History of the Earth

Today’s episode is from my book, What Things Are Made Of and the chapter that includes gold. THE LAST GREAT GOLD RUSH began in August 1896, when prospector George Washington Carmack and his two Indian companions, Skookum Jim and Tagish Charlie, found gold in the Klondike River basin in Canada’s Yukon Territory. Two years later, thousands of the gold seekers who had climbed the perilous slopes of Chilkoot Pass were gone, either back to the states or on to other diggings in Alaska. But that summer of 1898 saw a rush of another sort: the rush to understand Alaska’s resources. The 19-year-old United States Geological Survey dispatched four parties that spring, and the War Department sent two more teams accompanied by geologists. Among other things, they were to “observe and note all occurrences of valuable minerals, giving special attention to the presence or absence of gold, whether in placers or veins.” These early scientific expeditions guided later exploitation of Alaska’s mineral wealth, and established the careers of several USGS geologists. Spurr's 1898 geologic map of southwestern Alaska Josiah Spurr was just 28 years old when he led a reconnaissance in southwestern Alaska, but he knew what he was looking for: his previous geological experience in the Yukon gold fields prepared him for any exploration dealing with gold. Spurr’s 1425-mile Alaskan journey with a handful of other scientists in three lightweight cedar canoes resulted in new geologic and topographic maps covering a vast territory. Spurr’s report reflects the dangers of exploration as the 19th Century came to a close. The team arrived at Tyonek on Cook Inlet, on April 26, 1898, but could not head upriver until the ice began to break up on May 4. After reaching the Susitna River mouth, their intended portal to the interior, the weather forced a delay until May 20 when the river became sufficiently ice-free for them to travel. Even then, after they “had gone several miles, we were surprised by a solid wall of ice bearing swiftly down upon us, and we had only time to throw our load upon the banks and drag the boats out of the water before the ice jam swept past, piling over upon the banks in places and grinding off trees.” Spurr’s narrative reads more like an adventure story than a scientific document. The Spurr Expedition coined the term alaskite, a word for a particular light-colored granitic rock. The scientists observed considerable mineralization associated with southwestern Alaska’s granites, though gold occurred only sparingly. Nonetheless, in 2007 Alaska was the second-leading gold producing state in the U.S., with more than 700,000 ounces, mostly from mines near Fairbanks and Juneau. Alaska’s production is a distant second to Nevada, the heavyweight in the U.S. gold-mining industry with around 5,000,000 to 7,000,000 ounces per year. The United States exports gold, in a virtual three-way tie with Australia and South Africa for second, third and fourth place in the world after China. Josiah Spurr’s work on western U.S. mineral deposits gained him considerable fame, and he wrote a book on economic geology. In the 1940s his work focused on the moon – earning him a crater named Spurr to go with Spurr Volcano in Alaska and the mineral spurrite, a complex calcium silicate. Another 1898 Alaska explorer, Walter Mendenhall, became the fifth Director of the U.S. Geological Survey in 1930 and gave his name to a glacier near Juneau. Alfred Hulse Brooks, chief Alaska geologist for the USGS from 1903 until he died in 1924, explored Alaska’s interior Tanana River valley in 1898 when he was just 27 years old, and was honored by the naming of the Brooks Range in 1925. The rare mineral hulsite, an iron-magnesium-tin borate, discovered at Brooks Mountain on the Seward Peninsula, also bears his name. George Eldridge and Robert Muldrow led an 1898 expedition that accurately pegged the height of Mt. McKinley, or Denali, at 20,464 feet – remarkably close to today’s value, 20,306

Today I want to talk about some of the youngest minerals on earth – kidney stones. Some might argue whether these should be considered minerals at all, since part of the definition of a mineral is other than man-made, but that’s usually phrased as “naturally occurring” and these mineral deposits really are natural, even if they are not welcome. My first professional job was analyzing kidney stones. My mineralogy professor, Carl Beck at Indiana University, had died, and his wife asked me, his only grad student, if I would continue the analytical business he had been operating. I said yes, setting out on a four-year, 20,000-kidney-stone start to my geological career. First, please accept this disclaimer. I am not a medical doctor. I’m not even a geological doctor. So don’t take anything I say as medical advice. Apatite is a common mineral in nature. Chemically it is a complex calcium phosphate with attached molecules of hydroxyl (OH), fluorine (F), and sometimes other elements. Apatite is the fundamental mineral component in bones and teeth, and when apatite has fluorine in its crystal structure, it is stronger. This is why fluorine is added to water and toothpaste. In kidney stones, carbonate (CO3) substitutes for some of the phosphate, making a mineral that is relatively poorly crystallized. Its formula in kidney stones is usually given as Ca5(PO4,CO3)3(F, OH, Cl). Well-crystallized or not, apatite often forms the nucleus upon which other urinary minerals are deposited. It usually occurs as a white powdery mineral deposit, one of the most common components of kidney stones. Two minerals that are really common in human urinary stones but that are exceedingly rare in nature are whewellite and weddellite, calcium oxalates. Oxalate is C2O4, not too different chemically from carbonate, CO3, the common constituent of limestone. Whewellite (CaC2O4.H2O) is known to occur in septarian nodules from marine shale near Havre, Montana, with golden calcite at Custer, South Dakota, and as a fault filling with celestite near Moab, Utah. It is found in hydrothermal veins with calcite and silver in Europe, and it often occurs in association with carbonaceous materials like coal, particularly in Saxony, the former Czechoslovakia, and Alsace. Whewellite was named for William Whewell, an English poet, mathematician, and naturalist who is credited with the first use of the word ‘scientist,’ in 1833. It is one of the most common kidney stone minerals, where it typically occurs as small, smooth, botryoidal – which means like a mass of grapes –  to globular yellow-green to brown, radially fibrous crystals. Whewellite stones larger than ½ inch across are quite unusual. Often whewellite is deposited upon a tiny nucleus of apatite, which may form as build-ups on the tips of tiny papillae in the kidney. Those papillae are little points where ducts convey the urine produced by the kidney into the open part of the kidney. Weddellite, CaC2O4.2H2O, was named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea, off Antarctica. Unfortunately the sharp yellow crystals that urinary weddellite forms are often much larger than that, and they are frequently the cause of the pain experienced in passing a kidney stone. Rarely, weddellite crystals may occur that are nearly a half inch on an edge, but most are somewhat smaller. The yellow crystals are commonly deposited upon the outer surface of a smooth whewellite stone. Like whewellite, weddellite is a calcium oxalate. They differ in the amount of water that is included in their crystal structures, and this gives them very different crystal habits. Occasionally, weddellite partially dehydrates to whewellite, forming excellent pseudomorphs of grainy whewellite after weddellite's short tetragonal dipyramids. Together, apatite, whewellite, and weddellite are probably the most common urinary stones. Struvite is a hydrous magnesium ammonium phosphate, Mg(NH4)(PO4).6H2O, that forms

For 2014, the podcast consisted of daily productions averaging about 5 minutes each. The individual episodes for each month are now being assembled into packages for each month, representing various spans of time in the history of the earth. This episode is the first package, which combines the 31 daily episodes for January, covering the Precambrian – the time from the origin of planet earth up to about 543 million years ago. This episode is an hour and 15 minutes long and includes all the individual daily episodes from January 2014. If you listened to last year’s daily episodes, you’ll find that I’ve re-recorded some of them. Especially in early January, when I was just getting started, some of the episodes were technically lacking. I hope I have improved on that for this assembly of 31 episodes covering the Precambrian. Thanks very much for your interest in this project. As the year continues, there will be new episodes, maybe every week or two, in addition to these monthly assemblies. If you have questions or comments, please let me know, either here on the blog – there’s a page for Question of the Week – or contact me by email at rigibson at earthlink.net. I’ll try to respond. —Richard I. Gibson

Mammoths, mastodons, saber-toothed cats, American horses and camels, the giant bear, the Irish elk, Glyptodonts, which were giant relatives of the armadillo, the Megatherium, an elephant-sized ground sloth, giant kangaroos, a 7-foot-long beaver, and dozens of other large animals and animal groups, collectively called megafauna, died off during the Quaternary Period of the Cenozoic Era, many of them at about the time of glacial retreat, 10,000 to 12,000 years ago at the end of the Pleistocene Epoch. Some of them were relatively new to planet earth, and some species had been around for millions of years.  Don’t visualize this extinction as an instant in time, or really even a short period of a few hundred years. It’s true that there seems to be a spike in extinctions right at the time the glacial period was ending, 10 or 12 thousand years ago, but many animals, including for example the giant terror birds of South America, were becoming extinct at about the beginning of the Pleistocene glaciation, 2.5 million years ago.    Mammoth (left), American mastodon (right) Image by Dantheman9758, used under Creative Commons license.  And how do we know something was completely extinct? We can only go by the fossil record, and fossils of modern large animals are rare. Even for a particular species, extinction can take thousands of years, winding down to the last mastodon, or passenger pigeon, or whatever. Mammoths got started about five million years ago, in the Pliocene, and were mostly gone at about the end of the glacial period, 12,000 years ago. But small populations survived on isolated Arctic islands such as St. Paul, off Alaska, and Wrangel, off Siberia, as recently as 3700 years ago. Different species in different niches disappeared sooner or later, depending on circumstances. The pygmy mammoth probably survived on the Channel Islands of California until about 11,000 years ago. So, why did the mammoths, giant sloths, and all the rest die? The first obvious explanation is the dramatically changing climate. Glacial environments certainly constrain food supplies and habitat, and other habitats, even in temperate zones, change. That could account for some of the extinctions early in the Pleistocene, 2 to 2½ million years ago. But why would so many go extinct at the end of the glaciation? Well, change is change, and warmer might not necessarily always be better for all species; forests supplanted grasslands in the north, perhaps denying mammoths their main food sources. It might be a simple matter of inability to adapt to the changes. Once a population becomes small, many things – disease, a big storm, a flood – could have been the final blow that ended a species that may have been in decline for some time. The second primary idea for the extinction of the Quaternary megafauna is still somewhat controversial, and involves hunting of the animals by predators – specifically, early humans. There’s no question that humans hunted and ate some of these animals – or hunted them to remove them as threats. Homo erectus was killing mammoths a million and a half years ago. The question really is, were humans the primary, or even a major, cause of the extinctions. There’s pretty good correlation between the arrival and expansion of humans in various areas with the demise of the large mammals. The correlation in time and space alone does not mean the one caused the other. Arguments against this hunting hypothesis include the idea that the small populations of humans would not likely have been enough to eradicate millions of animals. And while the correlations are best in the Americas and Australia, the extinctions take place at about the same time in Eurasia, where these animals had been exposed to human threats for far longer. The bottom line is that the ultimate cause of extinction of so many large mammals is uncertain. Humans may have had a role, either large or small, and climate factors are likely to have been important, a

So we’ve made it to the end of our geological year, covering 4.6 billion years of geologic time. 366 episodes, close to 180,000 words and a total of about 25 or 30 hours of programs. I hope you’ve enjoyed it! The podcast will not end, but the structure will change as we go into next year. It won’t be daily any more, sorry to say – there were times when it was really touch and go in terms of me getting the episodes out, but I’m happy to say I managed. Thanks for your interest – that was my main motivation once things got going.  I’m not certain exactly what sorts of topics next year will bring. I’m not going to try to cover “current events” in any particular way because there are many good blogs and podcasts that do that. I expect I may do a few more that use my own work as a basis, and some posts will probably be based on topics in my other book, What Things Are Made Of. I want to try to do a few interviews with geoscientists working on interesting topics, and that may give it a Montana-centric flavor, but we’ll try to make things pertinent to a wide audience. Since I only covered 366 topics – and they were selected largely based on my own prejudices – there’s certainly plenty more that we can look at. Feel free to submit questions or suggestions, either through the blog or email me at rigibson at earthlink.net. You’ll get a spamblocker message, but I’ll find your email. I’m not going to make any promises, but I am going to shoot for at least one episode every week or ten days next year.  I also will be assembling the existing podcasts into single recordings. I’m not sure how it will work in terms of file sizes, but I’m hoping that I can make each month of the previous series into one or two packages – without the repetition of the intro music and exit tagline. I’ll try to edit the episodes into those assemblies early in the coming year and make them available in the usual way, through the blog.  To close the year, I thought I might address what has been called the Sixth Extinction. Of all the mass extinctions in earth history, only five have been really, really devastating. Those are the ones at the end of the Ordovician, in the Late Devonian, the biggest of all at the end of the Permian, the Triassic-Jurassic extinction, and the one that ended the Cretaceous Period and the Mesozoic Era. The case has been made that we are presently in the midst of another mass extinction, the Sixth great one. There is an entire book, titled The Sixth Extinction: An Unnatural History, by Elizabeth Kolbert. It just came out in 2014, and I recommend it highly. The New York Times Book Review listed it as one of the ten best books of 2014. The book makes the case for a present-day massive die-off of organisms, ranging from bugs and bats to corals and rhinos. We’ve seen in the podcasts in this series that things like climate change certainly affect extinction rates, and there is no question that earth’s climate is changing at high rates right now. There’s also no question that human activities are affecting many of the things that contribute to climate change. It’s happening. Kolbert integrates our knowledge of past extinctions, such as the disappearance of the ammonites, which you have heard about in this podcast series, as lessons for the present. We don’t really know how many species of plants and animals exist on earth today, although we have good ball-park estimates. New species, even new large animals, are being found all the time. So it’s hard to say with certainty what kind of extinction rate is in progress, but some estimates say that as far as we can tell, extinction rates today are as much as 1,000 times those that typified most of earth’s history. We might argue about whether what’s happening now is on a scale comparable to the Big Five Extinctions, but at some scale, an extinction event is assuredly in progress. The big, charismatic animals, like rhinos, elephants, tigers, and whales, that need extensive spaces for thei

Through the year, we’ve talked about events that broke apart and combined the various tectonic plates on the earth, but today, as we’ve almost reached the present, I wanted to just summarize the way things are today.  First, I know I talked repeatedly about oceanic crust and continental crust. They are quite different from each other, in density, thickness, and mechanical behavior, and those differences drive subduction and plate tectonics. But the two types of crust also move together, a lot. The North American Plate includes all of the North American continent – except the bit of California west of the San Andreas Fault – but it also includes the oceanic crust beneath the North Atlantic Ocean, all the way out to the Mid-Atlantic Ridge. Iceland straddles the mid-ocean ridge, a pile of volcanic material erupted because of a hotspot at depth, but the west half of Iceland is part of the North American Plate, and the east half is on the Eurasian Plate.   Depending on exactly how you want to define “major,” there are 9 to 16 major plates. Africa has two sub-plates – Arabia, which is tectonically separated from Africa, but only by the width of the young Red Sea, and the Somalia Plate, breaking away along the East African Rift. The Somalia Plate is certainly separating from Africa, but in many ways and in many places, they’re still attached to each other too. Then there’s the North American and South American Plates, both of which include the continent and the western half of the Atlantic Ocean. There is only a vague boundary between the North and South American Plates, because they are to a large extent moving together at a similar speed and direction. There is an extensional rift between North America and Greenland – but it failed, and Greenland is now completely attached to North America, and is moving with it. Australia and India have their own plates and include more oceanic crust than continental, but like the Americas, they are almost locked together now that India has collided with Eurasia and slowed down. Antarctica also has its own plate, and except in some small sections, Antarctica is entirely surrounded by oceanic rifts. Everything else is pulling away. How can that be possible? You can’t pull away everywhere. Well, you can, for a while – the excess is taken up by collisions elsewhere on the globe, but eventually, even if the active rift between West and East Antarctica opens up, part of the Antarctica Plate will start colliding somewhere. The Pacific Plate is entirely oceanic – no continental material except the bit of California west of the San Andreas Fault, which is traveling with the Pacific Plate. And the Pacific Ocean is really underlain by two large plates – the larger Pacific, and east of the East Pacific Rise spreading center, the Nazca Plate which is subducting beneath South America to lift up the Andes. The eastern half of the oceanic plate in the North Pacific was called the Farallon Plate, but it has been almost entirely subducted beneath North America. That subduction made for the various mountain-building events that created the Rockies, the Sierra Nevada, and the Coast Ranges. The other big plate is Eurasia – Europe, and Asia except for Arabia and India and the far eastern tip of Siberia, plus the east half of the North Atlantic Ocean. The smaller plates make for some interesting geography and tectonic activity. Arabia, breaking away from Africa, makes the Red Sea, and colliding with Asia makes the mountains of Turkey, Iraq, and Iran, and the Caucasus. The Philippine Plate is a small oceanic plate in the western Pacific Ocean. The volcanoes of the Philippines, Taiwan, and southern Japan are the result of the Philippine Plate subducting beneath the complex eastern margin of Asia, and the Mariana Trench – the deepest point on the ocean floor – is on the opposite side of the Philippine Plate, where the Pacific Plate is subducting beneath it. When two oceanic plates collide as they are doing

I know that I’ve implied that the change from the Pleistocene glacial period to the warmer Holocene was quite abrupt, about 10 to 12 thousand years ago. And it was, generally speaking, but it wasn’t a particularly smooth change. Dryas octopetala, photo by Jörg Hempel, used under Creative Commons license. Toward the end of the glacial time, as the continental ice sheets were melting back quite rapidly, various things happened to tweak the climate from one that was warming to one that was cooling again. Three of these cooling episodes are called the Dryas – Younger Dryas, Older Dryas, and Oldest Dryas. The Dryas is an Alpine and tundra-loving shrub of the rose family, the national flower of Iceland, which typifies these cool periods. The peak of glaciation, with glaciers as far south as the Ohio and Missouri Rivers in North America and covering the British Isles in Europe, was about 21,000 or 22,000 years ago. The warming and melting that began by about 20,000 years ago was interrupted by the Oldest Dryas interval, which lasted from about 18,000 to 14,700 years ago. It appears to mirror the overall trends of the ice ages – a gradual fall in temperatures to a low point, followed by a relatively abrupt warm up over a short time span. The temperature estimates for all these events are based largely on measurements of oxygen, nitrogen, and argon ratios, which are proportional to temperature, from gases trapped in ice in Greenland and Antarctica, but they are supported by other lines of evidence too. During each of the Dryas periods, much of Europe was tundra or taiga – Arctic conditions, but that does not mean lifeless. The taiga or boreal forest is one of the largest biomes on earth today, supporting vast forests and wide diversity of large animals, from caribou and yaks to bears and many birds. The treeless tundra is less biodiverse, but still not really barren. After a fairly short warming period, fewer than 1,000 years, the Older Dryas cooling took place for a short time, from about 14,100 to 13,900 years ago, only a couple centuries. Its expression is largely European, so the changes may not have been global in scope. The Younger Dryas is the best-known of these cool periods. It lasted from about 12,800 until about 11,570 years ago. It seems to have ended in a step-wise manner, in increments of 5 or 10 years over as short a period as 50 or so years. The end of the Younger Dryas is dated by various means quite accurately, to between 11,545 and 11,640 years ago, with 11,570 a common estimate. The Younger Dryas, like the earlier events, was felt most strongly in Europe, though there is evidence for it in the Pacific Northwest of the United States. Scandinavia and Finland were under ice sheets – still, or again. Britain was largely tundra or taiga, as was most of what is now the North Sea, which was dry land supporting an extensive flora and fauna. By now, you can probably guess at some of the speculated causes for the Younger Dryas. It’s been suggested that there was some impact at about 12,900 years ago that initiated the cool period, but I think that idea has been largely discredited. There was a decent-sized eruption of a volcano at Laacher See, near Koblenz in Germany, also at about 12,900 years ago. It was comparable to the eruption of Mt. Pinatubo in 1991, and while it may have had some effects, it’s pretty hard to see it as THE single cause of a 1,300-year cooling event. I think the most likely cause is some change in the fundamental heat engines of the Northern Hemisphere. The focus of this line of reasoning is the circulation of warm waters to the north in the Atlantic Ocean – specifically, the Gulf Stream and the more important deep-water exchange that keeps the North Atlantic warmer. This works because of the variable density of sea water at different temperatures, so it sets up a continuous cycle of circulation and exchange. For the Younger Dryas, the idea is that this circulation was shut down because of an influx of

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

Today’s glacial topic is lakes again – but this time, it’s lakes that are gone. Glaciers were constantly melting, and the runoff, from on top of them, beneath them, along them, and in front of them had to go somewhere. Often, it just ran in rivers away from the glaciers, and the sediment carried by that flow created extensive deposits called outwash, glacially-derived sediment plains and mounds and more. But depending on the topography around a glacial front, you might get a lake pooling there, especially at times when glaciers were retreating. As I mentioned yesterday, the terminus of a stagnant glacier could result in a pile of sediment called a moraine, and combined with pre-existing topography, could easily form a lake. This happened repeatedly, and we recognize lakes by their typically thin-bedded sedimentary layers. In some cases, the layers represent annual sequences like tree rings – a winter layer of minimal sediment and a summer layer when more melting brought more sediment into the lake.  Yosemite Valley, California, is one of the best examples of a glacially-carved valley anywhere. It has the characteristic U-shape, with steep walls ground away by glacial ice. Yosemite Valley contained more than 3,000 feet of ice at the peak of the most recent glaciation. As the transition to the present interglacial period happened, probably 11,000 to 9,000 years ago, the valley floor became the site of a lake about six miles long, with the terminal moraine at the valley mouth impounding it. The situation was much like that of the Finger Lakes in New York, except in high mountain country where the bedrock is the granitic rocks of the Sierra Nevada Batholith. So the floor of Yosemite Valley today is quite flat because of those lake sediments – it’s not really truly U-shaped. You don’t have to have a moraine to dam a river. A tongue of ice can do it, too. That’s what happened repeatedly in northern Idaho about 13,000 to 15,000 years ago, where an ice dam 2,500 feet high impounded the ancestral Clark Fork River. The resulting Glacial Lake Missoula was 2,000 feet deep and more than 225 miles long, in many branches into the mountain valleys of Montana.    From time to time, with the changes in the climate and the build-up of pressure behind the ice dam, the ice would melt and erode until the lake began to empty catastrophically. The water volume is estimated at 60 times the discharge of the Amazon River, draining the entire lake in days to weeks. All that water poured out over eastern Washington state, where it carved the landscape into what’s called the Channeled Scablands today. There are features all over this country reflecting these events, from giant ripple marks at Camas Prairie, Montana, to dry waterfalls in Washington and wave-cut shorelines in the hills around Missoula, Montana that represent multiple lake levels. There were many fillings and emptyings of Glacial Lake Missoula, probably at least 40 times.  There’s lots more to this fascinating story, of course, and lots to see in the area where it happened. I think your best source for detailed information about it is a book by David Alt titled Glacial Lake Missoula and its Humongous Floods (Mountain Press, 2001). Glacial and Pluvial Lakes (and route of Bonneville and Missoula Floods) Map by Fallschirmjäger, used under Creative Commons license  There are a couple more big lakes that developed in the west. We call these pluvial lakes, meaning rain-derived, because they were not the direct result of snow and ice melt. Much of western Utah was covered by Lake Bonneville. The Great Salt Lake is a remnant of this lake, but Lake Bonneville covered more than 10 times the area of the Great Salt Lake. The Wasatch Mountains at Salt Lake City show two prominent horizontal lines which are shorelines of Lake Bonneville. It was more than 1,000 feet deep, and like Glacial Lake Missoula, it had a catastrophic emptying. The Bonneville Flood was about 14,500 year

from NASAThe origin of the Great Lakes goes back hundreds of millions of years, to the Silurian and Ordovician Periods of the Paleozoic and even longer ago. You may recall the resistant Silurian rocks that surround the Michigan Basin – but today, the important rocks are the underlying strata, the less resistant beds. Lake Superior is underlain by the Mid-Continent Rift System, established more than a billion years ago (we talked about it January 26 and 27). There’s another fundamental break, called the St. Lawrence Rift, that formed about 570 million years ago in very late Precambrian time. It underlies the St. Lawrence River valley in Canada, where it is still seismically active, and it may extend as a weak zone southwest to where Lakes Ontario and Erie are today. And finally, around the Lower Peninsula of Michigan, alternating layers of high and low resistance were laid down over Paleozoic time, and warped into the bowl-shaped Michigan Basin.  All these things gave glaciers some paths to follow that were easier than others.  The Pleistocene continental ice sheets had several points of origin. In eastern Canada the Laurentide ice sheet was centered in northern Quebec, but the ice flowed thousands of miles to the south and west. When it reached the area of today’s Great Lakes, it went into the low-lying areas underlain by weaker rocks, digging those areas into basins where ice thickness was somewhat greater. The ice covered everything around Michigan, including the resistant Silurian rocks, but it eroded more in the weaker zones. There’s one little area in Wisconsin where two of the main lobes of the ice sheet came together but left a small zone ice-free, but completely surrounded by ice.   from US Army Corps of EngineersGlaciers came and went multiple times. When you hear about glaciers “retreating,” don’t think of it as backing up. With minor exceptions, glaciers don’t back up. Glacial retreat just means that the rate of melting at the ice front exceeds the rate at which the ice is advancing. In the Great Lakes area, advances and retreats eventually made the old basins noticeably lower than the surrounding highlands, and those lowlands were filled with glacial meltwater. The modern lakes are fed by precipitation including snowmelt, as well as by the rivers in the region. They are the largest volume of surface fresh water on earth, accounting for about 21% of the total. While we’re on the topic of lakes, the Finger Lakes of New York also owe their origin to the Pleistocene glaciation. It’s basically the same thing as the Great Lakes, but on a smaller scale. Pre-existing river valleys were widened and sculpted by valley glaciers, and natural dams were built that created the Finger Lakes. Those natural dams are called moraines, and they represent a place where a glacier was neither advancing nor retreating, but in a continuous state of melting in place. The ice carried with it all sorts of debris – rocks and soil – which was deposited into a mound called a moraine. When that pile was at the terminus of a long valley glacier, it ended up serving as a dam to create a lake in the valley where the glacier used to be. * * * Today’s anniversary is one that many of you probably recall – the earthquake and ensuing tsunami on December 26, 2004, which killed an estimated 230,000 people in 14 countries. The earthquake was on the subduction zone off the northern coast of Sumatra, in Indonesia, where the Indian-Australian Plate, which is oceanic in that area, is subducting beneath an extension of the Eurasian Plate into southeast Asia and Indonesia. Most of the western and southern islands of Indonesia are volcanic, the magmatic arc that forms above the subduction zone. It’s complicated somewhat by the larger islands, especially Sumatra, having elements of more rigid crust, possibly even some small continental blocks. The quake’s magnitude of about 9.2 is the third largest ever recorded. More than 1,600 km, 1,000 miles,

There have been at least five major glacial periods in earth’s history – two in the Precambrian, one at the end of the Ordovician, one during the Carboniferous, and now. “Now” is perhaps an overgenalization, but it’s not all that certain that the most recent ice age is over. It started about 2.6 million years ago, and occupied the Pleistocene Epoch of the Quaternary Period of the Cenozoic Era. It ended – if it has ended – about 12,000 years ago, which is taken to be the end of the Pleistocene and the start of the Holocene, the present-day epoch of geologic time.  I say we don’t know if it’s over because the 2.6 million years saw at least four major and several minor pulses of glaciation that alternated with times called interglacials, when the ice was much less extensive. We may be in the early stages of an interglacial period, or the whole cycle may be over.  Maximum glaciation (ice shown in black) by Hannes Grobe/AWI, used under Creative Commons license.    At the peak, glacial ice reached as far south as the Missouri and Ohio Rivers and New York City in North America, and across the British Isles, central Germany, and most of the plains of western and central Russia. Continental ice sheets as much as 3 to 4 km thick (2 to 2½ miles) were prevalent in the northern hemisphere, which seems to have been affected more than the southern hemisphere, though the Antarctic Ice Sheet expanded. Mountain glaciers in southern South America, New Zealand, and in the northern hemisphere also grew significantly. Because so much water was locked up in the ice, sea level fell about 120 meters, or 390 feet. Huge areas of what are now the continental shelves, under water, were dry land, including the Bering Sea off Alaska, which created the famous Bering Land Bridge between North America and Asia. What caused it? Over the course of these podcasts, you’ve heard about ideas of global cooling related to volcanism that might have led to glacial periods, as well as ocean circulation, the rise of plants that changed greenhouse conditions, and other things as possible causes for the sporadic glacial times through earth history. The bottom line is, we really don’t know for certain. The modern glacial period, the past 2½ million years, may be just the culmination of cooling that dates to 34 million years ago, when the Antarctic Ice Sheet got going. One factor that has been promoted as a cause for ice ages is the variations in earth’s orbit. Regular cycles in the tilt of the earth’s axis and in the shape of the obit around the sun produce regular variations in temperature – but that happens all the time, and there have only been a handful of glacial periods over billions of years, so there must be more to it. Pleistocene ice extent (USGS)Ocean currents and the arrangements of continents may have a role. I mentioned the opening of the Drake Passage, creating a truly circumpolar oceanic current around Antarctica, as a possible factor in the growth of ice there. In the modern Northern Hemisphere, the changing arrangements of continents has made the Arctic Ocean an almost landlocked sea, and combined with the growth of the Isthmus of Panama about 3 million years ago that drastically changed global circulation patterns, maybe that all created the tipping point that led to the onset of glaciation. A decrease in carbon dioxide levels would reduce the greenhouse effect and global average temperatures would be cooler. Such changes can be observed in ice cores, and there was indeed a general, slow, but significant decrease in CO2 in the atmosphere through most of the Cenozoic until recently. That would still call for a threshold or tipping point because the onset of significant continental glaciation was pretty abrupt, about 2.6 million years ago, and it is hard to explain the changing CO2 levels in ways that would account for the rapid changes from glacial to interglacial periods. And even within predominantly glacial or non-glacial times,

The end of the Paleocene and start of the Eocene was apparently a stimulating time, in terms of evolutionary diversity for the mammals. The oldest known primate-like animal dates to this time, from lake beds in France and from North America, about 56 million years ago. Plesiadapis was a small lemur-like animal adapted to climbing trees and eating plants, although it may have been an opportunistic omnivore. Its ancestors are uncertain, but it probably evolved in North America and migrated to Europe. Mesopithecus (Gaudry, 1867)By Oligocene time, the lineages of both the Old World and New World monkeys had been established. Many fossils have come from the East African rift valleys, also the source of many hominid fossils. The lake deposits there preserve these rare remains relatively well.  The Miocene, around 12 to 18 million years ago, saw the divergence of groups such as the great apes, orangutans, and gibbons. By very late Miocene, 5 to 7 million years ago, many monkeys were probably quite recognizably modern in appearance. There’s an excellently preserved example called Mesopithecus, found in Greece, that lived about that time and likely lived on fruits and leaves. It was about 16 inches long. The primitive primates of Madagascar, lemurs and their kin, present a challenge to plate tectonics. Madagascar was already separated from the other continental masses by Paleocene, when these animals developed. So how did they get there? In fact there are only five orders of land mammal on Madagascar today. The favored idea has usually been rafting – drifting of land animals on floating mats of water-borne debris, though that idea relies on a lot of luck. The luck depended on a relatively short window of geologic time, between 60 and 20 million years ago, when the distances and currents would make the rafting idea plausible. The best estimate for the colonization of Madagascar by primates is probably early to middle Eocene time, 40 to 52 million years ago.  Alternatives to the rafting idea include the presence of an ancestral primate on Madagascar, but none have been found, and “island hopping,” crossing the distance in short spurts to intervening islands – but the likelihood of such islands is low. I don’t think there is unanimous agreement as to how primates got to Madagascar, but for now I think the rafting idea is most likely, even if it was actually pretty unlikely. Just more likely than the other possibilities. Once they got there, the lemurs and other primates probably survived because they lacked the competition from the more advanced monkeys that developed in Africa by Oligocene time. —Richard I. Gibson Lemur origins  Reconstruction of Mesopithecus from A. Gaudry, Animaux Fossiles et Géologie de l’Attique. Recherches faites en 1855–1856, 1860 (1867), public domain.

Today’s episode is a follow-up to the Basin and Range discussion the other day, and a follow-up to the Sierra Nevada Batholith which we talked about back in the Jurassic, in October. The Sierra Nevada Mountain Range in California coincides pretty well with the Jurassic-Cretaceous Sierra Nevada Batholith, the deep roots of the subduction and magmatic arc system that was established out there by at least 150 million years ago.  But the modern range is vastly younger than that. The Sierra Nevada is basically one huge fault block, tilted to the west, with the fault marking its eastern front. It’s a big normal fault, with the mountains up to the west and the region to the east dropped down. That fault is essentially the western margin of the Basin and Range Province, the extended, faulted suite of uplifts and basins that continues all the way east to the Wasatch Mountains in central Utah.  The action on the Sierra Nevada Fault started about the same time as the Basin and Range became active, in the Miocene Epoch, 15 to 20 million years ago. And most of the uplift is in the past 5 million years. There’s easily 10,000 feet of displacement on the fault and probably quite a bit more.  Intermountain Seismic Belt marked by dashed black line.It may be that the size and strength of the huge granitic batholith provided a mechanical boundary to the part of the crust that was able to break into the Basin and Range Province. The other side of the Basin and Range, the Wasatch Mountains, mark the western edge of the Colorado Plateau, which is also a region that is fundamentally different in terms of the nature of the crust from the Basin and Range. The Colorado Plateau’s crust is thicker, cooler, and mostly older than the Basin and Range. So, thinking back to the episode a couple days ago, this helps us look at the unique broken nature of the Basin and Range as likely connected to its mechanical properties, such as thickness, composition, and age. Today, there is some seismic activity within the Basin and Range, but it’s pretty spotty. In California, the main tectonic focus is along the San Andreas Fault Zone. But the Sierra Nevada Fault is almost certainly still active. The uplift of the Sierra Nevada isn’t simply on the one fault, and in 1872, a magnitude-7.8 quake hit the Lone Pine area, in the Owens Valley east of the Sierra. The movement wasn’t on the Sierra Nevada Fault, but on the nearby and related Owens Valley Fault, which is part of the system of faults continuing to uplift the Sierra Nevada. Some segments had vertical offset of as much as 20 feet. On the other side, the Wasatch Mountain Front, which forms the backdrop of Salt Lake City and all of central Utah, is part of an earthquake belt called the Intermountain Seismic Zone. Its heritage is also about the same timing as the Basin and Range, the Miocene, but there has been some level of discontinuity there, at the western edge of the Colorado Plateau, for many tens of millions of years. The activity along this belt is usually nothing like that on, say, the San Andreas Fault, but great earthquakes do occur, including the Hebgen Lake Quake in 1959, which had a magnitude of about 7.4. The Intermountain Seismic Belt goes from southern California, through southern Nevada and central Utah, eastern Idaho and western Wyoming, through western Montana and into Canada. If you drive Interstate 15, you’re pretty much following this zone. There hasn’t been a big earthquake on the Salt Lake City segment of the Wasatch Fault for about 1,300 years. That doesn’t mean one is due; typically, the historical record of earthquakes is far too short to infer any kind of regular periodicity, and it can be challenging to date prehistoric quakes with accuracy. Nonetheless, seismologists always try to figure this out for general hazard preparedness, and there do appear to have been sizeable earthquakes on some part of the long Wasatch Fault zone about every 350 years. A big one will happen there

Smilodon, the famous saber-toothed cat, was not a tiger nor even closely related to tigers. They were felids, cats in general, but they branched off from the families that include all modern cats pretty early in the development of the feline families. They probably diverged sometime during the Miocene, more than 10 million years ago.  Smilodon and dire wolves (drawing by Robert Horsfall, 1913)Smilodon’s scientific name doesn’t mean “smiling tooth,” but rather comes from the Greek meaning “carving-knife tooth,” and they did certainly have huge sharp canine teeth up to 11 inches long. The animal was about the size of a lion, and was probably the last in a lineage of similar carnivorous animals. It lived during the glacial period, the Pleistocene Epoch, from about 2.5 million years ago until 10,000 years ago. We’ll talk a bit more about the Pleistocene extinction of the saber-toothed cat and other large animals in a few days. Smilodon is the state fossil of California. Lots of Smilodon fossils have been found in the famous La Brea Tar Pits in Los Angeles, along with a hugely diverse fauna. The presumption is that animals were attracted to the tar mistaking it for water or simply crossed it inadvertently, and became trapped. The asphalt, the tar, in the tar pits, is basically a heavy oil, a natural seep that has been active for many thousands of years. Oil, migrating from depth, has reached the surface. The material is less fluid than most oil, but more fluid than, say, the tar sands of Alberta. It’s not really a surprise that we’d find oil through the spectrum of possibilities, from highly liquid to completely solid, as in oil shale. There is a real oil field at depth at the La Brea site, the Salt Lake Oil Field, where oil is trapped in Miocene and Pliocene sediments in anticlines and fault blocks. The buoyant oil continued to migrate upward along one fault to reach the surface and form the tar pits. The material is heavy and viscous because the volatile portions have evaporated at the surface, leaving the tar behind. Brea is Spanish for tar. The oldest fossils in the La Brea Tar Pits date to about 38,000 years ago – and many of them are extinct. The assemblage is huge, and includes extinct bison species, llamas, cheetahs, bats, sloths, mastodons, more than 100 bird species, snakes, insects, and plants. It is truly a lagerstätte, one of those amazing places where fossils are preserved in extraordinary detail. —Richard I. Gibson Drawing of tar pits by Robert Bruce Horsfall, 1913 (public domain)

We’re getting really pretty close to the present, geologically speaking, and most of the rest of the episodes will focus on the most recent 10 million years of earth history. If our calendar was at a proper scale, these last 10 million years would have to be squeezed into the last 18 hours of December 31. We’ll spread it out some.  Today I have two different water-related topics to discuss, both from near the end of the Miocene epoch, about 5 or 6 million years ago. The Messinian age is the last subdivision of the Miocene Epoch, from 7.2 to 5.3 million years ago. Near the end of this time, the Mediterranean Sea dried up.  Mediterranean Sea bathymetry map from NASA (public domain) annotated by Gibson. Heavy black lines outline deepest parts which might have remained lakes or brackish areas when the rest of the Mediterranean was dry land.It was actually multiple events in which the sea evaporated, leaving at most some large lakes in the deepest portions of the ocean basin. It was mostly in the latest part of the Messinian age, about 5.9 to 5.3 million years ago. The Strait of Gibraltar, the Mediterranean’s only connection to the Atlantic, closed as a result of ongoing tectonic activity there, essentially part of the Alpine Orogeny that reflects the complex interactions between North Africa and the complex bits of southern Europe. With the Mediterranean cut off from oceanic circulation, evaporation could have become significant. The climate generally was not especially hot – in fact the planet was approaching the glacial period. But it’s been suggested that the evaporation took the lead because precipitation decreased, mostly due to orbital cycles that affected rainfall. The argument is a little bit circular, as it calls on reduced solar energy as a cause of lower evaporation in the Atlantic, and therefore less rainfall in Europe, and therefore more evaporation in the Mediterranean. There is no clear consensus on the role of climate changes in this event. But it definitely did happen. Vast deposits of salt and gypsum dating to the Messinian age – named for Messina, Sicily, where such deposits are found – show it clearly. Drilling in the modern sea has also revealed evaporites in the deep-sea sediments as well. In places, the salts are interbedded with typical marine sediments including foraminiferal oozes, indicating that there were repeated times of drying and return of oceanic conditions. A completely different line of evidence for the Mediterranean being dry at this time is submarine canyons, which must have been cut by rivers flowing across dry land, but which are now far below sea level. At the end of the Messinian, the end of the Miocene Epoch, the Mediterranean basin filled for the last time. The dam at Gibraltar was breached. There is debate as to whether the refilling was a gentle process or catastrophic – it might have taken place over many hundreds or thousands of years, although the image of a gigantic waterfall half a mile high with the water of 1,000 Amazon Rivers gets your attention. The only evidence regarding the nature of the refilling is in structures cut into the sediments around Gibraltar, beneath younger rocks – and the information there has been interpreted both ways. But fast or slow, the Mediterranean did refill. Saturated thickness of Ogallala aquifer, by Kbh3rd, used under Creative Commons license. The second water topic from 6 million years ago that I have for today is the largest ground-water aquifer in the United States. The Ogallala Aquifer underlies much of the high plains, from Wyoming and South Dakota to West Texas and New Mexico. It contains about 30% of the groundwater in the U.S., and supplies water for irrigation, industrial uses, and drinking water. Deposition of the rocks of the Ogallala Formation, which contain the aquifer, began in very late Miocene time, about 6 million years ago, and continued through the Pliocene, until about 2 million years ago when gentle uplift changed th