- BASIC STRUCTURE:
The rocks near the Earth's surface make up the Crust. The most abundant rocky materials in the crust are silicates.
Beneath the crust lies the Mantle, which makes up about 84% of Planet Earth by volume.
Below the mantle, the innermost 15% (by volume) of Earth's interior is called the Core, made mostly of iron and nickel under very intense pressure.
The density of materials increases from the surface toward the Earth's center.
Much of the evidence for this structure comes from studying earthquakes and the waves they produce.
Scientists can measure how long it takes for the wave motions of earthquakes to reach different recording stations.
From this, they can calculate
the velocity (speed) of wave transmission, which can be used to measure the density of the material that the wave passes through.
When a wave passes from a low-velocity medium into a high-velocity medium (or the reverse), the wave is bent, or refracted. This allows scientists
to detect boundaries between different materials.
- CRUST:
The Crust of the Earth contains mostly silicate minerals. The continental crust has an average density of about 2.7 g/cm3, close to the density of granite.
Below the oceans, the crust has a higher density, approximately 3.0 g/cm3, closer to the density of basalt.
The crust is about 32 km thick beneath the continents but only 8 km thick beneath the oceans.
- MANTLE:
The Mantle has a higher density than the crust, between 3.5 and 5.5 g/cm3
The sharp boundary between the crust and the mantle (discovered by Andrijs Mohorovic) is called the Mohorovicic discontinuity.
The Upper Mantle is divided into the Rigid Mantle, about 100 km thick or less, and the Asthenosphere.
The Asthenosphere, which extends down to a depth of about 410 km, is close enough to the melting point of its rocks that is flows very slowly ("plastic flow").
The slow movement within the asthenosphere is responsible for Plate Tectonics or Continental Drift (Topic 50).
A Transition zone extends from a depth of 410 km to about 660 km.
Below this transition zone lies the Lower Mantle, extending to a depth of about 2900 km.
A boundary layer, about 200 km thick, separates the Lower Mantle from the Core.
Scientists think that the mantle contains mostly silicate minerals with high magnesium content:
Olivine and Pyroxenes (which together make Peridotite), and lesser amounts of garnet and spinel.
(There is still some uncertainty about the mineral makeup of the mantle, especially the lower mantle.)
Scientists estimate the mantle's temperature to vary from about 1000o C in the Upper Mantle to about 3700o C at its greatest depth, near its boundary with the core.
- CORE:
The Core lies at the center of the Earth and makes up about 15% of the planet's volume. It has a radius of about 3500 km.
Scientists think that the core is made mostly of Iron and Nickel.
The Outer Core is mostly liquid iron and nickel, at a temperature varying from about 4500o C (near the top) to about 5500o C near the bottom.
The Inner Core is made of solid iron and nickel under tremendous pressure (about 3.6 million atmospheres), at a temperature estimated at 5200o C.
Scientists think that most of the Earth's magnetic field is generated by the iron in the core, spinning around the Earth's axis of rotation.
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