How are Igneous Rocks Identified?

Identifying igneous rocks and determining whether they are intrusive (formed inside the earth) or extrusive (formed on the surface from lava or magma) relies upon observing a variety of features and properties.

Texture

The texture of an igneous rock provides clues about how it was formed. Intrusive igneous rocks cool slowly beneath the surface, allowing mineral crystals to grow large. As a result, intrusive igneous rocks have visible crystalline textures. For example, granite has an interlocking texture with large crystals of quartz, feldspar and mica. Extrusive igneous rocks cool much more rapidly on the earth’s surface, resulting in small or microscopic mineral crystals. Basalt and obsidian are examples of extrusive igneous rocks with very fine-grained textures. Pumice has a porous, frothy texture due to gas bubbles formed in the lava.

Mineral Composition

The minerals present in an igneous rock provide insights into its origins. Felsic igneous rocks such as granite contain high levels of silica and minerals such as quartz, feldspar and muscovite mica. Mafic igneous rocks like basalt have less silica but contain higher percentages of magnesium and iron, resulting in dark-colored minerals like olivine and pyroxene. Ultramafic rocks are the highest in magnesium and iron with minerals like peridot and augite. Knowing the expected minerals in each type of igneous rock aids identification.

Layers and Flows

Another way to identify extrusive igneous rocks is by their layering or flow patterns. As successive lava flows cool, they form layers on top of each other. This layered appearance is visible in rocks like rhyolite. Obsidian and pumice commonly display flowing patterns made as the lava moved before it solidified. The glassy texture of these rocks also indicates rapid cooling. Intrusive igneous rocks do not display layers or flow patterns.

Location

The location where an igneous rock is found provides clues about how it was formed. Intrusive igneous rocks are found in areas where ancient magma cooled beneath the surface, often in large batholith formations. Extrusive igneous rocks are located where ancient volcanoes erupted onto the surface and lava flows spread across the landscape. Knowing where a rock was originally located helps determine whether it is intrusive or extrusive.

Grain Size

Under a microscope, the individual mineral grains in igneous rocks can be measured to determine average size. Intrusive rocks have larger crystals, while extrusive rocks have smaller grains that are difficult to distinguish with the naked eye. A technique called point counting involves randomly selecting points on a microscope slide and identifying the mineral grain located at each point. The average grain size can then be estimated statistically.

By considering texture, mineral content, location, layers and flows, and grain size, geologists can identify key characteristics to determine whether unfamiliar igneous rocks are intrusive or extrusive. These clues enable confident identification of igneous rock samples and provide insights into the environments in which they were formed.