Rocks Formed by Pressure

Pressure and Heat

• The three types of rocks are classified by how they were formed. "Metamorphic" means to change form, and a metamorphic rock is transformed by great pressure or great heat. These rocks were originally either sedimentary or igneous rock. However, the movement of the Earth's crust changed the rock into another type, according to the Rocksforkids website. This type of rock is the least common of the three types of rocks.
  
  

Marble

• An example of metamorphic rock is marble, which is originally sedimentary rock but transforms over time into a metamorphic rock. When the Earth's crust moves, rocks are squeezed and the resultant heat causes the rock to change form. Squeeze your hands together and see if you can feel the pressure as well as the heat that results. This is the same thing that happens when rocks are squeezed.
  
  

Types of Metamorphic Rock

• Metamorphic rocks that are foliated feature banding and layers. An example of this is schist, a common metamorphic rock, as well as mica, which is the most common mineral. Slate turns into shale during the metamorphic process. It splits apart into flat slabs that are smooth. Gneiss is a metamorphic rock made up of alternating layers of minerals; it looks streaky as a result. When a metamorphic rock is non-foliated, it is not layered. Marble falls into this category and it is transformed into limestone during the metamorphic process. Another non-foliated rock is quartzite. It is extremely hard.
  
  

Foliation

• Foliation happens when rocks are altered by regional metamorphic compression. Foliation is the penetrative planar fabric found in the rock. The "fabric" is made when the rock is in its original formation. Penetrative fabric means that it is found throughout the rock, usually all the way down to the grain scale -- also called particle size, the diameter of the individual grains of particles or sediment found in a rock.
  
  

Igneous and Sedimentary

• The two other kinds of rocks include igneous, which means that it is the result of heat or fire. When a volcano erupts, liquid rocks spew from the cone and new igneous rock is created. When it is liquid and inside the earth, this rock is called magma. When it gets hard inside the crust, it becomes granite. The liquid that comes out of a volcano is lava. When it cools and hardens, it is referred to as pumice, lava rock or obsidian, depending on what it looks like.
  
  The third kind of rock is sedimentary, which are rocks that start out as layers of mud and sand at the bottom of the ocean or a lake and are transformed into rock. Sandstone and shale are sedimentary rocks. You are likely to find fossils in sedimentary rock.