Montana Governor and First Lady's Math and Science Initiative
Elkhorn Volcanoes & the Boulder Batholith
Jefferson City Rest Area on I-15 south of Helena
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.
The 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.
Elkhorn Volcanoes & the Boulder Batholith Road Sign
A Brief History
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.
The Unveiling of the Elkhorn Volcanoes Road Sign
 Jim Lynch, Director Department of Transportation, First Lady Nancy Schweitzer |
June 2006 |
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