Question

What is the cause of upthrust?

Answer 1

As the liquid contained in a vessel exerts pressure at all points and in all the possible directions and the pressure at a point in the liquid is same in all directions. The pressure increases with depth inside the liquid.
If a block is immersed in a liquid, the pressure exerted upwards on the lower surface of the block is more than the pressure exerted downwards on the upper face of the block. 
Thus there is a pressure difference between the lower and upper face of the block. 
The difference between the pressures due to liquid on the two faces of the block causes the net upward force (i.e., up thrust) on the body.

Answer 2

Answer:

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Explanation:

Upthrust force is caused due to difference of pressures between lower and upper surfaces of the immersed body in a fluid. Upthrust force acts on lower surface of the immersed body in upward direction.