which is also the largest asteroid).
The Venus image is false color and shows topography with the veil of Venusian atmosphere stripped away.
Unfortunately, the image of Venus of what the colors mean except that they show structure. The bright elongated horizontal region at the center is Aphrodite Terra which is one of the three Venusian terrae which are continent-like highlands.
The Mercury image is rather old since it cowlick of unimaged surface which has since been imaged by the NASA MESSENGER probe (MESSENGER Team Releases First Global Map of Mercury, 2009dec15
The Moon should be shown too since it also counts as a large rocky body. Its radius is 0.271 Earth radii which is NOT so much smaller than Mercury's radius of 0.3829 Earth radii.
Credit/Permission: NASA,
2009
(uploaded to Wikipedia
by User:Weirdoinventor,
2009) /
Public domain.
Image link: Wikipedia.
And apparently, its primordial-radiogenic heat geology is somewhat less than that of Earth. It's surface is renewed more often than once per gigayear, but this is about 5 times slower than the renewal time for most of the Earth's surface (CW-34).
Mars probably had primordial-radiogenic heat geology comparable to Earth's once. Now it is much slower, but probably NOT dead as older textbooks used to say.
Volcanic activity probably still occurs at a slow rate (HI-189; Baker, V. R. 2005, Nature, 434, 280).
They still have radioactive isotopes releasing heat energy, but it heat conduction to the surface releases it to the surface so quickly that it CANNOT drive primordial-radiogenic heat geology.
But they are all smaller than the Moon and long ago lost nearly all their residual heat energy.
Like the Moon and Mercury, they still have radioactive isotopes releasing heat energy, but it heat conduction to the surface releases it to the surface so quickly that it CANNOT drive primordial-radiogenic heat geology. .
As mentioned above, aluminum-26 (half-life 0.717 Myr) may have driven the primordial-radiogenic heat geology even on rapidly cooling small rocky bodies like asteroids.
The asteroids are now nearly geologically dead, except for impact geology, space weathering, and diurnal temperature cycle weathering. All very slow processes.
This is important for life on Earth. As dangerous as volcanoes and earthquakes are, we need primordial-radiogenic heat geology.
If primordial-radiogenic heat geology turned off, Earth's continents would erode and wash away leaving us with an ocean planet or WATER WORLD, but without Kevin Costner (1955--).
Also volcanic outgassing sustains the carbon dioxide (CO_2) over geological time scales. Without CO_2 gas in the atmosphere, life as we know it would cease.
Then we'd have an ocean planet, unless the water all disappeared first.
The idea of continenal drift (a key component of plate tectonics) was introduced in the early 20th century by Alfred Wegener (1880--1930) and only slowly gained traction.
The full theory of plate tectonics only emerged in the 1960s through the work of many including Canadian John Tuzo Wilson (1908--1993).
In one sense, it is surprising that it was so late since it is arguably the biggest of Earth's geological processes.
But the fact is that the motions of plate tectonics are excruciatingly slow on the human lifetime time scale---as most geological processes are---and the tectonic plates themselves are pretty thoroughly camouflaged.
So plate tectonics is NOT at all obvious.
Nowadays plate tectonics is a very robust theory. It's truth.
Of course, endless refinements continue to appear---and many of those have uncertain status: very strongly supported, possible, probably wrong, etc.
Here we can only give a simplified outline of plate tectonics.
To start with heat energy flows from hot to cold spontaneously.
The temperature of the interior of the Earth rises going inward up to maybe 5000 K in the center.
Thus heat flows outward in the Earth.
Some of the flow is by heat conduction: one atom's thermal kinetic energy being directly transferred to another or by FREE MOVING ELECTRONS in conductors.
This is why metal spoons are NOT good for stirring boiling water.
Answer 1 is right.
Material that is plastic can convect though very slowly compared to liquids. A plastic in this sense is a material that deforms permanently under an applied force (Wikipedia: Plasticity).
Actually, the combination of the Earth's rotation and the convection in the liquid iron outer core causes electric currents that drive a dynamo effect that creates the Earth's magnetic field. We discussed this briefly in IAL 8: The Sun. Somehow macroscopic kinetic energy gets turned into charge separation energy and then electrical currents flow which in turn generate magnetic fields.
As a recapitulation on convection, see the figure below.
In the asthenosphere (i.e., the lower
mantle), it is also believed that
the plastic mantle convects and the
convection cycle drives
plate tectonics.
It is thought there are huge if slowly cycling
convection cells in
mantle convection---the
time scale of a complete cycle thought to be
∼ 200 Myr
(see Wikipedia: Mantle convection:
Speed of convection)
See the cartoon of mantle convection
and mantle convection
videos in the figure below
and the computer simulation
of mantle convection in the figure below that.
The surface manifestation of the asthenosphere
convection cells
are the tectonic plates which consist of two
main regions:
oceanic basins and relatively high continents.
The surface manifestations of the boundaries between
asthenosphere
convection cells
are the
tectonic plate boundaries
There are three kinds as illustrated in the figure below.
Very hot lava
flows up from the deep asthenosphere and fairly
quiescently (SWT-571)
creates new crust in a relatively narrow valley
called a rift.
But volcanoes are possible too as in
Iceland (which
straddles a divergent boundary) demonstrates.
The rifts are typically bounded by twin ridges.
Most divergent boundaries are under the oceans and
are marked by giant mountain chain systems split by central rifts.
These systems are called mid-ocean ridges.
Basalts to have small crystals and be dark
(SWT-568).
As the material moves away from the mid-ocean rift (being
dragged by the motion of the underlying asthenosphere
convection cell), it cools
and grows more dense and at a convergent boundaries (i.e.,
subduction zones) it can slides
under another plate.
At oceanic
convergent boundaries there is usually a deep
oceanic trench.
Mariana Trench in Western Pacific Ocean is
an example: as mentioned above it is the deepest trench and drops to
to 11,035 meters below mean sea level
(Oceanclopedia: Mariana Trench).
Between continental tectonic plates there are no trenches
(SWT-594).
On subduction some subducting matter heats up from contact with
the asthenosphere and then becomes low
density molten rock
that rises and emerges as
lava
that forms volcanoes
SWT-594).
Volcanic mountain chains often form where a oceanic plate subducts
below an oceanic or continental plate: e.g., the Andes of South America and
the Cascades of Washington State
(SWT-594).
Caption: Zagros Mountains from space.
They straddle the Iran-Iraq border.
They are a fold mountain systems.
There is no perfect folding, of course. But you can mountains are stretched in one direction.
Those mountains are the upfolds.
Credit/Permission: NASA,
1992
(uploaded to Wikipedia
by User:Amizzoni~commonswiki,
2006) /
Public domain.
There are also FAULT-BLOCK MOUNTAINS that occur when
the crust
is fractured and one side is uplifted.
The Sierra Nevada Mountains in California are FAULT-BLOCK MOUNTAINS
(SWT-607).
Granites tend to have large crystals from slow cooling of
lava or magma
(CW-195).
They tend to be lighter in color ???? than basalts and less dense.
The continental material rose up above the oceanic crust because
of its lower density---which is
NOT full explanation of why
continents
form. (We discuss continents a little bit further below
in the section The Past and Future of Plate Tectonics.)
Note both continental and oceanic crusts are immensely complex
with all kinds of variations: the distinction into basaltic
and granitic just gives the main trend.
For example, the North American Plate and the Pacific Plate
have a transform boundary in California: the
San Andreas Fault.
Earthquakes near transform boundaries
are common
(SWT-595).
In simple terms, the tectonic plates try to slide past each other, but
are caught on each other by friction and other
forces??? and elastic potential energy builds
up until there is a sudden release of stored potential energy as
kinetic energy.
Everything moves and shakes and then
the transform boundary comes into
a new relatively stable configuration---for awhile.
The tectonic plate boundaries,
particularly on dry land, are NOT always obvious since surface material can camouflage them.
Of course, before modern geology,
they were NOT recognized at all as
tectonic plate boundaries.
The tectonic plate boundaries
do move around over geollgical time scale:
hence continental drift.
Tectonic plates grow, shrink, and move.
The tectonic plates have been largely mapped.
There are 17 or 18 (depending on how you count them)
minor plates:
some very large and some quite
See 15 of these tectonic plates
(including all the
But there are a few number that cross land for part of their length.
For example, the Mid-Atlantic Ridge (a divergent boundary)
crosses Iceland.
Credit/Permission:
U.S. Geological Survey (USGS),
before or circa 2005 /
Public domain.
The figure below shows that an actual
space image
of Iceland just shows the ice-cap and
the
tectonic plate boundaries
is NOT obvious.
Caption:
Iceland: Mid-Atlantic Ridge, volcanoes,
glaciers.
Credit/Permission: NASA,
before or circa 2005 /
Public domain.
But geologists can locate
tectonic plate boundaries
on the ground.
Caption: "Rock outcrop in Iceland,
a visible surface feature of the
Mid-Atlantic Ridge,
the easternmost edge of the North American plate.
A popular destination for
tourists
in Iceland."
You wouldn't know this is part of a
tectonic plate boundary
without other geological evidence.
It could be any old rock outcrop.
Credit/Permission: User:Pmarshal,
2006 /
Public domain.
Caption: Krafla volcano in
1984.
Krafla is on the
Mid-Atlantic Ridge as it crosses
Iceland.
Credit/Permission: Michael Ryan,
U.S. Geological Survey (USGS),
1984
(uploaded to Wikipedia
by User:Peko,
2006) /
Public domain.
Caption: "Lava flow during a rift eruption at
Krafla volcano,
northern Iceland,
1984."
Krafla is on the
Mid-Atlantic Ridge as it crosses
Iceland.
See also the eruption image at
USGS Krafl site.
Credit/Permission: Michael Ryan,
U.S. Geological Survey (USGS)
1984
(uploaded to Wikipedia
by User:Galar71,
2007) /
Public domain.
See Volcano videos
below/at link:
Caption: San Andreas Fault.
The San Andreas Fault
is one of the relatively few places where a
tectonic plate boundary
crosses land.
Lower California is will one day become an island.
For more information on the
San Andreas Fault, see
USGS The San Andreas Fault site
Credit/Permission:
U.S. Geological Survey (USGS),
circa or before 2005 /
Public domain.
But from the air, its linear form can be picked out.
See the two figures below.
Caption: San Andreas Fault.
This is an aerial view of the San Andreas Fault splitting
Carrizo Plain in the Temblor Range east of San Luis Obispo, California.
The San Andreas Fault is one of the relatively few places where
a tectonic plate boundary
crosses land.
Lower California is will one day become an island.
For more information on the San Andreas Fault, see
USGS The San Andreas Fault site
Credit/Permission: USGS.
circa or before 2005 /
Public domain.
Caption: "San Andreas Fault
in the Carrizo Plain, aerial view from 8500 feet altitude'' (November 2007nov16).
Double-click to see the high-resolution version.
From the air, the San Andreas Fault
is a striking geological feature---a sort of long double ridge with a narrow valley between the ridges.
But on the ground, it may just be landscape as far as the geologically challenged can see.
The tectonic plates
are jammed together and erosion has probably mostly covered the
surface fault---but miles down it reaches.
Credit/Permission: ©
Ian Kluft AKA User:Ikluft,
2007 /
Creative Commons
CC BY-SA 3.0.
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The ocean crustal material is dominated by basalt rocks.
These are relatively metal-rich (i.e., Mg, Al, Ca, Fe rich)
silicates that formed by fast cooling from
lava or magma
(CW-39, 195;
SWT-569).
The plate material at the present time
is created at about 2--4 cm/yr as as sea-floor and
satellite measurements show
(Se-434).
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The oceanic trenches
are deepest between two oceanic tectonic plates and less
deep between oceanic and continental tectonic plates.
The emergence is often eruptive.
When subduction occurs between two continental tectonic plates
the subducting plate often causes the overlying crust to buckle and fold
creating
fold mountain systems
like the Alps, Himilayas,
Appalachians, and Zagros Mountains
(SWT-594--595)
See figure below.
Image link: Wikipedia.
Fold mountain systems
can probably happen at other places than plate
boundaries. The Alps seem a bit remote from the boundary between
the African and Eurasian tectonic plates.
Near convergent boundaries,
stresses can build up that can be released suddenly in earthquakes.
The continental and continental shelf crustal material is dominated by
GRANITES.
These are relatively less metal-rich silicates than
basalts
and hence have more Si and O in their composition
(CW-39).
php require("/home/jeffery/public_html/astro/earth/geology/plate_tectonics/usgs_001_plates.html");?>
Most of the tectonic plate boundaries
are under the ocean.
Caption: Iceland straddling tectonic plate.
Image link: Itself.
Download site:
USGS: Understanding plate motions.
Image link: Itself.
Download site: NASA: Visible Earth
Alas, a dead link.
Image link: Wikipedia.
And there are frequent volcanic eruptions in Iceland
along the tectonic plate boundary.
See the two figures below.
Image link: Wikipedia.
Image link: Wikipedia.
php require("/home/jeffery/public_html/astro/earth/geology/volcano_videos.html");?>
A closer-to-home boundary is the transform boundary
between the North-American and Pacific Plates that runs up
through the Gulf of California and through California as the
San Andreas Fault before heading out to sea at San Francisco.
EOF
Image link: Itself.
Download site:
U.S. Geological Survey (USGS)
The San Andreas Fault is NOT obvious from the ground, unless
one knows the geological features to look for.
Image link: Itself.
Download site:
USGS:
photographer Robert E. Wallace, USGS
Image link: Wikipedia.
Form groups of 2 or 3---NOT more---and tackle homework 11 problems 11--17 on plate tectonics.
Discuss each problem and come to a group answer.
Oh, 5--10 minutes.
See solutions 11.
The winners get chocolates.
php require("/home/jeffery/public_html/astro/videos/ial_0000_standards.html");?>
php require("/home/jeffery/public_html/astro/videos/ial_010_solar_system_formation.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_hot_2.html");?>
We can make some general remarks to start with.
Ocean basin crust is typically about 200 Myr old (The Mountains of Wisconsin site).
Some continental material is significantly older: some is more than 3 gigayears old.
The oldest rock is zircon crystals from Western Australia that is dated to 4.4 Gyr (Wikipedia: Oldest dated rocks: Oldest terrestrial material).
We know that ocean basin crust is created at
divergent boundaries.
How is the continental crust created?
The continents were created by two uplifting processes:
accretionary wedges
and volcanism:
Caption: An accretionary wedge
at a subduction zone.
Continents
are built up by
accretionary wedges
and volcanism.
Credit/Permission:
United States Geological Survey (USGS),
USGS Earthquake glossary,
before or circa 2009
(uploaded to Wikipedia
by User:Miya,
2009) /
Public domain.
Both sedimentary rock from
accretionary wedges
(granitic rock)
and rock from
volcanoes that go into making
the continents are less dense than
oceanic sedimentary rock and deeper rock???.
Thus at convergent boundaries,
where continental and oceanic tectonic plates
meet the
continents tend to be the winners---they
tend to stay on top???.
So
continents tend NOT be subducted and recycled in
the giant mills of mantle convection.
The subduction rate of the continents is thus
much slower than that of the ocean basin crust.
This is why
continents are generally older than
ocean basin crust and
why
parts of continents can be very old.
Of course, continents do erode as well as get built
and so their surfaces can't last forever.
Their bits get washed in the ocean basins.
But it does seem that the total land area of the
continents has NOT changed radically in the last
225 Myr or so despite the
continents being shifted around a lot (see below).
So continent creation by
accretionary wedges
and volcanism and destruction by
erosion (and maybe sometimes when
continental tectonic plates meet at
convergent boundaries
by subduction)
must be roughly in balance????.
But the creation and destruction rates are both slower than for
ocean basin crust,
and so the continents
on average are much older than the
ocean basin crust.
The early past of the tectonic plates
gigayears ago is hard to trace.
Their motion, often called for historical reasons continenal drift,
is uncertain.
It is NOT really possible
I think to trace where the continents
were to such early times as 3 Gyr ago.
But some parts of the modern continents were around then.
The constant crustal creation and subduction has moved the
continents around too much to trace their locations back to 3 Gyr.
It has been possible to trace the continents back to 220 Myr ago
(i.e., 220 million years ago) using radioactive and magnetic
dating with some certainty
(CW-41).
Movements back to 600 Myr ago are known roughly
(WB-91).
Caption: A replica
of the fossil of the
dromaeosauridae
Sinornithosaurus
(specimen NGMC 91, nicknamed Dave) at the
American Museum of Natural History
in New York City.
Specimen NGMC 91 is about 75 cm long.
Sinornithosaurus was a small
feathered dinosaur.
Some Sinornithosaurus types may have
been able to glide
(see Wikipedia: Sinornithosaurus:
Feathers).
Sinornithosaurus
is NOT in the lineage of
birds---the
dinosaurs of our day.
Credit/Permission:
© User:Dinoguy2,
before or circa 2006 /
CC BY-SA 1.0.
Caption: Continenal drift
since 250 Myr Before Present (BP).
The geologic periods seen
are the
Permian
(298.9 ± 0.2 -- 252.2 ± 0.5 Myr BP),
Triassic
(252.2 ± 0.5 -- 201.3 ± 0.2 Myr BP),
Jurassic,
(201.3 ± 0.6 -- 145 ± 4 Myr BP)
Cretaceous,
(145 ± 4 -- 66 Myr BP)
and
Quaternary
(2.588 ± 0.005 -- present Myr BP)---our brief shining hour
upon the stage.
Continenal drift is, of course,
a major manifestation of
plate tectonics.
We see the breakup
of the
supercontinents
Pangaea,
Laurasia,
and Gondwana.
For further explication, see the
continental drift
videos in
Plate tectonics videos
below/at link:
Caption: The breakup
of Pangaea.
Credit/Permission:
U.S. Geological Survey (USGS),
2005
(uploaded to
by User:Nicke L,
2006) /
Public domain.
Continenal drift still goes on and can be monitored by precise
satellite measurements.
For example, the Atlantic Ocean is widening at about 3 cm/yr
(Ze2002-158).
Pointing out this factoid seems to be
de rigueur
when discussing plate tectonics: the textbooks all seem to mention it
(PF-95).
What is in store for the future?
Plate tectonic modeling suggests that most of the
continents
will ram together again to create a new supercontinent in
about 250 Myr
(WB-92).
Is there any fundamental reason for the arrangements of
continents and
tectonic plates as we see them?
Probably NOT.
Initial conditions on the early Earth were probably set randomly
and then evolved in a deterministic fashion, but over
the gigayears the motion may be so
super-sensitive to initial conditions that it may NOT be predictable in practice from
the initial conditions.
Image link: Wikipedia.
The first dinosaurs appeared about 250 Myr ago
and they extincted about 65 Myr ago
(Cox-250).
The image and animation below illustrate what we know of the
past of continenal drift.
Image link: Wikimedia Commons.
php require("/home/jeffery/public_html/astro/earth/geology/plate_tectonics/plate_tectonics_videos.html");?>
Credit/Permission:
USGS,
before circa 2005 /
Public domain.EOF
Image link: Itself.
Download site:
USGS: Historical perspective.
Image link: Wikipedia.
Question: What also grows at about 3 cm/yr?
Answer 2 is right.
Form groups of 2 or 3---NOT more---and tackle homework 11 problems 11--17 on plate tectonics.
Discuss each problem and come to a group answer.
Oh, 5--10 minutes.
See solutions 11.
The winners get chocolates.
php require("/home/jeffery/public_html/astro/videos/ial_0000_standards.html");?>
php require("/home/jeffery/public_html/astro/videos/ial_010_solar_system_formation.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_easter_bunny_2.html");?>
There are many other geological processes---and a lengthy discussion is beyond our scope---but we should mention a few main ones in brief.
Weathering of rock, erosion, and sedimentary rock formation which I collectively call EROSION GEOLOGY.
There is also hotspot volcanism (which is a feature of primordial-radiogenic heat geology and impact geology (i.e., cratering and weathering by impactors from space).
Liquid water can dissolve rock or provide a solution of other chemicals that dissolve rock.
Solid water (ice) can cause FROST WEDGING: water freezing in crevices expands and fractures.
Glaciers as they move can also break up rock. See figure below.
Wind is a much weaker than
water
in weathering in general, but
it is important particularly in arid areas like the American Southwest where
water weathering is comparatively week.
Wind weathering is very important on
Venus
and Mars
where there is no liquid water.
php require("/home/jeffery/public_html/astro/earth/geology/glacier_johns_hopkins.html");?>
Wind, especially when carrying dust or sand particles, can abrade rock surface.
Mars may have a
liquid water subsurface at least some of the time.
Intermittently and briefly, there may even be surface liquid water
today---but NOT likely.
See IAL 14: Mars: the Red Planet.
Liquid water can easily move dissolved material and fine particles. Larger particles or pebbles can be rolled along. Floods can move large objects up to the size of boulders.
Glaciers can move small and large rock fragments and boulders too.
Wind is much less powerful, but it can blow dust and sand around. In arid regions this is an important process.
Water usually and wind probably on average move rock material downhill depositing it lower regions and ultimately on the oceanic seafloor.
Both answers seem right to me.
The Earth would become an ocean planet eventually---if the water lasted so long.
The Mississippi Delta was created by erosion followed by geological deposition.
See the figure of the Mississippi Delta below.
php require("/home/jeffery/public_html/astro/earth/geology/mississippi_delta.html");?>
Metamorphic rock is formed from pressure and/or heating of igneous rock or sedimentary rock. But NOT heating to the point of melting.
The main rock types are related by the rock cycle which in earliest form was worked out by James Hutton (1726--1797), a Scottish physician and geologist (SWT-565).
Caption: The rock cycle of the Earth.
Credit/Permission: © David Jeffery,
2004 / Own work.
Image link: Itself.
But most sedimentary rock is mostly silicates (CW-199).
Answer 1 is right.
Common cementing agents are silica (SiO2), iron oxides, and calcium carbonate (CaCO_3) (SWT-577).
Calcium carbonate is particularly important.
This is a major process by which CO_2 is removed quasi-permanently from the atmosphere.
Only quasi-permanently since volcanic action can free CO_2 again by volcanic outgassing during lava flows.
Note the process requires liquid water.
But it's much more important than 5 % implies.
Sedimentary rock is the covering layer of about 75 % of the continents and apparently even more of the ocean basins (SWT-576).
Most of North America is covered by a veneer of sedimentary rock a few kilometers thick. In many parts of the continental interior sedimentary rock is almost all you see.
This is a pretty common sight in the American west. A famous example is the Grand Canyon (Se-437).
Caption: A view of the Grand Canyon presumably.
There was no caption at the USGS Grand Canyon site.
Credit/Permission:
U.S. Geological Survey (USGS),
before or circa 2004 /
Public domain.
Image link: Itself.
Studying the layers of rock (i.e., the strata), particularly sedimentary rock, is stratigraphy.
Studying the strata is a main way to learn about geological and biological history.
But there are places where the tectonic plates ride over point-like hotspots in the asthenosphere.
At a hotspot, a volcano can arise that is then shifted away from the hotspot by tectonic plate motion and becomes extinct.
The theorized origin of the hotspots are mantle plumes of hot rock that arise from the boundary of the outer core and the mantle.
However, since circa 2013, mantle plumes seem to be becoming a consensus theory:
But "the plume deniers are fading," says retired geochemist Stanley Hart (c. 1935--). "The major forces are getting older, and none of the young guys are picking it up." Plume critics aren't writing peer-reviewed papers, he says, so "I don't argue with them anymore; it's a distraction." (Slightly edited.)
Most seamounts are volcanic seamounts.
Caption: A diagram illustrating hotspot geology driven by mantle plumes.
A mantle plume from the outer core-mantle boundary rises (probably in a somewhat complicated fashion) through the asthenosphere (lower mantle in red), lithosphere (upper mantle in yellow) and the crust (dark yellow).
The asthenosphere and mantle plume are both red in the diagram. It may be that there is a broad pool of hot rock in the asthenosphere brought there by a superplume from the outer core-mantle boundary.
The surface and near surface manifestation of the mantle plume is a hotspot.
The lava from the mantle plume builds volcanoes.
As the crust slides over the hotspot due to plate tectonic motion, a series of volcanoes can be created.
The volcanoes become extinct after moving off the hotspot and erode away.
If the hotspot is in the ocean, the eroding volcanoes will eventually submerge and become volcanic seamounts. Seamounts are submerged oceanic mountains, most of which are volcanic seamounts.
Credit/Permission: User:Los688,
2008 /
Public domain.
Image link: Wikipedia.
There are lots of hotspots and therefore mantle plumes. See the diagram just below.
Caption: Mantle plume locations suggested by Gillian R. Foulger, 2010, Plates vs Plumes: A Geological Controversy.
Credit/Permission:
Gillian R. Foulger,
2011 /
Public domain.
Image link: Wikipedia.
The most famous hotspot is the one that has created the Hawaiian Islands and Emperor Seamount Chain (SWT-571).
The figure below shows images of the Hawaiian Islands and the Hawaii hotspot.
Another well known hotspot is the Yellowstone hotspot which creates a geologically active area in the vicinity of Yellowstone National Park under which the Yellowstone hotspot is centered---you know, Old Faithful, etc.
php require("/home/jeffery/public_html/astro/earth/geology/hawaii_hotspot.html");?>
Now there are two kinds of geologic craters that are quite different and have to be distinguished: volcanic craters and impact craters. Volcanic craters are on top of volcanoes and result from primordial-radiogenic heat geology. Impact craters arise from space debris (meteoroids, asteroids, and comets) falling on Earth.
We will discuss impactor physics and the distinction between impact and volcano craters in IAL 12: The Moon and Mercury and the current impactor danger to the Earth in IAL 16: Asteroids, Meteoroids, and Target Earth.
Most rocky/icy bodies in the solar system are heavily impact cratered, but Earth is NOT even though it has been impacted at a similar rate.
The absence of obvious impact craters is because of two reasons.
Thus, impact geology has been comparatively unimportant Earth since the heavy bombardment: during the heavy bombardment, impact geology was important on Earth.
Currently, there are 188 confirmed Earth impact craters as of late 2015 and other probable or possible ones (see Earth Impact Database).
But most of these craters are NOT obvious the way craters on the Moon are.
Erosion has degraded their appearance in many cases and in many cases they are pretty much buried by sediment.
There is a fairly obvious impact crater in Quebec. See the figure below.
php require("/home/jeffery/public_html/astro/earth/geology/crater_manicouagan.html");?>
There is hidden, but important,
impact crater off the
Yucatan Penisula.
For elucidateion, see the maps in two figures below.
Caption: "This shaded relief image of Mexico's Yucatan Peninsula show a subtle, but unmistakable,
indication of the Chicxulub impact crater.
Most scientists now agree that this impact was the cause of the
Cretaceous-Tertiary (K-T) extinction event,
the event 65 million years ago that marked the sudden extinction of the
dinosaurs as well
as the majority of life then on Earth."
You'd never know the Chicxulub crater was there
without some authority to tell you.
Credit/Permission: NASA,
2000
(uploaded to Wikipedia by
User:David Fuchs
2010) /
Public domain.
There are a few impact craters that look like impact craters.
The most famous is Meteor Crater (AKA
Barringer Crater) near
Winslow, Arizona.
Meteor Crater is about 50,000 years old, has a diameter of
about 1.2 km, and is about 180 meters deep.
The Meteor Crater impactor
was an iron-rich
meteoroid of about 50 m in diameter.
The impact energy was equivalent to about that
of 20-megaton H-bomb (FK-362).
The Meteor Crater impactor
itself was fragmented and spread about in the crater or
ejected out. There is no single big meteorite to be found it seems.
Credit/Permission:
US Geological Survey (USGS),
before or circa 2005 /
Public domain.
Caption: Meteor Crater in
Arizona.
More on Earth craters can be found
at the Geological Survey of Canada's
Earth Impact Database.
Credit//Permission: NASA,
before or circa 2005 /
Public domain.
Small impact craters get erased pretty quickly.
They can be just muddy holes in the ground like the
Carancas impact event crater.
The Carancas impactor
hit near Carancas, Peru,
2007 Sep15.
See the pictures at
Meteorite Recon: Carancas impact event (2007sep15).
See
Asteroid impact videos
below/at link:
Form groups of 2 or 3---NOT more---and tackle
homework 11
problems 15--19 on
plate tectonics and
volcanism
Discuss each problem and come to a group answer.
Oh, 5--10 minutes.
See solutions 11.
The winners get chocolates.
php require("/home/jeffery/public_html/astro/maps/map_mexico_cia.html");?>
Image link: Wikipedia.
Caption:
A USGS educational picture of Meteor Crater in Arizona.
Image link: Itself.
Download site:
USGS:
MeteorCrater.
Image link: Itself.
Download site: NASA: Visible Earth.
Alas, a dead link.
php require("/home/jeffery/public_html/astro/asteroid/asteroid_impact_videos.html");?>
EOF
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_swiss_3.html");?>
Group Activity:
php require("/home/jeffery/public_html/astro/videos/ial_0000_standards.html");?>
php require("/home/jeffery/public_html/astro/videos/ial_010_solar_system_formation.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_swiss_2.html");?>
The table below specifies this composition.
_________________________________________________________________________ Earth Atmosphere Composition _________________________________________________________________________ Gas Percentage by Mass Percentage by Number (%) (%) _________________________________________________________________________ N_2 (nitrogen) 75.52 77. O_2 (oxygen) 23.14 21. Ar (argon) 1.29 0.99 CO_2 (carbon dioxide) 0.05 0.033 Ne (neon) 0.0013 0.0018 He (helium) 7*10**(-5) 5.2*10**(-4) CH_4 (methane) 1*10**(-4) 1.5*10**(-4) Kr (krypton) 3*10**(-4) 1.1*10**(-4) H_2 (hydrogen) 5*10**(-5) O_3 (ozone) 4*10**(-5) N_2O (nitrous oxide) 3*10**(-5) CO (carbon monoxide) 1*10**(-5) NH_3 (ammonia) 1*10**(-6) H_2O (water vapor) 0.06 to 1.7 0.1 to 2.8 (Not counted in the atmosphere composition above.) _____________________________________________________________________________ References: Se-439, CW-296, and Cox-258 (but note Cox has the wrong exponents for some numbers). ____________________________________________________________________________Now lets expand a bit on the components of the composition of the Earth's atmosphere.
But N_2 is necessary for life: it is used making in many organic compounds.
Both biological and non-biological methods of fixing nitrogen (i.e., converted to ammonia or nitrates) exist: see, e.g., Wikipedia: Nitrogen cycle.
The main respiration reaction is C_6H_12O_6 + 6O_2 = 6CO_2 + 6H_2O + released energy where C_6H_12O_6 is glucose which is a sugar.(Fundamentals of Geology).
Photosynthesis provides plants with energy and also substance---their mass largely comes from carbon from the air (Photosynthesis: Discovery). The latter point means that the most biomass that is NOT water (e.g., much of us) comes from carbon from the air via photosynthesis. Plants photosynthesize, animals eat plants, then animals eat animals---and that's the food chain for you.
The main photosynthesis reaction is 6CO_2 + 6H_2O + light energy = C_6H_12O_6 + 6O_2 where C_6H_12O_6 is glucose which is a sugar.(Fundamentals of Geology).
The Earth's carbon cycle is illustrated in the figure below.
Most basically because salt sea water
gets converted into fresh water on land through the hydrological cycle:
i.e., evaporation from the sea and rain onto land.
Ozone absorbs ultraviolet light from the Sun that is dangerous
to organic life.
Caption: "Levels of ozone (O_3)
at various altitudes, and related blocking of several types of ultraviolet radiation.
The ozone concentrations
shown are very small, typically only a few molecules O_3
per million molecules of air. But these
ozone
molecules are vitally important to life because they absorb the biologically harmful ultraviolet radiation from the Sun.
There are three different types of ultraviolet (UV) radiation, based on the wavelength of the radiation.
These are referred to as UV-a, UV-b, and UV-c. The figure also shows how far into the atmosphere each
of these three types of UV radiation penetrates. We see that UV-c (red) is entirely screened out by
ozone
around 35 km altitude. On the other hand, we see that most UV-a (blue) reaches the surface, but it is
NOT
as genetically damaging, so we don't worry about it too much. It is the UV-b (green) radiation that can
cause sunburn and that can also cause genetic damage, resulting in things like skin cancer, if exposure to it is prolonged.
Ozone screens out most UV-b, but some reaches the surface. Were the ozone layer to decrease, more UV-b radiation would reach the surface,
causing increased genetic damage to living things."
DU/km are Dobson units per km---these are rather obscure units
related to ozone concentration.
Credit/Permission: NASA,
before or circa 2011
(uploaded to Wikipedia
by User:Hardwigg,
2011) /
Public domain.
Chlorofluorocarbons (CFCs) used in refrigeration and air condition
escape into the atmosphere and destroy ozone
(Se-440). They are being phased
out.
Unfortunately, H_2 (molecular hydrogen) can also destroy ozone.
If we convert to a hydrogen economy, then H_2 leakage may become
a significant environmental problem. The issue has been debated
intensely recently.
php require("/home/jeffery/public_html/astro/earth/atmosphere/carbon_cycle.html");?>
This variability is also vital for life on Earth.
Water vapor
is also the most important greenhouse gas
(SWT-507).
carbon dioxide)
is only the 2nd most important greenhouse gas.
Image link: Wikipedia.
Ozone at ground level produced by combustion in
industry and automobiles is also a pollutant and has
negative effects on the respiratory system.
See
How Ozone Pollution Works.
php require("/home/jeffery/public_html/astro/earth/atmosphere/atmosphere_structure.html");?>
Thus, the Earth's atmosphere has motion, both relatively steady and strongly vargying, and heat flow
The figure below illustrates the dynamic Earth atmosphere.
php require("/home/jeffery/public_html/astro/earth/atmosphere/hadley_cell.html");?>
It is the insulating effect of a planetary atmosphere that keeps the planet surface at a higher temperature than it would have if it simply re-emitted absorbed star flux like a single temperature radiator: i.e., a blackbody radiator.
The figure below explicates the greenhouse effect by an everyday life analogy.
php require("/home/jeffery/public_html/astro/thermodynamics/heat_flow.html");?>
The mean temperature of the
airless Earth with the same
albedo as the actual
Earth
AIRLESS EARTH is calculated to be -18° C (255 K):
see the figure below.
php require("/home/jeffery/public_html/astro/earth/temperature_effective.html");?>
Question: Would the Earth be more or less suitable for
its current life if its mean temperature were -18° C?
The biosphere as it now
exists is set up for a higher mean temperature, and so
wouldn't work well if the mean temperature were -18° C.
Answer 1 is right essentially.
Penguins might prefer -18° C.
Currently, the worldwide average is 15° C (288 K) (see Wikipedia: Instrumental temperature record: Absolute temperatures v. anomalies).
It's the Goldilocks predicament--- the greenhouse effect has to be just right---or at least it should stay mostly the way it has been through most of human history.
Since an early point in the Industrial Revolution (which arguably started in about 1700) circa 1750, CO_2 in the atmosphere has been increasing.
Caption: An animation of the first kind of steam engine, the Newcomen engine.
The Newcomen engine has the essential ingredient of all heat engines: a hot reservoir (the fire), a cold reservoir (the ambient air), and a working substance (water both in liquid and steam form).
Heat engines turn heat energy into mechanical energy, but never with 100 % efficiency as dictated by 2nd law of thermodynamics.
From the primitive Newcomen engine sprang the glory and misery of the modern world.
Credit/Permission: User:Emoscopes,
2006 /
Creative Commons
CC BY-SA 3.0.
Image link: Wikipedia.
See Industrial Revolution videos below/at link:
In 1750, CO_2 abundance was about 280 ppm (parts per million in air) which is about what it had been for millennia (and all of historical human history until 1750) (James F. Kasting, 1997 or after, The Carbon Cycle, Climate, And The Long-Term Effects Of Fossil Fuel Burning, scroll down ∼ 25%
Historical, CO_2 abundance values can be determined from air bubbles and other inclusions in ice cores drilled in Greenland, Antarctica, and other cold places.
See Ice core videos below/at link:
So here's what we find and what we predict for CO_2 abundance:
Caption: The historical record back to 1000 CE is from ice core measurements.
Direct measurements have been done since about 1960 when abundance was 315 ppm. The last direct measurement on this plot was from 2000 when the abundance was about 370 ppm.
In various scenarios CO_2 abundance is seen to rise by varying amounts by 2100 The IPCC states that much of the variation is due to varying human response.
Actually many people think there is more uncertainty than the IPCC has found.
In 2011, CO_2 is/was about 390 ppm (Carbon dioxide in Earth's atmosphere) and since 1960 it has risen by about 22% (Wikipedia: Keeling curve). Probably by about 2015, the abundance will go above 400 ppm---which will be a bit of a psychological threshold.
Some people believe that anything above 350 ppm is dangerous---and we need to scrub the atmosphere.
In 2003, the increase was 3 ppm which is a record increase (Carbon Dioxide Reported at Record Levels). The recent average increase has been about 1.8 ppm
The increase amount does fluctuate, but it is plausible that the 2003 increase was caused by increased fossil fuel burning in Asia, particularly China and India.
Credit/Permission: © Intergovernmental Panel on Climate Change (IPCC),
from publication
Climate Change 2001 - Synthesis Report, IPCC, SYR-Fig. 9-1a,
2001 /
IPCC
with correct credit.
Image link: Itself.
Below is a Keeling curve (carbon dioxide (CO_2) abundance versus time plot) showing the period 1958--date & Time in detail.
php require("/home/jeffery/public_html/astro/earth/atmosphere/keeling_curve.html");?>
There is no real doubt that the CO_2 increase is primarily
caused by the burning of fossil fuels (i.e., coal, oil, and natural gas).
When we burn these fuels we release CO_2 where the carbon component has been locked up in the ground as organic fossil fuel for geologically long times: millions to hundreds of millions of years???.
Of course, CO_2 is always being released and absorbed from the atmosphere by a variety of processes which collectively form the Earth's carbon cycle:
SOURCES
SINKS
The Earth's carbon cycle is illustrated in the figure below.
php require("/home/jeffery/public_html/astro/earth/atmosphere/carbon_cycle.html");?>
So it is NOT as simple as we burn fossil fuels
and increase CO_2.
But nevertheless, some fraction of the CO_2 we produce stays in the atmosphere for centuries: about 65 % according to some calculations (James F. Kasting, 1997 or after, The Carbon Cycle, Climate, And The Long-Term Effects Of Fossil Fuel Burning, scroll down ∼ 10%
The SIMPLE prediction is that if you increase a greenhouse gas, you should increase the greenhouse effect and cause global warming. This prediciton was first made by Svante Arrhenius (1859--1927) in 1896.
But climate is NOT simple.
There are all kinds of complex FEEDBACK MECHANISMS and also other effects such as increasing dust pollution which can increase reflection of sunlight and cause GLOBAL COOLING.
Nevertheless, CLIMATE MODELING does predict that increased CO_2 combined with other effects will lead to global warming---but there are still great uncertainties.
In the 20th century---of fond memory---it seems that the GLOBAL MEAN temperature rose by 0.6 ± 0.2 K (see Wikipedia: Instrumental temperature record).
It is NOT easy to meaure GLOBAL MEAN temperature especially going back in time, and so that value may be more uncertain than indicated.
The GLOBAL MEAN temperature can be reconstructed from historical measurements and natural records like ice cores from ice caps in Greenland, etc. See figure below:
Caption: Annual global mean temperature relative to the mean 1961--1990 temperature.
The grey region on the lower plot indicates the range of uncertainty???. It is considerable, and so it is NOT certain that temperatures were generally cooler over the 2nd millennium.
The shape of this curve has been called the hockey stick.
Since the curve was first published in about 1999, there have been many other calculations with widely different results for the past millennium. They all pretty much agree, however, that the Earth after 1980 has been warmer than any time since 1000 AD (reporter, 2005, Science, February 11, 307, 828).
Credit/Permission: © Intergovernmental Panel on Climate Change (IPCC),
from publication
Climate Change 2001 - Synthesis Report, IPCC, SYR-Fig. 2-3,
2001 /
IPCC
with correct credit.
Image link: Itself.
The GLOBAL MEAN temperature varition does NOT show, of course, the complex local temperature variation.
We must look at other data representations for that. See figure below:
Caption: Annual temperature trends all over the Earth for 1976 to 2000.
The dots indicate the increase in annual temperature per decade averaged over the period 1976--2000.
As one can see, temperature change has NOT been uniform, but has been rather complex. Some regions have even gotten colder.
Non-uniform change is probably what we can expect for the future.
Credit/Permission: © Intergovernmental Panel on Climate Change (IPCC),
from publication
Climate Change 2001 - Synthesis Report, IPCC, Fig. 2-6b,
2001 /
IPCC
with correct credit.
Image link: Itself.
Is the temperature change of the 20th and early 21st centuries anthropogenic or just a natural fluctuation?
The Intergovernmental Panel on Climate Change (IPCC) has given their conclusion. See figure below:
Caption: A comparison of models with measured temperature.
The comparison shows models with natural, anthropogenic, and combined forcing for 1860--2000.
The grey region for the model results indicates the range of uncertainty in the model predictions.
An objection in the past to computer model climate predictions was that they could NOT fit the PAST, and so how could you believe them for the FUTURE.
Well now they can fit the PAST.
But how many free parameters have been used?
You can always fit any curve if you adjust the unknown controls (i.e., free parameters). I guess one could find out.
But in any case, it is significant evidence that global warming (AKA anthropogenic climate change) is occurring.
The evidence has firmed up since this figure was generated 5 or so years ago.
There is still lots of uncertainty though.
It seems every year or so, there's a wups moment when something is discovered NOT to have been modeled adequately.
But so far those wups moments have NOT changed the qualitative conclusions much.
Credit/Permission: © Intergovernmental Panel on Climate Change (IPCC),
from publication
Climate Change 2001 - Synthesis Report, IPCC, SYR-Fig. 2-4,
2001 /
IPCC
with correct credit.
Image link: Itself.
What about the future?
Well the IPCC predicts that GLOBAL MEAN temperature will rise by 1.4--5.8 K in the time periond 1990--2100 based on detailed computer modeling.
Their range (1.4--5.8 K) is uncertain partially because of unknown response of human society to global warming---this is one of those complex feedback mechanisms mentioned above.
But, in fact, the temperature rise may be much more uncertain than what the IPCC has predicted.
The IPCC does their best, but there are objectors to their predictions outside (and even inside) their ranks---but the objectors go both ways---from "why worry" to "apocalypse now".
See figure below for the range of predictions of GLOBAL MEAN temperature.
Caption: IPCC predictions for temperature increase.
The temperatures are relative to the 1990 global mean temperature.
Under various circumstances the mean global temperature is seen to rise by varying amounts in the next 100 years.
The IPCC states that much of the variation is due to varying human response.
Actually many people think there is more uncertainty than the IPCC has found.
Credit/Permission: © Intergovernmental Panel on Climate Change (IPCC),
from publication
Climate Change 2001 - Synthesis Report, IPCC, SYR-Fig. 9-1b,
2001 /
IPCC
with correct credit.
Image link: Itself.
Climate is complex and subject to many natural processes which are NOT yet fully predictable.
If there is significant future global warming, what are some possible consequences?
But the overall biosphere is likely to be loser in that complex ecosystems might be devastated.
There already has been some significant sea level rise since 1880 and more is expected. See the figures below.
Caption:
IPCC predictions for the rise in mean sea level.
They present 6 models that depend on varying circumstances many
of which are under human control. There also uncertainty envelopes.
Many people believe that the uncertainties are greater than the IPCC estimate.
Credit/Permission: © Intergovernmental Panel on Climate Change (IPCC),
from publication
Climate Change 2001 - Working Group I - The Scientific Basis, IPCC, WG1 TS Fig. 24 /
IPCC
with correct credit.
Some low-lying island nations (and
southern Florida) could suffer severely with the maximum
sea level rise predicted in some scenarios.
Naturally, these places are NOT happy about this prospect.
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Image link: Itself.
What should be done? This is a policy question.
This is usually a slow leaking process, but over millions or billions of years an atmosphere can be diminished.
The lightest particles have the highest speed, and so have the strongest tendency to escape. Note the Earth doesn't have much molecular hydrogen gas or helium gas.
Earth rock contains volatile element atoms trapped in the rock. Small amounts compared to the rock/metal mass of the Earth, but enough to give us atmosphere and oceans.
These volatiles are continually released by volcanic activity: they tend to be released when the rock is melted and spewed on to the surface of the Earth.
This process is called volcanic outgassing.
This theory has had its ups and downs in acceptance.
At present, theory that comets from the Kuiper belt brought most water to Earth seems to be gaining ground.
See the figure below illustrating volcanic outgassing.
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Other elements like
argon (which is a
noble gas: it is a monatomic
gas) and nitrogen in compounds must have been outgassed
(Ze2002-161).
But how did we get to the happy atmospheric state we enjoy now?
Happiness being having a warm puppy---no, happiness
is having liquid water.
The Goldilocks principle has helped us:
"Primordial Venus is too hot for
liquid water."
"Primordial Mars (or Mars
sooner rather than later) is too cold."
"But primordial Earth is just right."
And liquid water
is all important for life as we know it
and it exists only in a narrow temperature
range and only if pressure is greater than about 0.01 atmospheres
(i.e., 1 atmosphere is current Earth sea level atmospheric pressure
which is about 10**5 Pascals.)
See this water phase diagram.
See references
Water health FAQ Frequently Asked Questions
and
Water for human beings
As you know some
life has evolved to live outside of the
ocean, but it maintains
its own internal ocean.
Another reason for needing liquid water is get rid of the large
CO_2 abundance and to prevent a
runaway greenhouse effect
(similar to what happened on Venus).
The early Earth needed more
greenhouse effect than now since the
Sun was about 30 % less bright 4.6 Gyr ago
(WB-106).
So considerable atmospheric
CO_2 was needed to keep
Earth warm enough for
life and liquid water.
The theoretical result that the Sun was about 30 % less bright 4.6 Gyr ago
(WB-106) seems as sure as a purely theoretical result can be.
But maybe there is something we do NOT understand about the
early Sun or the early.
Or maybe we don't understand the early Earth well enough.
The problem with making the theories consistent on this point is called the
faint early Sun paradox.
This is what happened to Venus which
has surface atmosphere pressure about 92 times that of Earth,
an atmosphere that is 96.5 % CO_2 by (number???),
and a mean surface temperature of 735 K (i.e, 462 C)---which as people always
note is hot enough to melt lead
(melting temperature 600.61 K at
Earth surface pressure and probably a bit higher on
Venus).
After early times when a CO_2 atmosphere first built up,
it seems that carbonate rock production has usually
outpaced CO_2
volcanic outgassing.
CO_2, which was once a dominant atmospheric species, has
dropped to being a trace species.
400 Myr ago, CO_2 abundance was still about 20 times what it is today
(WB-62), but even that was only about
0.7 % by number of the atmosphere.
Now it is only about 0.033 % by number.
Anthropogenic release of CO_2 from fossil fuels created
by ancient life
CANNOT stop the return of the glaciers.
But over a millennium
or two the CO_2 abundance would come back to
about where it was before the
Industrial Revolution (starting circa 1700),
because the oceans tend to absorb it up to
some saturation limit (Ka-60%).
So anthropogenic global warming is only a temporary problem.
It's NOT going to save us from the next
glacial period
of the current ice age (i.e., the Quaternary glaciation)
which started circa 2.58 Myr ago and isn't over as far as we know
(WB-76).
We are just in an interglacial period
which started circa 12,000 years ago
and may well end in a few thousand more
(WB-77).
See the figure below for some details of the
Quarternary glaciation (AKA the current ice age).
On a much longer time scale than that of the current ice age,
the Sun is still brightening---as the figure below illustrates---and
thus increasing the heating the Earth.
Another doomsday scenario---there
are so many of them.
Where did the O_2 come from?
First, note that O_2 is a highly reactive gas and would relatively
quickly disappear over megayears or hundreds of megayears
if it weren't being continually produced
(HI-125).
It reacts to form oxides like rust
(HI-125).
Our current oxygen-rich atmosphere could never be established
from volcanic outgassing???.
Yes, plants gave rise to the condition that brought about
.
Of course, rather inert N_2, NOT
O_2, is the dominant species presently.
N_2 must have taken over as dominant species as
CO_2 dropped due to carbon becoming locked in
carbonate rock
and also due to O_2 production from plants.
Water was produced by
volcanic outgassing
or a hard comet rain as noted above and mostly
became our oceans.
What is the fate of water?
At present, up to about 12 km above sea level---the exact altitude varies with latitude---is
the troposphere.
The troposphere is
lower atmosphere in which our weather occurs
(WB-138).
The upper boundary of the troposphere is
the tropopause.
There is relatively little mixing of air through the tropopause.
At the top of the troposphere
the temperature is about 208 K
(WB-137) and
water vapor that
air can contain there is very tiny.
Most water will condense out and fall inside the troposphere.
Thus, convection cells
in the troposphere can't carry water up to great
altitudes above the troposphere.
But the order of one gigayear, the brightening of the Sun will cause the
mean global surface temperature to reach about 70 degrees C (or 340 K)
and then the troposphere will extend to over 100 km in altitude
(WB-129,138)
which means (although we omit the argument) that
water vapor will be convected up to over 100 km in altitude????.
At those altitudes, UV radiation from the Sun is available to break
H_2O into H and O.
A much greater fraction of H atoms will then escape the atmosphere by
having escape velocity than they do now.
Caption: A cartoon of Earth losing its water in the future.
Credit/Permission: © David Jeffery,
2003 / Own work.
The future conditions of the Earth after massive
H_2O loss begins are
hard to predict, but the oceans might be gone by 1.5 Gyr ????
(WB-142).
Some liquid water could remain due to the still-ongoing
volcanic outgassing
(WB-141).
And so some primitive life could persist in
ponds.
But sooner or later the Earth will get too hot for
liquid water and
life will end---at least
life as we know it.
Perhaps some exotic evolution will preserve complex life---or
hyper-intelligent beings of the future might transcend mere physical limitations.
When the liquid water gone or sufficiently gone,
CO_2---which disappeared
into calcium carbonate deposition mainly
in liquid water---could make a
comeback.
Volcanic outgassing
without competition from calcium carbonate
deposition could give the
Earth a rich CO_2
and then Earth would go the way of
Venus.
Of course, the end of the Sun's
main sequence life as discussed in
IAL 9: The Life of the Sun
spells the doom of the Earth anyway in 5 to 6 Gyr.
"Hm" said Goldilocks:
The temperature
of the early Earth did cool enough for
liquid water to
exist: thus much of the water vapor atmosphere became the
oceans.
Question: Why is liquid water important for
life as we know it?
All the answers are right and, in fact, are NOT independent.
Actually, getting the early Earth warm enough
for liquid water has proven a challenge for the
Earth modelers.
But volcanic outgassing just
keeps producing CO_2,
aeon after aeon.
An aeon is an indefinitely large period of time in
casual discourse.
In geology,
an eon it is formally the 2nd
largest division of geologic time comprising two or more eras and lasting half a gigayear or more.
In fact, Earth history seems to be divided into only
4 eons: going back in time we have the
Phanerozoic (0.570--present),
Proterozoic (2.50--0.570 Gyr),
Archean (4.0--2.5 Gyr)
and
Priscoan (4.57--4.00 Gry)
(Cox-249).
If there was no major quasi-permanent CO_2
... All hope abandon ye who enter here."
Such characters in colour dim I mark'd
Over a portal's lofty arch inscrib'd:
---Dante Alighieri (1265--1321), Inferno, transl. H. F. Cary
(Project Gutenberg's Dante's Inferno).
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But, as we noted above,
CO_2 dissolved in
liquid water can be broken up
and the carbon deposited in carbonate
rock (i.e.,
rock
containing calcium carbonate (CaCO_3)).
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If we burnt all oil, natural gas, and coal at once, then we'd
increase the CO_2 abundance by a factor of 8
(Ka-25%).
php require("/home/jeffery/public_html/astro/earth/geology/quarternary_glacial.html");?>
php require("/home/jeffery/public_html/astro/sun/sun_evolution.html");?>
The increased heating of
the Earth will increase rate of the chemical
weathering that locks CO_2 up in
carbonate rock.
Like most chemical reactions, chemical weathering should
speed up with increased heat.
So the CO_2 will continue to drop and it has been estimated
(with great uncertainty) that in 0.5 to 1 Gyr,
CO_2 will drop too
low for land plant photosynthesis and land
life will come to an
end (WB-109).
Question: What did give rise to most of the O_2 in the
atmosphere?
It is thought that O_2 began a rapid rise about 2.5 to 2 Gyr ago
(HI-125).
Answer 3 is right.
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Image link: Itself.
Then there are 1970s rock bands . . .
But to conclude, the Earth is an evolving place.
Anatomically modern humankind have been around only since ∼ 200--300 kyr BP (see Wikipedia: Anatomically modern human; Wikipedia: Jebel Irhoud), but even that has seen two or three interglacials including the present one (see Wikipedia: Interglacial).
But the really big changes, we only contemplate philosophically. They are outside of the time scale of human society.
We breach the time barrier to the far past and far future---deep time---only by scientific inference.
Form groups of 2 or 3---NOT more---and tackle
homework 11
problems 25--30 on
the greenhouse effect.
Discuss each problem and come to a group answer.
Oh, 5--10 minutes.
See solutions 11.
The winners get chocolates.
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Group Activity:
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php require("/home/jeffery/public_html/astro/videos/ial_010_solar_system_formation.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_fountain_2.html");?>