Omar Khalid
The asthenosphere is a hot, semi-fluid layer inside the Earth's mantle, extending from approximately 100 to 700 kilometres below the Earth's surface. It is composed of peridotite, a rock containing mostly the minerals olivine and pyroxene. The asthenosphere is where the mantle rock most closely approaches its melting point, with a small amount of melt likely to be present. This gives the asthenosphere a plastic-like quality, allowing it to flow slowly over geological timescales and enabling the movement of the more rigid lithospheric plates above it. This flow of the asthenosphere is believed to play a critical role in the movement of the Earth's tectonic plates.
| Characteristics | Values |
|---|---|
| Location | 80-200 km (50-120 miles) to 700 km (430-450 miles) below the Earth's surface |
| Composition | Peridotite, a rock containing mostly the minerals olivine and pyroxene |
| State | Semi-solid, semi-fluid, ductile |
| Velocity of seismic waves | Relatively slow |
| Velocity of S-waves | Slows down significantly at 62 miles (100 km) depth |
| Velocity of S-waves increases | At a depth of 155 miles (250 km) |
| Temperature | Hotter than the lithosphere |
| Pressure | Up to 24 gigapascals, approximately 240,000 times the atmospheric pressure at sea level |
| Role | Plays a critical role in the movement of tectonic plates |
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What You'll Learn
- The asthenosphere is a semi-fluid layer inside the Earth's mantle
- It is composed of peridotite, a rock rich in iron and magnesium
- The asthenosphere is involved in plate tectonic movement
- It is the most important source of magma on Earth
- The asthenosphere is where the mantle rock most closely approaches its melting point
The asthenosphere is a semi-fluid layer inside the Earth's mantle
The asthenosphere is sometimes referred to as a "plastic" layer because it is ductile and can deform and flow over geological timescales. This plasticity allows the more rigid lithospheric plates above it to move around, facilitating the movement of tectonic plates. The heat in the asthenosphere comes from the decay of radioactive elements and the Earth's core, and it plays a critical role in driving plate tectonics by lubricating the undersides of the tectonic plates.
The asthenosphere is where the mantle rock most closely approaches its melting point, with a small percentage of molten material present. This partial melting contributes to the mechanical weakness of the asthenosphere, making it less rigid than the lithosphere above it. Seismic studies have shown that S-waves slow down significantly as they reach the depth of the asthenosphere, indicating its semi-fluid nature.
The upper part of the asthenosphere is believed to be the zone upon which the rigid and brittle lithospheric plates of the Earth's crust move. The interaction of temperature and pressure in the asthenosphere causes the rock to become ductile, moving at rates of deformation that eventually result in thousands of kilometres of distance over time. This movement of the asthenosphere influences the convection currents that drive plate tectonics and the creation of new crust through volcanic activity.
The asthenosphere is also believed to be the repository for older and denser parts of the lithosphere that are dragged downward in subduction zones. The decompression melting of asthenospheric rock as it moves towards the surface is the most important source of magma on Earth, contributing to the formation of mid-ocean ridge basalt (MORB) and magmas that erupt in subduction zones and regions of continental rifting.
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It is composed of peridotite, a rock rich in iron and magnesium
The asthenosphere is a part of the upper mantle just below the lithosphere. It is involved in plate tectonic movement and isostatic adjustments. The lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, enabling the movement of tectonic plates. The asthenosphere is composed of peridotite, a dense, coarse-grained ultramafic igneous rock. Peridotite is composed of mostly the minerals olivine and pyroxene. It is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle.
Peridotite is a rock rich in iron and magnesium. It is a generic name for a number of different rock types. All of them are rich in olivine and mafic minerals. They are usually green in colour and have a high specific gravity for a nonmetallic material. Peridotite is classified as ultramafic because it contains less than 45% silica. It is high in magnesium (Mg2+) and has a typical magnesium number of 89. In other words, of the total content of iron plus magnesium, 89 mol% is magnesium. This is reflected in the composition of the mafic minerals making up the peridotite.
Olivine is the essential mineral found in all peridotites. It is a magnesium-rich silicate mineral (Mg2SiO4-Fe2SiO4) with the variable formula (Mg,Fe)2SiO4. Olivine is typically green in colour and has a glassy or granular texture. The abundance of olivine in peridotite can influence the overall composition of the rock. Pyroxenes, such as clinopyroxene and orthopyroxene, are also important minerals in peridotite. They are silicate minerals that can have a range of chemical compositions, but in peridotite, they are typically rich in iron and/or magnesium.
Amphiboles are another group of minerals found in peridotite. They have varying chemical compositions but often contain calcium, magnesium, and iron. Common amphiboles found in peridotite include tremolite and actinolite. In addition to these primary minerals, peridotite can also contain minor amounts of other minerals such as spinel, garnet, and chromite, among others. The compositions of peridotites can vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.
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The asthenosphere is involved in plate tectonic movement
The asthenosphere is a semi-fluid layer found in the upper mantle of the Earth, situated just below the rigid lithosphere. It is composed of peridotite, a rock containing mostly the minerals olivine and pyroxene. The lithosphere-asthenosphere boundary is conventionally taken at the 1,300 °C (2,370 °F) isotherm. The asthenosphere is almost solid, but a slight amount of melting (less than 0.1% of the rock) contributes to its mechanical weakness.
Heat from deep within the Earth is thought to keep the asthenosphere malleable, lubricating the undersides of Earth’s tectonic plates and allowing them to move. Convection currents generated within the asthenosphere push magma upward through volcanic vents and spreading centres to create new crust. The movement of the plates causes mountains to rise where plates push together and continents to fracture and oceans to form where plates pull apart.
The asthenosphere is also the most important source of magma on Earth. Decompression melting of asthenospheric rock creeping towards the surface occurs, erupting at mid-ocean ridges to form the distinctive mid-ocean ridge basalt (MORB) of the ocean crust.
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It is the most important source of magma on Earth
The asthenosphere is a mechanically weak and ductile region of the upper mantle of the Earth, lying below the lithosphere. It is believed to be much hotter and more fluid than the lithosphere. The upper part of the asthenosphere is a zone upon which the rigid and brittle lithospheric plates of the Earth's crust move about. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, enabling isostatic equilibrium and allowing the movement of tectonic plates.
The asthenosphere is almost solid, but a slight amount of melting (less than 0.1% of the rock) contributes to its mechanical weakness. More extensive decompression melting of the asthenosphere takes place where it wells upwards, and this is the most important source of magma on Earth. Decompression melting in upwelling asthenosphere likely begins at a depth of 100 to 150 kilometers (60 to 90 miles), where small amounts of volatiles in the mantle rock assist in melting. At a depth of about 70 kilometers (40 miles), dry melting conditions are reached, and melting increases substantially. This melting of asthenospheric rock creeping towards the surface is the most important source of magma on Earth.
Most of this magma erupts at mid-ocean ridges to form the distinctive mid-ocean ridge basalt (MORB) of the ocean crust. Magmas are also generated by decompressional melting of the asthenosphere above subduction zones and in areas of continental rifting. The asthenosphere is the source of mid-ocean ridge basalt (MORB) and some magmas that erupt above subduction zones or in regions of continental rifting.
Convection currents generated within the asthenosphere push magma upward through volcanic vents and spreading centres to create new crust. Convection currents also stress the lithosphere above, and the cracking that often results manifests as earthquakes.
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The asthenosphere is where the mantle rock most closely approaches its melting point
The asthenosphere is a hot, semi-fluid layer inside the Earth's mantle. It is located below the rigid lithosphere, extending from approximately 100 to 700 kilometers (62 to 435 miles) below the Earth's surface. The asthenosphere is where the mantle rock most closely approaches its melting point, with a small amount of melt likely present in this layer. This is due to the high temperatures and pressures at these depths, causing the rock to become partially molten and ductile. The presence of a very small percentage of melt in the asthenosphere is thought to cause the decrease in seismic wave velocity observed from the lithosphere to the asthenosphere.
The asthenosphere is composed of a type of rock called peridotite, which is rich in iron and magnesium. Peridotite is a dense, coarse-grained igneous rock that forms the bulk of the Earth's mantle. While the asthenosphere is mostly solid, it is able to flow due to the small percentage of molten material within it. This gives the asthenosphere a plastic-like quality, allowing it to deform and flow slowly over geological timescales. The plastic nature of the asthenosphere allows the more rigid lithospheric plates above it to move, playing a critical role in the movement of Earth's tectonic plates.
The heat in the asthenosphere is generated by the decay of radioactive elements and the Earth's core. This heat causes the asthenosphere to convect, with hot material rising and cooler material sinking, driving plate motion. The asthenosphere is also the source of mid-ocean ridge basalt (MORB) and some magmas that erupt above subduction zones or in regions of continental rifting. The decompression melting of asthenospheric rock as it creeps towards the surface is the most important source of magma on Earth.
The asthenosphere has been called the low-velocity zone (LVZ) due to the relatively slow passage of seismic waves through this layer. The lower boundary of the LVZ is at a depth of 180 to 220 kilometers (110 to 140 miles), while the base of the asthenosphere is at a depth of about 700 kilometers (430 miles). The LVZ exhibits high seismic attenuation, with seismic waves losing energy as they pass through the asthenosphere. The discovery of the LVZ through seismic studies provided evidence for the existence of the asthenosphere and revealed some of its physical properties, such as the decrease in seismic wave velocity with decreasing rigidity.
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Frequently asked questions
The asthenosphere is a hot, semi-fluid layer inside the Earth's mantle. It lies beneath the rigid lithosphere, extending from approximately 100 to 700 kilometres (62 to 435 miles) below the Earth's surface.
The asthenosphere is mostly solid, but it contains a small percentage of molten material, giving it a plastic-like quality. This allows the asthenosphere to flow slowly over geological timescales.
The asthenosphere is primarily composed of a type of rock called peridotite, which is rich in iron and magnesium. Peridotite is a dense, coarse-grained igneous rock that forms the bulk of the Earth's mantle.
The asthenosphere plays a critical role in the movement of Earth's tectonic plates. The rigid lithosphere moves along the top of the plastic asthenosphere, enabling isostatic equilibrium and allowing the movement of tectonic plates. This movement of plates is responsible for geologic features such as volcanoes, lava flows, and mountain-building.
