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. It stretches from about 80 to 200 kilometers (50 to 120 miles) below the surface down to around 700 kilometers (430 miles).

This region is almost solid rock, but it has a tiny amount of melted material. This makes it weak and able to flow very slowly. This slow movement helps Earth's lithospheric plates move.

When the asthenosphere rises, it creates magma. This process helps feed volcanic activity at mid-ocean ridges, subduction zones, and areas where continents are pulling apart. Learning about the asthenosphere helps scientists understand many features and processes that shape our planet.

Characteristics

The asthenosphere is a part of Earth's upper mantle just below the lithosphere. It helps tectonic plates move and shift land over time. It is made of rock called peridotite, which has minerals like olivine and pyroxene. The asthenosphere is soft and flexible because of the heat inside Earth.

Seismic waves, which are like echoes during earthquakes, move slower through the asthenosphere. This slower movement helped scientists find this layer and learn about it. The asthenosphere lets the rigid layers above it, the lithosphere, move slowly, like a flowing current deep inside our planet.

Boundaries

The asthenosphere is the layer just below the lithosphere. It starts about 80 to 200 kilometers (50 to 120 miles) beneath the Earth's surface and goes 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 clear. It 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 weaker than the rock above it because it has small amounts of melted material. Tiny bits of melt, along with water and carbon dioxide, make the rock more flexible. Even though there is very little melt, it still helps make the asthenosphere less strong.

The asthenosphere may also be weak because of how the rocks move 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 move 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 is called decompression melting and it 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. Small amounts of water and carbon dioxide in the rock help it melt. As the rock rises, 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.