Lower mantle

The lower mantle is the third and bottom-most layer of Earth's mantle. It lies between the transition zone and the outer core, stretching from about 660 to 2,890 kilometers below Earth's surface. This layer makes up around 56% of Earth's total volume.

Pressure and temperature in the lower mantle are extremely high, ranging from 24 to 127 gigapascals and 1,900 to 2,600 Kelvin. Scientists believe it is made mostly of materials called pyrolitic, with three main types of minerals: bridgmanite, ferropericlase, and calcium-silicate perovskite. These minerals change under the immense pressure, which can influence how heat moves through the Earth.

The lower mantle is important for understanding how our planet moves and changes. We learn about it by studying how seismic waves travel through it and by doing experiments that mimic these extreme conditions in laboratories.

Physical properties

The lower mantle is a deep part of Earth located between the transition zone and the outer core. It was once called the D-layer. Scientists have divided it into three main layers based on how seismic waves move through it.

The top layer is from 660 to 770 kilometers deep. Here, minerals change form, which affects how seismic waves travel. The middle layer, from 770 to 2,700 kilometers, shows a slow increase in wave speeds as the minerals are pressed together. The bottom layer, from 2,700 to 2,900 kilometers, marks the start of the outer core.

Temperatures in the lower mantle are very hot, ranging from about 1,960 K to 2,630 K. Heat moves mainly through convection, with very little from conduction or radiation.

Composition

The lower mantle is mostly made of three materials: bridgmanite, ferropericlase, and calcium-silicate perovskite (CaSiO₃-perovskite). Scientists have different ideas about how much of each material is there. One idea, called pyrolitic, suggests the lower mantle is made of 75% bridgmanite, 17% ferropericlase, and 8% CaSiO₃-perovskite. Another idea, called chondritic, thinks the lower mantle has more bridgmanite and CaSiO₃-perovskite.

Experiments and calculations have tried to figure this out, but there is still discussion about the exact mix. Some studies suggest a mix close to the pyrolitic idea matches certain measurements, while others suggest a mix with more bridgmanite fits better with different measurements.

Spin transition zone

In the lower mantle, certain minerals containing iron change their electronic structure. This change is called a spin transition. For one mineral, ferropericlase, this happens between pressures of 50–90 GPa. In another mineral, bridgmanite, part of the iron changes at 120 GPa, while another part changes between 30–70 GPa. These changes affect how these minerals behave, such as becoming denser. Scientists are studying how these spin transitions influence the movement and other properties of the lower mantle.

History

The word mesosphere comes from an old idea about Earth's layers. A teacher named Reginald Aldworth Daly from Harvard University thought Earth had three main layers: the lithosphere (which includes the crust), the asthenosphere, and the mesospheric shell. He believed the mesospheric shell started about 200 to 480 kilometers below the surface.

Later, some people used a new term, mesoplates, mixing "mesosphere" and "plate," to talk about how hot spots in the mantle might move.