Important Facts About Earth’s Mantle
The mantle is a highly viscous layer between the crust and the outer core. The boundary between crust and mantle is called Mohorovičić discontinuity after the name of Croatian geologist Andrija Mohorovičić who proposed this. No one has been able to physically drill into the mantle and there are no samples of the mantle with human beings as of now. Whatever information we have is based on indirect study, particularly of seismic waves.
Composition of the Earth’s Mantle
Similar to earth’s crust, Oxygen is most abundant element in Earth’s Mantle. The following table shows the composition of earth’s mantle.
| Element | Amount | Compound | Amount |
|---|---|---|---|
| O | 44.8 | ||
| Si | 21.5 | SiO2 | 46 |
| Mg | 22.8 | MgO | 37.8 |
| Fe | 5.8 | FeO | 7.5 |
| Al | 2.2 | Al2O3 | 4.2 |
| Ca | 2.3 | CaO | 3.2 |
| Na | 0.3 | Na2O | 0.4 |
| K | 0.03 | K2O | 0.04 |
| Total | 99.7 | Total | 99.1 |
Salient Features of Mantle
Earth’s mantle is a rocky shell about 2,890 Kms thick that constitutes about 84 percent of Earth’s volume. It is predominantly solid and encloses the iron-rich hot core, which occupies about 15 percent of Earth’s volume. The mantle is divided into sections viz.
- The Upper Mantle, which starts from the Mohorovičić discontinuity around 7 to 35 km, downward to 410 km),
- The transition zone (410–660 km)
- The Lower Mantle (660–2891 km).
The upper and lower mantle differentiate on the basis of seismic and chemical changes in the layer. These changes create different kinds of discontinuities in the mantle. For example:
- Hales Discontinuity is found in the upper mantle at depths of about 60 to 90 kilometers, a region in which seismic velocities change.
- Gutenberg Discontinuity or the core–mantle boundary (CMB) lies between the Earth’s silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km depth beneath the Earth’s surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth.
Convective Material Circulation in Mantle
Due to the temperature difference between the Earth’s surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle. Hot material upwells, while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones. The convection of the Earth’s mantle is a chaotic process, which is thought to be an integral part of the motion of plates. Here, we have to note that the Plate motion is different from the continental drift which applies purely to the movement of the crustal components of the continents.