The Relationship Between

        Plate tectonics and the carbon cycle are intertwined in several different ways. In many respects, it is plate tectonics that spurs on the recycling of carbon atoms.

 Convergent boundaries affect the carbon cycle in two ways: through subduction and eruption.

•  Subduction is the process by which continental crust slides beneath another portion of crust.  The subducting crust melts and becomes magma, the material that fuels volcanic eruption.  The melted crust contains carbon in the sediments and soils, thus recycling it through the mantle of the earth.
• The melted crust convecting through the mantle will eventually resurface in the form of lava during eruptions from volcanoes. These volcanoes were originally formed by tectonic forces--where there is an excess of magma below the crust due to subduction, it is forced to erupt.  The process of eruption includes degassing.  Degassing is where carbon dioxide is released into the atmosphere as the eruption occurs because the dissolved carbon in the magma is unstable and under pressure, and is therefore forced to leave the fluid.
• The recycling process can be seen in the diagram below.  The trenches are the areas of subduction where a "slab" of crust is pulled into the earth. This crust, containing carbon, is then recycled through the mantle and later released through a ridge, either convergent or divergent.

Plate tectonics and the carbon cycle also have a major effect on climate change. The stages of Snowball Earth of about 600 Ma are a prime example of this relationship.

• The breakup of Pangea about 770 Ma ago left many small continents scattered about the globe.  These broken areas of land became surrounded by plentiful sources of moisture (e.g. oceans).  Increased rainfall takes carbon dioxide out of the air, making the erosion and weathering of continental rocks occur at a faster rate. This in turn reduces the amount of carbon dioxide in the atmosphere which results in a fall of global temperature. As the temperature falls, glaciation occurs in the polar oceans. White ice has a high albedo and thus reflects more solar energy back into space. This creates a positive feedback which continues to reduce global temperature.

• As the cooling continues, the cold dry air eventually halts the further growth of glaciation, creating deserts. The air becomes so dry that next to no rainfall occurs so the carbon dioxide released through volcanoes is kept in the atmosphere. The atmospheric carbon then accumulates and begins to trap the infrared waves of the sun in the greenhouse effect, eventually increasing the global temperature.

• As the planet grows warmer, moisture from the sea ice refreezes at a higher elevation due to the difference in isostacy.  The open waters that are left around the equator absorb more solar energy and help to increase the global temperature.

• The large amount of carbon in the atmosphere can now combine with the water being evaporated into the atmosphere and form carbonic acid.  This rain erodes and weathers rock formation. Water then carries the bicarbonate and the other ions into the ocean where they form carbonate sediment.

The pictures below describe each step of the snowball earth:
 

                  Snowball Earth Prologue                                                       Snowball Earth at its Coldest

                Snowball Earth as it Thaws                                   Hothouse Aftermath

 
 

    Thus the effects snowball earth, characterized by large areas of glaciation, were eventually countered when volcanic activity and tectonic forces allowed further concentrations of carbon dioxide to build up.  Here the relationship can be seen how plate tectonics, through the formation of volcanoes, works with the carbon cycle: it is the tectonic forces which release carbon through degassing and entrap carbon during subduction.  This relationship has occurred most noticeably in the break up and formation of continents and the resulting effect on climate.
 
 
 

Resources:
          -  Carbon Cycle Modelling, ed. Bert Bolin. 1981, John Wiley & Sons
          -  Tectonic Uplift and Climate Change, ed. William F. Ruddiman. 1997, Plenum Press
          -  Natural Sinks of CO₂, ed. Dr. Joe Wisniewski & Dr. Ariel E. Lugo. 1992, Kluwer Academic Publishers
          -  GE 70A Reader by Mark Morris, Mark Harrison, and Stephen Mojzsis
          -  The Cosmic Perspective by Jeffrey Bennett, Megan Donahue, Nicholas Schneider, and Mark Voit, 1999 Addison Wesley Longman.
          -  http://www.sciam.com/2000/0100issue/0100hoffmanbox1.html