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You may vaguely recall learned about the earth in Earth Science classes as a child. But you probably forgot many of the details, and these help you to make sense of climate change, hurricanes, earthquakes, and volcanoes.
The earth is composed of three main layers: crust, mantle, and core. But the core has 2 distinctly different regions: the inner core and the outer core.
The diagrams at right and below show a schematic of the layers inside the earth.
The earth's crust is made up of about twelve plates, combinations of continents and ocean basins, which move around on the Earth's surface. `
through much of geologic time. The edges of the plates are marked by concentrations of earthquakes and volcanoes. Collisions of plates can produce
mountains like the Himalayas, the tallest range in the world. The plates include the crust and part of the upper mantle, and they move over a hot,
yielding upper mantle zone at very slow rates of a few centimeters per year, slower than the rate at which fingernails grow. The crust is much
thinner under the oceans than under continents (see figure above).
The mantle is a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle, which contains more iron, magnesium, and calcium than the crust, is hotter and denser because temperature and pressure inside the Earth increase with depth. As a comparison, the mantle might be thought of as the white of a boiled egg. Our knowledge of the upper mantle, including the tectonic plates, is derived from analyses of earthquake waves (see figure for paths); heat flow, magnetic, and gravity studies; and laboratory experiments on rocks and minerals. Between 100 and 200 kilometers below the Earth's surface, the temperature of the rock is near the melting point; molten rock erupted by some volcanoes originates in this region of the mantle. This zone of extremely yielding rock has a slightly lower velocity of earthquake waves and is presumed to be the layer on which the tectonic plates ride. Below this low-velocity zone is a transition zone in the upper mantle; it contains two discontinuities caused by changes from less dense to more dense minerals. The chemical composition and crystal forms of these minerals have been identified by laboratory experiments at high pressure and temperature. The lower mantle, below the transition zone, is made up of relatively simple iron and magnesium silicate minerals, which change gradually with depth to very dense forms. Going from mantle to core, there is a marked decrease (about 30 percent) in earthquake wave velocity and a marked increase (about 30 percent) in density..
The boundary between the crust and mantle is called the Mohorovicic discontinuity (or Moho); it is named in honor of the man who discovered it, the Croatian scientist Andrija Mohorovicic. No one has ever seen this boundary, but it can be detected by a sharp increase downward in the speed of earthquake waves there. The explanation for the increase at the Moho is presumed to be a change in rock types. Drill holes to penetrate the Moho have been proposed, and a Soviet hole on the Kola Peninsula has been drilled to a depth of 12 kilometers, but drilling expense increases enormously with depth, and Moho penetration is not likely very soon.
The core was the first internal structural element to be identified. It was discovered in 1906 by R.D. Oldham, from his study of earthquake records, and it helped to explain Newton's calculation of the Earth's density. The outer core is presumed to be liquid because it does not transmit shear (S) waves and because the velocity of compressional (P) waves that pass through it is sharply reduced. The inner core is considered to be solid because of the behavior of P and S waves passing through it.
The outer core is more fluid and comprised of mostly iron and nickel. It spins slightly slower than the inner core. The outer core, being
magnetic, is responsible for Earth's magnetic field. The iron inside the liquid outer core spinning produces
the electric currents that create the magnetic field. The earth's magnetic field, which shields the earth from much deadly cosmic radiation, such as
that from the sun.The magnetic field waxes and wanes, the poles drift and, occasionally,
flip. Included in the diagram is the movement of magnetic north from 1900 to 1996.
Down at the deepest layer, we find the earth's inner core, which is very dense, made of heavy metals, super heated and about 75% the size of the moon. It is thought to have concentrated there during the early period of earth's formation. The intense heat of the inner core is produced by the massive pressure and nuclear reactions. The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The inner core begins about 4000 miles beneath the crust and is about 800 miles thick. The temperatures may reach 9000 degrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure at sea level.
This heat keeps the outer core and magma liquid and
produces convection currents, which are also affected by the earth's spin. These convection currents result in tectonic plate movement. The dense core and its spin also help to produce stability of the earth's orbit and rotation, producing the stable days and nights. This inner and outer core duo is referred to as Earth's geodynamo.
Data from earthquake waves, rotations and inertia of the whole Earth, magnetic-field dynamo theory, and laboratory experiments on melting and alloying of iron all contribute to the identification of the composition of the inner and outer core. The core is presumed to be composed principally of iron, with about 10 percent alloy of oxygen or sulfur or nickel, or perhaps some combination of these three elements.
Many references are included by embedded links to the sources in the text above. Below are others: