 The butte is made up of a very dark rock called "Shokinite", named after the town of Shonkin (east of Great Falls) where it was first discovered. Shonkinite is an igneous rock that forms through the cooling of magma that originated beneath the Earth's crust, probably from a depth of over 25 miles. 
A closer look (right) reveals the black "augite"
crystals. These crystals formed before the
magma moved into the lacolith while the
magma was cooling at a slower rate (time for
the molecules to arrange
themselves into crystals). Since augite
has a higher freezing point than the other
minerals in the
magma, the crystals were surrounded by
magma for a time, allowing them plenty of
space to grow. Then the magma moved
("pulsed") from the magma chamber into the
laccolith, where it began to cool at a faster
rate. So, as the other minerals froze they
didn't get as much time to form nice big
crystals. Such a rock is called a
"porphyry".porphyry: (POR fah
ree) Check out this photo album from a hike onto Crown Butte with the Helena High Outdoors Club in the fall of 2015. |
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One last look |
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Start of the Crown Butte Virtual Field Trip |
Below: In some places on the butte,
the rock has weathered so that black crystals
of augite can be collected. |