Most of our knowledge about the structure of the earth comes from studying earthquakes. Every earthquake sends out waves in all directions like the way that dropping a rock into a lake sends out waves through the water. These earthquake waves are called seismic waves. Observing these seismic waves as they travel through the earth gives scientists an idea of the different materials that the waves move through.
There are two types of seismic waves: S-waves and P-waves. These waves behave differently when they pass through different types of materials. Just like a sound wave behaves differently when it passes through the water instead of air; seismic waves behave differently when they pass through different phases of matter. Scientists know that P-waves will travel through all types of materials but S-waves will not travel through liquid.
Earth is made of several layers. Each layer has its own characteristic properties. Scientists think about Earth’s layers in two ways – in terms of chemical composition and in terms of physical properties.
Based on chemical composition, Earth can be divided into three layers outward from the centre of the Earth: core, mantle, and crust.
The outermost solid layer of Earth is called crust. It lies above the mantle and is the Earth’s hard outer shell. The crust is the surface on which we are living.
The crust is 0-32 KM (0-19.8miles). In relation to the other layers, the crust is the thinnest and the least dense layer. It floats on the softer, denser mantle. The crust is made up of solid rock but these rocks are not the same all over the world.
There are two major types of crust:
Oceanic crust is a thin layer (about 5 km) found under the oceans. Even though it is relatively thin it is the densest type of crust and is made up of a metamorphic rock called basalt.
Continental crust makes up the continents and rests on top of oceanic crust. As compared to oceanic crust, continental crust is thicker (30 km). Continental crust consists of less dense rock such as granite. Even though the continental crust is less dense, it is much thicker than the oceanic crust because it consists of the rocks that make up the continents.
Because the earth is very hot inside, a current of heat flows from the core to the crust. This is called the convection current. This current cools as it rises closer to the surface of the earth. This convection current along the bottom of the crust causes the moving of the tectonic plates. The constant movement of the plates is called plate tectonics. The movement of these plates is very slow but when they bump together it causes an earthquake. The combination of convection currents from the mantle and the effects of the atmosphere make the crust about 0-1598 °F from the surface to the bottom of the crust. The crust and atmosphere are the coolest of earth’s layers.
The crust is brittle in nature. Nearly 1% of the earth’s volume and 0.5% of earth’s mass are made of the crust. Major constituent elements of crust are Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL.
The discontinuity between the hydrosphere and crust is termed as the Conrad Discontinuity.
The layer beneath the crust and above the core is the mantle. It is approximately 2900 km thick. Nearly 84% of the earth’s volume and 67% of the earth’s mass is occupied by the mantle. The mantle has an average density of 4.5g∕cm3. The density increases with depth because the pressure increases.
The discontinuity between the crust and mantle is called the Mohorovich Discontinuity or Moho Discontinuity.
The mantle mainly consists of solid rocks made from silicon and magnesium, and hence it is termed as SIMA. Deep into the mantle, the rocks consist of magnesium and iron. Another reason that the mantle gets denser with depth is that the rocks at this level contain iron and iron is denser than the materials at the upper layers of the mantle.
Earth’s mantle has different temperatures at different depths. The temperature of the mantle increases with depth. It ranges from 1598-3992°F. The highest temperatures occur where the mantle material is in contact with the heat-producing core. The mantle retains a lot of heat, which is circulated throughout the mantle in spaces called convective cells. The movement of heat can cause the plates of the seafloor and continents to shift. Over millions of years, the plates of the earth can move quite a lot. When these shifts happen quickly, we experience earthquakes.
This steady increase of temperature with depth is known as the geothermal gradient. The geothermal gradient is responsible for different rock behaviours. The different rock behaviours are used to divide the mantle into two different zones. Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot and soft but not molten. Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes. However, rocks in the lower mantle are soft and flow when subjected to forces instead of breaking.
The uppermost solid part of the mantle and the entire crust constitutes the Lithosphere.
The asthenosphere (in between 80-200 km) is a highly viscous, mechanically weak and ductile, deforming region of the upper mantle which lies just below the lithosphere. The asthenosphere is the main source of magma and it is the layer over which the lithosphere plates/continental plates move (plate tectonics).
The discontinuity between the upper mantle and the lower mantle is known as Repetti Discontinuity.
The portion of the mantle which is just below the lithosphere and asthenosphere but above the core is called Mesosphere.
The inner part of the Earth is the core. This part of Earth is about 2900 km below the Earth’s surface. The core is separated from the mantle by Guttenberg’s Discontinuity.
The core is composed mainly of iron (Fe) and nickel (Ni) and hence it is also called NIFE. The core constitutes nearly 15% of Earth’s volume and 32.5% of Earth’s mass. It is the densest layer of the Earth with its density ranging between 9.5 to 14.5g∕cm3.
After observing the speeds of P-waves and S-waves, scientists have concluded that the earth’s center is divided into two layers – the outer core and the inner core.
The outer core is a liquid because the temperature is high enough to melt the iron and nickel metals. The outer core begins about 2900 km below the surface and is about 2300 km thick. Because the Earth rotates, the outer core spins around the inner core and that causes the Earth’s magnetism. Magnetism has been used by sailors to find their way on Earth for thousands and thousands of years. Magnetism also influences particles outside the atmosphere of the Earth up to more than 60,000 km into space. The outer core is about 3992- 9032 °F. The density of the outer core is between 10 g/cm3 and 12.3 g∕cm3.
The inner core is 5150 kilometres (3200 miles) below the earth’s surface. One would still have to travel about 1300 kilometres (808 miles) more to reach the center. The temperature in the inner core is about 5000 – 6000 °C (9032 – 10832 °F). It is made from the same materials as the outer core but because of the high pressure, the inner core is solid. Here, tremendous pressure, produced by the weight of the overlying rocks, is strong enough to crowd the atoms tightly together and prevents the liquid state. This high pressure and the dense metals at the core make its density 13g∕cm3.
The discontinuity between the upper core and the lower core is called as Lehmann Discontinuity.
Earth is also divided into layers based on physical properties, such as whether the layer is solid or liquid.
The five physical layers are the lithosphere, asthenosphere, mesosphere, outer core, and inner core.
1. Lithosphere - The outermost layer of solid rock found at Earth’s surface is the lithosphere. It includes both the crust and the solid, the uppermost part of the mantle. It is relatively less dense than Earth’s other physical layers. The lithosphere is divided into pieces called tectonic plates.
2. Asthenosphere – The asthenosphere is found below the lithosphere and it is the layer of a weak or soft mantle made of solid rock that moves very slowly. It is located below the lithosphere. Tectonic plates move on top of the asthenosphere.
3. Mesosphere - The strong, lower part of the mantle is called the mesosphere. The rock in the mesosphere flows more slowly than rock in the asthenosphere. The mesosphere is much denser than the asthenosphere.
4. Outer core - The outer core is the liquid layer of Earth’s core. The outer core lies beneath the mantle and surrounds the inner core.
5. Inner core - The inner core is the solid, dense center of our planet. The inner core extends from the bottom of the outer core to the center of Earth.
The lithosphere and asthenosphere are not the same as the crust and mantle. The crust and mantle are compositional layers of Earth. The lithosphere and asthenosphere are physical layers. The lithosphere includes the crust and the solid, outermost part of the mantle. The crust is thinner than the lithosphere and contains rock material that is rich in silica and is much less dense than the rock material in Earth’s other layers. The asthenosphere is a semisolid layer between the solid lithosphere and mesosphere.
The asthenosphere is not a liquid. Rock material that makes up the asthenosphere is ductile, meaning that it can be stretched slowly. The asthenosphere is ductile because of the intense heat of Earth’s interior. As rock material in the lower portion of the asthenosphere is heated, it rises slowly. As it rises, it begins to cool and sink again. Thus, rock material in the asthenosphere circulates in enormous convection cells. These convection cells cause tectonic plate movement. Lithospheric plates which rest on the asthenosphere, are carried along as the asthenosphere slowly flows. The movement of lithospheric plates causes earthquakes and volcanoes.
