Wednesday, July 8, 2009

Fire Down Below II - a Geological History of the Colorado Plateau

Continuing a long-running adventure, we pick up the tale of the geology of my cherished corner of the United States, the Colorado Plateau....we've come through 2 billion years of the story, and are now only 20 million years from the end!

Volcanic activity had not been a prominent part of the geology of the Colorado Plateau for hundreds of millions of years, throughout Paleozoic and Mesozoic time, except for the volcanic ash that drifted in from elsewhere and ultimately provided much of the color of the Morrison and Chinle formations. That all changed in Cenozoic time. As outlined in yesterday's post, there was a vast outpouring of lava and tephra across much of the western United States around 30 to 20 million years ago, which resulted in the formation of numerous volcanic necks, calderas and the subject of today's post, laccoliths.

There are some real oddities in the landscapes that make up the Colorado Plateau, islands of rock with alpine snowfields and deep green forests that seem more like the Rocky Mountains than part of a desert. These strange out-of-place mountains include the La Sal Mountains (top photo), the Abajo Mountains, Navajo Mountain, the Henry Mountains (center photo), the Sleeping Ute Mountains (bottom photo), and a number of others. The highest, the La Sals, top out at over 12,000 feet. They are a beautiful addition to the landscape; imagine Delicate Arch in Arches National Park (top photo) without the dramatic backdrop of the La Sal Mountains in distance.

How can such mountains develop out of an otherwise flat landscape?

They aren't volcanoes, not in the sense that we normally think of them, but they are volcanic in origin. The mountains are made of a semi-coarse-grained igneous rock that has a number of names, but diorite will do for the moment. 'Coarse-grained' usually means the rock results from slow cooling of the magma miles underground, but the rocks forming the core of these mountains are sort of a hybrid, showing a fine enough texture that they were probably a few thousand feet below the surface, rather than several miles beneath the surface. G.K. Gilbert was studying exposures of these rocks in the Henry Mountains in the 1870's and he realized that although there were complexities, the rocks tended to squeeze between sedimentary layers, and to make room, they pushed the overlying layers upward into a domelike structure, much like a blister pushes the skin upwards. He called these igneous 'blisters' laccolites, and eventually they came to be known as laccoliths.

A simple laccolith, courtesy of

Navajo Mountain appears to be an ideal example of a simple dome-style laccolith. It certainly looks the part. The more complex mountain ranges include multiple laccoliths at different levels, sometimes branching out in several directions. They often probably constituted the plumbing systems of volcanoes that developed on a surface many thousands of feet above, but have since been eroded away.

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