December 19. A Hotspot Breaks Out

Columbia River Basalts (yellow) - see below for source.

During the Miocene epoch of the Cenozoic, about 16 or 17 million years ago, the Pacific Northwest of the United States was a busy place. 

A hotspot, a relatively small location where heat is focused upward from deep in the earth’s mantle, either reached shallow depths, or North America in its movement westward encountered one. At about what is now the common corner of Oregon, Idaho, and Nevada, the hotspot’s heat brought out lava – lots and lots of lava. The Columbia River Flood Basalts are comparable to those in Siberia, and the Deccan in India, and the Parana Basalts of South America. Over about two or three million years, 17 to 14 million years ago, something like 40,000 cubic miles of basalt was erupted, mostly in what is now Washington and Oregon. There are at least 300 individual flows stacked upon each other. In area and volume, the Columbia River basalts are tiny compared to the Siberian flows, and about one-third the size of the Deccan, but still pretty large, and they are among the youngest of these flood basalts. 

It looks like there was a north-northwest trending zone of weakness that extended away from the center of the hotspot – or maybe the hotspot was asymmetrical, or bigger than usual – so that the cracks through which the lavas came were focused to the northwest in Oregon and Washington. Another big crack extended to the south-southeast, through northern Nevada, producing the Northern Nevada Rift, a narrow zone of igneous rocks of Miocene age. Flood basalts didn’t flow there, though, perhaps because the region was a little stronger, a little more a part of the North American craton than the country in Washington and Oregon.

Hotspot origin of various features (see below for source)

At the site of the hotspot itself, that corner where Oregon, Idaho, and Nevada come together, huge explosive volcanism took place. While the flood basalts came out relatively quiescently, like the flows in Hawaii today, the center was a scene of violent activity. A caldera developed. This is a huge collapse feature that forms when a magma chamber erupts much of its lava, leaving a void behind. That empty space may collapse, with the surface rocks falling down into the old magma chamber. The first caldera related to this hotspot, near that corner of Oregon, Idaho, and Nevada, is about 35 miles across. As North America continued to move southwestward, the position of the hotspot was progressively further and further to the northeast. Today, it is under Yellowstone National Park – the Yellowstone Hotspot. 

The trace of North America’s movement over the hotspot is clearly defined by a series of calderas that get younger and younger as you go from the southwest corner of Idaho to Yellowstone. They are in the Snake River Plain of southern Idaho, which is covered by basaltic and other volcanics associated with the various calderas.

Ages of Yellowstone Hotspot Calderas (illustration by Kelvin Case at English Wikipedia, used under Creative Commons license)

Our other discussions of extensive volcanic events have often found some correlation between the volcanism and extinctions. Was there one with this one? About 14.5 million years ago, about 2 million years after the flood basalts started and while they were still in progress, there was a marked global cooling event that coincided with a major growth spurt in the Antarctic Ice Sheet. And it does correlate with an increase in extinction rates, though I don’t think we’d call it any kind of mass extinction like the great ones in earth’s history. This may have been mostly a result of the change from what’s called the Miocene climatic optimum, a warm period 17 to 15 million years ago, and part of a more general change to cooler conditions that eventually led to the ice ages. It’s not obvious that the Columbia River Basalts played a major role in this minor extinction event at 14 million years ago.

We’ll talk a bit more about Yellowstone in a few days when we talk about supervolcanoes. There is of course a vast amount of information about the Yellowstone Hotspot and the Columbia River Basalts. One of the best resources in my opinion is a book by Robert Smith and Lee Siegel, titled Windows into the Earth – the geologic story of Yellowstone and Grand Teton National Parks (Oxford University Press, 2000).

—Richard I. Gibson

Links and image sources:
Miocene climate

Hotspot breakout model and Columbia River basalt map both from Camp, V.E. and Ross, M.E., 2004, Mantle dynamics and genesis of mafic magmatism in the intermontane Pacific Northwest: Journal of Geophysical Research, v. 109.  doi:10.1029/2003JB002838, used under Creative Commons license 

Hotspot track illustration by Kelvin Case at English Wikipedia, used under Creative Commons license