Granite Beauties: The History of the Boulder Batholith
by Terry Habeger
Broadcast 8.10 & 8.13.2021

Outcrops of Butte quartz monzonite (a.k.a. Butte pluton) on Rampart Mountain, part of the Boulder Batholith. Photo by James St. John, CC 2.0.




My wife and I were driving from our home in Wisconsin toward Missoula to see our daughter and family. Near Butte, Montana, we found ourselves traveling through a boulder field. It was a wonder to behold! Seeing these boulders for the first time I had to ask myself, “What happened here?” I got my wife to look up from her Kindle – it had been a long day on the road – and she was equally taken aback by their varied shapes: a community of vertical and bulbous, fractured granite sculptures!

I learned that this boulder field is part of the Boulder Batholith. The term “batholith” comes from the Greek bathos meaning depth and lithos meaning rock. Batholiths, simply put, are the result of molten material rising toward the earth’s surface. This material hardens into granite, a coarse-grained igneous rock. Some of this rock finds its way to the surface. Batholiths are not noted as distinct areas unless they cover at least 40 square miles. The Boulder Batholith extends roughly from Butte to Helena and covers around 100 square miles, a relatively small area compared with other batholiths.

I like to think these boulders hear footsteps as hikers explore the beauty of their shapes and their glistening granite clothing. I was one of these hikers in Arizona, northeast of Cave Creek, when I first learned about batholiths. My fellow hiker introduced me to these granite beauties, and I had to know more. After some research I came to see the boulders as being “alive.” I wanted to trace their formation by comparing them to the stages my three children have gone through: birth, early childhood, adolescence, and adulthood.

So, when and how were they born? The Boulder Batholith is the result of around 78 million years of development. Dinosaurs were still dining on leaves when the magma that would become these boulders was birthed from earth’s molten core. By some small miracle of life, my wife gave birth to our three children. To develop into what we see today this magma had to pass through the thick stiff mantle, two thinner layers and then the continental crust. I like to imagine the magma pushing its way through the hard mantle, as my children explore their world in early childhood. Both somehow forged new pathways. In adolescence the hot magma finds its way upward. It settles in a pocket or pokes its nose toward the surface, while my children start to learn their own place in the world. As it ‘finds its place’ the magma loses some of its liquid form, hardening into plutons, but still lying below the surface. On their journey toward adulthood my children wear caps of exuberance and promise, while the plutons wear caps of other rocks and soils. After millions of years of erosion and continental uplift the plutons see the light of day as creviced, rounded, shapely granite boulders. I see that my children have become productive and responsible adults.

I am fascinated by the many layers that make up our earth. The layers are well defined in diagrams, but they do not show the action where the layers meet, how they are hot and active and constantly interacting. Cold material falls inward while hot material rises. For the earth and my children, “action” and “interaction” are perhaps key words in their development.

My wife’s Kindle remained asleep as we celebrated Montana’s beauty and the Boulder Batholith in particular. My daughter and family awaited us in Missoula!

Boulder Batholith, drawing by Terry Habeger.


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.

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