Episode 393 The Mountains of the Moon

Today we’re going to the Mountains of the Moon – but not those on the moon itself. We’re going to central Africa.

There isn’t really a mountain range specifically named the Mountains of the Moon. The ancients, from Egyptians to Greeks, imagined or heard rumor of a mountain range in east-central Africa that was the source of the river Nile. In the 18^th and 19^th centuries, explorations of the upper Nile found the sources of the Blue Nile, White Nile, and Victoria Nile and identified the Mountains of the Moon with peaks in Ethiopia as well as 1500 kilometers away in what is now Uganda. Today, the range most closely identified with the Mountains of the Moon is the Rwenzori Mountains at the common corner of Uganda, the Democratic Republic of Congo, and Rwanda.

This location is within the western branch of the East African Rift system, an 8,000-kilometer-long break in the earth’s crust that’s in the slow process of tearing a long strip of eastern Africa away from the main continent. We talked about it in the episode for December 16, 2014.

The long linear rifts in east Africa are grabens, narrow down-faulted troughs that result from the pulling apart and breaking of the continental crust. The rifts are famously filled in places by long, linear rift lakes including Tanganyika, Malawi, Turkana, and many smaller lakes.

Virunga Mountains (2007 false-color Landsat image, annotated by Per Andersson : Source)

When rifting breaks the continental crust, pressure can be released at depth so that the hot material there can melt and rise to the surface as volcanoes. In the Rwenzori, that’s exactly what has happened. The Virunga volcanoes, a bit redundant since the name Virunga comes from a word meaning volcanoes, dominate the Rwenzori, with at least eight peaks over 10,000 feet high, and two that approach or exceed 4,500 meters, 15,000 feet above sea level. They rise dramatically above the floors of the adjacent valleys and lakes which lie about 1400 meters above sea level.

These are active volcanoes, although several would be classified as dormant, since their last dated eruptions were on the order of 100,000 to a half-million years ago. But two, Nyiragongo and Nyamuragira, have erupted as recently as 2002, when lava from Nyiragongo covered part of the airport runway at the town of Goma, and in 2011 with continuing lava lake activity. Nyiragongo has erupted at least 34 times since 1882. The volcanic rocks of these and the older volcanoes fill the rift enough that the flow of rivers and positions of lakes have changed over geologic time.

Lake Kivu, the rift lake just south of the volcanoes, once drained north to Lake Edward and ultimately to the Nile River, but the volcanism blocked the outlet and now Lake Kivu drains southward into Lake Tanganyika. Local legends, recounted by Dorothy Vitaliano in her book on Geomythology, Legends of the Earth (Indiana University Press, 1973), tell the story of demigods who lived in the various Virunga volcanoes. As demigods do, these guys had frequent arguments and battles, which are probably the folklore equivalent of actual volcanic eruptions. The stories accurately reflect – whether through observation or happenstance – the east to west migration of volcanic activity in the range.

The gases associated with the volcanic activity seep into the waters of Lake Kivu, which has high concentrations of dissolved carbon dioxide and methane. Generally the gases are contained in the deeper water under pressure – Lake Kivu is the world’s 18^th deepest lake, at 475 meters, more than 1,500 feet. But sometimes lakes experience overturns, with the deeper waters flipping to the surface. When gases are dissolved in the water and the pressure reduces, they can abruptly come out of solution like opening a carbonated beverage bottle. This happened catastrophically at Lake Nyos in Cameroon in 1986, asphyxiating 1700 people and thousands of cattle and other livestock. The possibility that Lake Kivu could do the same thing is a real threat to about two million people.

The critically endangered mountain gorilla lives in the Virunga Mountains, which also holds the research institute founded by Dian Fossey.

—Richard I. Gibson

December 28. Supervolcanoes

The Pleistocene is justly famous for the glaciations, which certainly dominated things. But the world doesn’t stop for glaciers, and plenty of other things were going on. Like supervolcanoes.

A supervolcano is a big one, conventionally taken to be a single eruption of more than 1,000 cubic kilometers, or 240 cubic miles. That’s a volume vastly greater than even many significant, damaging eruptions – for comparison, Mt. St. Helens in 1980 ejected about 1.2 cubic kilometers (or less) of material. So a supervolcano would be at least 833 times that volume. 

In this comparison of "dense rock equivalent," Toba erupted more than 11,000 times the volume of Mt. St. Helens in 1980.

We’ve talked about a lot of volcanic events in this series, but most of them were likely to be many events over long periods of time, a million years or more, adding up to a lot. One exception might be the eruptions that created what is now the Ordovician Deicke Bentonite that we talked about March 24. That might have been a single eruption, and if it was, it might have been the largest in at least the past 600 million years. Its ejecta volume is estimated at 5,000 cubic kilometers or more. 

So these things were probably happening sporadically throughout earth’s history. The favored locations would be subduction zones or hotspots, places where heat can build up and pressures can increase to the point where the crust can’t contain them, and they erupt violently. 

During the Quaternary, we know of six eruptions with volumes of 1,000 cubic kilometers or more. The largest, at what is now Lake Toba in Sumatra, had a volume of 2,800 cu km and happened about 74,000 years ago. That eruption is linked to a controversial idea that the ensuing global winter lasted perhaps 10 years, and, based on genetic studies, might have reduced the existing human population of the planet to as few as 3,000 to 10,000 individuals. It is controversial, and 75,000 years ago the evidence of human life is spotty at best. Consider this to be another idea for which the jury is still out. 

The second largest supervolcano eruption was at Yellowstone. In fact two of the six Quaternary supervolcano eruptions were there, one at 2.1 million years ago, and the other at 640,000 years ago, with volumes of about 2,500 and 1,000 cu km respectively. There was another large eruption there about 1.3 million years ago, only about a tenth the size of the one at 2.1 million years ago.

The fourth Quaternary supervolcano was in Argentina, about 2.5 million years ago just as the Quaternary was starting, and the other two were in New Zealand, in the Taupo Volcanic Zone on the North Island. Those two eruptions were at about 254,000 and 27,000 years ago – the latter is the most recent supervolcano eruption that has occurred. Its volume, about 1,200 cu km, is still 1,000 times that of Mt. St. Helens in 1980.

Some supervolcanoes seem to work in ways that are different from regular volcanoes. To release the vast volumes, special conditions are required. Let’s use Yellowstone as an example – and in passing, make the argument for why such an eruption cannot be imminent there.

The mouths of supervolcanoes are much larger than the craters that form at the top of a standard volcano. A caldera is a collapsed region that has fallen into a magma chamber beneath it – a magma chamber that had to evacuate its magma in many small eruptions to allow for the collapse. When the crust over the chamber collapses, all the fractures cause a rapid release of pressure, and the confined, pressurized magma that’s still down there can come out, violently. It’s like a pressure cooker – the relief valve on the top is like the geysers at Yellowstone, releasing pressure and keeping things safe. Without that, the pressure could increase and ultimately blow the cooker apart.

Or think of an apple pie. You poke holes in the crust to allow steam, the pressure inside, to escape. If you didn’t, cracks might develop and some of the filling could escape, but the crust would still be there. But if enough of the filling escapes, say around the edges of the pie, the crust on top might collapse, cracking, and the instant reduction of pressure would allow the entire contents of the pie to explode up to the ceiling.

So my point is, you can’t really have a caldera collapse, which would make a supervolcano eruption, without emptying enough of the magma chamber for the crust to collapse into it. The last time any magma was erupted at Yellowstone was 70,000 years ago – and not much came out. I think we need to have a LOT more little eruptions – magma, not just the hot water – before anything like a major collapse is likely that would produce a supervolcano eruption. I live 120 miles away and I’m definitely not losing any sleep over it. Besides, if it does happen, there’s precious little we can do about it. Yellowstone’s supervolcano eruptions have deposited ash as far away as the state of Mississippi, so the area of devastation would be huge, and in a big way dependent on the wind directions at the time.

With only three data points for the present Yellowstone caldera eruptions, it’s irrational to see any predictable regularity to them, at least not more than ball-park figures like plus or minus 200,000 years, and even that could be far, far off in such a chaotic system.

Supervolcano eruptions happen. Don’t worry about it. Regular eruptions are far more frequent and we can plan for them, though even much smaller events can be incredibly disruptive, as the unpronounceable Icelandic eruption a few years ago proved. Its erupted volume was less than 1 cubic kilometer.

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December 28, 1835, was the birthday of Archibald Geikie, in Edinburgh, Scotland. He was an eminent scholar of Scottish geology, but he expanded his work on volcanics to include western North America as well. He is probably as well known for his popular writings about science as for his technical work. Also born this day, in 1894 at White Plains, New York, was Alfred Romer. His focus from his base at Harvard was in the field of vertebrate paleontology. He classified the labyrinthodonts, and the basics of his general classification of the vertebrates is still in use today, with modifications and expansions.

Also on this date, December 28, 1908, a strong earthquake hit Messina, Sicily, the location that the Messinian stage of the Miocene was named for. Messina and other major cities were practically destroyed, and at least 70,000 people were killed. There was a tsunami as well, and the final total death toll is put at about 123,000. The quake was a result of the ongoing Alpine collision between Africa and Europe. Africa is pushing northward and small blocks – Italy, Sardinia, and the oceanic crust in the Tyrrhenian Sea, are being forced over the leading edge of Africa. It’s a really complex zone, and Mt. Etna, the active volcano near Messina, is another consequence of this tectonic activity.

—Richard I. Gibson

Supervolcanoes 

November 19. Volcanoes in Texas

Eighty million years ago, while the Laramide Orogeny was getting underway in western North America, central and south Texas were parts of the shallow carbonate bank that developed on the North American side of the Gulf of Mexico. The Gulf had begun to form during the Jurassic as Yucatan pulled away from what is now Texas and Louisiana. These Cretaceous rocks are part of the Gulf Coastal Plain that we talked about a few days ago. 

Remember the rudists? The tubular clams a meter high that trapped sediment to help form reefs? They grew in this area too. It was overall a quiet, shallow sea, perhaps something like today’s Florida Shelf. The setting was not one where you’d expect volcanoes, but that’s what we got. 

In a long, linear zone east of a line from Waco to Austin and on south to San Antonio and Uvalde, Texas, there’s a string of more than 200 little volcanoes that erupted into the carbonate sediments. They’re basically piles of volcanic ash together with some basaltic flows, and some of them today are resistant enough to form mounds on the land surface. One well-known example is at Pilot Knob, not too far from Austin. Pilot Knob is about two miles across. 

Most of these volcanic bodies are buried in the subsurface and have no expression that we can see on the land, but most of them are basaltic and contain a lot of magnetite. This gives them a distinct expression in a magnetic map. That characteristic was useful in oil exploration, because the volcanics and the surrounding carbonate rocks contain oil in 35 or 40 of the known volcanoes. Something like 50 million barrels of oil have been produced from several accumulations since the first field was discovered in the 1910s.

Magnetic map of Uvalde and Medina Counties, Texas (from USGS). Most of the little pimple-like bumps represent igneous plugs of Cretaceous age.

The margin of the old Texas Craton, the Precambrian core of this region, is called the Balcones Escarpment, a topographic feature that follows a fault zone separating the older rocks of central Texas from the Cretaceous and younger rocks of the Gulf Coastal Plain. The Balcones Escarpment is followed by Interstate 35 from Waco to Austin to San Antonio, just about the same zone where the volcanic centers are found.

It’s not completely clear why these volcanoes erupted into this tectonically quiet region. Probably the best explanation is that there was a pulse of extension, pulling apart, that opened deep-seated faults and fractures through which the magma rose. The weakest zone was along the old break between the strong craton and the stretched crust to the south and east that was pulled and stretched by Yucatan’s departure in the Jurassic. The magmas are not all basaltic, which makes it difficult to say, for example, that they simply came from the mantle – there must have been some melting of other rocks and mixing of magmas to get the kinds of igneous rocks we see.

—Richard I. Gibson

Cretaceous volcanism in South Texas and oil 

Pilot Knob

Magnetic modeling  

Magnetic map (USGS OFR-02-0049)   and this one  

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.