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. These are repeated flow of energy and/or matter through different parts of Earth's system. Most ongoing natural processes are involved in cycles at some time scale or another. Each cycle involves various components:

• Reservoirs: the part of a system in which material is stored for a period of time, temporarily sequestered from interacting with the rest of the system.
  • The residence time is the average time a particle spends in a given reservoir
• Fluxes: processes that move material from one reservoir (the source) to another (the sink). Some fluxes might be continuous (major rivers and currents, for instance, are always flowing), while others are discontinuous (volcanoes only erupt on occasion).

Many systems have feedback loops: processes whose continued operation modify the future operation of that same process. We can think to two types of feedback:

• Negative feedback: or "dampening". As negative feedback operates, the intensity of the system decreases and may shut down.
• Positive feedback: or "snowballing". As positive feedback operates, the intensity of the system increases and may exceed the limits of that system.

A famous case of positive feedback:

Another case of runaway positive feedback is the climate of the planet Venus. Venus may have originally had liquid water oceans. As the early Sun increased in brightness, evaporation increased. H₂O is a greenhouse gas, so the atmosphere got hotter, leading to more evaporation. Water vapor high in the atmosphere got split into H₂ and O₂; the former was lost to space and Venus dried out. On Earth one major sink for CO₂ is chemical weathering and deposition: water breaks down minerals in the rocks, using up carbon dioxide in the process, and that weathered sediment (containg carbon) gets transported and sequestered from the atmosphere. On dry Venus the CO₂ could not be removed from the atmosphere via weathering, etc., so it built up in the atmosphere, ramping greenhouse effects up even higher. Because of this, average temperatures on Venus are 460°C; 40 C° hotter than Mercury even though it receives only 1/4 as much sunlight!

Finally, one last cyclical concept: Homeostasis. Processes in a system can operate together to maintain a stable condition. Often if there are a set of counteracting feedback loops in a system, these work to prevent extreme conditions from overpowering, and thus a "middle of the road" condition can be automatically maintained. (A standard thermostat operates this way.) Homeostasis is very common in biological systems, and are known to operate to some degree in other physical systems.

Some specific examples:
In the hydrologic cycle, 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 short term carbon cycle. The values in the boxes (reservoirs) are in gigatons of C; the values along the arrows (fluxes) are in gigatons of C/year. ote that the atmosphere contains only a tiny fraction of the amount of carbon that is found in the deep ocean.

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: 27 February 2014