The main cause of global convection currents is as a result of the uneven
heating of the Earth by the Sun.
Convection involves the movement of heat in a fluid without direct contact
with the heating surface unlike conduction which involves physical
contact.
Convection takes place in fluids ( water and air) and the heated fluid is
usually replaced by the cooler ones to be heated and the cycle continues.
The Sun not heating the earth surface equally due to its spherical shape is
the reason why global convection currents occurs.
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