Igneous Intrusive Bodies

In an early article on the Rock Cycle, I covered some of the basic igneous rock types and how they are formed. Unlike sedimentary rocks, igneous rocks are formed from the cooling of material known as magma from deep in the earth’s interior. The magma rises from the interior up into the lithosphere exploiting cracks, faults, and fissures in the crust. As the magma rises, it cools, forming different rock types and creating different features beneath the surface. At times these features, known as igneous intrusions, can be viewed at the earth’s surface, giving geologists the opportunity to study the process of how igneous rocks form and interact with pre-existing rock.

There are five basic types of igneous intrusions. Each is formed in the same way, by magma forcing it’s way up through the crust. The size and shape of these intrusions is dependent on a few factors. 1.) The size of the main magma body. 2.) The composition of the magma (some magmas are viscous and don't flow well or spread out, while other less viscous magmas form large, thin sheets). 3.) The geology of the rocks the magma is intruding into. All three of these affect the formation of igneous intrusions.

The dike is probably one of the most abundant of the igneous intrusions. Although not very large in size, dikes cross the areas between larger igneous bodies, feeding smaller intrusions from larger bodies. Dikes generally radiate out from the central cone and feed the lava that erupts from volcanoes. Dikes are tabular in shape and are discordant, meaning they cut across the bedding of the country rock. (Country rock is the rock that was there before the intrusion was emplaced.) Dikes follow lines of least resistance, and typically follow pre-existing joints and fractures. They can occur singly or in clusters called dike swarms. Good examples of dikes can be seen radiating away from an ancient volcanic neck near Shiprock, New Mexico and from the volcanoes that form the Spanish Peaks in southern Colorado.

Dikes are on the small end of the types of igneous intrusions. On the large end is the batholith. Batholiths are very large intrusive plutons with steeply dipping walls. (A pluton is a general term that can be applied to any intrusive igneous body regardless of its composition, shape, or size.) Batholiths generally lack a known floor; basically, there is no discernable bottom. Batholiths typically form from silica-rich rocks like granite. They can range in size from hundred’s to thousand’s of square kilometers in size. They can be concordant (parallel with existing structural bedding) or discordant. One of the most famous batholiths is the Sierra Nevada batholith in central California. The famous landmarks of El Capitan and Half Dome in Yosemite National Park are part of the Sierra Nevada batholith.

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