In what layer does plasticity and convection occur?

Mantle.

Explanation:

The mantle is present between the earth's crust and the core. It is not a static layer. The upper part of the mantle is called the asthenosphere which changes its shape with temperature and pressure. It is not a rigid layer. Any changes in the inner layer of the mantle cause the movements in asthenosphere plates. This is called the plasticity of the mantle. The convection is a process, where high temperature causes the movement of atoms. The atoms move from high-temperature region to the low-temperature region and thus a cyclic movement happens in the inner mantle. This causes the changes in the plates location of the upper mantle.

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when the displaced particles are perpendicular to the direction the wave travels, is called a transverse wave, whereas in longitudinal wave the displace practices are parallel to the direction of the wave.

in the given image:

first image shows - longitudinal wave

second image shows - transverse wave

third image shows - longitudinal wave

fourth image shows - longitudinal wave