I believe the answer is pressure, or else stress. The change is known as metamorphism when heat is involved (which it almost certainly is).
If the rock is in a river it gets smoothed.
Those would be the forces of gravity and self weight as well as in-situ stresses which may cause compression, tension or shear.
The answer is metamorphic change.
Stress
Weathering.
the force that acts on rock to change its shape or volume is stress.
gas!
Weathering is the process that results in the reduction of size and corresponding shape of a body of rock.
A force that acts on rock to change its shape or volume called stress. Tension is the stress that sketched rock so that is becomes thinner in the middle.
I believe pressure is the answer you are looking for here. Other forces that change the shape of rocks include weathering and erosion.
Pressure / stress.
Stress
all of the forces
pressure
The force which changes the dimension of the object such as length, volume or which changes the shape is known as deforming force
Erosion shapes the rock as wind, sand, water, and other forces of nature wear down the rock.
A gas can change shape but it doesn't change volume, a liquid also changes shape but doesn't change volume.
SHAPE
It changes shape and volume.
the force that acts on rock to change its shape or volume is stress.
gas
the answer is liquid... as my science teacher says.