March 24. Ordovician explosive volcanism

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.

Given all the tectonic events that have happened since, it won’t surprise you to hear that this is NOT one continuous sheet of bentonite today. It’s broken up, tilted, faulted, folded, eroded. So it took a lot of careful study, including painstaking geochemical work, to figure out that it really was all one sheet. One BIG sheet of volcanic ash.

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.

The possible volcanic island arc discussed in the text is not shown on this map. It would lie between Laurentia (the core of North America) and Avalonia, which includes terranes that today are in New England, maritime Canada, Newfoundland, and Great Britain. Avalonia is also a possible source for the Ordovician volcanism.

Where did they come from? No one is certain. Since they thicken across the United States to the southeast, it’s likely that the source, the volcano, was somewhere off the coast of what is now Georgia. Back in the Ordovician, that was the volcanic island arc that was just about to collide with North America to start the Taconic Orogeny. Beyond that was another complex terrane that we mentioned a few days ago – Avalonia, which included bits and pieces that are now attached to North America in New England, Nova Scotia, and Newfoundland, as well as in Great Britain and Ireland. Maybe the source was in that terrane. Last week I compared Avalonia to the western Pacific – Kamchatka, Japan, the Philippines. Plenty of big-time volcanoes there. Or you could think of it as similar to Indonesia – Sumatra, Java, and Borneo, which include both continental fragments as well as major volcanic zones. Sumatra has the remnants of a volcano at Toba, which exploded 75,000 years ago and has been suggested to have reduced global human populations to a few thousand. Then there’s Krakatau, whose explosion in 1883 was heard 3,000 miles away, and which affected sunsets around the world for years. And Indonesia also harbors Tambora. Its eruption in 1815 caused the famous “Year Without a Summer,” when it snowed in Washington, D.C., in June, and made the weather in Europe so miserable that a depressed Mary Shelly wrote her most famous novel, Frankenstein.

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.