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.

The next three types are usually found associated with a larger igneous pluton and are fed by feeder dikes between the batholith and the smaller structures. How these three types form are affected by the geology of the rocks the intrusion is moving into and all three are generally concordant. Sills are emplaced parallel to the bedding of the country rock. They are generally thin (but can still be tens to hundreds of meters thick) and occur shallowly. Sills are generally very large in area, either tens or hundreds of kilometers in size. For them to flow and form large sheet-like structures the magma must be very fluid and sills are generally formed from basaltic magma (low in silica content). A good example of a sill is the Palisades Sill in the state of New York.

The last two structures are similar and can sometimes be confused. Laccoliths are igneous intrusions that generally form a mushroom-shape. They are relatively small in width, only one to eight kilometers in diameter, but can be up to a thousand meters in thickness. They occur at shallow depths. They form as magma rises through the country rock and meets a more resistant rock, forcing the magma to spread laterally and forming a dome in the overlying strata. Most laccoliths are silicic or intermediate in composition (see my article on Igneous Rocks for more information on the composition of igneous rocks) and as such may not spread as far as less viscous magmas. This may account for the different between laccoliths and sills.

A Lopolith is generally a large structure, lenticular (lens-shaped) and has a centrally sunken basin or funnel-shaped intrusions. Their size ranges from tens to hundreds of kilometers in diameter, and their thickness varies but are upwards of thousands of meters in thickness or more. Lopoliths are usually composed of mafic (or basic) and ultramafic rocks. (See my article on Igneous Rocks for more information on the composition of igneous rocks.)

There are five basic types of igneous intrusions. Each is formed as magma from the earth’s interior rises through the earths crust, exploiting fractures, joints, and faults that are pre-existing in the country rock. Their shape is a reflection of the composition of the igneous rock being intruded, and the geology of the country rock. They can be relatively small, on the scale of a few kilometers, or very large, covering hundreds of kilometers. Understanding how large plutons are emplaced into the crust will give a geologist a better understanding into the nature of how the different types of igneous rocks are formed.