What is Rock Cycle? How does it keep the earth young?

Rocks undergo cyclic transformation. The continuous process of transformation of old rocks into new ones is known as rock cycle. The cycle may also pass through the process of formation of sedimentary, metamorphic and igneous rocks.

To keep the earth young, rocks melt again resulting in the formation of igneous rock. This disintegrated material again forms sedimentary rock, which takes hundreds and thousands of years.