Why? Because... While convection is generally discussed as a form of heat flow within liquids and gases, it can occur in solids too. For example, the temperature and pressure conditions in the Earth's mantle allow mantle rock to flow very slowly. Hotter rock gradually rises up and cooler rock gradually sinks down in large convection currents in the mantle.
We know that as we move <u>towards Earth's center</u>, the <u>temperature rises</u>. This rise in temperature causes rocks to melt (magma). The high temperature and melting of rocks <u>decrease the density of material </u>which makes it <u>move upward</u>. By <u>moving upward</u>, the molten rock <u>starts turning solid</u> and becomes <u>denser again</u>. Here, it <u>sinks back due to gravity</u>. This upward and downward movement of rocks due to density and temperature makes a convection cell.
PS: The formation of convection cells in Earth's interior is also the reason for plate tectonics.
