Non-foliated metamorphic rocks are rocks that have been changed by heat and pressure into rocks with a non-layered or banded appearance. Some examples of non-foliated metamorphic rocks include quartzite, marble, amphibolite and hornfels.
Both foliated and non-foliated metamorphic rocks are formed deep under the earth's surface over millions of years by the combination of heat, pressure and chemical processes. The amount of heat and pressure determines the final state of metamorphic rocks, as does their specific mineral contents.
Generally speaking, non-foliated metamorphic rocks were created under extremely high temperatures and very low, even amounts of pressure. These rocks typically cooled very slowly, allowing the minerals within them to form large, uniform crystals that are tightly packed together.
Different types of rocks and minerals can form non-foliated metamorphic rocks. Quartzite is made of metamorphosed sandstone, while marble is formed from limestone that was metamorphosed. Hornfels have no specific composition while amphibolites are formed from hornblende and plagioclase.
Although non-foliated rocks may have small bands of different colors, the crystals that form them generally have no visible organization. On the other hand, foliated metamorphic rocks have very obvious bands of layers that formed as a result of a more rapid cooling or higher pressure. Some examples of foliated metamorphic rocks include gneiss, schist, slate and soapstone.