By Afroz Ahmad Shah
As I have discussed in my previous article, that through the vast amount of evidences obtained from the earthquakes waves, meteorites, which fall on Earth, magnetic fields, some rare exposures of the deeper section of the Earth’s layers, and other physiochemical properties demonstrate that Earth is made up of three major layers, the crust, the mantle and the core. This stratification within the Earth was achieved through differentiation, which is a process by which materials are arranged in a sequence according to their densities. The Earth’s uppermost layer, the Crust is lighter and less dense. It constitutes 1% of the Earth’s volume. Its thickness varies under the continents and oceans. It is thickest under the mountains, where it can reach up to a thickness of about 75 kms. The crust is composed of granite (an igneous rock, formed within the crust from a molten material). However, the oceanic crust is thin and could be as thin as 5 kms deep. It is composed of basalt (an igneous rock, formed from volcanic eruption) whereas the Mantle is relatively dense and the Core is very dense (Figure B). Most of the Earth is Mantle, which constitutes about 82% of its volume and ~68% of its mass. The Core constitutes 16% of the Earth’s volume and ~32% of its mass, which is due to its high density. The Crust is a very thin layer and constitutes only ~ 1% of the Earth’s volume. Its thickness varies from oceans to continents.
Based on physical properties, the earth is divided into a number of layers:
Lithosphere (litho=rock, sphere=sphere): This includes the Crust and upper part of the Mantle (Figure). It varies in thickness under oceans (about 10km) and in continents (about 300km).
Asthenosphere: Within the upper mantle, there is a major zone where temperature and pressure are just right so that part of the material melts, or nearly melts. Under these conditions rocks lose much of their strength and become soft and flow slowly. This zone of easily deformed mantle is known as the Asthenosphere (weak sphere). This boundary is sharp and it’s not due to changes in chemical properties of rocks; however, it is a major mechanical boundary, where it is characterized by changes in rock’s mechanical properties (Figure).
A low-velocity-zone, which is a major discontinuity, is located at about 100 to 250km below the surface. This zone was recognized by a German seismologist, Beno Gutenberg in 1920s. This zone became prominent when a sudden change in seismic velocity was recognized, where in fact, a general trend is that velocity should increase with depth in mantle. This region is characterized by about 6% slower travel time of the seismic waves. This is explained by assuming that the mantle is very near its melting point or even partially molten with perhaps 1% to 5% liquid. A thin film of liquid around the mineral grains may slow both S and P waves. Moreover; rocks near their melting points are very weak and ductile. The low-velocity zone is embedded within the asthenosphere, which plays a key role in the motion of plates at Earth’s surface. If Earth lacked a weak, ductile asthenosphere, the upper part of the mantle would be directly tied, frozen, to the lower part of the mantle, and plate motions would be prohibited. Apparently, the asthenosphere effectively decouples the moving lithosphere from the lower part of the mantle.
The rocks below the asthenosphere are relatively stronger, which is suggested by an increase in seismic wave travel velocity. This is because the pressure increases, which offsets the effect of increase in temperature, forcing the rock to be stronger than the overlying asthenosphere. The region between the asthenosphere and the core is the Mesosphere (middle sphere).
Earth’s core marks a change in both chemical composition and mechanical properties. On the basis of mechanical behavior alone, the core has two distinct parts: a solid inner core and a liquid outer core. The outer core has a thickness of about 2270 km compared with the much smaller inner core, with a radius of only about 1200km. The core is extremely hot and heat loss from the core and the rotation of Earth probably causes the liquid outer core to flow. This circulation generates Earth’s magnetic field.
Afroz Ahmad Shah is a research fellow at Earth Observatory of Singapore, Nanyang Technological University, Singapore.