The Relationship Between
Plate Tectonics and the Carbon cycle
tectonics and the carbon cycle are intertwined in several different ways.
In many respects, it is plate tectonics that spurs on the recycling of
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
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
Cycle Modelling, ed. Bert Bolin. 1981, John Wiley & Sons
Uplift and Climate Change, ed. William F. Ruddiman. 1997, Plenum Press
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
Cosmic Perspective by Jeffrey Bennett, Megan Donahue, Nicholas Schneider,
and Mark Voit, 1999 Addison Wesley Longman.