History of the Earth

Trilobites continued a remarkable diversification during the Silurian. Some, such as Staurocephalus murchisoni, had these big bulbous things like noses at the front of their bodies. It’s actually the glabella, which in turn is the central, axial portion of the cephalon, or head. The sketches here give you an idea how bizarre it looks.  Staurocephalus murchisoni (left); Deiphon forbesi (right) Many trilobites have a simple dome-like glabella, slightly raised above the side parts of the cephalon, or head, where the eyes are. The glabella apparently covered and contained some of the trilobite’s forward digestive organs – the stomach, which was above and forward of the mouth. Exactly how it worked isn’t clear, at least not to me after checking around – if any trilobite specialist hears this and can enlighten us, please contact me at rigibson@earthlink.net. But at a first pass, it seems that the glabella is more or less the stomach, or maybe part of the intestine. It’s been speculated that larger glabellas indicate more complex food sources, which some infer to mean that the trilobite in question might have been carnivorous – but that seems to be a bit of a stretch to me. How and why did these bulbous glabellas evolve into knob-like protrusions extending away from the animal’s body? Beats me. —Richard I. GibsonSee also: Richard Fortey, Lifestyles of the trilobites (PDF) The drawings above are from a 19th century textbook.

In the April 4 episode, I mentioned that the supercontinent Gondwana was situated in the southern hemisphere, partly on the South Pole, during the Silurian. The continents of Laurentia – which is North America – Baltica (Europe) and Siberia were all converging to some degree, and all were pretty much in the tropics, near the Silurian equator. Once the glacial epoch at the start of the Silurian was over, warm, shallow seas prevailed on those continents. A lot of the sediments that were deposited in those seas became limestone, and many of them are remarkably fossiliferous. In Wales, where the Silurian got its name, and southwest England, the Silurian Much Wenlock limestone is one of the richest fossil assemblages in the world. It’s named for the village of Much Wenlock in Shropshire, so called to distinguish that village from the nearby Little Wenlock. More than 600 different species have been found, and most of them are invertebrates typical of warm, shallow seas. Crinods, brachipods, corals, bryozoans, and trilobites make for a diverse collection – it must have been something like a modern tropical lagoon, supporting lots of life. We’ll talk more about corals, which began to make reefs during the Silurian, tomorrow. Wenlock limestoneIt probably won’t surprise you to hear that the Wenlock limestone was laid down during the Wenlock Epoch, the second subdivision of the Silurian Period. The time span for the Wenlock is about 423 to 428 million years ago, a total of 5 million years. The Wenlock limestone was deposited in the early part of that time. The geological term for a package of rocks with similarities in time and composition is a formation, which is often, but not always, one particular kind of rock like limestone. The Wenlock is subdivided into three members that reflect slightly different environmental conditions. Some parts of it contain silt, finer grained than sand, which would probably mean that there was an influx of sediment being eroded off a land area at some distance from the site of deposition. Coarser sediments, like sand or gravel, would have been dumped closer to the source area, such as along a beach. So the silty sediment in the Wenlock limestone would be a ways out into the sea, perhaps a half mile or a few miles from shore, maybe carried there by storm action. A study by Ratcliff and Thomas in 1999, linked on the blog, determined that the Wenlock was deposited in water with a variety of energies operating – parts of it were laid down below any wave action, while some was within the zone where storm waves could operate. Some of it was deposited well above the wave base, meaning the water was in motion almost continuously. Lots of living things like crinoids, fastened to the water bottom, like that kind of water, where fresh nutrients are moved in all the time. Such water would be the opposite of stagnant – the waves and moving water would be oxygenating it as well as bringing in food particles. A great place for animals that can’t move around. Because of the abundant diverse fossils, some have called the Much Wenlock the most famous rock formation in England. It was the fossils in these rocks, together with additional work, that led Murchison to define the Silurian. Today, it’s considered to be a lagerstätten, a German term meaning ‘fossil ore body’ because of its richness and completeness. The Burgess shale is another lagerstatten. * * * April 6, 1916, is the birthday of Vincent McKelvey, in Huntington, Pennsylvania. He joined the US Geological Survey in 1941, and served as its 9th director, from 1971 to 1977. He was well known for his scientific studies of phosphates and deep-sea mineral deposits. —Richard I. Gibson Photo by Rept0n1x via Wikimedia Commons, under Creative Commons license. Lapworth Museum collections  Ratcliff and Thomas (1999) Much Wenlock limestone

The low sea levels of the Ordovician glacial period rebounded in the Silurian as the ice melted. Although much of Gondwana was still over the south pole, it appears that conditions were not right for glaciers, and the ice caps, if any, were much smaller during most of the Silurian than they were during the Ordovician glacial time. Map by Ron Blakey, via Wikipedia, used under CC-A-SA license. Still, sea level wasn’t quite as high as during much of the Ordovician, at least in parts of North America. We’ll discuss some of the variations later in the month. The Silurian map above shows Baltica, the Precambrian core of Europe, getting pretty close to North America, and the long narrow microcontinent, Avalonia, is getting caught between them. Siberia is to the northeast, and Gondwana is on the margins of the globe in the map above. Plenty of fodder for mountain building. We’ll talk about that, too, later in April as we get further into the Silurian Period. —Richard I. Gibson Map by Ron Blakey, via Wikipedia, used under CC-A-SA license.

If there’s any animal that in my mind is the hallmark of the Silurian, it’s the eurypterids. The name means “broad wing,” for the paddle-like appendages they used to swim. They’re called sea scorpions – and they do look sort of like scorpions – and like scorpions they are arthropods, the group that includes insects, crabs, and trilobites. But they are most closely related to modern horseshoe crabs and ultimately to spiders rather than scorpions. Eurypterids were predators, and with the largest reaching a length of two and a half meters – more than eight feet, the largest arthropods that ever lived – they were probably at the top of the food chain during the Silurian Period. A more typical size would be 20 centimeters, or 8 inches, but they were still formidable attackers in shallow waters, both in the oceans and in lakes. More than 240 species of eurypterids have been described, and all are extinct. The group got started in the late Ordovician, managed to survive the end Ordovician extinction, and proliferated during the Silurian. They are so common in Silurian rocks of New York that they became the state fossil of New York in 1984. Eurypterids had strong legs – strong enough to possibly scuttle around on land, like crabs today, but these were fundamentally aquatic critters. Many varieties had a long, sharp, pointed tail, called a telson, which might or might not have been used to inject venom into victims. There’s no hard evidence for that. Eurypterids survived until the extinction at the end of the Permian, giving them a range of about 210 million years. They declined in diversity after the Silurian. —Richard I. Gibson Reconstruction from Ernst Haeckel's Kunstformen der Natur (1904; public domain).  Photo of one of the largest eurypterids  More information

The Silurian was named in 1835 by Roderick Murchison, who took the name from the Silures, an ancient Celtic tribe of south Wales. The Silures began fighting the invading Romans about 48 AD. Their guerilla war was successful for something like 30 years, and it wasn’t until about 78 AD that the Silures were incorporated into Roman Britain. It’s not clear whether they eventually just acquiesced or were conquered in a military campaign. The oldest sedimentary rocks of Britain, called the Primitive Series, were subdivided into the Cambrian and Silurian by Adam Sedgwick and Roderick Murchison, respectively. We talked in late February about the controversy those two friends became embroiled in over which rocks were in which period, a 40-year-long argument that was finally settled by the establishment of the Ordovician Period, also named for a Welsh tribe. —Richard I. Gibson Map from Wikipedia, used under GNU free documentation license. 

The Silurian Period as it is now defined spans the time from 443 million years ago – the end Ordovician extinction – to 416 million years ago. The error bars on those dates are a million or so years. The Silurian covers just 27 million years, the shortest period we’ve defined. Where the Cambrian and Ordovician were more or less divided into early, middle, and late epochs, the Silurian has four major subdivisions: Llanderovy, Wenlock, Ludlow, and Přídolí. The first three are named for rocks found in Wales, Scotland, and England, respectively, and the Pridoli is from the Czech Republic. Each epoch is separated from the next by breaks in the rock record, some of which coincide with relatively small extinction events, but the real definition of the epochs is based on specific graptolite zones. You can hear more about graptolites in the podcast for March 8.  * * * Charles J. Vitaliano, a geology professor at Indiana University, one of my own college professors, was born April 2, 1910, in New York City. He specialized in igneous and metamorphic rocks and mineralogy as well as field geology. —Richard I. Gibson Time column from Wikipedia

It’s the end of March, and the end of the Ordovician Period. If you’ve been following along, you know that this time, 443 million years ago, is marked by a huge mass extinction – by some estimates, second only to the Great Dying at the end of the Permian in terms of its decimation of life on earth. More than 100 families of marine animals went extinct. Families are groups of genera which are groups of species – so this was a LOT of diverse fauna. More than half the species of bryozoans, which had just appeared early in the Ordovician, were destroyed. Trilobites, brachiopods, and nautiloids were severely impacted. Something like 60% of all marine species died. Ordovician lifeAs near as we can tell, it seems that the extinction event was worse among animals that lived in shallow tropical seas, which supports the idea that the glaciation going on at the same time was an important factor. Cooler conditions, cooler water, fewer tropical animals. Also, when water is tied up in ice on land, sea levels drop, so the shallow seas that predominated over much of North America retreated. There were many fewer niches for life to occupy. And in fact there are two phases to the extinction, which seem to be related to these two different but related causes. We talked about some of the factors that may have led to the glacial period. The position of the supercontinent of Gondwana over the south pole, the possibility that life on land was removing CO2 from the atmosphere, reducing greenhouse conditions, and the cooling effect of some of the greatest volcanic episodes known in earth history. There isn’t much doubt that whatever caused the glaciation, the consequences for life were pretty dire, and generally, it’s the cooler conditions and lower sea level that probably did the killing of marine life. All of the phyla of life survived and came back after the glacial period, in the Silurian that starts tomorrow. I think it’s important to note that this is the only glacial period that is closely connected with a mass extinction, unless the modern glacial period has been a factor in the fairly recent destruction of a great many species. The jury is still out on whether or not it was climate change or human activity that eliminated things like mastodons, giant sloths, and saber-toothed tigers. The end Ordovician glacial epoch was relatively short-lived, spanning just a few million years, and the extinction event at 443.4 million years ago is very much a sharp spike in terms of species loss. I think it’s fairly well accepted that cooling and loss of shallow-water niches were the ultimate primary causes of the extinction, and those things were caused by the glaciation. There are lots of potential factors that might have caused the glaciation, as we discussed March 28. Tomorrow, the Silurian begins. * * * March 31, 1850, is the birthday of Charles Doolittle Walcott, in New York Mills, New York. Walcott was the geologist who discovered and described the Burgess Shale in British Columbia back in 1909. You can hear more about Walcott in the podcast for February 12. —Richard I. Gibson Photo by Ryan Somma under Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)  

When oceans close, and force island arcs, oceanic floor, and what ever else is out there to collide with continents, as happened in the Taconic Orogeny, there’s usually enough subduction or partial subduction that some materials reach temperatures and pressures at which they melt. The molten rock generated by tectonic activity is what makes the volcanoes in a volcanic island arc – which might not be a string of islands; the Andes is a fine example of a volcanic arc related to subduction that is not islands. Remember that we can have magma flowing on the surface as volcanic lava flows and ash falls, but we can also have molten material down inside the earth, never reaching the surface. We called such rocks plutonic the other day, because they form at depth, down in the realm of Pluto. They can also be injected through and between pre-existing rocks, cutting across them. Then, they’re called intrusive – a fairly straightforward word meaning that the magma, the molten material, is forced into other, solid rocks. Think of the neck of a volcano, or better, the deeper roots where magma is pushing here and there, wherever it can find a weak zone to send a narrow, vein-like shoot through. Those weak zones might be bedding planes in a sedimentary rock, and intrusive igneous rocks that are parallel to bedding in sedimentary rocks make a structure called a sill. If the igneous rocks cut across the older rocks, the feature is called a dike. Dikes and sills were forming at depth during the Taconic Orogeny, on both sides of the closing ocean. On the side where parts of Great Britain were found, the microcontinent of Avalonia, the same thing was happening. Some of the intrusive rocks there formed dikes made of dolerite. That’s the British term; Americans call it diabase, and it’s a fine-grained version of gabbro – or a coarse-grained version of basalt, depending on how you look at it. It’s the intermediate variety of those rocks that are relatively low in silica, as compared to silica-rich granite, and relatively rich in iron and magnesium. After they were formed in the Ordovician mountain-building event about 450 million years ago, the dolerites of Wales sat there – tossed around by later tectonic activity, but still there, until erosion exposed them. Today, they crop out in the Preseli Hills of Wales, a bluish rock. By using careful geochemical analysis, those outcrops have now been identified as the source of the bluestones at Stonehenge, 250 miles away. The bluestones at Stonehenge are not all dolerite, and some 20 different rock types have been identified, but all are from the Ordovician outcrops in Wales. Some are volcanic equivalents of the dolerites, and the location they came from was not pinpointed until 2011. They’re from the same general area of the Preseli Hills in Pembrokeshire, in far southwestern Wales. It isn’t clear whether the stones were transported from Pembrokeshire to the Salisbury Plain by the builders of Stonehenge, or by glacial movements thousands of years earlier. —Richard I. Gibson Photo by Richard Gibson References and further reading: The Earth Story’s nice essay with good links.  Another piece in Stonehenge rock source puzzle  Stonehenge mystery

The ends of the periods of geologic time tend to be marked by major events – extinctions, glaciations, and mountain-building, at least in Britain and the United States, where most of the big packages of geologic time were first described. The Ordovician is no exception. The Taconic Orogeny is the mountain-building event – that’s what orogeny means – that began toward the end of the Ordovician Period in eastern North America. This was the first significant pulse in the construction of the Appalachian Mountain system, a process that took many millions of years. The Taconic Mountains today lie along the eastern border of New York, and in western Vermont, Massachusetts, and Connecticut, but the Taconic Orogeny really extends south into Georgia. The collision that caused the mountain uplift involved a complex mess of masses, including some small continental blocks, an extensive island arc system, maybe some slices of oceanic crust, and the sediments that were among all those pieces – but most simplistically, it was probably a volcanic island arc. I’ve used the analogy of the modern western Pacific, and I think that’s a reasonable view. Something like Kamchatka-Japan-Taiwan-Philippines or Indonesia, slamming into a rigid North American continental block, over a period of several million years. And not at the same time, and not in the same way everywhere along its length. There’s evidence for the beginning of the convergence even in Cambrian and early Ordovician times, in the types of sedimentary rocks we find, but the effects really started to be felt in late Ordovician time. The culmination of the Taconic Orogeny, the uplift of a significant mountain range along the east side of North America, was near the end of the Ordovician, something like 455 to 445 million years ago. The evidence is in the modern Taconic Mountains and in the subsurface elsewhere, where we see the deeply eroded roots of that Ordovician mountain range. The Queenston Delta, which we discussed on March 21, was one of the most significant consequences of the Taconic Orogeny, at least in North America. Ebenezer Emmons, a geologist in New York in the 1840s, defined the Taconic Orogeny. He had already described the Potsdam Sandstone and other specific elements of early Paleozoic geology. Emmons ascribed his Taconic System of rocks to the Cambrian – a view vehemently opposed by James Hall, the State Geologist of New York. Hall felt that the rocks belonged to the Silurian. This bitter controversy – called the greatest in American geology – was analogous to the debate going on in Britain, between supporters of Sedgwick and Murchison over the same part of the geologic section. In New York, Emmons was banned from practicing geology; he sued Hall for slander and libel, but lost, and moved to North Carolina. In 1888, 25 years after Emmons’ death, he was ultimately vindicated, as the new Ordovician Period was defined, putting Emmons’ rocks into the Cambrian as he had originally claimed. But it wasn’t until 1903 that the U.S. Geological Survey formally accepted the Ordovician Period. The present-day Taconic Mountains are mountainous not because of the Taconic Orogeny. The Ordovician mountains were long since eroded to low hills, or less. Later events have pushed the rocks there back up. Building the Appalachians was a long process which we will touch on for many months – many million years – to come. —Richard I. Gibson Callan Bentley’s outstanding animation Another useful link from Paleontological Research Association Emmons image – public domain Cross sections from USGS – public domain

I’ve mentioned the late Ordovician glaciation several times in recent posts, suggesting things that might have been factors causing it. One possible factor is the fact that the largest continent, Gondwana, which included most of Africa, South America, India, Australia, and Antarctica, was located over the south pole. That alone wouldn’t do it unless the climate was cold enough. Upsala Glacier, Argentina During most of the Ordovician, planet Earth was in greenhouse conditions, with high concentrations of carbon dioxide in the atmosphere. We mentioned two things that might have affected that – the presence of life on land, taking more and more CO2 out of the atmosphere, and the tectonic activity that made a high mountain range in what is now eastern North America. Erosion of that mountain range, to create the immense Queenston Delta, might have produced enough sediment to have an effect on the atmosphere so that CO2 was reduced. Both of those ideas are in the “might have” category, but something certainly did affect CO2 concentrations. And then we talked about the humongous volcanic eruptions, the Deicke and Millbrig and others near the beginning of the Late Ordovician epoch, around 455 to 457 million years ago. Cooling because of high concentrations of dust in the atmosphere was certainly likely, and the timing is good with respect to the glaciation. The glaciation seems to have been at its height about 440-455 million years ago, which includes the first part of the Silurian – the boundary with the Ordovician is put at 443 million years ago. There’s some evidence that the glaciation might have started as long ago as 460 million years. The evidence about CO2 values isn’t speculative. There is abundant evidence in carbon isotopes to indicate significant changes in water temperature – a pretty significant disruption of the carbon cycle, enough to imply cooling and a reduction of the greenhouse effect. In addition to the ideas that life and erosion helped remove CO2 from the atmosphere, another idea is that volcanism helped. But wait, you say – I thought volcanoes added CO2 to the atmosphere. Well, they do. But earlier in the Ordovician, extensive basaltic eruptions – when they were finished – created large expanses of solid basalt, which reacts and erodes relatively quickly, and might have been a factor in CO2 removal. That’s another pretty speculative possibility. Glacially-deposited rocks of Late Ordovician age are common across what is now the Sahara Desert, from Morocco to Ghana to Libya, and they’ve also been found in South Africa, Brazil, Arabia, Germany, Nova Scotia, and Newfoundland. The ages of all those deposits are not absolutely determined, but they are all of Late Ordovician or Early Silurian age, coinciding nicely with and defining the glacial period. There’s evidence that this glacial event ended quite abruptly, for reasons that seem to me highly speculative and hardly able to explain it. One suggestion is that once the ice sheets reached their limit, they essentially collapsed – which to me doesn’t adequately explain why they then retreated to practically nothing. CO2 levels increased in the Silurian, but I haven’t seen a good explanation for why. The Ordovician-Silurian glaciation is the only one associated with a mass extinction event – one of the largest in earth’s history. We’ll talk more about that at the end of the Ordovician, in a few days. —Richard I. GibsonFurther reading: The carbon cycle Photo by longhorndave via Wikipedia, under cc-by-2.0

Yesterday we talked about some common rock types, granite and basalt and some others. Today, let’s talk about a very special rock type, but one that’s specific to the Ordovician. Kukersite is oil shale – one of the richest oil shales in the world, with as much as 40% organic material in it. It’s found in Estonia. First, let’s distinguish between oil shale and shale oil. Shale is a very fine grained rock, solidified from mud. Oil shale is a rock – not liquid – in which a lot of organic material, called kerogen, has been incorporated with the sediment. Shale oil, like the famous Bakken in North Dakota, is entirely different – it’s liquid oil, trapped in very tiny, very poorly interconnected spaces, in contrast to a good oil reservoir like a sandstone with lots of open pore space and plenty of interconnectedness to allow the oil to flow. Some of the Bakken isn’t even in shale, but it in dolomite. The main difference is that oil shale is a rock that does not contain ANY liquid oil. To get the organic matter out, you have to mine the rock, then cook it. The organic stuff might burn directly, like it does in peat or coal, or you can use a distillation system to convert the solid kerogen to liquid oil. See also this post. The geology of the Ordovician kukersite in Estonia suggests that there were episodic periods when the waters there were anoxic – lacking in oxygen. The alternation with periods of more oxygenated water resulted in maybe 50 separate beds of kukersite, separated from each other by layers of limestone and other rock. The individual kukersite layers are typically around 20-30 centimeters thick, but some are a couple meters thick. The setting was probably something like a low, flat tidal zone along the sea coast that was periodically cut off from circulation, so that stagnant, swampy conditions developed. What was in those swamps? This was the Ordovician, so don’t visualize the Okeefenokee Swamp, with big trees, lily pads, and such. The life that put its organic matter into these sediments was algae. In fact, algae and other microbial plants are the greatest contributors to the source rocks that make all oil and natural gas. The volume of dead animals was usually far too small to add significantly to the stuff that would become oil – so forget that attractive idea of dinosaurs in your gas tank. It was algae. The Estonian kukersite – a rock that burns – was known as long ago as 1700, or even earlier, but its commercial development began in 1918 in the face of fuel shortages resulting from World War I and the Russian Revolution. Production has continued pretty much up to this day, with both open-pit mines and underground mines. It’s an important energy resource for Estonia. In 2011, Estonia manufactured about 11,000 barrels of oil per day from its oil shale – a bit less than half of its needs. 90% of Estonia’s electricity comes from plants fueled with oil derived from oil shale. It’s important as a local resource, but producing oil from oil shale is expensive, given that you have to mine it, like you mine coal, then you have to input a lot of energy to heat it up to distill the oil out of it. We know how to do this technologically; the issue is the cost. Building a high-volume oil shale reduction plant might cost something comparable to an oil refinery – maybe several billion dollars or more, with many, many years before you get a real return on your investment. Conventional crude oil’s energy return on investment is 20 to 40 to one, while oil shale’s return is more like 2 to 1. You don’t make much money, and it takes a lot longer to make it. For an oil shale plant to be economic, I’ve read figures all over the map for a sustained price of oil, from $80 per barrel to $150 per barrel. I think something like a cost to produce of $90 to $100 per barrel for oil shale is reasonable. If that’s the production cost, then you have to add taxes, royalties, and so on, plus a reasonable 10% profit margin – and you end up with oil fro

Today, we take a break from the chronology of earth history to talk a little about rocks types: igneous, derived from molten magma, sedimentary, deposited as tiny broken fragments of other rocks, as sand, silt, or mud, or precipitated chemically, like limestone, and metamorphic rocks, which are the changed forms derived from anything else that’s been subjected to high temperatures or pressures or both. Let’s expand a little on igneous rocks. The word is from Latin, ignis, meaning fire, and it’s the same root that gives us words like ignition. These rocks come from fire, from volcanoes, but there’s more to it than that. When magma – molten rock – flows out on the surface of the earth as lava, it cools quickly. “Quickly” is relative, of course – in geological terms that could mean a million years, but when we talk about cooling of rocks, I’d day that generally means anything from hours to years, maybe even hundreds of years. When magma cools quickly, the stuff, the minerals, that are crystallizing from the magma don’t have much time to grow, so they are usually pretty small. Sometimes molten rock can cool so quickly that it solidifies into a glass – obsidian – where the crystals are so tiny that they are invisible. In fact, in natural glass, obsidian, it isn’t really crystalline at all. The stuff just solidified, but didn’t crystallize. Give it a little more time, say months or more, and tiny crystals can grow, like salt forming around a pan of evaporating salty water. They might still be microscopic, but they’re there, as crystals. Give it a long, long time – say, a few hundred thousand years – and the crystals have time to grow to be pretty big, visible to the naked eye, or maybe even a few inches long. How can it take a hundred thousand years for molten rock to solidify? Well, generally, it would have to be pretty well insulated to retain heat that long, so we’re talking about magma that is NOT erupted onto the surface of the earth, where it cools quickly, but magma that’s down inside the earth – maybe miles down, where it cools very very slowly. Small crystals, quick cooling. Big crystals, long time cooling. The size of the crystals is part of the texture of the rock, one of the most important things in giving it a name and figuring out its story. graniteThe other important thing about igneous rocks is their composition. You can mix up the chemicals in hundreds of different ways, depending on the abundance of the elements in the mix. But only a relatively few common minerals will form from whatever elements are present. So, if you have a lot of silicon in the melt, you’re probably going to get a lot of quartz, silicon dioxide, in the solidified rock. Quartz is the most common mineral in the earth’s crust, but there are rocks that have none. They’d often be rocks that solidified from magma with more iron and magnesium, but don’t visualize a simple one-to-one relationship between silica and iron. There’s way too much variety for that. But, as a first pass, we can think of two common igneous rock types – granite and basalt. Granite is silica rich, and it’s made of a lot of quartz and feldspar, another silica rich mineral that includes aluminum and other things. Basalt is silica poor, and contains more iron-rich minerals than granite. Granite is coarse-grained, telling us it formed down inside the earth and cooled over a long period of time. Basalt is fine-grained, telling us it cooled relatively quickly. basaltBut you can have the same composition as granite that cools quickly, such as in lava flows. That rock, fine-grained but silica rich like granite, is called rhyolite. And likewise, you can have magma of basaltic composition that cools over a long time within the earth. We call that rock gabbro. The two other terms I’m sure I’ll use in these presentations are volcanic – you know what that means – and plutonic, which means the rock formed inside the earth, in the realm of Pluto, so it took a long time to cool. Both granite

In 1883 pioneers of oil and gas exploration drilled more than 1,000 feet – a deep well in those days – to reach the Middle Ordovician Trenton limestone in northeastern Ohio, encountering a significant flow of natural gas. Within a few years, an extensive area of Trenton oil and gas production had been established in northwestern Ohio and eastern Indiana. It was one of the largest accumulations discovered in the United States before 1900. Most of the Ordovician production in this area has now been depleted, but in 1887 a group of small companies, threatened by John D. Rockefeller’s Standard Oil, based in Cleveland, joined forces in Findlay, Ohio, to form the Ohio Oil Company, known today as Marathon. Just two years after its founding, the Ohio Oil Company was gobbled up by Standard, in 1889, and remained part of the Standard Oil Trust until the breakup of Standard Oil in 1911. In 1930, the Ohio Oil Company acquired a small marketer, Transcontinental Oil Company, and also acquired Transcontinental’s brand, Marathon, which ultimately became the trademark of the original company. US Steel bought Marathon in 1982, but the combined company spun off the steel business in 2001 and now it’s Marathon Oil Company again, an important multinational oil company. The middle Ordovician limestones of Ohio and Indiana that held the hydrocarbons, oil and natural gas, that made Marathon, are part of the quiescent, warm, shallow-water deposition that dominated Ordovician times in what is now North America – at least until the action started in late Ordovician time, over in what is now the Appalachian Mountains. Production from the Trenton Oil and Gas Field peaked long ago, about 1902 to 1905. The estimated total production over time is a trillion cubic feet of natural gas and 105 million barrels of oil. For context, while that was a huge boom back in its day, 105 million barrels of oil is just five and a half days worth of US oil consumption today. Winter natural gas consumption in the United States is about 3 trillion cubic feet per month, so the Trenton Field would have provided three months’ supply at today’s rates. By no means was all of the oil and gas removed – it never is – and newer technology and higher prices have driven some rejuvenation of production in this area today. But the production is small, and expensive compared to that of the late 19th century. An interesting side light to the natural gas boom in Indiana in the 1880s was the development of a flourishing stained glass industry around Kokomo. The Opalescent Glass Company was established there in 1888 because of the abundant natural gas in the area, which served as a ready fuel for the glass kilns. They’re still in business today. —Richard I. Gibson Photo of gas flares from Trenton Field, Jan, 18, 1889, Leslie’s Illustrated Magazine (public domain) Oil & gas statistics from EIA  

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

When blocks of the earth’s crust collide, several different things can happen. When good dense oceanic crust impinges on relatively light continental crust, the denser one, oceanic crust, usually goes down under the lighter one. This process is called subduction, and it’s going on all over the world today. The continental crust above isn’t immune to effects – it can get uplifted, depressed, scrunched and broken, and volcanoes can and do pop up through the continental crust. Probably the best example of this today is the Andes Mountains along the west coast of South America, where the oceanic plate underlying the Pacific Ocean is diving down beneath the South American continental plate. If two relatively low density blocks – two continents, or a continent and something like an island arc – collide, then neither is likely to really descend beneath the other. This is a true head-on collision, and it can make some of the highest mountain ranges. This is what’s happening to day where the Indian continental plate collides with the Eurasian Plate. The Himalayas form. An obvious consequence of uplifted mountains is erosion – it starts as soon as rocks are above sea level. The Queenston Delta that we talked about the other day is the evidence of that kind of erosion from an uplifting mountain range. Sometimes there is so much erosion that the weight of the sediment is enough to bow down the crust itself, starting a trough-like depression along the mountain front. It can become a self-perpetuating thing, a depression, a basin, into which more and more sediment pours, and all that sediment keeps pushing the crust further and further down…. And so on. The weight of the stuff that’s colliding and being pushed up over the edge of the continent helps, too – all adding up to a physically low area to receive sediments. It’s called a foreland basin, because it’s in the foreland, adjacent to a rising mountain uplift. That’s what happened in eastern North America, where the Appalachian Mountains are today. We’ll be talking about the Appalachian Mountains for months – millions of years – as various things happen over time to contribute to their formation. But that’s getting started now, toward the end of the Ordovician, as the Taconic Orogeny gets started. During the Cambrian and early Ordovician, most of eastern North America was pretty stable, accumulating relatively thin uniform packages of sediment over pretty large areas. In the late Ordovician, as collisions began to bow down the crust, and lift up mountains to be eroded, changes began. Packages of sediment thicken noticeably to the east or southeast (see cross section above - the colored part is the Upper Ordovician), closer to the source area. More sediment is dumped close to the mountains than is carried hundreds of miles away from the mountain front, even though those far-flung sediments are part of the process as well. The mountains may have been in New England, extending down to Tennessee and Georgia, but the eroded mud made its way as far northwest as Ohio and beyond. The foreland basin that began during late Ordovician time is called the Appalachian Basin, and like I said, it will be months before we’ve heard the end of it. —Richard I. Gibson See also this excellent SmartFigure by Callan Bentley. One of the best visualizations of the tectonic development of the Appalachians that I've seen. Cross section from Harris & Milici, 1977, USGS Prof. Paper 1018. Map from USGS