The sequence of events (and resulting lithologies) described last lecture has been called a Supercontinent Cycle (or a Wilson Cycle, after J. Tuzo Wilson, who linked spreading centers and subduction zones).

Wilson Cycles are the largest scale manifestations of crustal cyclical processes. They represent the union of plate tectonics, orogenesis, volcanism, sedimentology, and other aspects of both continental and oceanic lithospheric geology (and ultimately links to mantle and, presumably, core geophysics).

Wilson Cycles have been in operation since the origin of modern-style tectonics, and will operate as long as the core continues to generate sufficient heat (i.e., as long as its fissionable "fuel" holds out).

Geological activity has three main sources of energy:

• Solar: the sun powers the weather, which acts to erode the continents and drive oceanic currents
• Gravity: rivers and glaciers flow down slope, eroding into rock and carrying sediment along
• Geothermal: heat generated inside the Earth by radioactive decay of naturally-occurring radioactive isotopes drives the mantle convection cells, which moves the plates along and serves to melt the crust above (generating magma)

The activities generated by these sources serve to drive geochemical cycles through Earth’s systems.

The different constituents of the Earth’s systems exist in various reservoirs (also called sinks) where they may remain for periods of time from just years (or shorter) to hundreds of millions of years (or longer), depending on the setting. These are the residence times of the element in that reservoir. Matter gets transferred from one sink to another through various fluxes. For a simple case, mountain glaciers and the ocean represent two different reservoirs of water, while stream systems (running from the glacier to the sea) and precipitation systems (bringing evaporated water from the ocean down as snow to add to the glaciers) are fluxes.

(Note: the old calcultations for the age of the Earth assuming that the oceans were originally fresh and measuring the flux of salt in stream systems actually was helping to calculate the residence times of Na and Cl in the hydrologic system, not the age of the Earth.)

Here is a more detailed one: the carbon cycle. The values in the boxes (reservoirs) are in grams; the values along the arrows (fluxes) are in grams/year. ote that the atmosphere contains only a tiny fraction of the amount of carbon that is found in carbonate rocks (like limestone) or organic carbon (soil, carbon-rich detrital sedimentary rocks, and especially fossil fuels such as coal and petroleum).

How plate tectonics drives geochemical cycles and global change:

• Subduction zones produces:
  • Magmas which can:
    • Erupt as volcanoes, releasing new lava and volatiles directly on the surface of the Earth and into the atmosphere, respectively
    • Cool before they reach the surface, producing new plutonic igneous rock
  • Metamorphism (due to the compression of the subduction and by contact with magma)
  • Uplift, and thus new source rock for erosion to act upon
• Weathering of bedrock:
  • Removes carbon dioxide from the atmosphere, as chemical weathering following the equations:
    • CaCO₃ + CO₂ + H2O -> Ca²⁺ + 2HCO₃^- (calcium ion and bicarbonate ion, respectively)
    • CaSiCO₃ + 2CO₂ + H2O -> Ca²⁺ + 2HCO₃^- + SiO2 (silica)
    • NOTE: The calcium and bicarbonate in the water are transferred down to the sea, where living things take them up to produce skeletons. These skeletons can be buried producing carbonate sediment (and potentially future carbonate rock).
  • In place weathering of bedrock—without transport, but with burial in place of lots of decaying plant matter—produces soil. Soil represents a medium-length residence time (thousands of years) reservoir for carbon.
  • Produces sediment that serves to bury organic material when streams flood, or at the mouth of the river systems.

Thus, changing rates of plate tectonic activity can greatly modify the fluxes of carbon (and other elements), as we'll see in our review of Earth History. Among these changes are the composition of the atmosphere itself!

Some other factors that plate tectonics will change over time:

• Position of the continents
  • Thus, the local distribution of land and water, and thus the regional albedo, and thus climate
  • Habitat ranges for organisms, including connections between different regions
  • Oceanic current patterns and the formation of deep water
• Sea level (due to changing activity of the mid-ocean ridges)
  • Thus, total planetary albedo, as well as regional distribution
  • Productivity (amounts of nutrients produced) as shallow seas come and go
• Rise & fall of mountain ranges
  • Changing amounts of volatiles from volcanoes
  • Changing climate due to presence or absence of mountain ranges
  • Changing amounts of weathering (removing carbon dioxide from atmosphere) and of availability of sediment (burying carbon away from atmosphere)

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Last modified: 18 February 2008