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 Formations 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.
To log this Earthcache please send me an email with the text "GC2BQCV Elkhorn Mountain Volcanoes” on the first line and the answers to the following questions (The answers can be found on the informational sign. Please do not post answers in your log.)
1. How many miles do the Boulder Batholith extend into the earth?
2. How many years ago did the Elkhorn volcanoes erupt?
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