Rocks
All rocks are formed from minerals. There are 3 main classifications of rocks they are igneous, sedimentary, and metamorphic. These rocks are formed under different conditions and posses certain characteristics. The characteristics a rock has depends upon the condition in which it formed. The rock cycle illustration below, from page 6 of the Earth Science Reference Tables, shows the process that each rocks type undergoes to form. An explanation to key identifiers for each rock will be discussed individually for each rock type.

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.

If an igneous rock is extrusive (volcanic) that means it formed on or close to the Earth’s surface where temperatures are cooler than deep within the earth. This would give the igneous rock samples small or possibly no crystal structure at all. The texture of these samples would range from fine (basalt) to glassy (obsidian).

The chart below, from page 6 of the Earth Science Reference Tables and shows the characteristics of the major igneous rocks that are to be identified. The color, density, and composition of igneous rocks depend on the mineral composition of each rock sample. To read the percentage of minerals in an igneous rock sample follow the column in which it is found directly down. Remember that not all samples of the same rock look the same so different percentages of minerals can be found for one rock.

Sedimentary Rocks

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).

Other Forms of Sedimentary Rock

Chemical: form from the evaporation and precipitation of dissolved minerals. The precipitation of a mineral forms rocks called evaporates, for example, limestone, rock salt, gypsum, etc.

Biological: form from the result of biological activity from millions of years ago. This includes all forms of flora (plants) and fauna (animals). The best example is coal which form from the deposits of plants and animals.

Metamorphic Rocks

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.

To read more about how igneous, sedimentary, and metamorphic rocks are formed visit Rock Hounds.