Across much of the eastern United States, from Minnesota to Georgia to New York, there are several thick layers in the Ordovician rocks that are bentonite. Bentonite – specifically, potassium bentonite – is a rock that’s the altered form of a volcanic ash fall. Such things are really pretty common in the rock record, given that there have been probably hundreds of thousands of volcanic eruptions over geologic time. What makes the Deicke bentonite – pronounced "dickie" – special is that in lots of places it’s around a meter thick. Volcanic ash does tend to erode easily, and it also compresses – so to have a meter-thick zone after 450 million years is remarkable, unless it was right next to the volcanic vent. So that fact that we have this kind of thickness spread out over thousands of square miles makes it doubly remarkable.
|Mt. Pinatubo's 1991 eruption was vastly smaller|
than the Ordovician eruptions discussed here.
There are actually two major and several minor bentonites close to each other in the Upper Ordovician, and as many as 16 others not to far away. The second-largest one is called the Millbrig. And if you need even more amazement, the probable equivalents of these layers are found in Europe as well, in England, Scandinavia, and Russia.
Together, they probably represent two of the largest – if not the largest – volcanic eruptions in at least the past 600 million years and probably quite a bit longer. The nature of the rocks, and their chemistry, suggests that it really was one or two events – erupted in a time span of days or weeks or months. That’s essentially instantaneous, geologically speaking. They’ve been estimated at volumes of 5,000 times the ash that came out of Mt. St. Helens in 1980 – or more.
There’s some reason to think that Avalonia was the host of the Deicke and Millbrig volcanoes. In England’s Lake District, the Borrowdale volcanics are lava flows of essentially the same age as the bentonites. That could make them the lava flows that came from the vents that put the ash all over. As it happens, I talked about the Borrowdale volcanics in a completely different context just a few weeks ago, in my first YouTube presentation based on my other book, What Things Are Made Of. The graphite that formed the basis for Europe’s pencil business in the 1700s is found in those rocks. The video is embedded at the bottom of this post.
There’s also another area, in New Brunswick today, that might have been the source of the volcanic eruptions.
It’s almost impossible to imagine the impact the Ordovician Deicke and Millbrig explosions would have had. Life on land would have certainly suffered – but remember, there was hardly any life on land yet, mostly just those moss-like plants we talked about on March 9. All that ash would have affected global temperatures, and might even have changed water chemistry, which in turn would have affected marine life.
We know the timing of the Deicke event really accurately, because the ash includes zircons, those tough little mineral grains that contain radioactive trace elements which give us ages based on their decay rates. So we know that the Deicke eruption was 457.1 million years ago, plus or minus 1 million years – really accurate for that long ago. This date was reported by Ryan Mathur in 2011 as well as by Samson and others in 1989. The Millbrig bentonite overlies the Deicke in the United States, but it is essentially the same age. They may represent episodes of the same event, but in any case they are almost certainly related to the same overall volcanic system.
If you’ve been going through these blogs and podcasts sequentially, you know that I’ve mentioned a couple of possible causes for the glaciation at the end of the Ordovician, which probably was a major factor in the end Ordovician mass extinction. I doubt if you’ll be surprised that we can now add another possible factor in both the glaciation and the extinction: unprecedented explosive volcanism during the Late Ordovician.
Besides this interesting story, what good is bentonite? In the United States, about a fifth of it is used in muds for oil and gas well drilling. Bentonite is mostly a mixture of clays, which can take up the fluids used in oil exploration, and it helps control underground pressures and strengthens the drill hole wall. Almost half the world’s commercial bentonite production comes from the United States, mostly from Wyoming and Montana where the stuff is much younger, much less consolidated than the Ordovician bentonites of the east. Bentonite is also used as absorbents like kitty litter, and to help pelletize iron ore for smelting. All told, the U.S. uses about a million tons of bentonite every year.
Thanks to Steve Henderson for pointing this topic out to me.
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As if that volcanic story’s not enough, today is also a pretty cool geological birthday – John Wesley Powell, the one-armed veteran of the Civil War who took the first exploring expedition by boat through the Grand Canyon, was born on this day in 1834. He became the second director of the U.S. Geological Survey.
—Richard I. Gibson
Selected references and further reading
Ryan Mathur’s abstract
USGS reference descriptions
Huff et al., 2010, Ordovician Explosive Volcanism, Geological Society of America Special Paper 466.
Samson et al., 1989, Origin and tectonic setting of Ordovician bentonites in North America: Isotopic and age constraints: Geo. Soc. America Bulletin, v. 101, p. 1175.
Did intense volcanism trigger the first Late Ordovician icehouse? Werner Buggisch et al., Geozentrum Nordbayern, Universität Erlangen Nürnberg, Schlossgarten 5, D-91054 Erlangen, Germany. Pages 327-330.
Map by Ron Blakey, via Wikipedia, public domain.