Elkhorn Volcanoes and the Boulder Batholith
I-90 Mile Post 169
The story of the Elkhorn Mountains Volcanics and the Boulder Batholith is the story of how molten magma or melted rock rising up through the crust of the earth just kept coming and coming and coming–from about 81 to about 74 million years ago. Magma that reached the surface created violent explosions, hurling chunks of rock, cinders, and volcanic ash into the air. At times it “rained” droplets of melted rock. Great clouds of volcanic ash were carried by the wind to the east and buried many animals. The dinosaur fossils in the Two Medicine Formation near Choteau owe their preservation to eruptions of the Elkhorn Mountains Volcanics. The volcanic field was enormous–about 100 miles in diameter and up to 3 miles thick. After the pile of volcanic rocks got so thick, magma stopped going all the way to the surface and just accumulated near the bottom of the pile. So much magma intruded at this level that it formed a body of granitic rock that now extends from Helena to Butte and is known as the Boulder Batholith. You are now in the middle of the Boulder Batholith and the Elkhorn Mountains Volcanics.
Magma was generated because one of the tectonic plates under the Pacific Ocean was subducted under Western North America. Approximately 10,000 miles of the Farallon Plate moved under Montana. This is one of places where it was easy for magma to reach the surface.
The magma contained many metals. As the granite formed cooling cracks, hot solutions squirted into the cracks to form veins with copper (near Butte) and gold (in this area). Millions of years later weathering of the granite allowed gold in the veins to wash down to the gravels in the valley floor. A gold dredge chewed through the gravels of Prickly Pear Creek beginning in 1938. Mounted on a barge floating on a 30-foot-deep pond, the machinery consisted of a chain of buckets that dumped the gravel on a maze of screens and sluices inside the dredge. In its wake, the dredge left behind a “churned up” landscape. In some places scrub brush holding the gravel piles together is still the only vegetation. Although the dredge shut down permanently in the mid-1960’s, it was not until the early 1970’s that a South American mining company purchased it and moved it to Bolivia.
