Know About Transition Zone

We all have come across and learned about the earth’s internal structure. We have studied that the earth has three main layers: the crust, the mantle, and the core. The core has two parts: the outer core and the inner core. The inner part is hot (yellow), and the outer is molten (orange). The mantle part is red, and the crust part is brown and thin. However, this article is about a different segment of the mantle – the transition zone. Let us understand what the transition zone is and other aspects related to it.

The Transition Zone

The part of the earth’s mantle located in the middle of the upper and lower mantle, between a depth of 410 and 660 km, is known as the transition zone. The transition zone and the earth’s mantle primarily consist of peridotite, which is an ultramafic igneous rock. 

The transition zone is covered by seismic discontinuities between depths of 410 and 660 km. The seismic discontinuities are considered to be due to olivine-β-olivine and post-spinel change of phase, respectively. The space of the transition zone (between 410 and 660 km) is also a measure of thermal anomalies because the approximates are affected less by upper mantle velocity design, and it can be more reliable.

Discontinuity depths are defined by identifying reflected waves or changes at the discontinuities and measuring their timings.

Observing the Transition Zone

The transition zone has an area between 410 and 660 km of the earth’s mantle. Within this area, sudden jumps in seismic velocity are observed, primarily due to constant change of phase in mantle minerals.

Two main velocity discontinuities occur at the mantle depths of 410 and 660 km. The depth at 410 is the top of the transition zone, and the base is located at 660. The discontinuities are not quick and occur over a limited area in depth. These discontinuities can also arise because of variations of chemical compositions at these depths of the mantle; phase changes due to high pressures seem more explanatory.

High-pressure observation has revealed that olivine, the primary mineral in mantle peridotite (a rock rich in olivine mineral), changes to the spinel structure at high-pressure/ temperature at a depth of 410 km and further to perovskite and magnesiowustite at a depth of 660 km. As the lithosphere decreases, the temperature is colder than that at the upper mantle. The depths when these discontinuities occur are changed, as identified by thermal modelling and high-pressure experiments.

This finding provides enough assistance to the hypothesis that the upper and lower coverings of the transition zone are determined by phase transformations. Garnet and pyroxene, which are the other parts of mantle peridotite, also go through phase transformations at the depth but at moderate speed, and discontinuities do not occur when seismic velocity changes with depth range. Pyroxene changes into garnet from pressure around 350-500 km depth. Ca-perovskite begins to separate from garnet at a pressure of 580 km, and the leftover garnet gets mixed with perovskite at 660-750 km. This leads the lower mantle to consist mainly of the perovskite phase. 

Conclusion

The mantle layer of the earth’s internal structure has a complex structure. The mantle transition zone is the layer between two discontinuities in seismic speed waves located at the estimated depths of 410 km and 660 km. It was discovered in the 1960s that there are sudden jumps in seismic velocity at these depths. It happens because of olivine, the dominant mineral in mantle peridotite, a rock.