August 2. Permian time

Permian time scale (from Wikipedia)

Today first, an update. There’s a big bend in the Appalachian Mountains in eastern Pennsylvania and southern New York, where the general trend of the range changes from about east-northeast to more north-south. A new study by geologists at the College of New Jersey and the University of Rochester suggests that there is an ancient volcanic block there that was thick and strong which forced the developing mountain belt to swing around it. It was probably a rift system during Proterozoic time, and the volcanic rocks that filled the rift served as the buttress to constrain the geometry of the Appalachian mountains. Link to a paper about this work. And a press release.

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Now, on with the Permian. It’s August 2, and today’s episode is a short one to outline the time scale for the Permian. As I indicated at the end of the Pennsylvanian, the start of the Permian is defined by international agreement as the first appearance of a particular species of conodont in the rocks of the Ural Mountains, Russia. The time of that occurrence is about 299 million years ago.

As is common, the international subdivisions differ from those used in the United States. The thickest section of Permian rocks anywhere is in West Texas and New Mexico, and in the U.S. the subdivisions are, from oldest to youngest, Wolfcampian, Leonardian, Guadalupian, and Ochoan. Internationally, there are three epochs in the Permian, each with two to four subdivisions or ages. The ages range from two to 10 million years each, adding up to a total of about 48 million years.

The end of the Permian, at 251 to 252 million years ago, coincided with the largest mass extinction in earth history. That’s a big enough deal that it marks not only the end of the Permian, but also the end of the Paleozoic Era, which we’ve been moving through since the first of February. We’ll talk about the extinction at the end of this month.

—Richard I. Gibson