Asthenosphere

The asthenosphere is a special part of Earth's interior located in the upper mantle. It sits just below the lithosphere, the rigid outer layer of our planet, and stretches from about 80 to 200 kilometers (50 to 120 miles) below the surface down to around 700 kilometers (430 miles). Although its lower boundary isn't clearly defined, the asthenosphere is important because it behaves in a unique way compared to the layers above and below it.

This region is almost solid rock, but it contains a tiny amount of melted material—less than one-tenth of a percent—which makes it mechanically weak and ductile. This slight melting allows the asthenosphere to flow slowly over very long periods, which is essential for movement in Earth's lithospheric plates.

When the asthenosphere rises, it undergoes more decompression melting, creating magma. This process is the primary source of magma on Earth, feeding volcanic activity at mid-ocean ridges, subduction zones, and areas where continents are pulling apart. Understanding the asthenosphere helps scientists explain many geological features and processes that shape our planet.

Characteristics

The asthenosphere is a part of Earth's upper mantle just below the lithosphere. It plays a key role in the movement of tectonic plates and how land shifts over time. Made mostly of a rock called peridotite, which contains minerals like olivine and pyroxene, the asthenosphere behaves in a soft, flexible way because of the heat deep inside Earth.

Seismic waves, which are like the Earth’s own "echoes" during earthquakes, move more slowly through the asthenosphere. This slower movement helped scientists discover this layer and learn about its properties. The asthenosphere allows the rigid layers above it—the lithosphere—to move around slowly, acting like a flowing current deep inside our planet.

Boundaries

The asthenosphere lies below the lithosphere, starting about 80 to 200 kilometers (50 to 120 miles) beneath the Earth's surface and extending down to around 700 kilometers (430 miles). This layer is weaker and more flexible than the lithosphere above it.

The boundary between the lithosphere and the asthenosphere is quite clear and may be linked to changes in how heat moves or in the rocks' properties. The lower edge of the asthenosphere is less certain but is often placed near a depth of 670 kilometers, where the type of minerals in the mantle changes. Main article: Lithosphere-asthenosphere boundary

Origin

The asthenosphere is thought to be weaker than the rock above it because of small amounts of melted material. Tiny bits of melt, helped by water and carbon dioxide, make the rock more flexible. Even though there is very little melt, it still plays a role in making the asthenosphere less strong.

Another reason the asthenosphere is weak may be how the rocks move past each other when pushed. This movement, helped by small amounts of water, makes the rocks easier to bend. Models show that temperature and how fast the rocks are moving also help create the weak asthenosphere, especially under the Pacific plate.

Magma generation

When rock from the asthenosphere moves closer to Earth's surface, it can melt and form magma. This process, called decompression melting, is the main way magma is created on Earth. Most of this magma erupts at mid-ocean ridges, forming the basalt that makes up the ocean floor. Magma can also form above subduction zones and in areas where continents are pulling apart.

Decompression melting usually starts about 100 to 150 kilometers below the surface, where small amounts of water and carbon dioxide in the rock help it melt. As the rock continues to rise, it reaches a depth of about 70 kilometers where it melts more easily. This melting changes the chemistry of the surrounding rock and helps form the lithosphere above.