These are formed as a result of sedimentary and igneous rocks being altered by chemistry, heat and pressure to form a new kind of rock. Some rocks that have been altered from sedimentary rocks are slate, mica, schist, gneiss, quartzite and marble. Some altered forms of igneous rocks are actinolite, chlorite, and amphibolite. There are several causes for rocks changing in this way. An increase in temperature and pressure caused by geological movement such as earthquakes or volcanic eruptions can change the rocks.
Water carrying chemicals can be the cause of new minerals in the rocks.
A very long period of time is necessary for these changes to occur. Completely new minerals can be formed that were not in the original rock or original mineral crystals. Original mineral crystals can be rearranged in shape, though not chemically altered. Water and carbon dioxide are frequently added or removed during metamorphism. In this way, new minerals are formed and recrystallization takes place.