Lecture 11: The Earth

Don't Panic

Sections

Return to Earth

A picture which never gets tired: "It is time for you to return to Earth"
( Andrei Tarkovsky's Solaris ).

Credit: NASA: Image #AS11-44-6549.

Credit: NASA: Image #AS17-148-22742.

One of the few images showing a full Earth. Blue oceans, anticyclones, To the south Antarctica and Madagascar. At the northern edge: India, Asia, Europe. Greece, Israel, Arabia, Egypt. Somewhere near the center is Oldavai Gorge.

Credit: NASA: Image #EL-1996-00155.

North America from Apollo 16, 1972 April 16.

You can see the Baja Peninsula (which is about 1200 km long), the Rocky Mountains, and, towards the top, Lakes Superior and Michigan.

Credit: NASA: Image #AS16-118-18873.

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. The I-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. UNLV is probably off the picture south of the word Las Vegas.

Credit: NASA: ISS EarthKam.

Shape, Size, Mass, Density

To very good approximation the EARTH is a sphere.

There are mountains and continents due to geological processes that we will discuss, but the range from highest to lowest points is only about 20 km.

Highest point: Mt. Everest rising to 8850 meters above mean sea level (Wikipedia: Mt. Everest; National Geographic: Press release on a new Everest height).

Lowest point: the Mariana Trench in Western Pacific Ocean dropping to 11,035 meters below mean sea level (Oceanclopedia: Mariana Trench).
Additionally, the Earth is slightly oblate (Cox-240).
- Equatorial radius: 6378.136 km
- Polar radius: 6356.753 km
- 6378.136-6356.753 = approx 21 km

But basically the EARTH is a sphere.

Gravity has pulled it into this shape.

Question:

Gas pressure.
The magnetic force of the Earth's magnetic field.
The pressure force of the solid and fluid components of the Earth.

Answer 3 is right.

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 gravity.

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.

TIDAL FORCE

CENTRIFUGAL FORCE

Question:

CENTRIFUGAL FORCE

The centrifugal force is a force that turns on whenever a system starts rotating. It trys to throw objects straight outward from the center of rotation. The centrifugal force of the Earth's own rotation causes the Earth to be oblate.
The centrifugal force is not a real force. It is just the tendency of a body to move in a straight line as experienced in a rotating frame. But the centrifugal force acts just like a body force (i.e., a force that pulls atom by atom) in that rotating frame. The centrifugal force of the Earth's own rotation causes the Earth to be oblate.

We can determine the mass of the Earth from law of gravity and Newton's 2nd law (i.e., F=ma).

Determining the Earth's mass.

Density is the ratio of mass to volume.

We can calculate the Earth's mean density. (Which is a very, very cruel density.)

Determining the Earth's mean density.

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 Interior of the Earth

The deepest drilling has only gone a few kilometers down.

And yet we think that we have a reasonable 1st order understanding of the interior structure of the Earth.

This understanding has had to be inferred from indirect evidence: density, primordial solar nebular composition, seismology, heat flow from the interior, and modeling.

DENSITY: 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.
COMPOSITION: 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.
CHEMICAL DIFFERENTIATION also has affected Earth.
For example, consider the most abundant dense refractory element iron?
- The mass fraction of iron in crustal rock varies, but is of order 10 % (Cox-253). (Another source gives 5 % [SWT-271]).
- But the primordial solar nebular composition makes iron as abundant as silicon (see Andrew Cowley's primordial solar abundance plot) which makes up more than 50 % of crustal rock by mass (Cox-253).
- Of course, the answer we already know. The denser substances mostly sank during the CHEMICAL DIFFERENTIATION phase of the early molten Earth.
- Thus we infer that the core of the Earth must be mainly iron even though we've never seen a sample of it and won't probably ever.
- Uncompressed iron density is about 8 g/cm**3. The center density of the Earth from modeling is estimated to be 13 g/cm**3. But the center pressure is estimated to be 3.6*10**6 atmospheres. Even iron compresses under such pressure.
SEISMOLOGY: By studying the propogation of seismic waves from EARTHQUAKES plus modeling the interior Earth layers (and their temperature, pressure, phase) can be inferred.
The phase information is particularly interesting: the seismic wave study allows us to see where the interior solid and where liquid.
HEAT FLOW AND TEMPERATURE: We can measure these directly at the surface and to some depth.
MODELING: One has to construct computer models of the Earth that match the observations from seismology and measured heat flow using the materials inferred from the primordial solar composition.
The Earth model must satisfy 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.

A cartoon of the interior of the Earth (HI-118; Ze2002-160).

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.

A cartoon of the Earth's interior temperature from modeling (Se-431).

The CRUST by direct inspection is mostly SILICATES: i.e., rock composed mainly of Si and 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).

The relative composition by mass of the Earth's crust (SWT-271).

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 CORE, liquid and solid, is probably mainly iron (Fe), but other materials like sulfur (S) and nickel (Ni) are also probably major components (CW-52).

There is also a second layering classification: lithosphere and asthenosphere.

The LITHOSPHERE is the rigid crust and upper mantle down to about 100 km (Se-433). The lithospheric material is relatively rigid. Lithos means stone in ancient Greek.
The mantle below the lithosphere is the asthenosphere. The material of the asthenosphere is technically solid, but it is believed to be sufficiently hot (i.e., more than 500 K: Se-431) that is rather plastic and can flow over long periods of time.

Heat Flow and Heat Sources

The atmospheric temperature, most??? of the oceanic temperature, and the very thin uppermost continental ground temperature are determined by heat from the Sun.

The EMR flux above the atmosphere impinging on the Earth is on average 1366 W/m**2 (Cox-340).

Question:

The solar constant.
The solar variable.
The solar wind.

Answer 1 is right, but as we saw in IAWL Lecture 8: The Sun, the solar constant does vary a bit both on short time scales and the time scale of the sunspot cycle of 22 years.

But once the very short-time variations (due to sunspots mainly???) have been smoothed away the sunspot cycle variations are only of order 0.1 %.

Of course, very long term variations probably 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 flux that is absorbed by the ground is about 700 W/m**2 (CW-46). The rest of the solar constant is reflected or absorbed in the upper atmosphere.

The flux absorbed by the ground is converted to heat energy which is eventually mostly radiated to the air again as infrared radiation.

The mean heat flow to the surface from the hot interior of the Earth is only 0.08 W/m**2 (CW-46).

Question:

The geothermal flux.
The solar flux.
Neither.

Answer 2 is right.

Clearly, geothermal heating is not what keeps us warm.


        700        pi*R**2             700           1
    ________   * ____________   =   _______   *  _________   .

      0.008       4*pi*R**2          0.008           4

CW-46

The geothermal energy on the continents and continental shelves is primarily due to radioactive materials in the crust itself. The outflow is about 0.057 W/m**2.

In the oceanic basins the heat flux is about 0.099 W/m**2 and comes from the deep interior.

What is the deep internal source?

Some of it is residual heat from formation and some is from radioactive sources past and present. The exact amount of each seems uncertain although probably comparable????.

Why is that past heat still there?

It just takes a very long time for heat to leak out from the interior of a massive object.

Size matters.

The residual/radioactive heat energy content is roughly proportional to volume or radius to the 3rd power.
But the rate at which heat flows out of sphere is roughly proportional to surface area or radius to 2nd power.

The cooling time scale grows as radius R according to this crude result.

The calculation ignores many effects: e.g., real structure and differences in composition, but it it gives the right direction of the effect.

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.

EARTH is the largest rocky body and has the most active RESIDUAL/RADIOACTIVE-HEAT GEOLOGY: i.e., volcanoes, plate tectonics, and earthquakes.
VENUS' radius is about 0.8 of the Earth's, and thus has a somewhat shorter time scale for cooling.
And apparently, its RESIDUAL/RADIOACTIVE-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' radius is 0.5326 of the Earth's (Cox-295), and thus has significantly shorter time scale for cooling.
Mars probably had RESIDUAL/RADIOACTIVE-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).
The MOON and MERCURY have radii of about that are about 1/4 of and 2/5 of Earth's, respectively. They seem to have been dead in regard to RESIDUAL/RADIOACTIVE-HEAT GEOLOGY for billions of years.
Some of the large ASTEROIDS it seems did have RESIDUAL/RADIOACTIVE-HEAT GEOLOGY at one time (Se-565).
But they are all smaller than the Moon and long ago lost nearly all their residual/radioactive heat energy and became nearly geologically dead except for METEORITIC GEOLOGY.

RESIDUAL/RADIOACTIVE-HEAT GEOLOGY

This is important for life on Earth. As dangerous as volcanoes and earthquakes are, we need RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.

If RESIDUAL/RADIOACTIVE-HEAT GEOLOGY turned off, Earth's continents would erode and wash away leaving us with a WATER WORLD.

And indeed RESIDUAL/RADIOACTIVE-HEAT GEOLOGY might slow tremendously and turn-off PLATE TECTONICS (which we discuss below) on the time scale of a gigayear---but this is very uncertain (WB-146).

Then we'd get a WATER WORLD, unless the water all disappeared first.

Plate Tectonics

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 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.]

The Earth being very opaque does NOT allow radiative transfer by electromagnetic radiation.

Question:

Yes, because some of the interior is liquid.
No, because some of the interior is liquid.
Yes, because the interior is entirely rigid solid.

Answer 1 is right.

It is believed that there is CONVECTION in the liquid iron outer core

Se-432

DYNAMO EFFECT

IAWL Lecture 8: The Sun

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.

Credit: U.S. Geological Survey (USGS). Their images are mostly public domain.

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 plate boundaries. There are three kinds:

DIVERGENT BOUNDARIES (or rifts) where new crustal material is being formed.
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 Iceland (which straddles a divergent boundary) demonstrates.
The rifts are typically bounded by twin ridges.
Most divergent boundaries are under the oceans, and hence one talks of MIDOCEAN RIFTS AND RIDGES.
The midocean ridges are giant submerged mountain chains split by midocean rifts.
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).
As the material moves away from the midocean 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.
CONVERGENT BOUNDARIES (or subduction zones) are where crust disappears. One of the plates slides beneath the other.
At oceanic convergent boundaries there is usually a deep 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).
The trenches are deepest between two oceanic plates and less deep between oceanic and continental plates.
Between continental 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).
When subduction occurs between two continental plates the subducting plate often causes the overlying crust to buckle and fold creating FOLD MOUNTAIN CHAINS like the Alps, Appalachians, and Himalayas (SWT-594--595).
Near subduction zones, stresses can build up that can be released suddenly in EARTHQUAKES.
TRANSFORM BOUNDARIES are where plates just slide against each other.
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 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 quakes and the boundary comes into a new relatively stable configuration---for awhile.

Credit: U.S. Geological Survey (USGS). Their images are mostly public domain.

The plate boundaries, particularly on dry land, are not always obvious since surface material can camouflage them.

But plates have been largely mapped. There are about 20 plates: some very large and some quite small (SWT-591).

I count only 15 on the map below.

Credit: U.S. Geological Survey (USGS). Their images are mostly public domain.

Most of the boundaries are under the ocean.

But a fair number that cross land.

For example, the Mid-Atlantic Ridge (a divergent boundary) crosses Iceland.

Credit: U.S. Geological Survey (USGS). Their images are mostly public domain.

An actual space image of image of Iceland just shows the ice-cap and the plate boundary isn't obvious.

Credit: NASA: Visible Earth.

But there are frequent volcanic eruptions in Iceland.

Krafla Volcano is on the Mid-Atlantic Ridge as it crosses Iceland.

Credit: Gudmundur E. Sigvaldason, Nordic Volcanological Institute, Reykjavik, Iceland . I assume that Dr. Sigvaldason will allow educational use of his image without express permission. It is displayed at this USGS site.

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.

The San Andreas Fault is one of the relatively few places where a 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: U.S. Geological Survey (USGS). Their images are mostly public domain.

The SAN ANDREAS FAULT is NOT obvious from the ground, unless one knows the geological features to look for.

But from the air, its linear form can be picked out.

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 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: U.S. Geological Survey (USGS): photographer Robert E. Wallace, USGS. USGS images are mostly public domain.

Credit: NASA: Visible Earth.

Past and Future of the Tectonic Plates

We can say some general things to start with.

Ocean basin material is typically about 200 Myr old (The Mountains of Wisconsin site).

Some continental material can be significantly older: some is more than 3 gigayears old.

The oldest rock is zircon crystals from Western Australia that is dated to 4.3 Gyr (CW-39).

The continents were created by two processes: ACCRETIONARY WEDGES and VOLCANISM.

ACCRETIONARY WEDGES: At a subduction sedimentary rock (which we discuss below) is scraped off the subducting plate and builds up as an accretionary wedge (The Mountains of Wisconsin site).
VOLCANISM: The volcanoes that form near a subduction zone come from molten subducted rock. Rock that is more rich in silicon-oxygen than basaltic rock melts more easily and is less dense. Thus, the volcanic rock produced by volcanoes tends to be more silicon-oxygen rich and less dense than the basaltic rock that goes to form most of the ocean basin crust ( The Mountains of Wisconsin site).

ACCRETIONARY WEDGES

VOLCANISM

Continents have a tendency to keep growing???, but there must be some limiting process to keep the whole Earth surface from becoming continental.

The early past of the tectonic plates gigayears ago is hard to trace.

It is not really possible I think to trace where the continents were to such early times as 3 Gyr ago.

The constant crustal creation and subduction has moved the continents around too much.

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).

Cox-250

Credit: U.S. Geological Survey (USGS). Their images are mostly public domain.

CONTINENTAL 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).

Question:

Human hair.
Human fingernails
Dandelions.

Answer 2 is right.

Pointing out this factoid seems to be de rigueur when discussing plate tectonics: the textbooks all seem to mention it (PF-95).

Plate tectonic modeling suggests that most of the continents will ram together again to create a new super-continent in about 250 Myr (WB-92).

Is there any fundamental reason for the arrangements of continents and plates as we see them?

Probably not.

Initial conditions on the early Earth were probably set randomly and then evolved in a mostly deterministic fashion, but over the gigayears the motion may be chaotic.

Other Geological Processes

Plate tectonics is the biggest of all geological processes.

It resurfaces the ocean floors.
It builds continents through accretionary wedges and volcanism.
It builds mountains through volcanism and folding and faulting processes.

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 RESIDUAL/RADIOACTIVE-HEAT GEOLOGY) and METEORITIC GEOLOGY (i.e., cratering by impactors from space).

Weathering of Rock

On Earth, the main culprit is water: liquid or solid.

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.

Credit: National Oceanic and Atmospheric Administration/Department of Commerce: Image ID: corp1862, NOAA Corps Collection; Location: Alaska Southeast; Photo Date: August 1991; Photographer: Commander John Bortniak, NOAA Corps.

Wind, especially when carrying dust or sand particles, can abrade rock surface.

Wind is a much weaker than water in weathering in general, but on planets like Mars and Venus where there is no liquid water, wind weathering must be relatively important.

IAWL Lecture 14: Mars: the Red Planet

Erosion

Apparently, geologists like to think of erosion as weathering PLUS downward transport (leveling) by liquid water, glaciers, and wind.

Liquid water can easily move dissolved material and fine particles. Larger particles or pebbles can be rolled along.

Glaciers can move small 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.

Question:

RESIDUAL/RADIOACTIVE-HEAT GEOLOGY

Nothing much would change anytime soon, but volcanoes, uplift and earthquakes would stop.
The continents would not be renewed and over hundreds of millions of years would wear away.

Both answers seem right to me.

The Earth would become a WATER WORLD eventually.

But although erosion ultimately trys to flatten everything, it does along the way create structures too: e.g., rivers, valleys, river deltas, alluvial (i.e., sedimentary) plains, and sedimentary rock (which we discuss below).

The Mississippi Delta is formed from alluvial deposits of soil from the Mississippi River. The Army Corps of Engineers and others spend much of the 20th century trying to straighten the channels in the Delta for flood control among other things. The result of straighter channels is less deposition and the delta is eroding away. The sediments that used to build up the Delta get deposit out in the deep Gulf of Mexico. By 2100 the New Orleans could submerged. So now people are unstraightening the channels.

Credit: NASA: ISS EarthKam.

Sedimentary Rock Formation

SEDIMENTARY ROCK is one three main classes of rock. The other two we somehow skipped mentioning before.

IGNEOUS ROCK froms from lava or magma (which is underground lava).

METAMORPHIC ROCK is formed from pressure and/or heating of igneous 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 (1727--1797), a Scottish physician and geologist (SWT-565).

The rock cycle for Earth.

But most sedimentary rock is mostly silicates (CW-199).

Question:

depositions of sediments of rock in water.
depositions of sentiments of rock in water.
secondary volcanic activity.

Answer 1 is right.

Common cementing agents are silica (SiO_2), iron oxides, and calcium carbonate (CaCO_3) (SWT-577).

CALCIUM CARBONATE (CaCO_3) is particularly important.

LIQUID WATER

CW-193

RC-88

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 outgassing during lava flows.

Note the process requires LIQUID WATER.

SEDIMENTARY ROCK

SWT-576

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.

SEDIMENTARY ROCK

This is a pretty common sight in the American west. A famous example is the Grand Canyon (Se-437).

There was no caption at the USGS Grand Canyon site.

Credit: U.S. Geological Survey (USGS). Their images are mostly public domain.

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.

Hotspot Volcanism

Volcanoes mainly form near plate boundaries where there is direct upflow in rifts or upflow from heating as most rock descends to the hot asthenosphere at subduction boundaries.

But there are places where the 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 plate motion and becomes extinct.

Hotspots can create chains of volcanoes: in the oceans these will become island and seamounts (submerged volcanic mountains).

The most famous hotspot is the one that has created the Hawaiian Islands and what is called the Emperor Seamount Chain (SWT-571).

The Big Island (Hawaii Island) is obvious. Oahu (with Honolulu with Pearl Harbor) is the 3rd island from the left.

Credit: NASA: Visible Earth.

Meteoritic Geology on Earth

METEORITIC GEOLOGY is caused by meteoritic impacts, large and small, and is evidenced by IMPACT CRATERS and

regolith

Now there are two kinds of geologic craters that are quite different and have to be distinguished: VOLCANO CRATERS and IMPACT CRATERS. VOLCANO CRATERS are on top of volcanos and result from residual/radioactive-heat geology. IMPACT CRATERS arise from space debris falling on Earth: meteoroids, asteroids, and comets.

We will discuss impactor physics the distinction between impact and volcano craters in IAWL Lecture 12: The Moon and Mercury and current impactor danger to the Earth IAWL Lecture 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.

Small impactors tend to burn up in the atmosphere, and so small craters and REGOLITH (fragments of rock broken up by micrometeoritic impactors) tend not to form on Earth. You can have some, of course, but they don't last long because of the 2nd reason.
Crustal renewal by plate tectonics and erosion tend to quickly eliminate craters, particularly the smaller ones.

VENUS

gas giants

Thus, METEORITIC GEOLOGY has been comparatively unimportant Earth since the heavy bombardment: during the heavy bombardment METEORITIC GEOLOGY was important on Earth.

Currently, about 170 craters are known on Earth and new ones are still being discovered (Reimold, W. U. 2003jun20, Science, 300, 1889).

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.

The Chicxulub crater straddles the northern coast of the Yucatan Peninsula with center just east?? of Progreso. It is centered near the village of Chicxulub. (Chicxulub is pronounced chick-shoe-lube I believe.)

The Chicxulub crater is 170 km in diameter and is the 3rd largest crater known on Earth. But it is entirely covered by sediments.

It was discovered by finding shock-exposed rock and subsequent geological investigation.

The Chicxulub impactor hit about 65 million years ago and probably the caused a mass extinction at the end of the Cretaceous period that included the extinction of the dinosaurs. The impactor may have touched off world-wide firestorms??? and caused dust in that atmosphere that a multi-year winter (Se-574).

Credit: Central Intelligence Agency (CIA); download site Perry-Casta~neda Library Map Collection University of Texas Austin. Most of the maps are in the public domain and can be downloaded. There are historical maps.

You may have thought the CIA were spys, but actually they make maps---I think, explains a great deal.

There are a few impact craters that look like impact craters.

The most famous is METEOR CRATER (or 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 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 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: U.S. Geological Survey (USGS). Their images are mostly public domain.

More on Earth craters can be found at the Geological Survey of Canada's Earth Impact Database.

Credit: NASA: Visible Earth.

Earth Atmosphere Composition

The composition of the Earth's atmosphere is a key component of the Earth and the

biosphere

_________________________________________________________________________

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_20 (water vapor)         0.06 to 1.7                   0.1 to 2.8 

_____________________________________________________________________________

References: Se-439, 
            CW-296, and
            Cox-258
            (but note Cox has the wrong exponents for some numbers).


____________________________________________________________________________

MOLECULAR NITROGEN (Ni_2): As you can see most of atmosphere is N_2 (diatomic nitrogen) which is chemically rather inert. We breathe it in and out all the time without any effect one way or the other.
But N_2 is necessary for organic life: it is used in many organic compounds.
Both biological and non-biological methods of fixing nitrogen (i.e., converted to ammonia or nitrates) exist: see, e.g., The Nitrogen cycle and Nitrogen fixation.

OXYGEN (O_2) is needed for respiration by animal and plant life.

     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).

CARBON DIOXIDE (CO_2) is vital for plant photosynthesis and also as a greenhouse gas. See, e.g., The Carbon Cycle.

      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).

WATER VAPOR (H_2O) is highly variable since it readily condenses and evaporates to liquid and solid form under Earth atmosphere conditions.
Water vapor is also the most important greenhouse gas (SWT-507). Carbon dioxide is only the 2nd most important greenhouse gas.
OZONE (O_3) although a trace gas is important in the stratosphere where it there is a concentration in the Ozone Layer at about 25 km (Se-439).
Ozone absorbs ultraviolet light from the Sun that is dangerous to organic life.
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.
NOBLE GASES argon (Ar), neon (Ne), helium (He), and krypton (Kr) are largely inert.
OTHER TRACE GASES probably have some role, but we ignore them now and I don't know anyway what those roles are.

The Greenhouse Effect

About 40 % of the solar EMR flux at the top of the atmosphere reaches the ground.

The other 60 % is reflected or absorbed in the atmosphere.

About 15 % of the solar EMR flux hitting the ground is just reflected.

Thus, about 25 % of the solar EMR flux is absorbed by the ground and heats it up.

Now all the heat energy going into the ground must come out again or the ground keeps heating up without limit.

The heat energy is radiated from the ground as INFRARED (IR) RADIATION.

IR RADIATION

The mean temperature of the AIRLESS EARTH is calculated to be 255 K or -18 degrees C (Ze2002-156).

Question:

Less.
More.
Neither more nor less.
Both more and less.

Anyway 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 degrees C.

Penguins might prefer -18 degrees C.

INSULATION

Water vapor is the major insulator, CO_2 is a secondary one (SWT-507), and there are a few other minor helpers---these are the famous GREENHOUSE GASES.

The GREENHOUSE GASES absorb IR emitted by the ground and warm up. This provides us with an extra warming layer and raises the near surface temperatures over what they would be absent the GREENHOUSE GASES.

Still all heat energy that comes in, goes out---but we reach a higher equilibrium temperature than without GREENHOUSE GASES.

A similar situation holds for a house and its insulation in cold winter conditions.

Steady-state for a heated house.

The GREENHOUSE GASES cause the mean Earth surface temperature to be a 288 K or +15 degrees C (Ze2002-156). This means much of the Earth is pretty comfortable for life: at least the kinds of life we currently have.

So the GREENHOUSE EFFECT is good.

Greenhouse: How Do Greenhouses Work?

The Greenhouse Effect and Climate Change

But if the GREENHOUSE EFFECT is basically good for us, why does it have such a bad rep?

It's the GOLDILOCKS SITUATION: GREENHOUSE EFFECT has to be just right---or at least it should stay mostly the way it has been through human history.

Since the Industrial Revolution circa 1750, CO_2 in the atmosphere has been increasing.

In 1750, CO_2 abundance was about 280 ppm (parts per million in air) which is about what it had been for millennia (i.e., all of historical human history at least) ( Ka-25%).

Historical, CO_2 values can be determined from air bubbles??? in ice cores drilled in Greenland, etc.

The historical record back to 1000 AD 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.

Under various circumstances the CO_2 abundance 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.

Currently, the CO_2 abundance is about 379 ppm and since 1960 it has risen by about 17.3 %. (PF-98; Carbon Dioxide Reported at Record Levels; Mauna Loa Observatory of NOAA).

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: Intergovernmental Panel on Climate Change (IPCC).

There is no real doubt that the CO_2 increase is 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.

SOURCES

Respiration.
Decomposition of organic materials including human deforestation.
Fossil fuel burning.

SINKS

Photosynthesis which locks carbon up in plants and the animals that feed on them.
Dissolution in the oceans ultimately leading to some carbon from CO_2 being bound up in sedimentary rock as calcium carbonate (CaCO_3) (CW-193).

So it is not as simple as we burn fuel 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 ( Ka-10%)

The NAIVE prediction is that if you increase a greenhouse gas, you should increase the greenhouse effect and cause GLOBAL WARMING.

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 it seems that the GLOBAL MEAN TEMPERATURE rose by 0.6 +/- 0.2 K (Ka-5%, NOAA: Global Warming Frequently Asked Questions ).

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.

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: Intergovernmental Panel on Climate Change (IPCC).

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: Intergovernmental Panel on Climate Change (IPCC).

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.

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.

Credit: Intergovernmental Panel on Climate Change (IPCC).

What about the future?

Well the IPCC predicts that GLOBAL MEAN TEMPERATURE will rise by 1.4--5.8 K from 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.''

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: Intergovernmental Panel on Climate Change (IPCC).

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?

Well global mean temperature rise can have a large range of varying consequences depending on region: some regions will get hotter than the average; some colder.
Ecologies will be disrupted which usually means there will be winners and losers in the natural world.
Agriculture might suffer overall although in some regions (e.g., Siberia) there might be pluses.
Probably some of the world's ice caps will partially melt increasing sea level by perhaps 0.09--0.88 meters in the time frame ( NOAA: Global Warming Frequently Asked Questions).
Some lowlying island nations could suffer severely with the maximum sea level rise. Naturally, they are not happy about this prospect.

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: Intergovernmental Panel on Climate Change (IPCC).

Some think that radical action should be taken.
Some think the science is still too uncertain to take radical action yet.
Some think we should just adapt come what may. This is the way we've always reacted to climate change in the past.

Intergovernmental Panel on Climate Change

NOAA: Global Warming Frequently Asked Questions

EPA's Global Warming Site

GLOBAL WARMING: Early Warning Signs

The Carbon Cycle, Climate, And The Long-Term Effects Of Fossil Fuel Burning

The Evolution of the Earth's Atmosphere

The Earth at formation was probably too hot and too often impacted by large protoplanets and planetesimals to hold much of an atmosphere or ocean.

The hotter gas atoms and molecules are the higher their speed. In the low density upper atmosphere, the hottest particles can reach ESCAPE VELOCITY and just fly off into space.
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.
Major impacts can literally shock the atmosphere off a planet (Ahrens lecture, 2003oct).

But after the Earth cooled off a bit and the last of the big impacts was over an atmosphere could grow up.

Where did it come from?

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.

The early very active volcanism of the Earth probably produced a dense early atmosphere of H_2O vapor (perhaps 58% by mass) and CO_2 (perhaps 24 % by mass) (

HI-124

[Recently, it has been argued that hydrogen gas (H_2) might have made up 30 % of the early atmosphere and that his gas would have promoted organic molecule formation (Hecht, J. 2005, New Scientist, April 16, 17).

A cartoon of volcanic outgassing.

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?

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.''

LIQUID WATER

And LIQUID WATER is all important for life 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.

Question:

Water is an important facilitator of biological processes and the medium in which many of them occur.
Much of living tissue is actually just water. Humans are about 70 % water by mass.
All life as we know it depends on liquid water and cannot exist in its absence.

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 an EXTREME GREENHOUSE EFFECT.

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.

But VOLCANIC OUTGASSING just keeps producing CO_2 eon after eon.

Cox-249

CO_2 SINK

EXTREME GREENHOUSE EFFECT

This is what happened to VENUS which now approximates hell.

---Dante Inferno, transl. H. F. Cary ( Project Gutenberg's Dante's Inferno).

Dante & Virgil in Hell.

LIQUID WATER

After early times when a CO_2 atmosphere first built up it seems that carbonate rock production has usually outpaced CO_2 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.

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%).

But over a millennium or two the CO_2 abundance would come back to about where it was before the Industrial Revolution (circa 1750), because the ocean tends 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 glaciation of current ICE AGE (the Pleistocene Ice Age) which started circa 2.5 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).

There are interglacials every 100,000 years or so (WB-76).

ICE AGES

In any case, there is probably more than one cause of the various ICE AGES.

Nine major ICE AGES have been identified in the geological record (Cox-251).

ICE AGE

This will increase rate of the chemical weathering that locks CO_2 up in carbonate rock.

WB-109

Another doomsday scenario.

Where did the molecular oxygen (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 outgassing.

Question:

It fell in from outer space.
Spontaneous generation.
It is a product of photosynthesis.

Answer 3 is right.

Yes plants gave rise to the condition that brought about herbivores.

The unhappy consequences for plants of photosynthesis.

HI-125

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 as noted above and mostly became our oceans.

What is the fate of water?

At present up to about 12 km above sea level is the TROPOSHERE.

The TROPOSHERE is lower atmosphere in which our weather occurs (WB-138).

At the top of the TROPOSHERE 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 TROPOSHERE.

Thus convection cells in the TROPOSHERE can't carry water up to great altitudes.

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 TROPOSHERE will extend to over 100 km in altitude (WB-129,138).

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.

A cartoon of Earth losing its water in the future.

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 transcent mere physical limitations.

Of course, the end of the Sun's Main Sequence life as discussed in IAWL Lecture 9: The Life of the Sun spells the doom of the Earth anyway in 5 to 6 Gyr.

Conclusion

We could go on discussing Earth topics---like the weather, but everyone talks about the weather.

Then there are 1970's rock bands . . .

But to conclude, the Earth is an evolving place.

HUMANKIND has only been around for a relatively brief phase (i.e., since about 160,000 years for sure [Gibbons, A. 2003jun13, Science, 300, 1641]; since about 195,000 years with some probability [Fleagle, J., et al. 2005, Nature]), but even that has seen two interglacials including the present one.

But the really big the changes we only contemplate. They are outside of the time scale of human society.

We breach the time barrier to the far past and future only by scientific inference.