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University of Missouri Department of Geological Sciences

Rock type:      Welded Tuff
Location:        Hughes Mountain, MO

gold line

This volcanic rock has very prominent horizontal layering, and vertical cracks that split the rock into

polygons. This outcrop in southeast Missouri is known as the Devil’s Honeycomb, and it is one of a

few places in the world where one can see columnar jointed rhyolite (other famous examples of

columnar jointing in less silica-rich rocks include the Devil’s Postpile in California; Devil’s Tower

in Wyoming, and Giant’s Causeway in Ireland). The rock contains flattened pumice clasts that

stand out best on weathered surfaces as darker orange/red lines. This rock formed from the

deposits of a large volcanic eruption around 1.47 billion years ago.

Welded Tuff Welded Tuff Welded Tuff

Photo by Alan Whittington, MU Geology

MU Geology Department
http://geology.missouri.edu/