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University of Missouri Department of Geological Sciences

Rock type:      Amphibole Granite
Location:        Knob Lick, MO

gold line

This igneous rock formed about 1470 million years ago when magma intruded a near-circular ring

fault at Butler Hill, Missouri. At that time, the local geology would have somewhat resembled

Yellowstone today, with a large caldera (a volcanic crater formed when the land surface sinks

into a magma chamber below) bounded by faults.

The minerals in this granite include quartz (glassy grey), feldspar (white and pink), and amphibole

(dark green), which indicates that the magma contained substantial amounts of dissolved water.

 The granite crops out at an abandoned quarry, where the granite can be seen to contain

inclusions of the overlying rhyolite. It is thought that the granitic magma intruded into overlying

volcanic rocks that had previously been erupted from the same magma chamber.

Amphibole Granite Amphibole Granite

Photo by Alan Whittington, MU Geology

MU Geology Department