June 30. The end of the Mississippian

As we discussed at the start of the Mississippian, it’s only in the United States that Mississippian and Pennsylvanian are treated as full periods of geologic time. In the rest of the world, the two together are called the Carboniferous.  

In the U.S. the distinction is made as much as anything because the two periods contain distinct kinds of rocks. The Mississippian is dominated by the shallow-water limestones we heard so much about this month, while the Pennsylvanian contains the coal that gives the Carboniferous its name. It’s true that the early Carboniferous in much of the rest of the world contains less-abundant coal than the later part of the period, the part that correlates with the Pennsylvanian in the United States, but it’s still treated as a single period worldwide.

I think the different terminology has to do with the history of usage as much as anything; in 1906, geologist T.C. Chamberlain pointed out that the rocks of Mississippian and Pennsylvanian age are not only significantly different in rock type but that there is a break – an unconformity – between them, so the two were recognized as periods in the U.S. That break more or less represents the onset of the Appalachian and Ouachita Orogenies which began to create significant uplifts in both the east and south, and to some extent the west of the North American Continent. The Variscan Orogeny was doing the same thing in southern and central Europe, but nonetheless, European geologists never embraced the idea of breaking the Carboniferous into two distinct periods. So, officially in terms of worldwide use, the Mississippian and Pennsylvanian are the equivalents of the Early Carboniferous and Late Carboniferous. Or you can call them sub-periods, or if you are in the United States, call them periods of geologic time. It really doesn’t matter as long as we can communicate!

So tomorrow we’ll enter the Pennsylvanian.

* * *

Today, June 30, in 1908, an object hit the earth near the Stony Tunguska River in Siberia. Something like 2000 square kilometers – 800 square miles – of forest were flattened. It could hardly have been in a more remote area, and consequently there were no known human fatalities. The explosion was almost certainly the result of the breakup in the atmosphere of a comet fragment or asteroid or meteor, although there have been any number of speculations as to the origin, ranging from black holes to alien space ships. The air burst probably occurred 5 to 10 kilometers above the surface, and the object was probably between 60 and 200 meters across, or about 600 feet at most. But it still represents the largest impact event on Earth in recorded history.

—Richard I. Gibson