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rocks can change from one type of rock to another, or not change at all
Earth's surface and inside the planet slowly change rocks from one kind to another.
Foliation
heat and pressure
metamorphic
metamorphic
The rock cycle
If by form you mean the composition and structure, the forces of heat and pressure can lead to metamorphism of a rock from one type into another.
yes, rocks change. metamorphic, sedimantary and igneous rocks can change into eachother.
Rock cycle
Metamorphic rocks are rocks that have "morphed" into another kind of rock. These rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you exam metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.
I believe pressure is the answer you are looking for here. Other forces that change the shape of rocks include weathering and erosion.