It reaches the location where warm mantle material rises, and the mantle convection cell is complete. Skip to main content. Search for:. Seismic waves travel outward in all directions from where the ground breaks at an earthquake. Seismograph stations measure the energy released by these earthquakes, but there are two that scientists are most interested with in regards to understanding the interior of the earth.
Primary waves also called P-waves are fastest, traveling at about 6 to 7 kilometers about 4 miles per second, so they arrive first at the seismometer.
P-waves bend slightly when they travel from one layer into another. Seismic waves move faster through denser or more rigid material. As P-waves encounter the liquid outer core, which is less rigid than the mantle, they slow down. This makes the P-waves arrive later and further away than would be expected. The result is a P-wave shadow zone. No P-waves are picked up at seismographs o to o from the earthquakes focus. The crust is very thin, relative to the radius of the planet.
There are two very different types of crust, each with its own distinctive physical and chemical properties. Oceanic crust is composed of magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro.
Sediments, primarily muds and the shells of tiny sea creatures, coat the seafloor. Sediment is thickest near the shore where it comes off the continents in rivers and on wind currents. Almost all of the earth is hotter, and the only other way to make a magnetic field is with a circulating electric current. Circulation and convection of electrically conductive molten iron in the Earth's outer core produces the magnetic field. To make the magnetic field, the convection must be relatively rapid much faster than it is in the plastic mantle , so the core must be fluid.
Louie, after a class chalkboard drawing by David Stevenson Because the Earth's magnetic field arises in the unstable patterns of fluid flow in the core, it changes direction at irregular intervals.
In recent geologic history it may have switched direction about every 20, years. Any kind of geologic deposit e. Geophysicists can measure the changes in direction to make a magnetostratigraphy for the deposit.
At oceanic spreading centers new ocean floor is being created constantly and slowly moved away from the rift. The farther the rock is from the rift, the older it is, and it will also show the magnetic reversals like a tape recording. Such maps show how the tectonic plates have re-arranged themselves over the last million years. Convection and the release of heat from the Earth's core drives further convection to release heat from the mantle. Convection in the mantle drives plate tectonic motions of the sea floor and continents.
It is possible to use P waves and S waves traveling through the mantle from earthquakes to map out this convection, much like a hospital CAT scan can map out bones and organs with x-rays. Seismology Lab ; used by permission In this view of a flattened-out mantle from the northwest, the blue blobs show where colder, denser material is sinking into the mantle.
Near the surface, most of the colder material is in the ancient roots of continental cratons. Subducting slabs of oceanic lithosphere also appear, being recycled into the mantle from oceanic trenches. Seismology Lab ; used by permission In this view from the southwest the red blobs are warmer plumes of less dense material, rising principally into the ocean-ridge spreading centers.
A huge plume seems to be feeding spreading at the East Pacific Rise directly from the core. Most of the heat being released from the earth's interior emerges at the fast-spreading East Pacific Rise. For comparison, the magnetic field of the Sun, which is also produces by convecting electrical charges in a rotating sphere, becomes magnetically unstable and reverses its magnetic field on a more regular basis, every 11 years.
Given that the inner core is a solid metallic sphere, made mostly of iron and nickel, surrounded entirely by liquid, it can be pictured as a giant ball bearing spinning in a pressurized fluid.
Detailed studies of earthquake waves passing though the inner core have found evidence that it is spinning — rotating — just slightly faster than the rest of the earth. The interior of the earth is not simply layered. Some of the layers, particularly the crust and lithosphere, are highly variable in thickness. The boundaries between layers are rough and irregular. Some layers penetrate other layers at certain places. For example, the lithosphere penetrates deep into the mesosphere at subduction zones.
Although it is still a matter of research and debate, there is some evidence that subducted plates may penetrate all the way into the lower mesosphere. If so, plate tectonics is causing extensive mixing and exchange of matter in the earth, from the bottom of the mantle to the top of the crust.
As another example, hot spots may be places where gases and fluids rise from the core-mantle boundary, along with heat. Studies of helium isotopes in hot spot volcanic rocks find evidence that much of the helium comes from deep in the earth, probably from the lower mesosphere. Attempts have been made to drill through the crust to reach the mantle, without success. Given the lack of actual pieces of the earth from deeper than the asthenosphere, how do we know about the internal layers of the earth, what they are made of, and what their properties and processes are?
There are two sources of rock samples from the lower lithosphere and asthenosphere, igneous rocks and fault blocks. Some igneous rocks contain xenoliths, pieces of solid rock that were adjacent to the body of magma, became incorporated into the magma, and were carried upward in the magma.
What region of Earth's interior do you think is the most rigid and brittle? Which region of Earth do you think is the hottest? Gutenberg discontinuity. Mohorovicic discontinuity. Also called the Moho. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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