Geology of the Bitterroot Mountains
by Mike Peterson
It is easy to imagine that Western Montana, dominated as it is by the Rocky Mountains, should be a great place to be a climber. I’m lucky enough to live in Missoula, where the beautiful granite cliffs of the Bitterroot Range are a mere 40 miles to the south. It’s always fantastic to wake up on a warm, sunny spring day, gather friends and gear, and take a day trip hiking and climbing through Kootenai or Blodgett Canyons. Several things come to mind as I look up at these cliffs, aside from the excited and nervous anticipation of soon being so high off the ground. As I contemplate the ascent, I often wonder how these natural playgrounds got here in the first place.
To understand where the sharp cliffs of the Bitterroots came from, one first needs to understand what’s going on inside the earth itself. The earth’s interior is made up of layers, like an onion. There is the outer crust, which is about four miles thick in the oceans and up to 20 miles thick on land. The solid crust is broken up into several plates that float on top of a hot, molten mantle. Under this layer of liquid rock is a dense, solid core. This core is under 360 billion times more pressure than the surface, and at 9,400 degrees Fahrenheit, it is certainly much hotter. The difference in temperature between the crust, mantle and core creates an effect where hot molten rock, called magma, slowly moves toward the surface in plumes, much like the wax of a lava lamp. This phenomenon, called convection, slowly moves the plates of the earth’s crust, grinding them against each other, causing volcanoes, earthquakes — and mountains.
The story of the Bitterroot Range begins relatively recently in earth’s history, roughly 70 to 90 million years ago. It was at this time that a vast plume of magma, heated by the earth’s core, rose into the earth’s crust. This formed what is known to geologists as a batholith, a mammoth underground lake of magma. The Idaho Batholith, as this one is known, stopped rising about ten miles below the surface of the earth. This intrusion of magma, which as a liquid is much more malleable than solid rock, severely weakened the crust of the North American plate above it. Grinding against the Pacific Ocean floor due to convection, the weakened plate buckled, slowly forcing the magma upward. The miles of rock covering the batholith gradually slid off over the next few million years as the magma slowly cooled to form granite. This same process of grinding and buckling plates is what has formed not only the Bitterroot Mountains, but many mountain ranges around the world.
The sheer cliffs of the mountains of the Bitterroots were formed as millions of years of exposure to water in all its erosive forms carved the rock. The most influential were the glaciers. Essentially, glaciers are rivers of ice, and these rivers are some of the most erosive forces in the world. They form when large amounts of snow fall in one area. As the snow deepens, the pressure builds, and the ice becomes malleable. This malleable ice acts as a liquid, flowing downward with the force of gravity. As it moves along, the ice picks up sediment of various size and grinds against the rock it comes into contact with. As opposed to rain, which slowly wears down rock to produce the smooth stones seen in streams and rivers, the grinding action of glaciers breaks and wears the rock down relatively quickly. This is what has given the Bitterroots and other glaciated ranges their sharp peaks and sheer cliffs, features that make them fantastic places to climb.
Every week since 1991, Field Notes has inquired about Montana’s natural history. Field Notes are written by naturalists, students, and listeners about the puzzle-tree bark, eagle talons, woolly aphids, and giant puffballs of Western, Central and Southwestern Montana and aired weekly on Montana Public Radio.
Click here to read and listen to more Field Notes. Field Notes is available as a podcast! Subscribe on iTunes, Google Play, or wherever you listen to podcasts.
Interested in writing a Field Note? Contact Allison De Jong, Field Notes editor, at adejong [at] montananaturalist [dot] org or 406.327.0405.
Want to learn more about our programs as well as fun natural history facts and seasonal phenology? Sign up for our e-newsletter! You can also become a member and get discounts on our programs as well as free reciprocal admission to 300+ science centers in North America!