August 30. Siberian basalts

Original extent of Siberian traps (see below for credit)

At just about the end of the Permian Period, one of the most voluminous volcanic eruptions of the past 500 million years occurred, in what is now Siberia. The rocks are largely basalts, relatively iron-rich, dark, fine-grained igneous rocks that erupted from many vents and perhaps some fissures scattered through the area. The eruptions spanned about a million years, 251 to 250 million years ago, and ultimately covered as much as 7 million square kilometers, about 2,700,000 square miles – about the area of the conterminous United States excluding Texas and California. Probably less than a third of the original extent is still preserved; the rest has been eroded away.

Flow after flow was erupted, along with explosive eruption of ash and other volcanic products. The flows are usually called “flood basalts” because they flooded out of the vents and fissures to cover vast areas. And the whole package is often referred to as the Siberian Traps – the word “trap” in geology comes from a Swedish word meaning “step,” because the piles of flows typically create a stair-step landscape.

There have been several huge eruptions of flood basalts through earth history, including 11 of various sizes in the past 250 million years. The Columbia Plateau in Washington and Oregon, the Deccan in India, and the Parana basalts in Brazil, are all huge, but the Siberian traps were the largest volcanic flows in the past half billion years.

What caused it? The ultimate cause of all the flood basalts is not completely certain. Ideas include active rifting, pulling apart of the crust, under special circumstances such as unusually thin crust. A mantle plume, a hot spot like those beneath Yellowstone and Iceland today, could have made it happen, especially if associated with rifting. It’s been suggested that an impact might have blasted the crust to the point that vast volumes of magma would erupt through the crater, but there isn’t much support for that idea.

The age of the eruptions has been pinned down quite accurately. The eruptions do have a moderately wide range, taken together, from about 256 to 246 million years ago, but the age dates really do strongly cluster at 251 to 250 million years ago. This is right at the Permian-Triassic boundary, the end of the Permian, when the greatest mass extinction in the history of life on earth occurred. It’s pretty much inescapable, given the extremely close correlation in time between the vast Siberian eruptions and the mass extinction event, that there was probably a connection between the two. But there’s more to it than a simple one-for-one correlation, and we’ll explore that in more depth tomorrow.

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Today is the birthday of William Stephens Twenhofel, August 30, 1918, in Madison, Wisconsin. His father, William Henry Twenhofel, has been called the patriarch of sedimentary geology: he pioneered the study of sedimentation as a subdivision of geologic science. The son, William Stephens, was a geologist with the U.S. Geological Survey who worked on ore deposits in Alaska, among many other things.

—Richard I. Gibson

Links and references:
Siberian Traps (Cowan)  
Siberian Traps ages 
Siberian Flood Basalts 
Multiple eruptions – not one
Extinction connection

Map by Jo Weber, under Creative Commons license.