Ozark Mountain geology- an ancient sea, volcanic islands, continental collision and erosion

Dave,
   We traveled through the Ozark Mountains in Missouri and Arkansas last summer. What is the geology of this beautiful wooded hill country? What are some geologic sites to visit when we return?
Arliss Clark
Detroit

Arliss,


The Ozark region

   I visited the Ozarks when I was 7 on a family vacation. One of my earliest geologic memories is finding weapons-grade flint in the Ozarks, and watching a guy make a small arrowhead out of a piece I found.  It is wonderful country for a vacation with the kids. Campgrounds and RV parks abound. Most of what follows is from a US Geological webpage and Charles Spencer's useful books, Roadside Geology of Missouri, published by Mountain Press Publishing Company.

PLACES OF GEOLOGIC INTEREST:
Arkansas is famous for its caves, many in Ozark limestone.
A guide to Arkansas geologic sites.
Johnson's Shut-ins State Park (in the Saint Francois Mountains, Missouri): refreshing swimming holes eroded in rocks erupted on ancient volcanic islands.
Elephant Rocks State Park also in the Saint Francois Mountains, Missouri: rounded rounded boulders of 1.5-billion-year-old granite.

   The Ozarks are a part of the Ouachita-Ozark Highlands and are actually an eroded plateau. Most of the high points are of similar elevations, separated by valleys. The rocks are mainly limestone (with interbedded chert) and sandstone deposited on an ancient sea floor during the Paleozoic era (from around 540 to 250 million years ago). At the time, only volcanic islands rose above the sea anywhere around here. These islands are represented by volcanic rock, mostly rhyolite from violently explosive eruptions and the granite that cooled beneath the volcanoes. The volcanics are really ancient: 1.5 to 1.4 billion years old! Not quite as old as the 3.6 billion year old Morton Gneiss I wrote about a month or so ago, but they are the oldest rocks in the southeastern USA. The island rocks are preserved in the Saint Francois Mountains at the northeast corner of the plateau, up against the Mississippi River, but they extend a great deal further, forming the continental crust beneath the sedimentary rocks in much of the region.

Volcanic ash from explosive eruptions 1.5 billion years ago is
now rhyolite rock at Johnson's Shut-ins State Park

   Thousands of feet of sea floor sediment was deposited all around and on top of the older volcanics. The flat-lying sediment became the Paleozoic sedimentary rocks that typify the Ozarks. These rocks are seen throughout the highlands. The limestone was once coral reefs; the sandstone was eroded from the volcanic islands and deposited on the floor of the now-disappeared Rheic Ocean.

The Ozark rocks (lower center) were uplifted in a plate collision.
Click photo to enlarge and see modern state boundaries.

   But then, a gradual but huge change occurred here. During the Mississippian Period (340 million years ago) two tectonic plates were converging. What was at the time the southeast margin of North America ran up against a plate that included much of South America and western Africa. These plates were coming together to form a supercontinent called Pangea, which included all the continental landmasses until its break up around 200 million years ago. The seafloor rocks were folded and faulted as the ocean basin contracted. The collision of the plates raised a huge rugged mountain chain, much as the Himalaya-Karakoram-Hindo Kush mountains are today forming during the collision of the Indian and Asian plates. These mountains stretched across the seam (the "suture zone") marking the area of collision, which ran more or less along the equator. Remnants are the Marathon Hills of east Texas, the Ouichitas in central Arkansas, and the Appalachians. The sedimentary rocks just to the north were uplifted and tilted about a degree or two towards the south to form a plateau. These are the rocks that underlie the Ozarks.

'Dumbo' is the largest eroded boulder at Elephant Rocks State Park.
It is eroded from granite that was the source of the
volcanic rocks in the Saint Francois Mountains.

   Pangea eventually broke apart to form the modern continental and oceanic plates. The Ozark rocks have been above sea level for the past 340 million years and subjected to the attack of erosion.
   The Ozarks are famed for their limestone caves and 'shut ins'. The latter are wonderful natural swimming pools eroded in the volcanic rocks at the eastern edge of the Ozarks in Missouri.