Common as Dirt

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Published on: Feb. 2, 1998

Last revision: Oct. 28, 2010

Some kinds of rocks are hard to get to know personally. Not so with chert. If your tire has ever been punctured by a chert sliver on an Ozark road, or if your air mattress has ever gone flat while camping on a gravel bar, you know what I mean.

Chert, also known as flint, is one of the most abundant rocks in Missouri, and is composed of one of the most common minerals on earth. In fact, in much of Missouri you could say that chert is "common as dirt."

Chert is a hard, fine-grained rock made up mostly of the mineral silica (SiO2). Silica itself contains the elements silicon and oxygen, two of the most abundant elements on earth. Silica is also the main ingredient in glass, obsidian, quartz, many kinds of sand and computer chips. It is a lesser component of numerous other rocks and minerals. Chert and flint are generic names for this siliceous rock, which comes in many colors and patterns.

Missouri chert is usually white, gray or tan, but almost any color is possible, depending on chemical variations in the stone and the details of its formation. Certain colors have their own special names. Jasper, for example, is reddish, orange or yellow chert. Agate is banded in many colors and is sometimes translucent, making it a favorite for jewelry and crafts. The Missouri General Assembly chose mozarkite, an attractive chert with a banded pattern of red, pink and gray, to be our state rock.

Most of Missouri is underlain by carbonate rocks-limestones and dolomites-that formed from sediments on the bottoms of ancient seas. This is the thick, layered rock we see along road cuts and river bluffs. Chert is associated with almost every type of limestone and dolomite in Missouri. It appears as globular-shaped "nodules" and "beads," and in pockets and veins of various sizes and shapes interbedded with the limestone.

Geologists are unsure exactly how the chert formed in the limestone. Two possibilities are that oceans deposited silica along with the calcium carbonate shells of marine life, and the chert formed into sedimentary rock along with limestone. Another theory holds that chert formed later, when silica-rich ground water permeated spaces in the limestone. Whatever its origin, chert exists in limestone, and chert nodules, veins and layers are easy to see in road cuts through chert-bearing carbonate rocks.

Chert is glasslike in its hardness and in its ability to withstand weathering.

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