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Saint Francois Mountains
I love rocks. Last week while traveling across the Ozark Dome in southeastern Missouri
I had the opportunity to explore the massive granite boulders piled in heaps at Elephant
Rock State Park near the small town of Graniteville, Missouri. The story behind these
rounded boulders, some as large as a house, requires a bit of time travel back when
the earth was young and new mountain ranges like the Appalachians and the Rockies
didn’t yet exist.
The granite now strewn about the surface there in Missouri ranges from almost 1.5
to 1.38 billion years old. As a point of reference, the earth is judged to be 4.5
billion years old, oldest life around hydrothermal vents first appeared about 3.5
billion years ago, oldest multicellular sea creature (a sponge) is 535 million years
old, and, of considerable importance to Arkansans, the Ouachita Orogeny occurred between
320 to 300 million years ago. So, the formation of the Saint Francos Mountains occurred
early in the development of North America.
There are two theories about how the old mountain chain formed. The most prevalent
seems to be that there was a volcanic hot spot, similar to the one that is perking
along in Yellowstone or the Big Island of Hawaii. The other possibility is that the
volcanism was a result of continental plate collision, much as is occurring in the
“ring of fire” surrounding the Pacific Rim. Though I’m hardly learned enough to have
an opinion, I favor the second theory because of a geological feature called the “Spavinaw
terrane.” This is an exposure of granite in Spavinaw, Oklahoma that has an identical
composition and is the same age as the youngest granite rocks in Missouri. Bore samples
from across northern Arkansas, western Missouri and Kansas show that the same granite
underlies the sedimentary deposits found across the same area.
Igneous rocks form from molten magma. Literally, the mantle or lower crust melt and
form a gelatinous pool or molten rock. If this happens close to the surface the molten
rock can escape through cracks and form a caldera, the open maw of a volcano. In the
case of the Saint Francos Mountains, at least four calderas have been identified,
but there could be others that are covered in sediments.
The magma that found its way to the surface cooled quickly, forming hard, weather-resistant
small-grain-sized rocks. A common one is called rhyolite, but others occur, depending
on what minerals were being pumped up at any given time. The eruptions of these volcanos
went on for about 100 million years, with several episodes of growth, then collapse
of the caldera as the magma-filled chamber below emptied.
Eventually the volcanos went dormant and the molten rock in the magma chamber began
to cool. But cooling takes a long time in this well-insulated subterranean world.
The rock chemistry of the “extrusive” rocks that were squeezed out on the surface
is the same as the “intrusive” rocks that cooled slowly below ground. But the intrusive
rocks cool more slowly, so crystals of silica, potassium and whatever else happens
to be there have more time to grow. The speckled look of the intrusive granite rocks,
such as the large boulders at Elephant Rock State Park, is the result of the large
grain size of these slowly cooled minerals. While these rocks were once buried deep
in the heart of the mountain chain, the erosive forces over the past billion plus
years wore the old mountain chain to a nubbin, leaving these remnants behind to ponder
what might once have been.
When South America collided with the North American plate during the Pennsylvanian
epoch, the Ouachita Mountains were pushed up and, further north, the Ozark dome lifted
up but not so much to buckle and form a proper range of mountains. The deep hills
and valleys we see today across the Ozarks is but surface erosion of a high place.
Such is the fate of all high ground. It will eventually be flattened by time and weather,
only to repeat the cycle time and time again.
Gerald Klingaman is a retired Arkansas Extension Horticulturist and retired Operations
Director for the Botanical Garden of the Ozarks. After more than two decades of penning the popular Plant of the Week column, he’s taking a new direction, offering views on nature as he pokes about the
state and nation. Views and opinions reflect those of the author and are not those
of the University of Arkansas System Division of Agriculture. If you have questions
or comments for Dr. Klingaman about these articles contact him at firstname.lastname@example.org.