Question

Tectonic plates float individually over the _____.
A) Crust
B) Mantle
C) Inner core
D) Outer core

Answer 1

Hint: The tectonic plates are huge pieces of rocks in the form of plates that make up the crust and the upper mantle. The entire crust along with the upper mantle forms the lithosphere of the earth. The tectonic plates are hence part of the lithosphere of the earth. In other words, we can say that the lithosphere itself is broken into tectonic plates.

Complete answer:
The earth is divided into layers i.e. the crust, upper mantle, lower mantle, outer core and the inner core. As we know that the lower regions of the crust and the upper mantle together form the tectonic plates, this knowledge can be used to identify over which layer they must be floating.

Now let us look into the given options:
Option A) Crust – The crust is the uppermost or the outermost layer of the Earth. It is rocky and made up of tectonic plates. These plates either form the continents and are called continental plates, or they may form the ocean bed and be called the oceanic plates. The crust itself is part of the tectonic plate, hence we cannot say that the tectonic plate floats over the crust. Therefore, this option is wrong.

Option B) Mantle – The mantle is the layer of silicate rock that behaves like a viscous liquid. Its uppermost layer forms the base of the crust and the tectonic plate. The lower mantle is the liquid region where the magma flows. On this sea of magma in the lower mantle, the tectonic plates float individually. Thus, this is the correct option.

Option C) Inner Core – The inner core lies below the outer core and is a solid hardball with a radius of 1220 kilometres. This region is also believed to be made of iron and nickel and some other alloys. This region is solid, unlike the mantle, because of the extremely high pressure from the weight of the earth. Since it is solid, tectonic plates cannot float on it. Thus, this option is also incorrect.

Option D) Outer Core – The lower mantle transitions into the outer core which is also a liquid layer. It is composed of iron and nickel, but the temperature here is so high that these metals exist in their molten state. This layer has no contact with the tectonic plates. Therefore, this option is incorrect.

Thus, the correct answer is Option (B) Tectonic plates float individually over the mantle.

Note: The sea of magma in the lower mantle, on which the tectonic plates float, is known as the asthenosphere. The upper boundary of the asthenosphere is called the lithosphere-asthenosphere boundary or LAB and is a well-defined region. The lower boundary of the asthenosphere, however, is not well defined and it merges into the lower mantle. The presence of this region was suspected in 1926 and confirmed in 1960.