Metamorphic Rocks are formed by changing the structure of pre-existing rock due to high pressure and temperatures deep within the crust of earth, or by chemically altering pre-existing rock. Metamorphic rocks are never melted in the process, just heated and squeezed such that the arrangement of the minerals in the original rock is changed. The degree of metamorphism depends on the amount of heat, pressure, and time applied to the original rock, and as such, one type grades into the next.

The type of rock that results from the metamorphic process depends on the original rock and the degree and type of metamorphism. For example, sandstone changes to quartzite, clay-rich sedimentary rocks change to schist, and granite changes to gneiss. Limestone is chemically changed to dolomite when some of the calcium carbonate is replaced with magnesium carbonate, and both can be heated and compressed to form marble.

	Sandstone changes to quartzite. Differences in the sand that formed the sandstone affect the nature of the quartzite, but generally, the original particles are still visible in quartzite. Wind-blown beach sands and dune sands form fine grained quartzite, while river sands form quartzite showing pebbles and other artifacts among the sand grains.
	Granite changes to gneiss (pronounced "nice"). When heated and placed under pressure, the original crystalline minerals (quartz, feldspar, and mica) begin to melt and segregate. When the rock cools, the result is separate layers of quartz and the other minerals, generally with fairly coarse grains. Gneiss usually shows these layers, and is referred to as foliated.
More to come ...	Clay-rich sedimentary rocks change to schist
More to come ...	Limestone is chemically changed to dolomite when some of the calcium carbonate is replaced with magnesium carbonate
More to come ...	Limestone and dolomite change to marble