Earth Science: The Core and Mantle Of The Earth
The interior of the Earth long presented an enormous puzzle to science. Seismic measurements helped to start unraveling the mystery and assisted the exploration of the structure and contours of the Earth’s interior.
The interior of the Earth is separated into three chemically distinct parts: the crust, mantle, and core, in a way similar to the composition of an egg. At the present time only the outer layer of the Earth-a relatively wafer-thin section-has been directly studied. The deepest boreholes have yet to reach one percent of the 3,959-mile (6,371 km) distance to the center of the Earth.
Instead, our best source of information about the Earth’s interior are the waves generated during earthquakes. The speed with which these waves travel depends on the temperature, pressure, and composition of rocks they penetrate. Wave measurements indicate that the physical characteristics of the Earth’s layers change dramatically in two regions. These discontinuities mark the delineation between the crust, mantle, and core of the Earth.
The Earth’s mantle
The Mohorovicic discontinuity characterizes the geological interface between crust and mantle. On average, the mantle is 1,770 miles (2,850 km) thick and makes up about 68 percent of the Earth’s mass. Its upper layer consists of rocky material, which—together with the solid crust-forms the lithosphere. Below that, with a thick- ness of roughly 250 miles (400 km), is the asthenosphere.
When temperatures reach about 2550°F (1400°C) and pressure reaches 200-350 kbar, the rocks turn into viscous magma that has a density of about 3.3 ounces per cubic inch (3.4 g/cm3). The increased pressure is responsible for the transition zone located at a depth of 250 to 560 miles (400 to 900 km), the mesosphere. Below the mantle, pressure rises to 1,450 kbar. Despite a temperature of 4892°F (2700°C), the mantle remains solid because of the pressure and has a density of 3.3 ounces per cubic inch (5.7 g/cm3).
The Earth’s core
At a depth of about 1,800 miles (2,900 km), the Wichert-Gutenberg discontinuity forms the boundary where the solid mantle rock and the molten iron core meet. The density rises to 5.5 ounces per cubic inch (9.5 g/cm3) and the temperature increases abruptly by about 1830°F (1000°C). In fact, heat is continuously exchanged between the core and mantle. The convection current this creates-hot material rising and cold material sinking—is thought to be the driving force of plate tectonics.
Convection currents in the outer core also create the Earth’s magnetic field. High pressure causes the liquid outer core to become solid at a depth of 3,200 miles (5,150 km). The rocks are compressed under more than 3,600 kbar, and the density is up to 7.8 ounces per cubic inch (13.5 g/cm3). Temperatures at the Earth’s center can reach 11,730°F (6500°C).
BASICS
P WAVES: The fast primary waves oscillate in the direction of the dispersal corridor and can disperse through solid, liquid, and gaseous material.
S WAVES: The slower secondary waves oscillate perpendicular to the P waves. They can only travel within a solid elastic mass.