Have there ever been volcanic eruptions in what is now southern Missouri/Northern Arkansas? (i.e. the Ozarks)

rocky The Librarian

Dear Librarian,

Yes, but it was a long time ago. Arkansas has less than 18 square miles (46 square km) of igneous rocks exposed at the surface. They have unusual (and to geologists interesting) compositions and are economically important but they are intrusive rocks that cooled inside the Earth. They are 85-100 million years old. I did find a reference to pyroclastic flow and ash fall deposits in Oklahoma and Arkansas. These volcanic deposits originated on land and accumulated in the adjacent ocean basin. The pyroclastic flow and ash fall deposits are about 330 million years old. There are beautiful Precambrian rhyolitic ash-flow tuffs exposed in Johnson Shut-ins State Park in the St. Francis Mountains of Missouri. These rocks are 1.4 billion years old but still preserve excellent examples of volcanic rock textures. As long as you're in the St. Francis Mountains stop and see the basalt dike swarm 14 miles south of Farmington on U.S. 67.

Thanks for a fun question. I've been to the Shut-ins and the basalt dikes. They're great.

Steve Mattox, University of North Dakota

Sources of Information:
Hebrank, A.W., and Kisvarsanyi, E.B., 1987, Johnson Shut-ins: A shut-in canyon exposing a sequence of Precambrian ash-flow tuffs, the St. Francis Mountains, Missouri: Geological Society of America Centennial Field Guide - North-Central Section, p. 169-172.

Howard, J.M., and Steele, K.F., 1988, Igneous rocks at Granite Mountain and Magnet Cove, Arkansas: Geological Society of America Centennial Field Guide - South-Central Section, p. 259-262.

Kisvarsanyi, E.B., and Hebrank, A.W., 1987, Roadcuts in the St. Francis Mountains, Missouri: Basalt-dike swarm in granite, Precambrian-Paleozoic nonconformity, and a Lamotte channel-fill deposit: Geological Society of America Centennial Field Guide - North-Central Section, p. 161-164.

Niem, A.E., 1977, Mississippian pyroclastic flow and ashfall deposits in the deep-marine Ouachita flysch basin, Oklahoma and Arkansas: Geological Society of America Bulletin, v.88, p. 49-61. 

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