Encyclopedic Entry

Changes in the mantle can cause changes in the deep crust, such as volcanic eruptions.

Photograph by Jim Cleveland, MyShot

Seeking the Holey Grail
Drilling all the way down to the Moho (the division between the Earth's crust and mantle) is an important scientific milestone, but despite decades of effort, nobody has yet succeeded. The United States and the Soviet Union both made great progress in the 20th century. A program called Project Mohole was begun by the U.S. in the 1950s, but it never reached a depth beyond about 183 meters (601 feet) under the seafloor.

In 2005, scientists with the Integrated Ocean Drilling Project drilled 1,416 meters (4,644 feet) below the North Atlantic seafloor and claimed to have come within just 1,000 feet of reaching the Moho.

The mantle is one of the three main layers of the Earth. It lies between the innermost layer, the core, and the thin outermost layer, the crust. The mantle consists of hot, dense, semisolid rock and is about 2,900 kilometers (1,802 miles) thick.

The mantle is divided into several layers.

Layers of the mantle

  • Lithosphere. The thin outermost shell of the upper mantle is similar to the crust, though cooler and more rigid. Together with the crust, this layer is called the Earth’s lithosphere.
  • Asthenosphere. The lithosphere is actually broken up into several large pieces, or plates. They “float” on a softer mantle layer called the asthenosphere. Their very slow motion is the cause of plate tectonics, a process associated with continental drift, earthquakes, volcanoes, and the formation of mountains.
  • Upper mantle. Below the asthenosphere lies another layer, stronger and more solid than the asthenosphere. All layers below the crust down to a depth of about 670 kilometers (416 miles) are known as the upper mantle.
  • Lower mantle. The rest of the mantle between the upper mantle and the core is known as the lower mantle. It is denser and hotter than the upper mantle.


Exploring the Deep Earth

Most of the Earth’s interior is much too deep for us to explore directly. Instead, scientists tell the mantle apart from the crust and core by measuring the spread of shock waves from earthquakes, called seismic waves. Two types of seismic waves pass through the Earth’s interior: P-waves, which represent vertical motion, and S-waves, which represent horizontal motion. Instruments placed around the world measure these waves as they arrive at different points on the Earth’s surface after an earthquake.

Seismic waves travel at different speeds and strengths through different material. For example, surface waves from a powerful earthquake near Northridge, California, in 1994 took 30 minutes to reach a point about 6,700 kilometers (4,163 miles) away, but it took P-waves only 10 minutes and S-waves just under 20 minutes to travel the same distance. Near the surface, P-waves travel about 6 kilometers per second through the ground. Below a certain point—an average depth of about 35 kilometers—they travel about 8 km/s, indicating that the waves have reached denser material at that point.

This abrupt divide between slower and faster speeds marks the boundary between the crust and the mantle. It is called the Mohorovicic discontinuity, or simply the Moho. At the base of the mantle, 2,900 kilometers (1,802 miles) below the surface, S-waves, which can’t continue in liquid, suddenly disappear, and P-waves are strongly refracted, or bent. This point, called the Gutenberg discontinuity, marks the end of the mantle and the beginning of the Earth’s liquid core.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

asthenosphere

Noun

layer in Earth's mantle between the lithosphere (above) and the upper mantle (below).

boundary

Noun

line separating geographical areas.

Encyclopedic Entry: boundary

continental drift

Noun

the movement of continents resulting from the motion of tectonic plates.

Encyclopedic Entry: continental drift

core

Noun

the extremely hot center of Earth, another planet, or a star.

Encyclopedic Entry: core

crust

Noun

rocky outermost layer of Earth or other planet.

Encyclopedic Entry: crust

dense

Adjective

having parts or molecules that are packed closely together.

Earth

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

Encyclopedic Entry: Earth

earthquake

Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

Gutenberg discontinuity

Noun

point between the Earth's mantle and the core below.

horizontal

Adjective

left-right direction or parallel to the Earth and the horizon.

lithosphere

Noun

outer, solid portion of the Earth. Also called the geosphere.

Encyclopedic Entry: lithosphere

lower mantle

Noun

bottom layer in Earth's mantle, closest to the core.

Moho

Noun

point between Earth's crust and the mantle below. Also called the Mohorovicic discontinuity.

Mohorovicic discontinuity

Noun

point between Earth's crust and the mantle below. Also called the Moho.

mountain

Noun

landmass that forms as tectonic plates interact with each other.

plate tectonics

Noun

movement and interaction of the Earth's plates.

P-wave

Noun

seismic shock wave that represents vertical motion. Also called a primary wave.

refract

Verb

to bend or change the path of something.

rigid

Adjective

stiff.

rock

Noun

natural substance composed of solid mineral matter.

seismic shock wave

Noun

vibration that travels from the focus of an earthquake in all directions.

seismic wave

Noun

shock wave of force or pressure that travels through the Earth.

S-wave

Noun

seismic shock wave that represents horizontal motion. Also called a secondary wave.

upper mantle

Noun

layer in Earth's mantle between the asthenosphere (above) and the lower mantle (below).

vertical

Noun

up-down direction, or at a right angle to Earth and the horizon.

volcano

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

Credits

Media Credits

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Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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