Vibrant, voluptuous, vast volcanoes part of Colorado's past

Spiller Peak, located in the La Plata Mountains, has a crumbly summit as a result of magma that cooled quickly. Enlargephoto

Photo/MK Thompson

Spiller Peak, located in the La Plata Mountains, has a crumbly summit as a result of magma that cooled quickly.

Here in Southwest Colorado, we don't often think of volcanoes. Some might think of volcanoes as far away forces creating islands in oceans or billowing toxic smoke across the skyline of some African city.

But present day volcanic activity is taking place not very far away in places like Wyoming and Washington state. Volcanic activity was once abundant in Southwest Colorado, perhaps right where you are sitting. It helped shape many of the landforms in the area.

There are three main types of volcanoes as they appear on the surface: shield volcanoes, composite cones and calderas. Shield volcanoes form when free flowing lava makes a low, shield shaped peak above the ground. Composite cones are built up by a series of thick flows and explosions. Calderas are large craters in the ground created when either an explosion or an underground leak drains the underground magma chamber. No two volcanoes are alike but all volcanoes form with some similarity to these three types.

There are visible signs of ancient volcanoes all around the south San Juan Mountains. Although many landforms were carved by glaciers, signs of volcanic activity are plentiful.

La Plata Mountains - The way these mountains formed is similar to that of the La Sal and Henry mountain ranges of Utah as well as the Sleeping Ute Mountain southwest of Cortez - by laccolithic intrusions. A laccolith is an intrusion of molten igneous rock that makes its way up from the mantle through cracks in the crust but does not break the surface. The intrusion pushes up the top layer(s) thus creating a mushroom of igneous rock below the surface - a pluton - similar to an above ground shield volcano. These intrusions in the La Platas had enough power to uplift the layers of sedimentary rock above, but not enough force to burst though the rocks like a major volcanic explosion. Both the top sedimentary layers and the rocks of the igneous intrusions are visible in the La Platas. Hesperus Mountain is most easily recognized by its thick sedimentary bands. Spiller Peak is mainly igneous. Its crumbly summit is a result of magma that cooled quickly.

Engineer and Graysill Mountains - Engineer Mountain is a sentinel of mountains in the area. This mountain and its neighbor, Graysill Mountain were formed by processes different from the mountains around them. The summits of Engineer and Graysill are made of volcanic sill. Volcanic sill forms from a horizontal intrusion - think "window sill." The igneous rock that comprises the summits of these peaks cooled quickly, creating the fragile rocks we now see piled in a loose jumble at the summits. Quick cooling also created the vertical columns of rock visible below the loose summits. According to this fact page about columnar jointing (, "The columns form due to stress as the lava cools. The lava contracts as it cools, forming cracks. Once the crack develops it continues to grow. The growth is perpendicular to the surface of the flow." This helps to explain why vertical columns formed due to a horizontal sill intrusion.

The history of many peaks in Southwest Colorado contains some sort of volcanic component. There is a caldera near Silverton. Much of the history of the San Juan Mountain range was affected by huge volcanic ash flows. Loads of volcanic history in Colorado are compiled here:

Interested in the current volcanic activity in the West? Mount St. Helens once embodied the stereotypical image of a composite cone until 1980 when it literally blew its top. The volcano now has a mile-wide crater in the middle and the summit elevation was reduced by over 1,300 feet. Mount St. Helens is still a very active volcano, but no eruptions similar to the one in 1980 are predicted anytime soon. Read more here:

Yellowstone National Park is a showcase for a wide range of ancient geologic activities. There are also geologic processes occurring at this very moment. Yellowstone's hot springs and geysers are heated by volcanic activity - a magma chamber - just below the surface of the land. Molten rock heats water in underground aquifers. When the water and steam create too much pressure in the aquifer, the water blows out of the geyser much like lava does in a volcanic explosion. For more information about volcanic activity in Yellowstone, watch this video:

We here at the SJMA hope you enjoy this information. You might now look differently at the amazing peaks around us. Enjoy that fresh mountain air!

MK Thompson is the Education and Program Assistant for the San Juan Mountains Association. SJMA is a nonprofit 501(c)(3) dedicated to public land stewardship and education. SJMA partners with the San Juan National Forest, BLM Tres Rios Field Office and Canyons of the Ancients National Monument, BLM, in addition to other organizations in SW Colorado.

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