Igneous Rocks- Igneous rocks form from the solidification of magma (molten rock). A reminder: magma is molten rock beneath the Earth’s surface and lava is magma that is on the Earth’s surface. The main characteristics of an Igneous rock are crystal size (grain size) and texture. The crystal size and texture of an igneous rock is dependant on where in the earth the rock formed.
If an igneous rock is intrusive (plutonic) that means it formed and cooled under the Earth’s surface (crust) and since the temperatures are greater at this level the rocks would take longer to cool. This would give the igneous rock samples large crystals along with a coarse texture due to the slow cooling motion. A few examples of large crystal, coarse texture igneous rocks are: granite and gabbro.
Sedimentary rocks are formed from the burial/compaction and cementing of earth materials. These earth materials (sediments) are carried by natural agents such as wind, water, and glaciers and then are deposited on land or underwater. Sedimentary rocks usually contain cemented rounded grains that may be the same size (sandstone) or different sizes (conglomerate) or in compacted layers (shale).
Metamorphic rocks are other rocks (igneous and sedimentary) that have undergone high amount of pressure and temperature deep within the Earth.
Metamorphism is the result of solidification of material under high temperature and pressure. There are two types of metamorphism, contact and regional. The difference between the two is simple. Contact will occur in a small area usually as a result of an igneous intrusion. Regional occur in a large area due to intense heat and pressure within the earth.
Metamorphic rocks are usually harder and denser than other rock types due to the intense heat and pressure they are exposed to. A great characteristic to recognize a metamorphic is to look for bands or foliation of the rock sample. The banding in metamorphic rocks has a stripped appearance. The thicker the bands the more intense the heat and pressure conditions were to form the rock.