IAL 11: The Earth

Don't Panic

Sections

  1. Return to Earth
  2. Shape, Size, Mass, Density
  3. The Interior of the Earth
  4. Heat Flow and Heat Sources
  5. Plate Tectonics
  6. Past and Future of the Tectonic Plates
  7. Other Geological Processes
  8. Weathering of Rock
  9. Erosion
  10. Sedimentary Rock Formation
  11. Hotspot Volcanism
  12. Impact Geology on Earth
  13. Earth Atmosphere Composition
  14. The Structure of the Earth's Atmosphere
  15. The Dynamic Earth Atmosphere
  16. The Greenhouse Effect
  17. The Greenhouse Effect and Climate Change
  18. The Evolution of the Earth's Atmosphere
  19. Conclusion

  1. Return to Earth

    2. See the figures below:

    The Earth from Apollo 17, 1972dec07 Caption: Earth from Apollo 17, 1972 Dec07.

    Credit/Permission: NASA, NASA: Image #AS17-148-22742, 1972 / Public domain.
    Image link: Itself.

     North America from Apollo 16, 1972 April 16. Caption: North America from Apollo 16, 1972 Apr16.

    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.

    nevada_lasvegas.jpg Caption: Las Vegas, Nevada.

    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.

    See Earth zoom in videos below/at link:

    Earth, the one and only.

    All other planets are failed Earths.

  2. Shape, Size, Mass, Density

    3. To very good approximation the Earth is a sphere.

    1. Peaks and Valleys:

      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:

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

        See Mount Everest videos below/at link:

      2. Lowest point: the Mariana Trench in the western Pacific Ocean dropping to 11,035 meters below mean sea level (Oceanclopedia: Mariana Trench).

    2. The Oblate Earth:

      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.

    3. The Spherical Earth and the Oblate Earth:

      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.

      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.

      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.

    4. The Earth's Mass and Density:

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

      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.

      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.

  3. The Interior of the Earth

    4. As mentioned above, the deepest point on Earth is the Mariana Trench which reaches to 11.035 km below mean sea level.

    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).

    And yet despite our measly direct observation of the Earth's interior, we think that we have a reasonable 1st order understanding of the Earth's interior structure.

    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:

    1. The Earth's 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 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.

    2. The Earth's 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.

      But what of the elements that stayed on Earth and chemically differentiated?

      For example, consider the most abundant dense refractory element iron (Fe):

      1. The mean mass fraction of iron in Earth's crust is ∼ 5 % (see Wikipedia: Crust: Composition).

      2. But the primordial solar nebula composition has iron slightly more abundant than silicon which has mass fraction ∼ 27.7 % in the Earth's crust (see Wikipedia: Crust: Composition).

      3. Where Devil is the iron that was once part of the Earth's surface near the time of formation?

      4. Of course, the answer we already know. The denser substances mostly sank---like Satan never to hope again---during the chemical differentiation phase of the early molten Earth.

      5. 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.

      6. 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. The center pressure is modeled to be 3.6*10**6 atmospheres (Wikipedia: Inner core) which is about 3.6*10**6 times surface air pressure. Even iron compresses under such pressure.

    3. Seismology:

      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:

        EOF

    4. Heat Flow and Temperature in the Earth:

      We can measure heat flow and temperature directly at the surface and to some depth (Wikipedia: Geothermal gradient).

    5. Other Evidences Too Numerous To Mention:

      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.

    6. Modeling:

      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.

    The result of all our evidence and modeling gives the Earth's interior structure as made of layers: the crust, mantle, liquid outer core, and solid inner core.

    These layers are shown in the figure below.

    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.

    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).

    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.

    1. The lithosphere is the rigid crust and upper mantle down to about 100 km (Se-433). The lithosphere material is relatively rigid. Lithos means stone in ancient Greek.

    2. The mantle below the lithosphere is the asthenosphere. The material of the asthenosphere is technically solid, but it is sufficiently hot (i.e., more than 500 K: Se-431) that it is rather plastic (in the sense of being permanently deformable: see Wikipedia: Plasticity) and flows over long periods of time.



  4. Heat Flow and Heat Sources

    5. The atmospheric temperature, most??? of the oceanic temperature, and the very thin uppermost continental ground temperature are determined by electromagnetic radiation (EMR) from the Sun (which becomes heat energy) and the greenhouse effect---which we discuss below in the section The Greenhouse Effect.

    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).

    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                 4
    4π*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.

    The flux absorbed is converted to heat energy which is eventually radiated to back to space as infrared (IR) radiation.

    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.

    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).

    Clearly, geothermal heating is NOT what keeps us warm.

    What is the deep internal source of heat?

    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 figure below explicates.

    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.

    Size matters.

    1. The residual/radioactive heat energy content of a spherical body is roughly proportional to volume or radius to the 3rd power.

    2. But the rate at which heat flows out of sphere is roughly proportional to spherical surface area or radius to 2nd power.

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

    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:

    1. Larger rocky bodies have lost past residual/radioactive heat energy more slowly.

    2. They lose current production more slowly. An asteroid has long-lived radioactive isotopes just like a large rocky body. But the heat energy just flows out rapidly by heat conduction and doesn't drive geological activity.

    The figure below compares the sizes of some rocky bodies. Their RESIDUAL-RADIOGENIC HEAT GEOLOGY scales roughly with their size.

    1. Earth is the largest rocky body and has the most active RESIDUAL-RADIOGENIC HEAT GEOLOGY: i.e., volcanoes, plate tectonics, and earthquakes.

    2. Venus's radius is about 0.8 of the Earth's, and thus has a somewhat shorter time scale for cooling.

      And apparently, its RESIDUAL-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).

    3. Mars's radius is 0.5326 of the Earth's (Cox-295), and thus has significantly shorter time scale for cooling.

      Mars probably had RESIDUAL-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).

    4. 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-RADIOGENIC HEAT GEOLOGY for billions of years.

      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 RESIDUAL-RADIOGENIC HEAT GEOLOGY

    5. Some of the large asteroids it seems did have RESIDUAL-RADIOGENIC HEAT GEOLOGY at one time (Se-565).

      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 RESIDUAL-RADIOGENIC HEAT GEOLOGY.

      As mentioned above, aluminum-26 (half-life 0.717 Myr) may have driven the RESIDUAL-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.

    Size matters: Earth's large size has maintained its RESIDUAL-RADIOGENIC HEAT GEOLOGY.

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

    If RESIDUAL-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.

    And indeed RESIDUAL-RADIOGENIC 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 have an ocean planet, unless the water all disappeared first.

  5. Plate Tectonics

    6. Plate tectonics is relatively new theory.

    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).

    Why did plate tectonics arise so late in the modern world?

    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.

    1. Heat Flow:

      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.

        Metals (in the ordinary sense of the word, NOT the astrophysical sense) are good heat conductors for the same reason they are good electrical conductors: they have free moving electrons. The electrons transport charge necessarily, but they also transport kinetic energy: i.e., heat energy.

        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: Can convection can happen inside the Earth?

        1. Yes, because some of the interior is liquid and some of it is rather plastic.
        2. No, because some of the interior is liquid.
        3. Yes, because the interior is entirely rigid solid.











        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).

      In the Earth, it believed that there is convection in the liquid iron outer core (Se-432) and in the Earth's mantle part which is the asthenosphere (see the subsection Convection in the Asthenosphere below).

      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.

    2. Convection in the Asthenosphere:

      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.

    3. Tectonic Plates and Plate Boundaries:

      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.

      1. Divergent boundaries 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 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.

        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 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.

      2. Convergent boundaries (or subduction zones) are where crust disappears in a process called . One of the tectonic plates slides beneath the other.

        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).

        The         oceanic trenches are deepest between two oceanic tectonic plates and less deep between oceanic and continental tectonic plates.

        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).

          The emergence is often eruptive.

          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).

        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.

          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.

          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).

        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).

          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.

      3. Transform boundaries are where tectonic 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 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.

    4. Where are the Tectonic Plates?

      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

      Most of the       tectonic plate boundaries are under the ocean.

      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.

      The figure below shows that an actual space image of Iceland just shows the ice-cap and the tectonic plate boundaries is NOT obvious.

      But geologists can locate tectonic plate boundaries on the ground.







      And there are frequent volcanic eruptions in Iceland along the tectonic plate boundary. See the two figures below.







      See Volcano videos below/at link:

        EOF

      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 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. See the two figures below.















        san andreas fault

        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.
        Image link: Wikipedia.

  6. Past and Future of the Tectonic Plates

    7. So we have heard of plate tectonics---what of the past and future?

    1. General Remarks:

      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).

    2. Continent Creation:

      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:

      1. Accretionary wedges: At a subduction, sedimentary rock (which we discuss below) is scraped off the subducting tectonic plate and builds up as an accretionary wedges (The Mountains of Wisconsin site).

      2. Volcanism: The volcanoes that form near a subduction zone come from molten subducted rock. This rock that is more rich in silicon-oxygen, melts more easily and is less dense than basalt rock. that forms most of the ocean basin crust. It tends to be granitic rock.

      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.

    3. The Past:

      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).

      The image and animation below illustrate what we know of the past of continenal drift.

      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).

    4. The Future:

      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).

    5. Why are They Where They Are?

      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.

  7. Other Geological Processes

    8. Plate tectonics is the biggest of all geological processes.

    1. It resurfaces the ocean floors.
    2. It builds continents through accretionary wedges and volcanism.
    3. 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-RADIOGENIC HEAT GEOLOGY) and impact geology (i.e., cratering and weathering by impactors from space).

  8. Weathering of Rock

    9. 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. See figure below.

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

    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.

  9. Erosion

    10. 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. 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.

    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 was created by erosion followed by geological deposition.

    See the figure of the Mississippi Delta below.

  10. Sedimentary Rock Formation

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

    There is, of course, all kinds of sedimentary rock.

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

    Wherever sediments can accumulate---but usually under water---compression and natural "cement" can squeeze them into a solid.

    Common cementing agents are silica (SiO2), iron oxides, and calcium carbonate (CaCO_3) (SWT-577).

    Calcium carbonate is particularly important.

    Sedimentary rock usually occurs relatively near the surface where it forms and is only about 5 % of the Earth's crust (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.

    If layers of sedimentary rock are cut through by erosion, then the layers are displayed.

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

    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.

  11. Hotspot Volcanism

    12. Volcanoes mainly form near tectonic plate boundary where there is direct upflow in rifts or upflow from heating as most rock descends to the hot asthenosphere in subduction zones (i.e., convergent boundaries).

    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.

    Hotspots can create chains of volcanoes: in the oceans these will become islands and volcanic seamounts.

    The figure below shows the creation of hotspot volcanoes by a mantle plume.

    There are lots of hotspots and therefore mantle plumes. See the diagram just below.

    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.

  12. Impact Geology on Earth

    13. Impact geology is caused by impactors, large and small, from space and is evidenced by impact craters and regolith (which is the broken up and shocked materials from micro-meteoritic impacts).

    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 RESIDUAL-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.

    1. Small impactors tend to vaporize due to atmospheric ram pressure 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.

    2. Crustal renewal by plate tectonics and erosion tend to quickly (on geological time scales) eliminate craters, particularly the smaller ones.

    Crustal renewal processes are geologically rapid on Earth, Venus (but less than on Earth), and on some of the moons of the gas giants, particularly Io and Europa. They are slow on other rocky/icy bodies.

    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.

    There is hidden, but important,     impact crater off the Yucatan Penisula. For elucidateion, see the maps in two figures below.

    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.

    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:

      EOF

  13. Earth Atmosphere Composition

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

    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.

    1. Nitrogen (N_2): As you can see most of atmosphere is N_2 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 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.

    2. Oxygen (O_2) is needed for respiration by animals and plants. 

           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).

    3. Carbon dioxide (CO2) is vital for plant photosynthesis and also as a greenhouse gas (see Wikipedia: Carbon cycle).

      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.

    4. Water vapor (H_2O gas) is highly variable since it readily condenses and evaporates to liquid and solid form under the conditions of the Earth's atmosphere.

        This variability is also vital for life on Earth.

        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.

      Water vapor is also the most important greenhouse gas (SWT-507). carbon dioxide) is only the 2nd most important greenhouse gas.

    5. Methane CH_4 is also an important greenhouse gas although only a trace gas.

    6. Ozone (O_3) although a trace gas is important in the stratosphere where 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.

        File:Ozone_altitude_UV_graph.svg

        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.
        Image link: Wikipedia.

      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.

        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.

    7. NOBLE GASES argon (Ar), neon (Ne), helium (He), and krypton (Kr) are largely inert chemically. So they do NOT do much. However the study of them in isotope analysis is often important in understanding planetary system processes.

    8. OTHER TRACE GASES probably have some role, but we ignore them now and I don't know anyway what those roles are.

  14. The Structure of the Earth's Atmosphere

    15. The structure of the Earth's atmosphere is illustrated in the figure below.

  15. The Dynamic Earth Atmosphere

    16. The Earth's atmosphere exhibits both large-scale fluid dynamics and large-scale varying thermodynamics.

    Thus, the Earth's atmosphere has motion, both relatively steady and strongly vargying, and heat flow

    The figure below illustrates the dynamic Earth atmosphere.

  16. The Greenhouse Effect

    17. What is the greenhouse effect?

    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.

    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.

    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.

    Penguins might prefer -18° C.

    Currently, the worldwide average is 15° C (288 K) (see Wikipedia: Instrumental temperature record: Absolute temperatures v. anomalies).

  17. The Greenhouse Effect and Climate Change

    18. But if the greenhouse effect is basically good for us, why does it have such a bad rep?

    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.

    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:

    Below is a Keeling curve (carbon dioxide (CO_2) abundance versus time plot) showing the period 1958--date & Time in detail.

    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

    1. respiration.
    2. Decomposition of organic materials including human deforestation.
    3. Fossil fuel burning.
    4. volcanic outgassing. A slow, long-term process.

    SINKS

    1. Photosynthesis which locks carbon up in plants and the animals that feed on them.
    2. 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). A slow, long-term process.

    The Earth's carbon cycle is illustrated in the figure below.

    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:

    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:

    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:

    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.

    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?

    1. 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.

    2. Ecosystems will be disrupted which usually means there will be winners and losers in the natural world.

      But the overall biosphere is likely to be loser in that complex ecosystems might be devastated.

    3. Agriculture might suffer overall although in some regions (e.g., Siberia) there might be pluses.

    4. Ocean water will expand with heating and 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).

      There already has been some significant sea level rise since 1880 and more is expected. See the figures below.

        /atmosphere/ipcc_sealevel_future.jpg

        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.
        Image link: Itself.

      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.

    What should be done? This is a policy question.

    1. Some think that radical action should be taken.

    2. Some think the science is still too uncertain to take radical action yet.

    3. Some think we should just adapt come what may. This is the way we've always reacted to climate change in the past.

    There are lots of sites on global warming: e.g.,

    1. Intergovernmental Panel on Climate Change, NOAA: Global Warming Frequently Asked Questions,
      GLOBAL WARMING: Early Warning Signs.

    2. The article James F. Kasting, 1997 or after, The Carbon Cycle, Climate, And The Long-Term Effects Of Fossil Fuel Burning is out of date, but presents useful discussion of the science and economics.

    3. Indoor carbon dioxide levels could be a health hazard, Guardian, 2019jul08: 1000 ppm could be bad for human and animal health, and many human and city environments could reach that level by 2100 all the time.

    4. Deepak Ray, Climate change is affecting crop yields and reducing global food supplies, 2019 July 9: Climate change is decreasing food yields and increasing world hunger since 2014 or so.

    5. See Global warming videos below/at link:

  18. The Evolution of the Earth's Atmosphere

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

    1. 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.

    2. 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.

    1. Where Did the Earth's Atmosphere 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.

        There is a theory that comets brought significant volatile material, particularly water, to the Earth in some early phase---"a hard rain fell".

        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.

      The early very active volcanism of the Earth (or maybe a hard comet rain) 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).

      See the figure below illustrating 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).

    2. Liquid Water:

      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:

      The temperature of the early Earth did cool enough for liquid water to exist: thus much of the water vapor atmosphere became the oceans.

      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.

    3. Another Reason for Needing Liquid Water:

      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.

        Actually, getting the early Earth warm enough for liquid water has proven a challenge for the Earth modelers.

        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.

      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 , eventually Earth would have had runaway greenhouse effect.

      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).

      Venus now approximates Hell.

      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)).

      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.

    4. The Return of the Glaciers:

      Anthropogenic release of CO_2 from fossil fuels created by ancient life CANNOT stop the return of the glaciers.

      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 (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).

    5. What's in Store in Next Gigayear for CO_2?

      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.

      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).

      Another doomsday scenario---there are so many of them.

    6. Where Did the O_2 Come From?

      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???.

      It is thought that O_2 began a rapid rise about 2.5 to 2 Gyr ago (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.

    7. The Fate of Water:

      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.

      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.

  19. Conclusion

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

    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.