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Earth's Mantle: Definition & Facts

Earth's Mantle: Definition & Facts
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  • 0:00 The Mantle Defined
  • 0:28 Mantle Facts
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Lesson Transcript
Instructor: Suzanne Rebert

Suzanne has taught college economics, geography, and statistics, and has master's degrees in agricultural economics and marine affairs (marine resource management).

What do we really know about that big, thick orange layer we see in so many cross-sectional diagrams of the Earth's interior? The mantle is a strange place, but it affects us all the time.

The Mantle Defined

The Earth's mantle is the thickest layer of the planet and accounts for about 84% of the Earth's volume and 67% of its mass. It lies between the crust and the core. The mantle begins at the boundary known as the Mohorovicic discontinuity and extends down to the core-mantle boundary, which is also called the Gutenberg discontinuity.

Mantle Facts

The upper mantle starts between 7 and 35 kilometers below the surface of the Earth. The boundary (the Mohorovicic discontinuity, or 'Moho') is shallowest in the ocean basins and deepest under the continents. In some deep parts of the ocean, mantle rock is actually exposed on the seabed.

The 'discontinuous' thing about the Moho is the velocity of seismic waves as they pass through rock. The primary or P-waves can move through the mantle rock much faster than through the more brittle rock of the crust. Thanks to the way they respond to different rock layers, seismic waves have been scientists' most important tool for exploring the inner structure of the Earth.

Composition

The mantle is composed primarily of magnesium, aluminum, silicon, and oxygen silicates. Some of the rock types that are common in the mantle include olivine, peridotite, pyroxene, and garnet. The mantle is denser than the crust but much less dense than Earth's core, which is a huge ball of hot metal, mostly iron and nickel.

Upper and Lower Mantle

The temperature at the top of the upper mantle ranges from 500 to 900 degrees Celsius and it increases with depth. Even though the uppermost part of the mantle is seriously hot, its behavior is still 'rocky' enough that it's considered part of Earth's lithosphere, together with the crust.

The part of the upper mantle below the lithosphere is called the asthenosphere, from asthenes, a Greek word for 'weak.' It gets its name from its ductile nature; at temperatures over 1300 degrees Celsius and super-high pressure, rock behaves very differently from what we're used to. It acts more like Silly Putty than a granite rock you could climb on or build with! Seismic waves slow down again in the asthenosphere.

The lower mantle is even hotter, stranger, and less well understood than what lies above it, but it appears to extend from a complex transition zone at the asthenosphere boundary (about 220 kilometers down) to the core boundary 2,890 kilometers below Earth's surface. The temperature at that amazing depth reaches 4000 degrees Celsius, but due to the extreme pressure, very little of the rock is liquid.

Mantle and Magma

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