Besides compositional classification, the Earth is separated into layers based on mechanical properties.

The topmost layer is called the lithosphere, composed of tectonic plates that float on top of another layer known as the asthenosphere. The term lithosphere is derived from the Greek words lithos, meaning rock, and sfaira, or sphere. The rigid, brittle lithosphere extends about 70 kilometers and is made up of Earth's crust and the upper part of the mantle underneath. It is broken into a mosaic of rigid plates that move parallel across the Earth's surface relative to each other.

The lithosphere rests on a relatively ductile, partially molten layer known as the asthenosphere, which derives its name from the Greek word asthenes, meaning "without strength." The asthenosphere extends to a depth of about 400 kilometers in the mantle, over which the lithospheric plates slide along. Slow convection currents within the mantle, generated by radioactive decay of minerals, are the fundamental heat energy source that causes the lateral movements of the plates on top of the asthenosphere. According to the plate tectonic theory, there are approximately twenty lithospheric plates, each composed of a layer of continental crust or oceanic crust. These plates are separated by three types of plate boundaries. At divergent boundaries, tensional forces dominate the interaction between the lithospheric plates, and they move apart and new crust is created. At convergent boundaries, compression of lithospheric plate material dominates, and the plates move toward each other where crust is either destroyed by subduction or uplifted to form mountain chains. Lateral movements due to shearing forces between two lithospheric plates create transform fault boundaries. Earthquakes and volcanic activities are mostly the result of lithospheric plate movement and are concentrated at the plate boundaries. Volcanic eruptions have severe effects on global climate.

The greenhouse effect, icehouse effect, and ozone depletion by far have gained the most attention in climate research and planning. In addition to lava and pyroclastic materials (fragments of hot and molten rocks), volcanoes emit a variety of gases such as water vapor, carbon dioxide (CO2), carbon monoxide (CO), chlorine, fluorine, and sulfur dioxide (SO2). Both CO2 and CO are greenhouse gases (GHGs) that contribute to global warming by creating a shield over the Earth that prevents heat from escaping into the atmosphere. In contrast, SO2 gas causes short-term cooling resulting from what is known as the icehouse effect.

In the lower atmosphere, SO2 gas is converted to sulfuric acid (H2SO4), which condenses to form a thick layer of sulfate aerosol. The suspended aerosols increase Earth's albedo by reflecting the Sun's rays back to space and cause cooling of the Earth's surface. An anomalous increase in SO2 layers in the atmosphere and decrease in average temperature correlates significantly with several volcanic eruptions. The 1991 eruptions of Mount Pinatubo in the Philippines were responsible for about a 0.5œ Celsius decrease in global temperature and an unusually cold summer in 1992 in the intermediate latitude of the Northern Hemisphere.

Although volcanic activity increases the global temperature by adding CO2 to the atmosphere, a much greater amount of CO2 is added to the atmosphere by anthropogenic activities each year (Schuiling, 2004). Research by Terrence M. Gerlach indicates that anthropogenic CO2 emissions are about 150 times greater than volcanic CO2 emissions. A small amount of global warming caused by the GHGs from volcanic eruption can considerably supersede the greater amount of global cooling caused by volcano-generated aerosol particles in the atmosphere. Without such cooling effect, global warming due to GHGs would have been more pronounced.

The lithosphere's plates move at a rate of about 3 centimeters per year (Gerlach, 2002). The distribution and relative movement of the oceanic and continental plates across the latitude also have profoundly affected the global climate. The major contributing factors are differences in surface albedo, land area at high latitudes, the transfer of latent heat, restrictions on ocean currents, and the thermal inertia of continents and oceans. According to the present configuration of oceans and continents, the low latitudes have a greater influence on surface albedo because the lower latitudes receive a greater amount of solar radiation than the higher latitudes.

Whereas the continents in higher latitudes receive lesser solar radiation and accumulate snow that consecutively increases the albedo and decreases the Earth's surface temperature, the latent heat of evaporation influences the surface temperature at lower latitudes, where there is greater oceanic surface. The evaporation of water from the oceanic surface, a dominant mode of heat transfer, results in greater heat loss in lower latitudes. Oceanic circulation is a primary mechanism by which the heat due to solar radiation is spread from equatorial to polar latitudes. The continents in between work as barriers that restrict the oceanic heat transport toward the poles, and can influence the area and thickness of polar snow cover. The thermal inertias of continents and oceans are different. The continents respond quickly if there is a change in solar input, whereas oceans have high heat capacity and act slowly. In addition to the present positions of the continents and oceans, the higher elevations due to mountain orogeny also control the global climate.

References

1. Gerlach, Terrence M., et al. "Carbon Dioxide Emission Rate of Kilauea Volcano: Implications for Primary Magma and the Summit Reservoir." Journal of Geophysical Research 107 (2002).

2. Schuiling, Roelof D. "Thermal Effects of Massive CO2 Emissions Associated with Subduction Volcanism." Comptes Rendus Geosciences 336 (2004).

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