History of the Earth

Today is a follow up to the discussion Colleen Elliot and I had about the Cambrian Explosion a few days ago. In 2004, A book by Oxford zoologist Andrew Parker was published entitled In the Blink of an Eye – how vision sparked the big bang of evolution. It offers arguments for the idea that the development of eyes led to increases in predatory behavior, and was the ultimate cause for the Cambrian explosion. Trilobite eye (NASA photo)First, Parker defines the explosion not as the proliferation of animals, but as the proliferation of calcareous and phosphatic shells and exoskeletons in animals. He makes the argument that with new discoveries, we see that most or all of the existing phyla of animals had already been present for some millions of years when the explosion happened. So what we see as an explosion is really just those organisms figuring out a new way of living, inside various types of armor. The focus of In the Blink of an Eye is why – why did this change happen? Parker objects to most of the possible causes that Colleen and I discussed the other day because they would probably have generated a gradual change, not an abrupt one as is observed. Or they were too far removed in time, as we mentioned regarding the end of Snowball Earth. So he makes the case for the development of eyes as the diving force behind the change. It’s not hard to imagine that if, suddenly, some animals became able to sense movement, or to recognize patterns that indicated other animals – FOOD! – that the “arms race” Colleen and I mentioned in passing could have developed. There is pretty good evidence that animals DID develop eyes at the time of the Cambrian diversification, about 542 to 530 million years ago. It’s reasonable, but in my opinion not absolutely conclusive that it might have driven the increase in diversity that we see happening then. For this to happen across species, in fact across phyla, at about the same time, there must have been some common reason. Maybe the development of eyes drove the Cambrian explosion, but why did eyes develop in so many kinds of animals at the same time? Parker analyzes this question carefully, and ultimately suggests that there could have been a dramatic, and sudden increase in the amount of light reaching those animals, so that sensitivity to light became a useful survival mechanism. What could cause a sudden increase in light on earth? Well, the sun could have suddenly become more luminous, or the atmosphere and oceans could have become more transparent. While those might seem far-fetched, there’s actually good reason to think they might have been possible, especially the changes on earth. There’s some evidence that the atmosphere, even while it was becoming increasingly oxygen-rich, might have been fog-like during the Precambrian. If something happened – a threshold was reached, or conceivably, a nearby supernova event drenched the earth in enough radiation to change it – it might have become more transparent. Or the water could have become more transparent because of changes in mineral content. Such changes certainly DID occur – but were they enough to suddenly change the amount of light reaching primitive animals, enough to make them all, suddenly, develop sensitivity that led to arms and armor as the world became an eat or be eaten kind of place? I don’t know. The Blink of an Eye is a fascinating idea, very much worth considering, and the book is a good read filled with lots of earth history. Personally I think the jury is still out on why the Cambrian Explosion happened, and it remains perhaps the most intriguing and poorly understood event in the history of life since it originated on earth. —Richard I. Gibson Further reading: In the Blink of an Eye (Amazon) In the Blink of an Eye (B&N)  Calcite in the oceans (NASA - source of trilobite eye photo) Trilobite eyes

During the early Cambrian, sea levels were rising. This produced a near-global transgression – which is not a sin, but rather it just means that the seas were advancing, covering more land area than they had previously. The opposite is a regression, when the seas become relatively smaller. So how does that happen? With minor exceptions, the volume of water on earth has been more or less constant, at least since pretty early in the earth’s history, back in early January sometime. But the volume of the ocean basins that hold the water can change and does.  One way that can happen is by rifting apart continents, as Rodinia was splitting into several smaller continents in late Proterozoic and early Cambrian time. The volume change comes about because of the mid-ocean ridges, the point where oceanic crust is pulling apart. Today, the oceanic ridge system is the longest mountain range on earth, and added together, it takes up a pretty notable volume of oceanic water. Enough that if there are a lot more oceanic ridges, it can result in sea level rise. Likewise, if there were a lot more oceanic trenches, very deep water, that could also accommodate at least a little more water than a flat ocean floor. We know from concerns today about sea-level rise that melting and freezing ice caps can contribute to sea level changes. And a small effect might even come about because of water temperature. Warmer water expands, if only a bit, but when the entire ocean expands, it can make a difference in sea level, especially on very low, flat shores. Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDLIn North America during the Cambrian, shallow seas covered a vast amount of the continent, all except high area on the Superior Craton and a long narrow peninsula called the Transcontinental Arch, extending from Minnesota southwest to what is now Colorado and New Mexico. Abbreviations in the middle Cambrian (about 500 million years ago) paleogeographic map above: NA-North America. B-Baltica (Europe). S-Siberia. SA-South America. AF-Africa. AUS-Australia. CH-China. ANT-Antarctica. Here is another version of a map showing the arch, and another one.   The sea took millions of years to transgress across North America during the Cambrian. As it progressed further and further, the shoreline beach also changed position. Consequently there was a lot of sand – sandstone today – that marks the base of the Cambrian across much of North America, but don’t think of it as one big beach – think of it more like a continuously migrating beach that, over millions of years, ended up depositing sand across many hundreds or even thousands of square miles of the continent. We’ll talk about some of those sandstones as we work our way through February. —Richard I. Gibson Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL.

Part of the early Cambrian explosion included the development of the archaeocyathids, somewhat conical animals with calcareous skeletons rather like corals. But they are not corals. Their affiliations are not 100% certain, and some scientists put them in their own phylum, but I think most taxonomists are classifying them as early sponges. Their name means “ancient cup” and some are long inverted cone shapes as much as 30 centimeters long, close to 12 inches, while others are like nested bowls. Some were solitary, but others grew together to make some of the earliest significant reefs on earth. They appeared about 525 million years ago, part of a famous fossil assemblage called the Tommotian fauna for the specimens found along the Tommot River in Siberia. They’ve been found all over the world, and they are distinctive enough that they serve as index fossils for the early Cambrian. That means rocks can be correlated across large distances using the particular varieties of archaeocyathids that are found in them. Just nine or ten million years after they appeared and proliferated, the archaeocyathids went into a sharp decline in numbers and diversity about 516 million years ago. Interestingly, this time is when more modern sponges began a rapid diversification, another pulse in the Cambrian explosion. The sponges that evolved then have survived with variations to this day, but the archaeocyathids were all extinct before the end of the Cambrian. —Richard I. Gibson Drawing of reconstructed archaeocyathids by Stanton F. Fink, via Wikipedia under GNU free documentation license.  

O Wonder! How many goodly creatures are there here! —Shakespeare The Cambrian Explosion was not a violent volcanic eruption, but rather the sudden appearance in the fossil record of abundant and diverse shelly fossils, the remains of relatively large animal life. Even Charles Darwin recognized it, and saw it as an important objection to his theory of evolution by natural selection. Like most geologic events the Cambrian explosion wasn’t instantaneous, but it did occur over a geologically short time, around 542 to 530 million years ago, right at the start of the Cambrian period of the Paleozoic Era. Today’s podcast is a discussion between me and geologist Dr. Colleen Elliot. Neither of us is an expert on paleontology, so this will be something of an exploration for all of us. Here are some definitions of terms non-geologists may be unfamiliar with in the discussion. Phanerozoic is the name of the eon, the largest subdivision of geologic time, that starts with the Cambrian period of the Paleozoic era. We talked about that a bit the other day. And the Great Unconformity is a break in the rock record that is nearly global in extent, a time before the first rocks of the Cambrian were deposited. It represents a time of near-global erosion of the older Precambrian rocks, and it means that the continents were standing above sea level so that they could be eroded. —Richard I. Gibson Links for further reading: McKenzie et al. on climate and evolution (January 2014) http://www.sciencemag.org/content/341/6152/1355 Smith and Harper on summarizing Cambrian explosion causes (Sept. 2013) http://www.astrobio.net/pressrelease/5758/oxygen-not-the-cause-of-the-cambrian-explosion https://www.pbs.org/wgbh/evolution/library/03/4/l_034_02.html http://ucrtoday.ucr.edu/9063

Almost from the start, geologists have argued, sometimes passionately, about exactly where the divisions of geologic time should begin and end. In the early days, when all ages were relative, meaning this is older than that, but we didn’t know absolute age dates closer than tens of millions of years or more, it was challenging to assign a particular time to the start of a period like the Cambrian. When I was in college, the start of the Cambrian was put at 600 million years ago, and honestly, it was a fairly arbitrary number even though it was generally accepted. My historical geology textbook, published in 1960, has a question mark after 600 million years. Much of geologic research is devoted to pinning down the details of ideas and concepts that were sketched out in the early 1800s. So with better age dates and much more information from the far corners of the world, the timing of the start of the Cambrian has been refined over time. When I wrote the history of the earth calendar in 1994, the generally accepted start of the Cambrian was about 575 million years ago. Today that date is 542 or 543 million years ago, but even now that’s not universally accepted and the USGS time scale here has slightly different dates. It isn’t that the time changes, but our way of looking at the data does change, and everyone doesn't quite agree on all the details. The Cambrian Period today is considered to have lasted until about 488 million years ago, for a total length of 54 million years, just 1% of Earth’s history. Yet we’re spending all of February on the Cambrian. Stuff happened! Like the months of the year, the periods of earth history are subdivided and given names to make it easier to refer to them. So we have Early, Middle, and Late Cambrian as general names, and in parts of North America those times have been related to packages of rocks called the Waucoban Series, Albertan Series, and Croixan Series. You can imagine that everything geologic didn’t happen simultaneously, or in the same way, all around the world, so there are dozens of regional variations in the nomenclature. The subdivisions are not arbitrary, but are often based on the fossils that can be found in each subset of the rocks. Fossils can be related to age, and can help correlate between rocks of different type that may be of the same age. There are ongoing attempts to create internationally acceptable time scales. So there is actually an International Subcommission on Cambrian Stratigraphy that works on this kind of thing, and at present the Cambrian is divided into four major subdivisions, based in part on the timing of major extinction events. I think we’ll settle for early, middle, and late. —Richard Gibson Further reading: http://tapestry.usgs.gov/ages/ages.html International Commission on Stratigraphy http://en.wikipedia.org/wiki/Cambrian

The Proterozoic is done. We’ve started a new era, the Paleozoic, which means “ancient life” in Greek. It’s part of an eon, the Phanerozoic, that includes all the eras and all the time beginning with the Cambrian and continuing up to the present day. Phanerozoic means “visible life”, reflecting the former thought that there was no life before this time. We know now that there was earlier life, and some of it, like the Ediacarans, was certainly visible, but the name continues to be used. It works if you don’t worry too much about the literal Greek meaning. The first subdivision of the Paleozoic is the Cambrian Period. Although there are older fossils, the Cambrian is the first time period when they are abundant, and this helped early geologists work out the sequence of rocks and the order in which they were laid down. A lot of that work was done in Great Britain. Adam Sedgwick at the University of Cambridge is considered to be one of the founders of modern geology. He studied rocks in Wales, which was called Cambria by the Romans, from the Welsh name for their land, Cymru. In 1835, he gave the name Cambrian to the older series of rocks in the mountains of central Wales. We’ll spend the month of February in the Cambrian.

By Richard I. Gibson Listen to the podcast: Transcript: Keweenaw copper districtThe mid continent rift system ruptured the crust of Proterozoic North America, allowing magmas to flow into parts of the rift. But don’t think of it as a simple low-lying trough: the flanks of the trough were probably fairly high mountains, and basins bordered the trough and mountains. Lots of sediment came into those basins. Coarse sediments, deposited relatively close to the mountains, included pebbles and cobbles that solidified into a rock called conglomerate, nicely exposed in the area around Copper Harbor, on Michigan’s Keweenaw Peninsula. In some places, the cement holding those pebbles together has been replaced by pure native copper that was brought to the surface by the volcanism associated with the mid-continent rifting about 1.1 billion years ago. On average, only about 1½% of the rock is copper, but it still adds up to one of the greatest copper deposits on earth. And unlike most of the world’s copper mines today, which are copper sulfides, the Keweenaw copper is pure – already in metallic form, called native copper. Relatively easy to mine, and because of the lack of sulfur, environmental problems are much less serious than in copper sulfide mines where sulfur generates sulfuric acid and pollution that can be challenging to deal with. Native copperThe first copper rush to northern Michigan was in 1843-1846, and by about 1870, 95% of America’s copper came from this area. But after decades of mining, the area was surpassed by western mines in Butte, Montana, and in Utah and Arizona in the late 19th century. The copper industry in Michigan was pretty much dead by the 1970s, but relatively high copper prices mean that there are several projects in the works today to rejuvenate copper mining there. Photo by Jonathan Zander under GNU free documentation license. Map from Michigan Tech Useful links for further reading: http://www.nps.gov/history/history/online_books/geology/publications/pp/754-b/sec1.htm http://pubs.usgs.gov/of/1999/of99-149/ http://www.geo.mtu.edu/~raman/SilverI/BlackLavas/Stratigraphy/Stratigraphy.html http://www.geo.mtu.edu/~raman/SilverI/BlackLavas/Copper_Mining.html

By Richard I. Gibson Here is the podcast: Green is the area where there is basalt in the subsurface, more or less.Transcript: I think most Americans have a sense of what Iowa is like today – relatively flat, low hills, scenic river valleys, thick fertile soils supporting lots of farmland. A billion years ago it was quite a different place. More like East Africa, minus the plants and animals. A billion years ago, North America was trying to split apart, just as East Africa is today. The split, called the mid-continent rift system, extends from central Oklahoma through eastern Kansas, then diagonally across Iowa, along the Wisconsin-Minnesota border and into Lake Superior. It swings around to head south beneath the lower peninsula of Michigan before it ends around Detroit. Visualize fissures spewing basalt magma along most of that zone for 15 to 20 million years. Like Iceland’s volcanoes, filling a trough as much as 40 miles wide and more than a thousand miles long with basalt lava flows. The piles of lava flows added up to 2 to 10 miles of basalt and other volcanic rocks. This rift was a zone like the mid-Atlantic ridge where heat moving up from the earth’s mantle splits the crust. The best modern analog for the mid continent rift system is East Africa, the Red Sea, and the Gulf of Aden – three branches that form what’s called a triple junction. The third arm of the system in North America headed north from Lake Superior beneath lake Nipigon in Ontario. Magnetic map of IowaThe mid-continent rift system failed, for reasons that are not completely clear, but we’ll talk about one possible factor in a couple days. So it never reached the stage of true ocean crust, like the Red Sea, but was always like the complex system of troughs, mountain chains, and volcanoes that mark the East African rift today. Gravity map of IowaHow do we know it’s down there? The best evidence comes from remote sensing – measurements of the earth’s gravity and magnetic fields. Those basalts in the trough are very dense and very magnetic compared to other rocks, so they produce a dramatic signature in gravity and magnetic maps. The maps of Iowa show a long curving band of gravity and magnetic highs extending from near the southwest corner of the state to the northern boundary with Minnesota. And further north, along the continuation of the zone, the rocks actually crop out on the surface around Duluth Minnesota, and on Lake Superior’s Isle Royale and the Keweenaw Peninsula of Michigan. This all happened over a geologically short period, 20 million years or so, about 1.1 billion years ago. The consequences are still evident today, in the presence of Lake Superior in the basin that sits on the rift, and in the mineral resources created by the volcanism. We’ll talk about that tomorrow. There are a lot of good online resources about the mid-continent rift, so be sure to check the links below for more information.  http://en.wikipedia.org/wiki/Midcontinent_Rift_System http://crustal.usgs.gov/projects/midcontinent-rift-minerals/index.html http://www.nature.com/news/geology-north-america-s-broken-heart-1.14281 http://www.gravmag.com/gmprimer.shtml http://www.igsb.uiowa.edu/browse/rift/mrs.htm All maps from USGS.

By Richard I. Gibson Here is the podcast: Grypania spiralisWhen I wrote the book back in 1994, the oldest eukaryote organisms – single-celled creatures with a distinct nucleus – were thought to be algae from the Beck Spring Dolomite of Eastern California. They are about 1.3 billion years old, and some of those algal cells were large enough to be megascopic - seen with the naked eye. In 2004 S. Sarangi and colleagues at the National Geophysical research Institute in Hyderabad India reported an age of 1.6 billion years for the megascopic alga Grypania spiralis. Some dates from Michigan have suggested that Grypania is as old as 2.1 billion years. Understanding the timing and worldwide extent of things like megascopic algae has implications for understanding the evolution of the atmosphere and its oxygen content. Scientists who work on these problems try to correlate the timing of global algal blooms with evidence of oxidation in banded iron formations to get at the timing of changes in atmospheric oxygen. Photo by Xvazquez, via Wikipedia under GNU free documentation license. 

By Richard I. Gibson Here’s the podcast: Transcript: St. Mary Lake, Glacier National Park. Photo by David Restivo, NPSWe’ve talked briefly about continental crustal fragments large and small drifting around the earth’s surface, a process which continues today. Inevitably, at some times a lot of the continents might come together to create what we’ve come to call a supercontinent. While we won’t encounter the best known supercontinent, Pangea, until August, supercontinent assemblies have happened repeatedly in the past. The Sioux Quartzite was deposited in North America during the time that the supercontinent called Columbia is thought to have existed, around 1.7 to 1.5 billion years ago. We know something more about the break up of that possible supercontinent than its assembly, and there is abundant evidence for a break, separating what is now western North America and eastern Siberia, around 1.4 billion years ago. That break created a huge basin in what is now the northwestern United States and nearby parts of Canada. It was filled with as much as 50,000 feet of mostly fine-grained sediments, mostly mud and silt that lithified into shale, mudstone, and a few sandstones and limestones. We call this thick package of rocks the Belt Supergroup, named for occurrences in the Big and Little Belt Mountains of Montana. Armored mud balls in Grinnell Formation, Glacier Park. Photo by Richard Gibson.The Belt rocks are beautifully exposed in Glacier and Waterton National Parks, where they display red and green colors sculpted by glaciers that were vastly younger than the rocks themselves. The rocks contain details that give us lots of information about the early earth. For example, thin layers of coarse grains indicate that storms probably stirred up the sediment on the floor of a shallow lake or sea. It may seem obvious that rain fell on the ancient earth, but we KNOW it did because of raindrop impressions found in some of the Belt rocks. One of the most remarkable things about the Belt rocks is the immense span of time that they represent – maybe as much as 250,000,000 years of relatively undisturbed deposition. Understanding the Belt is challenging, to say the least, and has been the life work of many geoscientists. From Idaho State UniversityLots of geologists try to figure out how the earth’s early continents were combined before they broke up – this has implications for resource exploration as well as pure science – and a lot of effort went into the question of what continent separated from western North America to create the Belt Basin. Antarctica, Australia, and Siberia were popular candidates. But thanks especially to the work of Jim Sears and his students at the University of Montana, and Paul Link at Idaho State, I think it’s pretty well settled now that it was Siberia that was once where western Washington and Oregon are today.

By Richard I. Gibson Here’s the podcast: Transcript: Sioux quartzite After the Trans-Hudson and Penokean mountain building episodes, which we talked about a few days ago, weathering attacked the mountains and eroded them. After about 150,000,000 years, much of what is now the southern edge of the Superior Craton was a low-lying, subsiding area crossed by braided streams. These rivers flowed between about 1.76 billion and 1.63 billion years ago The streams carried quartz grains, sand, that created a deposit as much as 3,000 meters thick in what is now Minnesota, South Dakota, Iowa, and Nebraska. That’s nearly two miles of Proterozoic sandstone, so intensely cemented and lithified that it’s called quartzite. The package is called the Sioux Quartzite, and Sioux Falls drops over a resistant escarpment in this rock. Similar rocks are found on the surface around Baraboo, Wisconsin, as well as in Arizona and New Mexico. It’s likely that much of North America was a low shield being eroded by shallow streams around 1.7 billion years ago. The countryside around those rivers would have been bleak by modern standards – no life at all, no trees, no grass, no plants, no animals. Not even any soil as we would recognize it, since modern soil contains a lot of organic matter. Just loose rocks and grains of resistant quartz. The Sioux Quartzite is so thick and resistant that the area where it crops out has been a persistent relatively high area for much of earth history. Any sediments that were laid down across it during high stands of seas have been stripped off so that today Sioux Falls and the surrounding area are a window into the past. The Sioux Quartzite is a pretty, pinkish rock that has been used in many historical buildings in the city of Sioux Falls and surrounding areas. Geologic map of China showing location of 1556 quake.Today, January 23, is also the anniversary of the most deadly earthquake in human history. In 1556, in Shaanxi, north central China, a quake estimated at a 7.9 magnitude killed at least 830,000 people. 7.9 isn’t that intense, as quakes go, but at the time many of the people there were living in caves dug into soft earth, and many died in the collapse of those caves. The quake was ultimately a result of the interaction of India and Eurasia. The geologic map shows the area around Xian to be near the intersections of several major faults, which were strained and continue to be strained by the northward push of India even though its collision was hundreds of kilometers to the southwest. Quartzite photo by Andrew Wickert, via Wikipedia under Creative Commons license.

By Richard I. Gibson Listen to the podcast: We know there was some free oxygen in the atmosphere by 2.3 or 2.4 billion years ago, but it took until around 2 billion years ago, after 700,000,000 years of work by the cyanobacteria, for there to be enough oxygen in the atmosphere to think of it as relatively oxygen rich. For a while, a few hundred million years, the highly reactive oxygen given off by photosynthetic organisms probably combined with iron dissolved in the early oceans, so oxygen didn’t accumulate in the atmosphere. It produced thick iron oxide deposits like those in Minnesota, which we’ll talk more about tomorrow. Was the air breathable, if we went back in a time machine 2 billon years? No one knows for sure—there is no good way to reliably estimate the percentage of oxygen in the early atmosphere. But probably not. The concentration of oxygen may not have been great enough for another billion years or more for oxygen dependent animals to evolve, but it did happen eventually. Obviously! What was a boon for oxygen-based life was a crisis for the original anaerobic life that didn’t need oxygen. Today such life is limited to a few small niches such as the reducing environments in swamps and deep oceans and near volcanic vents. Free oxygen is poisonous to the bacteria that got things started for life on earth, but the crisis for them probably was the impetus that allowed for multicellular plants and animals to develop toward the end of the Proterozoic era. Atmosphere photo from NASA (public domain).

By Richard I. Gibson Here’s the podcast: The Ediacara Hills of South Australia contain some remarkable fossils discovered in 1946, but largely ignored. They were thought to be of Cambrian age, and they were vague enough that it was challenging to determine what they were. With improvements in age dating, these fossils were eventually determined to be about 560 to 600 million years old – the Precambrian, a time when no one thought life existed, at least not as large animals. Dickinsonia costata was as much as a meter across, or more.They appear to be soft-bodied organisms, preserved as imprints in fine sand. There are disk-like shapes, and ovoids, with radiating textures. Some frond-like fossils have a quilted appearance, and there are many other varieties. We don’t know what they are. Early interpretations suggested they might be jellyfish, or worms, or sea anemones. They have a wide range in size, from a few millimeters to more than a meter across. Some researchers say they aren’t animals at all, but are some kind of fungus. Or they might belong to a phylum, or even a kingdom, that has no modern representatives.  Examples have been found in Newfoundland, England, Namibia, and elsewhere, so they appear to have been pretty common in the oceans that appeared after the glaciations and possible snowball earth ended 650 million years ago, perhaps 50 million years before the Ediacarans appeared. And interestingly, the Ediacaran life seems to disappear just before the Cambrian Explosion of life, about 530 million years ago. Whatever the Ediacarans are, they represent an important step in the evolution of multicellular life on earth. Figuring out what they were, or what they are related to, is one of the ongoing challenges of geology and biology. With the Ediacaran fossils, we reach the end of the Precambrian. The Precambrian represents 88% of Earth history, but we’ve compressed it into the month of January alone. The rest of the year will cover the most recent 500 million years of Earth history, because we know a lot more about it. And because it contains cool things like fossils and mountain ranges and sedimentary basins and coal swamps and dinosaurs. Tomorrow the Cambrian begins. The Wikipedia article is a good overview, and has extensive links to scientific literature. Photo by Verisimilius from Wikipedia under GNU Free Documentation license.

By Richard I. Gibson Here’s the podcast: Snowball earth is the controversial idea that the Earth was entirely, or almost entirely, covered with ice and snow during three or four periods during the late Proterozoic, from about 650 to 775 million years ago. While there is abundant evidence that can be interpreted to suggest the existence of a snowball earth, much of that evidence can be interpreted in other ways as well. The podcast outlines some of those lines of evidence. Below are some links for further reading. http://www.snowballearth.org http://en.wikipedia.org/wiki/Snowball_Earth http://www.amazon.com/Snowball-Earth-Maverick-Scientist-Catastrophe/dp/1400051258 http://www.sciencedaily.com/releases/2011/10/111012083450.htm Modern glacier photo by Dirk Beyer, under GNU free documentation license. 

By Richard I. Gibson Here’s the podcast: Orange = Grenville rocksIn North America the primary mountain building event associated with the assembly of the Rodinia supercontinent was the Grenville Orogeny. It’s named for a village in Quebec where the mountain roots are exposed today, part of a wide belt in eastern Canada that extends into the United States in the Adirondacks of New York, and in the subsurface beneath central Ohio and Kentucky, where the Grenville Front is clearly marked in geophysical data. It continues southwest into West Texas and Mexico, and rocks that record the Grenville event are found in Scotland and South America too. What was colliding with North America to make this mountain range? That’s still an area of active research, but we think several long linear belts – possibly complicated island arcs like Japan – as well as some small continental fragments impacted to help create the supercontinent of Rodinia. Some models show blocks that are now parts of South America and South Africa impinging on North America, but we really aren’t sure. Grenville rocks in Canada are typically high-grade metamorphic rocks, indicating that there was plenty of heat and pressure involved in the collisions. The compression that created the Grenville overlapped in time somewhat with the extension that was creating the Mid-Continent Rift that we talked about a few days ago. How can you have compression and extension at the same time? It’s not hard at all. Today, the Red Sea is forming by pull-apart on the west side of the Arabian Peninsula, while a thousand kilometers away, on the east side of Arabia, active continent-continent collision is causing the earthquakes of Iran. Further reading: see the links and references in the Wikipedia article. Map by G. Mills (public domain) after Tollo and others, 2004, Petrologic and geochronologic evolution of the Grenville orogen, northern Blue Ridge province, Virginia, in Proterozoic tectonic evolution of the Grenville orogen in North America, Geological Society of America Memoir 197.