History of the Earth

As near as I can tell in the original daily series in 2014, I never addressed the topic of turbidity currents and their sedimentary product, turbidites. But they account for the distribution of vast quantities of sediment on continental shelves and slopes and elsewhere. You know what turbid water is: water with a lot of suspended sediment, usually fine mud particles. In natural submarine environments, unconsolidated sediment contains a lot of water, and when a slurry-like package of sediment liquifies, it can flow down slopes under gravity, sometimes for hundreds of kilometers. It isn’t correct to think of these streams of water and sediment as like rivers on the sea floor. Rivers transport sediment, whether boulders or sand or silt or mud, through the traction, the friction of the moving water. Turbidity flows are density flows, moving because the density of the water-sediment package is greater than the surrounding water. That means they can carry larger particles than usual. Turbidite formation. Image by Oggmus, used under Creative Commons license - source Sometimes a turbidity flow is triggered by something like an earthquake, but they can also start simply because the material reaches a threshold above which gravity takes over and the material flows down slope. The amount and size of sediment the flow can carry depends on its speed, so as the flow diminishes and wanes, first the coarse, heavier particles settle out, followed by finer and finer sediments. This results in a sediment package characterized by graded bedding – the grain size grades from coarse, with grains measuring several centimeters or more, to sand, 2 millimeters and smaller, to silt and finally to mud in the upper part of the package. Repeated turbidity flows create repeated sequences of graded bedding, and they can add up to many thousands of meters of total sedimentary rock, called turbidites. Other sedimentary structures in turbidites can include ripple marks, the result of the flow over an earlier sediment surface, as well as sole marks, which are essentially gouges in the older finer-grained top of a turbidite package by the newest, coarser grains and pebbles moving across it. There are variations, of course, but the standard package of sediment sizes and structures, dominated by the graded bedding, is called a Bauma Sequence for Arnold Bouma, the sedimentologist who described them in the 1960s. Turbidity currents are pretty common on the edges of continental shelves where the sea floor begins to steepen into the continental slope, and repeated turbidity flows can carve steep canyons in the shelf and slope. Where the flow bursts out onto the flatter abyssal sea floor, huge volumes of sediment can accumulate, especially beyond the mouths of the great rivers of the world which carry lots of sediment. When the flow is no longer constrained by a canyon or even a more gentle flow surface, the slurry tends to fan out – and the deposits are called deep abyssal ocean fans. They are often even shaped like a wide fan, with various branching channels distributing the sediment around the arms of the fan. The largest on earth today is the Bengal Fan, offshore from the mouths of the Ganges and Brahmaputra Rivers in India and Bangladesh. It’s about 3,000 km long, 1400 km wide, and more than 16 km, more than 10 miles, thick at its thickest. It’s the consequence of the collision between India and Eurasia and the uplift and erosion of the Himalaya. The scientific value of turbidites includes a record of tectonic uplift, and even seismicity given that often turbidity currents are triggered by earthquakes. They also have economic value. Within the sequence of fining-upward sediments, some portions are typically very well-sorted, clean sandstones. That means they have grains of uniform size and shape and not much other stuff to gum up the pores between the sand grains – so that makes them potentially very good reservoirs for oil and natural gas. You need the proper arrangements of source rocks, trapping mechanisms, and burial history too, but deep-water turbidites are explored for specifically, and with success, in the Gulf of Mexico, North Sea, offshore Brazil and West Africa, and elsewhere. The Marlim fields offshore Brazil contained more than 4 billion barrels of producible oil reserves when they were discovered in the 1980s. Ancient turbidites sometimes serve as the host rocks for major gold deposits, such as those at Bendigo and Ballarat Australia, which are among the top ten gold producers on earth. —Richard I. Gibson

Today we’re going back about 280 million years, to what is now Uruguay in South America. 280 million years ago puts us in the early part of the Permian Period. Gondwana, the huge southern continent, was in the process of colliding with North America and Eurasia to form the supercontinent of Pangaea. South America, Africa, Antarctica, India, and Australia had all been attached to each other in Gondwana for several hundred million years, and the extensive glaciers that occupied parts of all those continents were probably still present in at least in highlands in southern South America and South Africa, as well as Antarctica. But the area that is now in Uruguay was probably in cool, temperate latitudes, something like New Zealand or Seattle today. The connection between southern South America and South Africa was a lowland, partially covered by a shallow arm of the sea or perhaps a broad, brackish lagoon at the estuary of a major river system that was likely fed in part by glacial meltwater from adjacent mountains. We know the water was shallow because the rocks preserve ripple marks produced by wave action or currents. The basin must have been near the shore because delicate fossils such as insect wings and plants are among the remnants. It looks like this shallow sea or lagoon became cut off from the ocean, allowing the waters to become both more salty, even hypersaline, and anoxic, as the separation restricted inflows of water, either fresh or marine, that could have continued to oxygenate the basin. In the absence of oxygen, excellent preservation of materials that fell to the basin floor began, and there were few or no scavenging animals to disrupt the bodies. The rocks of the Mangrullo Formation, as it’s called today, include limestones and siltstones, but the most important for fossil preservation are probably the extremely fine-grained claystones and oil shales. These rocks contain some of the best preserved fossil mesosaurs known anywhere. That’s mesosaurs, not the perhaps more well-known mosasaurs, which are large whale-like marine reptiles that lived during Cretaceous time. Here, we’re in the Permian, well before the first dinosaurs. Mesosaur by Nobu Tamura (Creative Commons license & source)  Mesosaurs were aquatic reptiles, and they are the earliest known. They evolved from land reptiles and were among the first to return to the water to adopt an aquatic or amphibious lifestyle. They were once thought to be part of a sister group to reptiles, a separate branch of amniotes, which are animals that lay their eggs on land or bear them inside the mother, like most mammals do. In that scheme, mesosaurs and reptiles would have diverged from a common, earlier ancestor. But more recent studies categorize them as reptiles that split off from the main genetic stem early in the history of the class, so they’re pretty distant cousins to dinosaurs and all modern reptiles, but they’re still reptiles. There is ongoing debate among evolutionary paleontologists as to exactly where mesosaurs fit. The fossils in Uruguay are so well preserved that we can identify the gut materials of mesosaurs, and we know they mostly ate crustaceans, aquatic invertebrates related to crabs, shrimp, and lobsters. The preservation is so exceptional that in some cases, soft body parts are preserved including major nerves and blood vessels in mesosaurs and stomachs and external appendages in the crustaceans. The earliest known amniote embryos also come from these fossil beds. Mesosaurs had a short run in terms of their geologic history, only about 30 million years. They were extinct about 270 million years ago, well before the great extinction event at the end of the Permian, 250 million years ago. But the presence of coastal-dwelling mesosaurs in both South America and Africa was a contributing idea in the early development of the theory of continental drift, since it was presumed that they could not have crossed the Atlantic Ocean as it is today. —Richard I. Gibson Links: Piñeiro et al. 2012 Environmental conditions  Paleogeography from Ron Blakey 

Today’s episode focuses on one of those wonderful jargon words geologists love to use: Ophiolites. It’s not a contrived term like cactolith nor some really obscure mineral like pararammelsbergite. Ophiolites are actually really important to our understanding of the concept of plate tectonics and how the earth works dynamically. The word goes back to 1813 in the Alps, where Alexandre Brongniart coined the word for some scaly, greenish rocks. Ophiolite is a combination of the Greek words for snake and stone, and Brongniart was also a specialist in reptiles. So he named these rocks for their resemblance to snake skins. Fast forward about 150 years, to the 1960s. Geophysical data, deep-sea sampling, and other work was leading to the understanding that the earth’s crust is fundamentally different beneath the continents and beneath the oceans—and we found that the rocks in the oceanic crust are remarkably similar to the greenish, iron- and magnesium-rich rocks that had been labeled ophiolites long ago and largely ignored except by specialists ever since. Those rocks that form the oceanic crust include serpentine minerals, which are soft, often fibrous iron-magnesium silicates whose name is yet another reference to their snake-like appearance.  Pillow basalts, iron-rich lava flows that solidify under water with bulbous, pillow-like shapes, are also typical of oceanic crust. The term ophiolite was rejuvenated to apply to a specific sequence of rocks that forms at mid-ocean ridges, resulting in sea-floor spreading and the movement of plates around the earth. The sequence usually but not always includes some of the most mantle-like minerals, such as olivine, another iron-magnesium silicate, that may settle out in a magma chamber beneath a mid-ocean ridge. Shallower, relatively narrow feeders called dikes toward the top of the magma chamber fed lava flows on the surface – but still underwater, usually – that’s where those pillow lavas solidified. There are certainly variations, and interactions with water as well as sediment on top of the oceanic crust can complicate things, but on the whole that’s the package. So why not just call it oceanic crust and forget the jargon word ophiolite? Well, we’ve kind of done that, or at least restricted the word to a special case. Pillow Lava off Hawaii. Source: NOAA.  The word ophiolite today is usually used to refer to slices or layers of oceanic crust that are on land, on top of continental crust. But wait, you say, you keep saying subduction is driven by oceanic crust, which is denser, diving down beneath continental crust, which is less dense. Well, yes – but I hope I didn’t say always. Sometimes the circumstances allow for some of the oceanic crust to be pushed up over bits of continental crust, despite their greater density. One area where this seems to happen with some regularity is a setting called back-arc basins, which are areas of extension, pulling-apart, behind the collision zone where oceanic crust and continental crust come together with the oceanic plate mostly subducting, going down under the continental plate. It took some time in the evolution of our understanding of plate tectonics for the idea to come out that you can have significant pulling apart in zones that are fundamentally compression, collision, but they’re recognized in many places today, as well as in the geologic past. It seems to me that back-arc basins are more likely to develop where the interaction is between plates or sub-plates that are relatively weak, or small, and more susceptible to breaking. An example is where two oceanic plates are interacting, with perhaps only an island arc between them. The “battle” is a closer contest than between a big, strong continent and weaker, warmer, softer, oceanic crust, so slices of one plate of oceanic crust may be squeezed up and onto the rocks making up the island arc. This happens in the southwest Pacific, where the oceanic Pacific Plate and the oceanic part of the Australian Plate are interacting, creating back-arc basins around Tonga and Fiji and elsewhere. It also happens where continental material is narrower, or thinner, or where the interaction is oblique or complex. One example of this today is the back-arc basin in the Andaman Sea south of Burma, Myanmar, where the Indian Ocean plate is in contact with a narrow prong of continent, Indochina and Malaya. We’ve now recognized quite a few ophiolites on land, emplaced there long ago geologically. At Gros Morne National Park in Newfoundland, the Bay of Islands ophiolite is of Cambrian to Ordovician age. The area is a UNESCO World Heritage Site for the excellent exposures of oceanic crust there, not to mention fine scenery. On Cyprus, the Troodos Ophiolite represents breaking within the Tethys Oceanic plate as it was squeezed between Gondwana, or Africa, and the Anatolian block of Eurasia, which is today’s Turkey. The Troodos Ophiolite is rich in copper sulfides that were probably deposited from vents on a mid-ocean ridge. In fact, the name Cyprus is the origin of our word copper, by way of Latin cuprum and earlier cyprium.   On the island of New Caledonia, east of Australia and in the midst of the messy interactions among tectonic plates large and small, the ophiolite is rich in another metal typical of deep-crust or mantle sources: nickel. There’s enough to make tiny New Caledonia tied with Canada for third place as the world’s largest producer of nickel, after Indonesia and the Philippines. There’s a huge ophiolite in Oman, the Semail Ophiolite, covering about a hundred thousand square kilometers. It’s one of the most compete examples anywhere, and it was pushed up on to the corner of the Arabian continental block during Cretaceous time, around 80 million years ago. Like the one in Cyprus, this one is also rich in copper as well as chromite, another deep-crustal or mantle-derived mineral. The Coast Range Ophiolite in California is Jurassic, about 170 million years old, and formed at roughly the same time as the Sierra Nevada Batholith developed as a more standard response to subduction. It’s likely that western North America at that time was somewhat like the southwestern Pacific today, with strings of island arcs, small irregular continental blocks, and diverse styles of interaction – the perfect setting for a long band of oceanic crust to be pushed up and over other material. The whole thing ultimately got amalgamated with the main North American continent. I talked a bit more about these events in the episode on the Franciscan, November 7, 2014. —Richard I. Gibson LINKS:  Nice images from Oregon State  Oman Virtual Field Trip 

As the name implies, mud volcanoes are eruptions of mud – not molten rock as in igneous volcanoes.  They’re found all around the world, amounting to about a thousand in total number known. The one thing they have in common is hot or at least warm water, so they occur in geothermal areas especially, but they also are found in the Arctic. They range in size from tiny, just a few meters across and high, to big things that can cover several square miles. In Azerbaijan some mud volcanoes reach 200 meters, 650 feet, in height, and around the world many of them do have conical, volcano-like shapes. But there are others that are just low mounds, more like a shield volcano. A little (15-cm) mud volcano in New Zealand. Photo by Richard Gibson. The mud is often enough just a slurry of suspended fine-grained sediment that mixes with the hot water. And by hot water, we don’t necessarily mean incredibly hot – mud volcano temperatures as cold as a couple degrees Centigrade are known, but most are associated with temperatures approaching the boiling point of water.  In some places, like Yellowstone, the water is acidic which helps it dissolve rocks down to the tiny fragments in mud, and in other places it may just be the weathered soil and debris picked up by the water that makes the mud. Mud volcanoes can erupt violently, or seep slowly, and emissions can last from minutes to years. I think it’s fair to think of some of them as geysers in which the water contains a lot of sediment, while others are more like thick, viscous muddy warm springs. Besides water and fine sediment, mud volcanoes often contain natural gas – most commonly methane, but sometimes carbon dioxide, nitrogen, or other gases. The pressure of these gases is often the driving force behind eruptions, and with a hydrocarbon gas like methane present you might think mud volcanoes would be associated with oil and gas fields, and you’d be right. The hundreds of mud volcanoes in Azerbaijan and in the adjacent Caspian Sea are in the midst of the first great oil province to be exploited, and some of the petroleum deposits there are related to structures in the rocks and sediments caused by the upward force of the mud, which can bend its confining rocks as it rises, just as a salt dome can do. And since methane is flammable, often enough there are flames associated with mud volcanoes. In 2001, near Baku, Azerbaijan, flames shot 15 meters, near 50 feet, into the air. Gobustan in Azerbaijan is a World Heritage Site for its abundant rock carvings dating to 5000 to 20,000 years ago or more. The flaming methane eruptions of mud volcanoes in Azerbaijan have been linked to the development of the Zoroastrian religion, and in fact the name Azerbaijan derives from words meaning Land of the Eternal or Sacred Fire. The most destructive mud volcano eruption in recent years was on the island of Java, in Indonesia, in May 2006. It erupted in the middle of a rice paddy, and ultimately killed 20 people, caused nearly 3 billion dollars in damage, and displaced 60,000 people. The mud it erupted covers about seven square kilometers, nearly three square miles, and in 2018 it continues to erupt something like 80,000 cubic meters of mud every day – that’s almost 3 million cubic feet, 32 Olympic swimming pools each day. What caused the violent and extensive eruption of the Lusi Mud Volcano, also called the Sidoarjo mud flow, on Java is not clear. It may be simply part of the ongoing natural tectonic and magmatic processes in the region, which is dotted with many real volcanoes, the kind that carry molten rock to the surface as lava, and there’s a fault system that may provide a conduit for hot water from a volcano about 50 kilometers away. Lusi may be an entirely natural phenomenon. But there are also interesting possible trigger mechanisms. One suggests that a large earthquake two days before the mud volcano erupted changed the plumbing system enough to spur the eruption. That’s reasonable, since we know that earthquakes can have significant effects on geyser systems. Old Faithful in Yellowstone changed its eruption period following the strong Hebgen Lake earthquake in 1959. The other possible trigger is nearby drilling by a gas exploration company, which may have encountered an open pocket of gas or some other feature that ultimately may have allowed enough pressure to build up to make the mud volcano erupt. Good science on all sides of this issue have not resolved its origin with certainty, but on the whole I think the consensus is that the mud eruption was indeed triggered by the drilling. Studies continue, and there are legal cases in progress too, of course. Sidoarjo Mud Flow, Indonesia, 2008 NASA image created by Jesse Allen, using data from NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team. Caption by Michon Scott, based on interpretation by Geoffrey S. Plumlee, U.S. Geological Survey Crustal Imaging and Characterization Team. Source  Another mud volcano that was recently in the news is in Taiwan. Taiwan has at least 17 mud volcanoes which have been known for centuries, and the flammable natural gas associated with them was used in brick-making in southern Taiwan. The gas is probably methane, and it sometimes ignites naturally. The Wandan mud volcano in this area has a sporadic history, dormant for 9 years in the 1980s but erupting with damage in 2011 and 2016. Taiwan is on the subduction zone between the Philippine plate and Eurasia, complicated by a change in orientation of the subduction zone where Taiwan sits. This complex tectonic setting, together with the heat liberated by subduction, is probably the ultimate cause of the earthquakes, geologically recent volcanism, and the mud volcanoes on Taiwan. Mud volcano eruptions are probably no more predictable than real volcanoes or earthquakes, but their similarity to geysers might give at least an element of predictability to them. A mud volcano that erupted in Trinidad in February 2018 seems to have a period of about 25 to 30 years, but that’s obviously a pretty wide range. The most recent event at Trinidad’s Devils Woodyard mud volcano covered an area about 100 meters across and tossed mud six meters into the air. Like the features in Azerbaijan, the mud volcanoes in Trinidad are closely associated with hydrocarbon deposits, including Trinidad’s famous pitch lake – thick tarry oil at the surface of the land. Most of the hot mud activity in Yellowstone isn’t really what you’d call mud volcanoes. It’s more boiling mud-rich hot springs like the Fountain Paint Pots, but every now and then they can make small cones, less than a meter high, and in the past there have been mud-rich geyser eruptions at Yellowstone. By some estimates there are many more mud volcanoes on the sea floor than there are on land. The known offshore mud volcanoes are often associated with methane hydrates – methane gas frozen into ice in the sediment beneath the sea floor. So it would be no surprise that as those ice-methane complexes melt they might drive the development of mud volcanoes underwater. —Richard I. Gibson Links: Trinidad  Taiwan  Indonesia 

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.Drawing by FunkMonk (Michael B. H.) used under Creative Commons license.  Running time, 1 hour. File size, 68 megabytes. This is an assembly of the episodes in the original series from 2014 that are about Triassic and Jurassic vertebrates. As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog. Thanks for your interest and support!

You may have seen some of the spectacular images of the earth in southern Algeria, curves and colors like some Picasso in the opposite of his cubist period. If you haven’t, check out the one from NASA, below.  The ovals and swirls, with their concentric bands, are immediately obvious to a geologist as patterns of folds, but not just linear folds like many anticlines and synclines form. These closed ovals represent domes and basins – imagine a large scale warping, both up and down, in a thick succession of diverse sedimentary rocks, like sets of nested bowls, some of them right-side up and some inverted, then all sliced off halfway through. But “obvious to a geologist” has plenty of limitations in a space image. Without knowing more information, it’s difficult to be sure if an oval is a basin or a dome. And you can speculate, but without some ground truth, it’s challenging to be sure what the rock types are. Ahnet-Mouydir, Hoggar Mountains, Algeria. NASA image - source This area, called the Ahnet-Mouydir, on the flank of the Hoggar Mountains close to the middle of the Sahara Desert, is remote, inhospitable, and arid, and called the “land of terror” for a reason. The rocks represent a thick sequence of marine sandstones, shales, and limestones, spanning a huge range of ages, from at least the Ordovician to the early Carboniferous – 150 million years or more, a great chunk of the Paleozoic era. The core of the Hoggar Mountains is an old Precambrian block, not as big as the cratons and shields that form the hearts of most of the continents, but otherwise similar. It might have been something like a microcontinent that became amalgamated into the growing supercontinent of Gondwana about 600 million years ago. After that amalgamation, seas came and went much like they did in western North America throughout much of the Paleozoic era, laying down the sediments that became the rocks we see today in the northern Hoggar Mountains. That’s all well and good – but here’s the next question, how did the rocks get deformed into these oval domes and basins? If you imagine the kinds of collisions that are typical on earth, you think of linear or curvilinear things – island arcs, edges of continents and such – that when they collide, are likely to make linear belts of deformation. This is why so many mountain ranges are long, linear features, and the folds and faults that make them up also tend to be linear. Domes and basins happen, but that seems to be almost all we have here in these mountains. We have to look for a deformational event that is later than the youngest rocks deformed. So if some of these rocks are as young as early Carboniferous, about 340 million years old, the mountain-building event that fills the bill is the Hercynian Orogeny, where ‘orogeny’ just means mountain-building. The Hercynian, at about 350 to 280 million years ago, represents the complex collision between Gondwana and the combined North America and Europe, which were already more or less attached to each other. The leading edge of Gondwana that collided was in what is now North and West Africa, and the collision produced mountain ranges all over – the Alleghenies in the central Appalachians in North America, and a complex swath of mountains across central Europe, from Spain, across France to northern Germany and into Poland, as well as elsewhere. In Africa, the most intense squeezing was at the leading edge, in what is now Morocco and Mauritania, colliding with North America, and northern Algeria, impacting Iberia. The basins and domes of southern Algeria that we’re trying to understand are 1500 kilometers or more from that leading edge of continental collision. So I think – and full disclosure, I’ve never really researched this area in detail – that what must have happened is that that distant hinterland wasn’t pushed into tight, linear belts like those we find along the lines of collision, but the force was enough to warp the sediments into these relatively small domes and basins. Alternatively, it might be possible that the brittle Precambrian rocks beneath the sedimentary layers broke from the force of the collision, so that the sedimentary layers draped over the deeper brittle surface like a carpet lying over a jumble of toy building blocks – some high, some low. The latter idea, that the brittle basement rocks were broken and pushed upward with the sedimentary layers draped over them is supported by research published in the journal Terra Nova in 2001. Hamid Haddoum and colleagues studied the orientations of folds and faults in this area, trying to figure out the orientations of the stresses that caused them. Their data show a shortening direction – which means compression, or squeezing – during early Permian time oriented about northeast-southwest. That is consistent with the collision that was happening at that same time between what is now Senegal and Mauritania, in westernmost Africa, and the Virginia-Carolinas region of what is now the United States. Haddoum and his colleagues show cross-sections with basement upthrusts, basically high-angle reverse faults where older rocks are squeezed so much that they are pushed up and over younger rocks. This is quite similar to the Laramide Orogeny in the western United States about 80 to 50 million years ago, but this compression was happening about 280 million years ago as the supercontinent of Pangaea was assembled during the early Permian Period. Both represent deformation at relatively great distances from the lines of continental collision. In the case of the Laramide in western United States, one idea for transmitting the stress so far from the collision is that the subducting slab of oceanic crust began to go down at a relatively gentle angle, even close to horizontal, creating friction and stress further away from the subduction zone than normal. Whether that’s the case here in southern Algeria isn’t clear for this Hercynian collision. I wouldn’t think of this area as high mountains, such as those that must have formed along the lines of Hercynian collision. Maybe more like warped, uplifted plateaus – but whatever they were, they were certainly subject to erosion. Erosion probably wore the domes and basins down to a common level, so that the nested bowls were exposed in horizontal cross-section – which for geologists is the equivalent of a geologic map. And that’s what the beautiful photos reveal. The area might have been planed off even more by Permian glaciers during and after the Hercynian mountain-building events. But then, during the Mesozoic era, seas returned to the region and all this mess of eroded domes and basins was buried beneath even more sediments. Sometime relatively recently, during the Cenozoic era, the past 65 million years, everything was uplifted at least gently, so that the highest parts – including today’s Hoggar Mountains, were stripped of the younger Mesozoic sedimentary rocks, revealing the much older Paleozoic rocks in the domes and basins. —Richard I. Gibson Link:  Haddoum, H., Guiraud, R. and Moussine-Pouchkine, A. (2001),Hercynian compressional deformations of the Ahnet–Mouydir Basin, AlgerianSaharan Platform: far-field stress effects of the Late Palaeozoic orogeny.Terra Nova, 13: 220–226. doi:10.1046/j.1365-3121.2001.00344.x

Ganoid fish from an old textbook (public domain) Running time, 1 hour. File size, 70 megabytes. This is an assembly of the 15 episodes in the original series from 2014 that are about Paleozoic vertebrates. I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog. Thanks for your interest and support!