Unit 3.2: Mantle

• The mantle is by far the largest layer which accounts for about 83% of Earth’s volume. It is located between the thin crust and the core which extends to depths of 2 900 km.
• Mantle


  This layer is rich in magnesium oxide that distinguishes it from the crust.
• Magnesium oxide is resistant to changes when exposed to intense pressure and temperature.
• Magnesium Oxide


  At a certain point when melting, this compound transcends from being an electrical insulator to a conductor.
• The bulk composition of the upper mantle is believed to be peridotites
• Peridotites


  dark-colored ultramafic igneous rocks
• Peridotites are believed to be economically significant rocks which have ores of chromium from chromite it contains. They can also be a source of diamonds
• Peridotites


  Most importantly, these rocks show potentials in sequestering carbon dioxide which humans can intentionally do to solve the drastically occurring climate change.
• The lower mantle is dominated by the mineral, bridgmanite, a type of perovskite.
• Bridgmanite


  This mineral can only exist under high-temperature conditions of at least 2 100 degrees Celsius, and high pressures of 240 metric tons per square centimeters, which is about 240 000 times the atmospheric pressure at sea level. It is in a fluid state and moves slowly in the mantle.
• When Earth experiences ground movements and shaking during earthquakes, body waves are produced.
• p-waves


  can travel in liquids and solids
• s-waves


  travel through solids only
• Scientists who study the behavior of body waves have found out that there is a decrease in the velocity of the waves in the upper mantle and increases as the waves reach the lower mantle. These findings suggest that the mantle may be divided into layers
• Seismic studies have revealed that the mantle consists of layers including the lithosphere, asthenosphere, and mesosphere.
• lithosphere


  is comprised of the crust and the uppermost part of the mantle. This layer is rigid (breaks under stress) and extends to depths of up to 150 km and has an average thickness of 100 km.
• asthenosphere


  lies below the lithosphere. Here, rocks have plasticity and have the ability to flow. This allows the plates above this layer to move.
• mesosphere

  is also called the lower mantle. This layer lies beneath the asthenosphere. This layer is composed of semi-solid ultrahigh-pressure silicates
• Mantle convection refers to the transfer of heat from the mantle to the crust through the movement of mantle materials.
• Mantle convection


  The upward and downward motions of materials due to temperature differences allows the formation of convection currents.
• The temperature from the crust to the lower part of the mantle is generally increasing.
• Geothermal Gradient


  rate of increase in temperature with respect to depth
• The geothermal gradient is around 15-30 °C/km but actually varies per region.
• Geothermal Gradient


  This temperature gradient increases slowly through the mantle and dramatically increases at its base
• Gutenberg discontinuity


  core-mantle boundary