How to Identify Rocks

Rocks make up the Earth’s surface and there are three main types including igneous, sedimentary, and metamorphic.

Rocks make up the Earth’s surface and there are three main types including igneous, sedimentary, and metamorphic. These three types of rocks generally move through a cycle, known as the Rock Cycle (shown below). Hot molten rock under the Earth’s surface creates igneous rocks, which can either cool underground or aboveground. These igneous rocks can then be exposed to the surface elements by tectonic processes, and be exposed to the atmosphere causing breakdown. The breakdown of rocks creates sediment, which eventually gets moved, deposited, and can be buried to form sedimentary rocks. Then, these sedimentary rocks can become further buried and succumb to high heat and pressure causing mineral changes in the rock. These mineral changes causes the formation of metamorphic rocks. If buried deep enough, rocks again melt into magma and become igneous rocks once more. Rocks are a great indication of the Earth’s geologic past, and can be used as a guide to interpret and understand current environments.

The rock cycle

Igneous Rocks
Extrusive Igneous

The word ‘igneous’ comes from the Latin word ‘ignis’ meaning fire (World Atlas, 2020). These rocks come from the molten magma, or heated liquid rock, beneath the Earth’s surface. The location that igneous rocks cool determines their grain size and rock type. Igneous magma that bubbles up to the earth’s surface is known as lava. Lava cools very rapidly as it is exposed to the atmosphere and becomes an extrusive igneous rock. This rapid cooling creates a fine-grained texture, meaning that individual grains are not visible to the human eye. A common extrusive igneous rock is basalt. Basalt is typically dark gray in color due to the high amounts of metals including iron and magnesium, and appears as a solid dark gray matrix. It can also have a texture with holes that appears like swiss cheese due to the gas bubbles that were trapped during cooling. This texture is called ‘vesicular’ and is pictured in the vesicular basalt below.

Intrusive Igneous

Contrarily, igneous rocks that have long timeframes to cool have a coarse-grained texture, meaning individual mineral grains are typically visible to the human eye. These rocks also begin as magma, but enter into the subsurface, such as through faults or fractures. Because they exist in the subsurface and are not exposed to the atmosphere, they tend to cool very slowly. This slow cooling allows for certain minerals to crystallize and grow. In general, the longer a rock has to cool, the larger the mineral grains will be. A common slow-cooling igneous rock is granite as shown below, which is mostly composed of light-colored minerals like quartz and feldspar, with minor amounts of dark minerals like biotite and amphibole.

Igneous granite rock

Sedimentary Rocks

Sedimentary rocks can form from the breakdown of any other type of rock that is exposed to elements at the Earth’s surface. Rocks that are exposed to air, wind, and water eventually will break down through a process called weathering. Weathering can be physical, such as the forces of rain and ice, or chemical, such as the breakdown of rocks from acid. As rocks break down from weathering, they release small particles (sediments) that are transported through erosion and deposited to different areas. Sedimentary particles can be moved through various means, such as by water (rivers and oceans) or wind. After accumulated sediments settle in an area, they will eventually compact as they are buried. Buried sediments become increasingly stuck together in a process called lithification, which slowly converts loose sediments into hard rock. A common sedimentary rock is a conglomerate, which is comprised of rounded gravel or pebble pieces that are cemented together with other smaller particles or minerals, as shown in the picture below.

Conglomerate sedimentary rock

Other identifying features of sedimentary rocks are bedding planes and fossils. Because sedimentary rocks form in layers of compacted sediment, they often will exhibit parallel layering called bedding. Shale can exhibit this bedding as it forms through the accumulation of clay and silt particles. In addition, sedimentary rocks can have fossils, such as the leaf fossil shown in the shale below. Plants and animals that lived long ago can be preserved in the rock record and allow humans to interpret what living things were abundant on the planet during different time periods.

Fossil in a shale sedimentary rock

Alternatively, sedimentary rocks can also form through biological and chemical processes. One example of this is the rock limestone, which is largely made up the mineral calcite. Limestone often contains fossils as it forms in shallow ocean environments. These shallow warm sea environments allow for animals, like mussels and clams, to make their shells from calcite in the seawater. Hence, often limestone contains many fossils, as shown in the mussel imprint below.

Fossil in limestone

Metamorphic Rocks

Metamorphic rocks are those that have been altered in some way by heat and pressure deep in the Earth’s crust. Metamorphic rocks can originate from any type of rock: igneous, sedimentary, or another metamorphic rock. These rocks are subjected to high heat and pressure through burial or tectonic processes and become altered. For example, a shale, or fine-grained sedimentary rock made up of clay, can go through various stages of metamorphosis. A low-grade metamorphosis, or low quantities of heat and pressure, converts this shale into slate. Slate appears similar to shale, but is more compact and hard. Increasing stages of metamorphosis with higher heat and pressure include phyllite, schist, and gneiss. A schist is pictured below, which often has a very shiny look due to the large quantities of the mineral mica. Certain minerals, like the garnet shown below, can only form in metamorphic rocks.

Schist metamorphic rock

Garnet metamorphic mineral

An interesting concept in metamorphic rocks is known as foliation. This occurs because the rocks become heated and pressurized enough to cause individual minerals to align perpendicular to the direction of pressure. This causes the distinct banding of light and dark mineral layers in gneiss rock, shown in the image below.

Gneiss metamorphic rock