Plutonic Rocks

Intro

The Rocky Mountains of Colorado boast spectacular views of numerous plutonic (or intrusive) rocks. These rocks were formed long ago as magma rose from deep sources and solidified before making it all the way to the surface. As the magma rose, in many places, it brought with it precious minerals such as gold, silver, lead, and molybdenum (used in hardening steel). After millions of years, erosional processes stripped off the overlying rocks, exposing them as we see today.

Alkalic Complexes

Alkalic complexes in Colorado’s Wet Mountains and the Powderhorn district near Gunnison contain carbonatite and a variety of other exotic rock types. Both complexes contain abundant rare earth elements. Indeed, Powderhorn (also known as Iron Hill) contains the nation’s largest volumetric deposit of rare earths as well as the world’s largest titanium deposits.

Batholiths

A batholith is an intrusive mass of solidified magma usually granitic in composition and defined as having a minimum size greater than 40 mi² (100 km²) and no known floor. These are emplaced deep in the crust and are irregular in shape.

Proterozoic granitic batholiths are common throughout Colorado’s Rocky Mountains. It was generally thought that the batholiths fell into three age groups: the Routt plutonic suite at about 1.7 billion years ago, the Berthoud plutonic suite at about 1.4 billion, and the Pikes Peak batholith at about 1.0 billion. With more age dating, and more advanced dating techniques, we are discovering that the intrusions were more spread out in time.

Dikes

Dikes are formed when magma (a mixture of molten material and crystals) rises from below and cuts across pre-existing strata. The magma may follow pre-existing cracks or faults, or may create its own path upward. The magma crystallizes underground and becomes a dike, which is a plutonic or intrusive rock.

Erosion cuts into the earth and allows us to observe the dikes. The magma in a dike may or may not have reached the surface. If the magma pours out onto the surface then it becomes a volcanic or extrusive rock. All extrusive rocks must of necessity have intrusive feeders, usually dikes or plugs.

Colorado is home to examples of every known type of dike structure.

Laccoliths

Laccoliths are mushroom-shaped bodies with a flat floor and a domed roof. Thus, they appear to have begun forming in the same way as sills; however, as magma continued to intrude, it pushed up the overlying layers rather than continuing to spread out laterally. Colorado has many laccoliths and is the first place in the world where they were recognized and named. Corry (1988) lists more than 80 by name and location, and it is estimated that there are between 8-10,000 globally.

Magma that moves upward and cuts across pre-existing layers of rock forms what are generally known as plutons. The largest plutons are batholiths, such as the granitic rocks of Pikes Peak, which are part of a 1,300-square-mile batholith. Smaller plutons take a variety of shapes, each with its own name, such as stocks, plugs, dikes and sills.

Plugs

A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. If rising volatile-charged magma is trapped beneath a plug, the resulting extreme build-up of pressure can sometimes lead to an explosive eruption. Eons later, erosion can reveal the more resistant plug forming residual landforms. For example, Edinburgh Castle in Scotland is built upon an ancient volcanic plug.

Sills

Sills form by magma squeezing between, and parallel to, the layers of the host rock, rather than cross-cut existing layers of strata as dikes do.

Colorado has sills of many different sizes and ages intruded into an astonishing variety of surrounding rocks. One of the largest is Archuleta Mesa in southern Colorado near the New Mexico border.