How to Identify Rocks
The Differences between Igneous, Sedimentary and Metamorphic Rocks
Nov 19, 2008
When it comes to the three rock types, it's all in the name. The names themselves - igneous, sedimentary and metamorphic - are descriptions of how the rocks formed. Understanding how the rocks physically or chemically formed can be the first step in being able to identify rocks in the field. It also helps to have the right equipment. Geologists commonly use rock hammers and rock lenses, or small magnifying glasses, to aid in the identification of rocks and rock minerals; but the most important tool is a good eye.
The composition of rocks varies according to its minerals, which are the building blocks of rocks. The types of minerals that make a rock depend largely on the source of the rock - igneous rocks have more magma-related minerals like olivine and biotite; sedimentary rocks have more sediment-related minerals like quartz; metamorphic rocks have all these minerals, but they may look different due to re-crystallization. Having a rock lens or a magnifying glass can help you see these minerals better when looking at a rock.
Identifying Igneous Rocks
Igneous rocks form directly from magma. There are two kinds of igneous rocks - intrusive and extrusive - and they look very different from each other. Intrusive igneous rocks form below the surface of the Earth as magma chambers rise slowly through the surface. Because the magma cools slowly, mineralization happens slowly and the minerals are allowed to grow big enough that they can be seen with the naked eye. A very common type of igneous intrusive rock is granite, which is light colored and contains light-colored minerals such as quartz and feldspar.
Igneous extrusive rocks form from the fast cooling of magma that breaks the surface of the crust through a volcanic eruption or a rift. Since this rock forms very quickly, the minerals have less time to form, often resulting in very small minerals that cannot be seen with the naked eye. Common types of igneous extrusive rocks are pumice and basalt. These rocks can also have vesicles because of the trapping of gas bubbles in the magma as it quickly solidifies to a rock.
Identifying Sedimentary Rocks
Sedimentary rocks form from the sediments, or clasts, of other eroded rocks. As wind and water erode and deposit pieces of other rock formations, they are compacted and cemented, and thousands to millions of years later, become a rock of their own. These clasts can range in size from clay-size to boulder-size. In between you get sand, pebble and cobble-size clasts. Very fine clasts, such as clay and sand, form claystone and sandstone. Coarser clasts, such as pebbles and cobbles, form a rock called a conglomerate. In conglomerates, it is usually easy to identify the different kinds of clasts that make up the rock because they are visible to the naked eye.
Identifying Metamorphic Rocks
Metamorphic rocks form as a result of other rocks being reburied and subject to intense heat and pressure. During this process, partial melting of the minerals occurs and re-crystallization results in the formation of a new rock. Identifying metamorphic rocks can be more challenging than identifying igneous or sedimentary rocks because they can still look similar to their source rock. Gneiss (pronounced like "nice") is a kind of metamorphic rock that is easily identified because it exhibits a feature called "foliation". Foliation occurs when the source rock is squeezed under such intense pressure, that the partially melted minerals in the rock align with eachother to form bands that are then folded.
Having a pocket guide for rocks and rock minerals can also assist a field geologist in identifications. These kinds of guides can be found in most bookstores in the reference or natural science sections.
Source:
Dietrich, Richard V. and Skinner, Brian J., Rocks and Rock Minerals. NY: John Wiley and Sons, 1979.
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