The Boulder Bath O’Lava
The Richest Hill on Earth…the largest placer deposit in history…Southwest Montana has more than its fair share of mineral wealth. Ever wonder why? The Montana Department of Transportation has a couple of Geological Roadsigns answering that very question, and I’ve summarized them here:
The Elkhorn Mountains northwest of Townsend were not always the peaceful mountain slopes you see today. 81 to 74 million years ago, the Elkhorns were an explosive area of volcanoes. Magma that reached the surface erupted in violent eruptions that rained ash and rocks for miles. In fact, the volcanic field covered a diameter of over one hundred miles. These explosions buried countless animals, including the dinosaurs whose fossils have been found in the Two Medicine formation near Choteau. So many volcanoes exploded that in some places, the volcanic field got up to three miles thick. Volcanoes kept erupting, but the debris was so thick magma could no longer reach the surface of the earth. The magma at these lower levels cooled very, very slowly. As it cooled, it turned into granite, forming the huge granite slab that extends from Helena to Butte. Geologists have dubbed this slab of rock the Boulder Batholith: “boulder” refers to the huge granite boulders strewn across the region, and “batholith” means a slab of cooled magma.
As the magma cooled, it cracked. Extremely hot solutions of melted minerals filled these cracks, and became part of the batholith. Southwest Montana’s mining boom, easily the richest in the state, was (and still is) fueled by these veins of gold, silver and copper in the Boulder Batholith. Where these veins were close to the surface, weathering caused the minerals to wash into the gravels of stream beds, forming placer deposits of precious metals. Elsewhere, the veins remained deep below the earth’s surface, waiting to be mined.
Whew, that’s just about as much geology as I can handle, but it does help explain the history of Southwest Montana.
Tags: geology, intro science, highway markers, roadtrips, Boulder, Townsend, mining
