How is magma part of the rock cycle?

The Rock Cycle follows a series of steps that may simplistically be considered as :- Plutonic rocks either erupting or emerging; followed by a mountain buildingprocess; followed by a surface weathering process which produces sediments that eventually find their way to the abyssal deeps of the ocean floor.

These sediments may be consumed by subduction zones at some plate boundaries, and the sediments become deeply buried and heated. The high temperatures and pressure cause Metamorphic processes to proceed, thus making new rock minerals from the sedimentary mix. These can be investigated to determine the conditions at which these new minerals were formed.

Some of these metamorphic rocks are borne directly to the surface again as new mountain ranges, for these sedimentary rocks are lighter than plutonic ones are. Otherwise, the descending rocks will be eventually absorbed into the magma. To begin the cycle over again.

Because of the sedimentary rocks being lighter than the magmatic ones, they 'float' on the magma, forming long lasting continental plates. Some of the older ones date to about 4 billion years old.