Summary: 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
