History of the Earth

Echinoids are another class of echinoderms. Echinoderms today are probably most familiar as starfish, but they also include sand dollars and sea urchins, and those groups are part of the class Echinoidea. Fossil echinoids are common in carbonate rocks from the Mississippian Period. The oldest known echinoids are from the late Ordovician. Most of the classes of echinoderms appear to have become established during the Ordovician, and many, including echinoids, have survived to the present. photo by Debivort via Wikipedia, under GFDLModern sea urchins have a globular body with five-fold symmetry, typical of all echinoderms, and a forest of spines encrusting the body. Fossil echinoids usually show only the body, often with a distinct 5-point star design on top. Ancient echinoids probably had spines as well, but they are not usually preserved intact with the body.  Echinoids began to decline during the Pennsylvanian or late Carboniferous period, which we’ll cover next month, and by the Permian extinction there were only six species that we know about, and only two survived into the Triassic. But that was enough to give rise to the modern varieties of echinoids, which total about 950 species today. * * * On June 6, 1912 and for several days thereafter, Mt. Katmai in the Alaska Peninsula erupted. The series of eruptions devastated the area and created the landscape known today as the Valley of 10,000 Smokes, for the many steaming and smoking fumaroles and vents that were formed, including one large volcano called Novarupta. The summit of Mt. Katmai collapsed to make a caldera more than two miles across. The eruption was related to the subduction of the oceanic Pacific Plate beneath Alaska, and it was probably the largest volume of material erupted in the 20th century, at about 11 to 13 cubic kilometers, or about 3 cubic miles of ash and lava. That volume is about 30 times the volume erupted by Mt. St. Helens in 1980. The only other 20th century eruption that comes close was that of Mt. Pinatubo in the Philippines in 1991, which is also estimated at about 11 cubic kilometers of ejected material. Today, the Valley of 10,000 Smokes is part of the Katmai National Park and Preserve. —Richard I. Gibson photo by Debivort via Wikipedia, under GFDL

First, an update. Some new work seems to have pinned down the cause of one of the multiple mass extinctions during the middle and late Cambrian period. The extinction at 510 to 511 million years ago correlates well with voluminous volcanism in Australia. I’ve put a link on the blog episode for February 28 to an article about this. Now, back to the Mississippian. It’s June 5, and today’s topic is blastoids. Blastoids are a class of echinoderms, quite similar to crinoids. Crinoids have decreased tremendously from their peak during the Paleozoic, but they’re still with us today. Blastoids however are extinct – they didn’t make it past the Great Dying at the end of the Permian Period. But since they began during the Ordovician, or possibly in the late Cambrian, blastoids had a run of more than 230 million years. Photo by DanielCD via Wikimedia Commons under GFDL. Like crinoids, blastoids were animals that had a plant-like stalk and a system of holdfasts, root-like structures that held them to the sea floor. Unlike crinoids, blastoids’ skeletons were held together by solid interlocking calcareous plates. In most crinoids, their bodies were held together by muscular tissues, so after death, crinoids tended to fall apart. Blastoids are often found intact. It also seems that blastoids and crinoids may have had different systems for moving water through their bodies to provide oxygen to the animal. The pentagonal symmetry typical of echinoderms is usually well displayed by blastoids, and the body fossils often look like a flower bud or some kind of nut. There’s a wide range in size, of course, but some of the most common genus, Pentremites, are on the order of a half inch to an inch long. Blastoids reached their peak of diversity and numbers during the Mississippian, and in some places, such as the fossil locality known as Pentremites Hollow, near Bloomington, Indiana, they are exceedingly abundant. —Richard I. Gibson Photo by DanielCD via Wikimedia Commons under GFDL.

We’ve talked about limestone quite a few times, and we’ll talk about it a lot this month. I thought we should focus a bit on the rock itself. I imagine most people have a concept of limestone. It’s a rock, not a mineral. A mineral is a compound with a distinct chemical composition as well as a specific crystalline structure, and a rock is an aggregate of minerals.    Sometimes, a rock might be just one mineral, and that’s often the case with limestone. It’s usually mostly the mineral calcite, CaCO3, calcium carbonate. Calcium, carbon, and oxygen are all pretty common in the earth’s crust, and carbon and oxygen are obviously also in the atmosphere. They all also get into the hydrosphere, the world’s oceans and other waters. Limestone quarry in Italy Photo by Michael J. Zirbes via Wikimedia Commons, under Creative Commons license. As a sedimentary rock, limestone is often mostly grains of calcite that are cemented together – often by more calcite. The grains can come from several different sources – they might be broken pieces of older limestone, or they might be broken pieces of the calcareous shells produced by a great many organisms including clams, snails, crinoids, bryozoans, and many more. Obviously you could not get broken shells until shelly animals had evolved, and that didn’t really happen in large volumes until the Cambrian Explosion that we talked about in February. But there are Precambrian limestones too. It’s also possible for calcium carbonate to precipitate directly from water that’s saturated with calcium that can react with carbon and oxygen – an inorganic process not related to life. That chemical process happens today, in places like caves. Stalagmites and stalactites are chemically precipitated calcium carbonate, usually resulting from pre-existing limestone being dissolved by water. Calcite is easily soluble in acids, and in fact geologists use the fizzy reaction between calcite and weak hydrochloric acid to test a rock or mineral for the presence of calcite. But there are acids in nature as well, including the acids produced by chemical weathering of rocks and by the reaction between rain and the carbon dioxide in the atmosphere. The latter is called carbonic acid, and it’s really a very weak acid, but a weak acid is enough to dissolve limestone when you’re talking about millions of years. The earth has had acid rain – slightly acid rain – pretty much forever. Modern acid rain that comes from human pollutants can be much more significant over shorter periods. So we can get limestone, calcite, deposited in vast layers through both chemical precipitation and as a result of the activity of marine organisms that secrete calcium carbonate to make their shells, which can become the main part of some limestones. Usually the rock is a combination of both factors. Something like 10% of all sedimentary rocks are limestones, but in some places it can seem that they are a lot more than that. In part that’s because in arid country, such as western North America, limestone does not dissolve as much as in areas where it rains a lot. It’s just that carbonic acid reaction again – lots of rain, more weak acid, more dissolution of limestone. In arid country, limestones often form prominent ridges and cliffs. Limestones can be pretty complex rocks, including grains that are really broken shells, as well as little grains that are chemically precipitated calcite forming tiny round balls maybe a half-millimeter across. Those things are called oolites – from the Greek word for “egg” because they are round or oval – and they often show concentric layers of calcite deposited on some nucleus such as a sand grain. As they get swirled by waves, they roll around but grow as thin layer after thin layer of calcite is deposited. While I said earlier that limestone is usually mostly calcite, that’s by no means the only thing that you can get in limestones. Sometimes you get calcium carbonate with a different crystal form –

Today I want to talk about a bryozoan, a colonial animal somewhat like corals but probably more closely related to brachiopods. You may recall that they were the only phylum that, so far as we know, was NOT established before or during the Cambrian explosion but developed later, during the Ordovician. And they do survive to this day.   During the Mississippian, the warm shallow seas of North America harbored plenty of life, including diverse bryozoans. But the screwiest of all is a group named Archimedes.   Archimedes was the Greek philosopher who invented the screw as a way to lift water. Archimedes wortheniFossils of the bryozoan genus Archimedes look just like screws. In life, the individual zooids that comprised the colony had lattice-works extending away from the long, spiraling skeletal substrate of the colony, but the fossils usually preserve only that skeletal backbone, and it looks like a screw. The name was first applied by the American geologist David Dale Owen in 1828. They’re really pretty common, and you find them typically something like one to four inches long, or shorter broken fragments. Screws can be left-handed or right-handed, and so are Archimedes bryozoans. They began and were abundant during the Mississippian and survived about 108 million years, until the mass extinction at the end of the Permian Period. Archimedes bryozoans also look an awful lot like fusilli pasta, and when you get a lot of geology students together in a place like geology field camp and serve it to them, you can bet that they’ll start calling it Archimedes noodles. I’ve had it dozens of times. * * * Two geological birthdays today. James Hutton,, who formulated the theory of uniformitarianism and many of the concepts that laid the groundwork for the modern science of geology, was born June 3, 1726, at Edinburgh, Scotland. Check the posts for March 7 and May 4 for more about him. Lee Suttner, one of my professors of geology at Indiana University, was born June 3, 1939, in Wisconsin. If I’m a decent teacher of geology, I owe it to Lee for sharing his style of asking leading questions, rather than telling. It’s a style I admire and try to emulate when I can. Happy birthday, all. —Richard I. Gibson David Dale Owen and the naming of Archimedes Drawing from an old textbook (public domain)

The time span of the Mississippian is about 36 million years. It began with the end of the Devonian about 359 million years ago and it ended about 323 million years ago. As with all the subdivisions of geologic time, there are error bars on these dates, in this case about a half million years, plus or minus, on both ends.   Mississippian time (from Wikipedia)In Europe, the Early Carboniferous, the equivalent in time to the Mississippian, is divided into three ages which correspond to three stages in the rock record. The Tournaisian and Viséan are named for rocks in Belgium and the Serpukhovian is from outcrops in Russia. They span from 7 to 17 million years each. In North America, there are four subdivisions of Mississippian time. From oldest to youngest they are Kinderhookian, Osagean, Meramecian, and Chesterian. These names are used a lot in the literature on Mississippian rocks, and I may use them too – but I will try to avoid too much jargon and I’ll try to always refer any names like that to the part of the period we’re in, and about how many million years ago it was. As I’ve said before, international agreements are often needed to assign specifics to the breaks in geologic time. Since rocks in one part of the world don’t necessarily record the same events as other parts – or the same event may span some time and may occur at one time here, and another there – because of that, it’s not surprising that we really cannot make an exact, world-wide time scale that applies everywhere. But things do usually work out so that they’re pretty close. —Richard I. Gibson

…or is it the Mississippian? The next period of the Paleozoic Era is called the Carboniferous, which means carbon-bearing in reference to the coal beds in the upper part of the system in many parts of the world. The name was invented by British geologists William Conybeare and William Phillips in 1822, making it the first to be established of the names we use today for the geologic periods.   Now we have a nomenclature problem to deal with. In the book I put together in 1994, the month of June corresponds with the Mississippian Period. In the United States, the Carboniferous of Europe is divided into two distinct time spans, the Mississippian and the Pennsylvanian. Technically, in terms of international geologic names, the Mississippian and Pennsylvanian are sub-periods of the Carboniferous, but in part because of long-standing usage, in the United States the two are treated as full-fledged periods of geologic time. In the U.S., the period takes its names from rocks of this age exposed along the Mississippi River, especially in western Illinois where these rocks are hundreds of feet thick. For purposes of the original book and for these podcasts, I’m using U.S. nomenclature, so the month of June is the Mississippian and July will be the Pennsylvanian. But technically, they are parts of the Carboniferous Period. Sometimes you’ll see the Mississippian equated with early Carboniferous and the Pennsylvanian with the late Carboniferous. It’s all a matter of human convenience, and in this case, a matter of some notable differences between the rocks of this time period in the U.S. versus those of Europe. —Richard I. Gibson

We’ve talked this month about the abundance of life in the Devonian – the spreading of plants across the land, and even the first forests. Both diversity and abundance resulted in oil and natural gas source systems that add up to one of the largest in all of geologic history.  It all came to a rather screeching halt with two major extinction events toward the end of the Devonian.  The first occurred before the end of the period, at the boundary between the last two subdivisions of Devonian time, the Frasnian and Fammenian stages, at about 374 million years ago. A second event took place at the end of the Devonian, about 359 million years ago. Together these two events wiped out something like 75% or more of all species – some estimates say as much as 87%. Depending on how you look at it – diversity, abundance, marine, non-marine, the end Devonian extinctions might be second to the end Permian extinction, but in any case, the end of the Devonian is one of the “big five” mass extinctions in earth history. It’s quite clear that the extinctions were associated closely with anoxic periods in the world’s oceans – times when the oxygen content of the water decreased drastically. That gives us a starting point for trying to determine the causes. One possibility is climate change. You may recall that western Gondwana was situated over the south pole during much of the Devonian, but there’s not much evidence for glaciation until late in the period. A Late Devonian glacial epoch would have decreased sea level, and as we’ve heard in previous extinctions, that reduces shallow water niches for life as well as cooling the average water temperature. Life that lives in warm, shallow, well-oxygenated water suffers. Another possibility is that all the life on land – plants including trees, for the most part – would have resulted in a dramatic increase in the volume of nutrients washing from the land into rivers and ultimately into the sea. Enough nutrients could have led to immense algal blooms that could have produced eutrophication – stagnation – in shallow or restricted seas. Plants on land would have accelerated chemical weathering of rocks as they form soil. By burying organic matter, it would have been a form of carbon sequestration, and reducing carbon dioxide in the atmosphere would have reduced the greenhouse effect and resulted in cooling. That in turn might be tied to the glaciation, or the glaciation might have begun for some other reason such as changes in the earth’s orbit, but it might have been accelerated by the removal of CO2 from the atmosphere by land plants. There’s some speculation that an impact or impacts from an asteroid or other bolide might have triggered, or at least contributed to, the climatic effects that resulted in extinctions, but even though there are a couple known impacts at about the right time, they really don’t seem to have been big enough to have had the dramatic effects that are observed in the fossil record. Nor would meteoric impacts have necessarily produced the global oceanic anoxia that we know was present and the timing isn’t really right, either. The anoxic conditions contributed to the organic-rich black shales we’ve talked about this month, so to an extent, the extinctions that led to trapping of organic matter in the rocks helped generate some important oil source rocks. All the phyla that we have today survived the extinctions in the Late Devonian, but several sub-groups did not. The arthrodires, the huge predatory fish, went extinct, and so did the primitive armored fish, placoderms and ostracoderms. Corals suffered badly and while most groups survived, abundance decreased so that reef-building pretty much ceased and there were no substantial reef ecosystems for the next 100 million years.  The late Devonian extinctions seem to have clearly come in at least two distinct pulses, each one lasting a million years or so, but as more information has become available, it is also loo

Do you remember conodonts, the tiny tooth-like fossils that are often the only remnants of an eel-like animal? We first talked about conodonts in March, during the Ordovician, but they were abundant in Devonian time as well. Like ammonites, conodonts are so specific in nature that they serve as excellent index fossils, and because they are tiny, often no more than a millimeter long, they can be identified from cuttings in oil and gas well drilling. They’re important to the science called biostratigraphy, which helps oil explorationists know exactly where they are as the well drills down.   We’ve also talked about chert, really fine-grained silica, and how it can preserve even microscopic fossils. Combine chert with conodonts and you’ve got something to hang your hat on, in terms of detailed stratigraphy. Caballos novaculite ridges (USGS photo). There are several layers of mostly chert in the United States, including the Arkansas Novaculite and the Caballos Novaculite. Novaculite is the rock name given to a special kind of chert that is hard, tough, and dense. Its broken edges can be sharp, and the name comes from Latin meaning “razor stone.” Native Americans valued novaculite as a resource for making projectile points. Chert is definitely a sedimentary rock, but most geologists would consider novaculite to be a very low-grade metamorphic rock, where heat and pressure have tightened the crystalline structure of the silica even more than in typical chert. Novaculite such as that from the Devonian of Arkansas has been used for whetstones and abrasives. In West Texas, the Caballos Novaculite serves as a good reservoir for oil and natural gas where it is fractured in the subsurface. These novaculite beds are generally a lot thicker than the chert beds and nodules we talked about earlier this month. Those discontinuous layers might be a few inches thick, typically, while the Arkansas and Caballos Novaculite can be as much as 60 feet of almost nothing but silica. One possible origin for the novaculites is thick accumulations of the shells of diatoms – planktonic or floating algae whose cell walls are made of silica. Even though they are microscopic, these algae in their billions could create quite a layer of silica on the sea floor as they died over many tens and hundreds of thousands of years. —Richard I. Gibson USGS Photo from U.S. Geological Survey Professional Paper 187.

Today is my 150th daily podcast. I very much appreciate your continued interest. As I said back on episode 100, I’ll be trying my darndest to keep the daily postings going, but just be aware that the summer is my busy season doing history tours and such. Please accept my apologies in advance if I miss a day or so here and there. And also, let me encourage you to post questions on the Question of the Week page on the blog, and also if you have suggestions for ways I can improve the program, feel free to post a review on iTunes or send me an email at rigibson@earthlink.net. GoniatiteI think we’ve only talked about cephalopods once in this exploration of the history of the earth. Cephalopods are mollusks, a diverse group that includes clams and snails as well as cephalopods. In contrast to clams and snails, the cephalopods have an internal shell or none at all. They also have an array of arms or tentacles extending from their heads. Cephalopods today include octopuses, squid, and cuttlefish, and the chambered nautilus, a spiral-shelled marine invertebrate. Spiral cephalopods became abundant during mid- to late Devonian time, around 400 million years ago. They included the first ammonites, coiled animals that lived in chambers within a spiral shell. Each chamber was separated from the next by a wall, called a septum, that had amazingly complex convolutions. Some of the traces of septa on fossil shells’ surfaces, called suture patterns, form an intricate fractal design and can be used to identify various species. This helps make ammonites excellent index fossils, fossils that are characteristic of a specific and typically short interval of geologic time. Ammonites get their name from the Egyptian god Amun, who was often portrayed with tightly coiled ram’s horns, which resemble ammonites. Early ammonites such as those from the Devonian often have simpler suture patterns than later species. The commonest general type from the Paleozoic is called goniatitic, for the genus Goniatites. Most Devonian goniatites have a gentle waving or zigzag suture pattern rather than the incredibly complex patterns that came later. Ammonites survived until the extinction at the end of the Cretaceous, a run of about 330 million years. We’ll talk about them several more times in the course of our journey through the history of the earth. * * * Today’s birthday is Hollis Hedberg, born May 29, 1903, in Falun, Kansas. Hedberg worked in the petroleum exploration business and he made major contributions to understanding sedimentation and stratigraphy. He was employed mostly by Gulf Oil Company, and worked extensively in Venezuela. Gulf Oil named one of its marine seismic exploration vessels for him. —Richard I. Gibson Goniatite photo by Rama under terms of the CeCILL license.  

Antler OrogenyToward the end of the Devonian Period and continuing into the Mississippian that followed, parts of western North America collided with something, perhaps a volcanic island arc like the modern West Indies. It does not appear to have been the intense kind of mountain-building event that results from continents colliding. There isn’t much evidence of extensive metamorphism and igneous intrusion. The best evidence is found in the state of Nevada, where coarse conglomerates indicate that there was an uplift in what is now central to east-central Nevada. Something was nearby, and eroding to make those thick, coarse conglomerate layers. To the east, the Devonian rocks are the typical Paleozoic carbonate shelf deposits, the same kinds of limestones we’ve heard about for much of the Ordovician, Silurian, and Devonian across the middle of North America. But to the west the rocks are more siliceous, those coarse conglomerates, and there are some interbedded volcanic rocks. The western rocks are thrust eastward on top of the eastern rocks in the Roberts Mountains Thrust Fault, a major feature which is additional evidence for the tectonic activity we call the Antler Orogeny. Could an impact event have triggered some of this tectonic activity? About 367 million years ago, Late Devonian time and about the time the Antler Orogeny was getting underway, something did crash into the waters that were forming the Devonian Guilmette formation, near the town of Alamo, Nevada. There is no crater for this impact, but the rocks record the results of the impact. Smashed and deformed rocks, huge broken blocks forming a rock called breccia, shocked quartz grains, and high iridium levels. There are even deposits that can reasonably be interpreted as tsunami deposits. This is some relatively recent work, mostly in the 1980s and 1990s. John Warme was one of the first geologists to recognize the nature of these rocks and to interpret them as a bolide impact. But one impact, even one that made impact breccias and the rest, really could not have made a mountain range. We really have to seek tectonic causes for something that’s hundreds of miles long. I said earlier that it might have been a volcanic island arc that was colliding. But that’s definitely not certain. There are various theories to put the Antler Orogeny into a plate tectonic context, but there are problems with all of them. It’s just not clear what was west of what is now central Nevada back in the Devonian. One reason for that is that much later, a lot of stuff was added to western North America – including most of California. And that process obliterated some of the rocks and structures that were there previously. As always, research continues. * * * Glacial Lake AgassizToday’s birthday is Jean Louis Rodolphe Agassiz, born May 28, 1807, in Motier, Switzerland. Louis Agassiz worked extensively on fish, both modern and fossil, and his reports published in the 1830s and 1840s established him as a renowned scientist. But he is probably best known as the first to propose scientifically that the earth had had an ice age. He worked extensively on the glacial deposits of both Europe and the United States. Glacial Lake Agassiz, of which Lake Winnipeg in Manitoba is a remnant, is named for him. —Richard I. Gibson Glacial Lake Agassiz map from Upham, Warren, "The Glacial Lake Agassiz". Plate III. Monographs of the en:United States Geological Survey: Volume XXV, 1895.  Antler orogeny map drawn by Richard Gibson.

I think it’s time to visit another lagerstatten – one of those amazing collections of fossils that are remarkable in their preservation and completeness. This time, it’s the Hunsrück slate, in southwestern Germany near the border with France and Luxembourg. Brittle star from Hunsruck slate. Photo by James St. John (creative commons license) Slate is a hard, dense rock with fine partings that make it come apart easily into flat slabs. Huge flat slabs of black slate used to make chalkboards in schools, and smaller pieces have been used for centuries to make roofing shingles. Slate is a metamorphic rock, formed by subjecting fine-grained shale to heat and pressure. The main difference between shale and slate is often just how indurated it is, how hard and tightly bound together the tiny grains are. The heating and pressure might drive off any water in the rock, and might mobilize the chemicals just enough to make the rock hard, rather than crumbly. So how do you get fossils in slate, a metamorphic rock? Well, there are all levels of metamorphism, and even the process of going from loose sediment to solid rock might be called really low-grade metamorphism, but we usually reserve the word to mean a later cooking or pressurizing that changes the minerals at least a little. But not necessarily a lot. The Hunsrück slate was laid down as mud and clay that became shale in a sea along the southern margin of the Early Devonian continent of Europe. As we’ve learned, fine-grained sediments are great for preserving details in fossils, and dark shales often indicate anoxic conditions, where even fragile organic structures might not oxidize and decay. The Hunsrück fossils include more than 260 animal species, and even soft parts are preserved. Most of the kinds of marine life of the Devonian are present, including spectacularly preserved crinoids and brittle stars with long delicate arms intact, as well as fish, trilobites, and sea cucumbers. The strange and extinct carpoids, a kind of echinoderm, are also found with outstanding preservation. The process of turning fossil-rich shale into slate must have been quite gentle, as metamorphism goes. The shale dates to the Emsian Stage of the Lower Devonian, about 400 million years ago. Sometime during the Carboniferous Period, perhaps 80 to 100 million years later, the rocks were metamorphosed to slate without significant destruction of the fossils. —Richard I. Gibson Photo by James St. John under Creative Commons license.   Carpoids

In a group as diverse as fishes, it’s no surprise that they developed differing kinds of scales. You recall the placoderms and ostracoderms, which had bony plates covering parts of their bodies. That model was unsuccessful – but flexible bodies, covered in scales, not only let fish survive for 400 million years, but might have led to the development of a distinct neck – an important anatomical feature for living on land. Devonian ganoid fish (Osteolepis) The ganoid fishes where bony fishes abundant during the Devonian, and they have many modern descendents including sturgeons, paddlefish, and gars. If you’ve ever gone fishing in the waters of Arkansas and caught a gar, you know how bony they are. One of my few clear memories from the time I was around 7 years old is fishing with my grandfather. I caught a gar that seemed to be as long as the boat – in reality it was probably 3 feet long – with a sharp nose, silvery scales, and lots of teeth. My grandfather fought with it for what seemed like an hour – probably 2 minutes – until he broke it in half. Fishermen in the backwaters of Arkansas in the 1950s didn’t like gars. Scales are similar to human hair and fingernails. They’re composed of collagen, the structural protein that makes up lots of connective tissues in animals, combined with calcium phosphate, the mineral apatite, which is found in bones and teeth. Some scales also include calcium carbonate. In ganoid fishes, much of the mineral matter in the scales is ganoine, glassy, rod-like calcium phosphate. That’s what makes gars so shiny and silvery. Other types of fish scales are somewhat more bony in the case of primitive fish like coelacanths. Most common modern fish have the overlapping flexible scales that you’re probably familiar with. —Richard I. Gibson Ganoid fish drawing from an old textbook (public domain)

Shales like the Marcellus Shale are usually deposited out in quiet, relatively deep water. Closer to shorelines, and on shorelines, you get coarser sediments including sand. Sands on beaches tend to get washed and winnowed by wave action, so unless there is some source for other materials than the quartz grains in the sand, you can end up with a rock, sandstone, that’s really pretty pure quartz, silicon dioxide. Photo by Kevinaj, public domain via Wikipedia. Caudy’s Castle (Oriskany Sandstone), West VirginiaIn what is now Pennsylvania, Maryland, West Virginia, and Virginia, during the Devonian, such a sand was laid down early in the period, around 400 million years ago. The clean quartz sand forms a layer that is 100 feet or more thick, spread over a large area. The purest portions were mined historically for making glass, which requires quartz of high purity. Even a trace of iron, less than a fraction of a percent, will color glass. The historical name for these glass sands was Oriskany, but that’s really a large group of related, similar, but not necessarily exactly equivalent sandstones. The glass sand miners didn’t care of course – they just went after the sand that gave them the best glass. These sandstones in the subsurface can also serve as excellent reservoirs for natural gas. —Richard I. Gibson Photo by Kevinaj, public domain via Wikipedia.  Caudy’s Castle (Oriskany Sandstone), West Virginia. 

Today is a break from the Devonian, and a thanks to listener Thatcher Hogan for suggesting the Adirondacks as a topic. I mentioned the Adirondacks back in the Precambrian, because the rocks are part of the Grenville Terrane which collided with the Superior Craton, the ancient core of North America, about 1.1 billion years ago. But the modern Adirondacks are much younger than that old mountain-building event. The rocks were buried deeply – perhaps as much as 15 miles below the surface – during the Grenville Orogeny. They were also intruded by various magmas. The result of the high pressures and temperatures was a wide variety of both igneous and metamorphic rocks. The Adirondacks of northern New York are a strange little range, almost circular in shape. It’s really a large dome, a circular geological uplift. The oldest rocks, uplifted the most, are in the center, with younger rocks draping the flanks of the dome. And this uplift is really quite young, beginning around 5 or 10 million years ago – just yesterday, geologically speaking – and continuing to the present. So the present mountains have nothing to do with the ancient Grenville mountains, and also nothing to do with the Appalachians – which today are relatively low, eroded hills, a remnant of the mountain building events of the Ordovician, Silurian, Devonian, and Carboniferous. So why are the Adirondacks there? The circular dome shape suggests some kind of force pushing up from depth, and you get domes where things like magmatic intrusions, cylinders of molten rock, like the neck of a volcano but down within the earth, rise. They push the rocks above them up like your fist pushing up in the middle of a blanket. Salt, which is not usually molten in the earth, can flow plastically under pressure, and salt domes can do the same thing to the rocks they rise up against and through. The Precambrian core of the Adirondacks is exposed because those old rocks were uplifted, along with a thick pile of younger sedimentary rocks. It’s probable that sedimentary rocks, including the Cambrian Potsdam sandstone (February 19) and strata from the Ordovician, Silurian, and Devonian were laid down over the region where the Adirondacks now stand. But those rocks were eroded off the rising Adirondacks, mostly in the past 5 or 10 million years or so. It would have to be something big to rise from great depth to produce the huge dome at the Adirondacks. Not a salt dome, and not the small uplift around a rising magmatic intrusion. Those kinds of things make domes that are maybe one to 5 miles across, maybe 10 miles at most. The Adirondack Dome is 160 miles in diameter. To be honest, we really don’t know why the Adirondack Dome began to rise, and why it continues to rise – by some estimates, one of the fastest-rising mountain ranges on earth, perhaps as fast as 1 or 2 millimeters a year, which is actually incredibly fast. There is controversy over uplift rate estimates, so stay tuned for more research on that. The best guess – and it really is a guess – is that there was a hotspot beneath the Adirondacks. Hotspots are regions of relatively low-density mantle, many tens of miles within the earth, that tend to rise buoyantly through denser parts of the mantle. Such a blob, pushing up, could make the broad dome that we see in the Adirondacks. Hotspots are well known, especially those that get shallow enough that reduced pressure allows the hot rocks to melt. Then you can get volcanoes. There is a hotspot beneath Hawaii, one under Iceland, and one under Yellowstone. There are a few dozen around the world. Out in the Atlantic Ocean there is evidence for a hotspot that the Atlantic Oceanic Plate has been moving over for some time. The track is represented by seamounts, essentially flat-topped eroded volcanoes that might have once been something like today’s Hawaiian chain. The hotspot that made those volcanic seamounts is called the New England Hotspot or Great Meteor Hotspot. Don’t let that name thro

The long-lasting complex collisions that created the Appalachian Mountains and their equivalents in Greenland, Britain, and Scandinavia continued into the Devonian. You may recall that we called that the Caledonian Orogeny, or mountain-building episode near the end of the Silurian last month, when Greenland and Scandinavia collided, as well as the bits of North America and Europe that would become most of the British Isles. The long, narrow microcontinent called Avalonia, for rocks in the Avalon Peninsula of Newfoundland, did not encounter North America at the same time in all places. South of Maritime Canada, a lot of the accretion of Avalonia to North America didn’t happen until the Middle Devonian and later. It’s usually called the Acadian Orogeny, for Acadia, the French name of Nova Scotia where some of the colliding happened. It was likely underway in the northern terranes by late Silurian, but it culminated in a major crunch in the Devonian. Bits of Avalonia or related terranes were accreted to North America as far south as Georgia and Alabama, and some of those rocks are in the subsurface today, while some are exposed in the Appalachian Mountains. This collision was another one that wasn’t really head-on, but more oblique in nature, involving some strike-slip faulting like that along the Pacific Coast of Canada today. But there was also enough subduction to generate magmas that are scattered through the Acadian mountain belt. Besides the well-known orogeny in eastern North America, there was some ongoing tectonic activity in parts of Europe at about the same time. In Europe, it’s called the Variscan or Hercynian Orogeny, just to confuse matters, and there, it continued well into the Carboniferous Period that followed the Devonian. This was the result of Africa, or really Gondwana, or even more accurately, some pieces of Gondwana moving to collide with Europe. The diagram shows the 200-million-year sequence of events that add up to the Appalachian Mountains, spread over at least four distinct tectonic collisions spanning parts of 5 periods of geologic time. It’s not over yet. —Richard I. Gibson Diagram from USGS