Plate Tectonics and Climate

The outer layers of the Earth are broken into about a dozen large tectonic plates, extending to about 60-100 miles (100-160 km) beneath the surface. Each of these plates may be made of oceanic crust and lithosphere, continental crust and lithosphere, or an oceanic plate with a continent occupying part of the area of the plate. Plate tectonics describes processes associated with the movement of these plates along three different types of boundaries: divergent, convergent, and transform. At divergent boundaries the plates move apart from one another, and molten rock (magma) rises from the mantle to fill the space between the diverging plates. This magma makes long ridges of volcanoes along a midocean ridge system that accounts for most of the volcanism on the planet. These volcanoes emit huge quantities of carbon dioxide (C02) and other gases when they erupt.

There have been times in the history of Earth that midocean ridge volcanism was very active, producing huge quantities of magma and C02 gas, and other times when the volcanism is relatively inactive. The large quantities of magma and volcanism involved in this process have ensured that variations in midocean ridge magma production have exerted strong controls on the amount of C02 in the atmosphere and ocean, and thus, are closely linked with climate. Periods of voluminous magma production are correlated with times of high atmospheric Co2, and globally warm periods. These times are also associated with times of high sea levels, since the extra volcanic and hot oceanic material on the seafloor takes up extra volume and displaces the seawater to rise higher over the continents. This rise in sea levels in turn buried many rocks that are then taken out of the chemical weathering system, slowing down reactions between the atmosphere and the weathering of rocks. Those reactions are responsible for removing large quantities of Co2 from the atmosphere, so the rise in sea level further promotes global warming during periods of active seafloor volcanism.

Convergent boundaries are places where two plates are moving toward each other or colliding. Most plate convergence happens where an oceanic plate is pushed or subducted beneath another plate, either oceanic or continental, forming a line of volcanoes on the overriding plate. This line of volcanoes is known as a magmatic arc, and specifically as an island arc if built on oceanic crust or an Andean arc if built on continental crust. When continents on these plates collide, the rocks that were deposited along their margins, typically underwater, are uplifted in the collision zone and exposed to weathering processes. The weathering of these rocks, particularly the limestone and carbonate rocks, causes chemical reactions where the Co2 in the atmosphere reacts with the products of weathering, and forms new carbonate (CaC03) that gets deposited in the oceans. Continental collisions are thus associated with the overall removal of Co2 from the atmosphere and help promote global cooling.

Transform margins do not significantly influence global climate since they are not associated with large amounts of volcanism, nor do they uplift large quantities of rock from the ocean.

The timescale of variations in global Co2 related to changes in plate tectonics are slow, and they fall under the realm of causing very long-term climate changes, in cycles ranging from millions to tens of millions of years. Plate tectonics and movement of continents has been associated with glaciations for the past few billion years, but the exact link between tectonics and climate is not clearly established. Some doubt remains as to why global temperatures dropped, inducing the various glacial ages. The answer may be related to changes in the natural (nonbiogenic) production rate of carbon dioxide—the number one greenhouse gas. C02 is produced in volcanoes and in the midocean ridges, and it is lost by being slowly absorbed into the oceans. Both of these processes are very slow—about the right timescales to explain the Great Ice Ages. One theory is that more carbon dioxide is produced during times of faster-than-aver-age rates of seafloor spreading and the subsequent increase in volcanism. During times of rapid spreading, the higher volcanic activity, coupled with higher sea levels and reduced chemical weathering of rocks, may promote global warming by enriching the Co2 content of the atmosphere. Similarly, global cooling may result from stalled or slowed spreading.

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