History of the Earth
Richard I. Gibson
We've concentrated the history of Planet Earth into one year. Follow the geology podcasts chronologically from the origin of the Earth to the origin of Mankind.
All episodes
Best episodes
Top 10 History of the Earth Episodes
Goodpods has curated a list of the 10 best History of the Earth episodes, ranked by the number of listens and likes each episode have garnered from our listeners. If you are listening to History of the Earth for the first time, there's no better place to start than with one of these standout episodes. If you are a fan of the show, vote for your favorite History of the Earth episode by adding your comments to the episode page.
Episode 395 Connections
History of the Earth
04/10/18 • -1 min
This episode is about some of the interesting connections that arise in science.
We’ll start with me and my first professional job as a mineralogist analyzing kidney stones. My mineralogy professor at Indiana University, Carl Beck, died unexpectedly, and his wife asked me as his only grad student to carry on his business performing analysis of kidney stones. Beck had pioneered the idea of crystallographic examination to determine mineralogy of these compounds because traditional chemical analysis was misleading. For example, some common kidney stones are chemically calcium phosphates and calcium carbonates – but they are hardly ever calcium carbonate minerals. That makes a big difference in terms of treatment, because calcium carbonate minerals can be dissolved with acids, while calcium phosphate cannot. The carbonate is actually part of the phosphate mineral structure, partially substituting for some of the phosphate. Other subtleties of mineral crystallography can distinguish between different minerals and can point to specific kinds of treatments, more than just chemistry can.
One of the most common minerals in kidney stones is called whewellite – calcium oxalate, CaC2O4 with a water molecule as part of its structure. In kidney stones it usually forms little rounded blobs, but sometimes the way the mineral grows, it makes pointy little things called jackstones, for their similarity to children’s’ jacks. And yes, those can be awfully painful, or so I’m told. Whewellite is really rare in the natural world beyond the urinary system, but it does exist, especially in organic deposits like coal beds. Whewellite was named for William Whewell, spelled Whewell, a true polymath and philosopher at Cambridge University in England during the first half of the 19th century. He won the Royal Medal for his work on ocean tides and published studies on astronomy, economics, physics, and geology, and was a professor of mineralogy as well.
Mary Somerville, 1834 painting by
Thomas Phillips - source Whewell coined many new words, particularly the word “scientist.” Previously such workers had been called “men of science” or “natural philosophers” – but Whewell invented the new word scientist for a woman, Mary Somerville. Somerville researched in diverse disciplines, especially astronomy, and in 1835 she became one of the first two female members of the Royal Astronomical Society, together with Caroline Herschel, discoverer of many comets and nebulae.
In 1834 Somerville published “On the Connexion of the Physical Sciences,” a synthesis reporting the latest scientific advances in astronomy, physics, chemistry, botany, and geology. William Whewell wrote a review in which he coined the word scientist for Somerville, not simply to invent a gender-neutral term analogous to “artist,” but specifically to recognize the interdisciplinary nature of her work. And even more, according to Somerville’s biographer Kathryn Neeley, Whewell wanted a word that actively celebrated “the peculiar illumination of the female mind: the ability to synthesize separate fields into a single discipline.” That was what he meant by a scientist.
Somerville was born in Scotland in 1780 and died in 1872 at age 91. Her legacy ranges from a college, an island, and a lunar crater named for her to her appearance on Scottish bank notes beginning in 2017. Besides the mineral whewellite, William Whewell is also memorialized in a lunar crater and buildings on the Cambridge campus, as well as in the word scientist, included in the Oxford English Dictionary in 1834, the same year he coined it. He died in 1866.
—Richard I. Gibson
LINK: Article about Whewell and Somerville
Episode 390 Mud Volcanoes
History of the Earth
03/06/18 • -1 min
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.
Episode 389 Vanadium
History of the Earth
02/27/18 • -1 min
Vanadium is a metal, and by far its greatest use is in steel alloys, where tiny amounts of vanadium improve steel’s hardness, toughness, and wear resistance, especially at extreme temperatures. As I reported in my book What Things Are Made Of, more than 650 tons of vanadium was alloyed with iron to make the steel in the Alaska Pipeline, and there’s no good substitute for vanadium in strong titanium alloys used in jet planes and other aerospace applications.
Vanadium isn’t exactly one of the well-known elements, but in terms of abundance in the earth’s crust, most estimates indicate that there’s more vanadium than copper, lead, or tin. But it’s difficult to isolate, and it wasn’t produced chemically as a chloride until 1830, when Swedish chemist Nils Sefström named it for the Norse goddess of beauty, Vanadis, perhaps better known as Freyja. It wasn’t until 1867 that pure vanadium metal was isolated by British chemist Henry Roscoe, whose work on vanadium won him the name of the vanadium mica roscoelite.
As a mineral collector, I’m attracted to vanadinite, lead vanadate, because it forms beautiful hexagonal crystals, often bright red and so abundant from one lead-mining area of Morocco that excellent specimens can be had without mortgaging your house. Some vanadinite crystals are like perfect little hexagonal barrels, and others can form needle-like spikes around a central crystal, making the whole thing look like a cactus with caramel-orange spines.
Some of the vanadium for making steel alloys comes from primary mined vanadinite, but much more was once produced as a by-product of phosphorous manufacture, because it’s commonly associated with phosphate rock. And today, a lot of the world’s vanadium comes from refining crude oil and from fly ash residues, which are products of coal combustion. I got curious about why vanadium metal is so closely connected with these organic deposits.
Crude oil actually has lots of trace elements in it, including metals like gold, tin, and lead, but by far the most abundant are nickel and vanadium, as much as 200 parts per million nickel and 2000 parts per million vanadium in some crude oils, especially heavy, tarry oils like those found in Venezuela. In some oil, the nickel and vanadium can add up to 1% by weight of the oil, an incredibly huge amount. Refining Venezuelan crude gave the U.S. a lot of vanadium back in the late 20th century. But why is it in there?
Oil and coal are both the result of decaying and chemically changing plant matter. Forget dinosaurs; virtually all oil, natural gas, and coal comes from plants – usually marine algae for oil and gas and more woody, land-based vegetation for coal. There’s a class of organic molecules called porphyrins. I’m no organic chemist, but these complex hydrocarbon molecules, made of carbon, hydrogen, oxygen, and nitrogen have boxy ring-like structures with open space in the centers. Chlorophyll and hemoglobin are related chemicals, both of which contain metals in the middle of the structure, magnesium in chlorophyll and iron in hemoglobin. The vacant holes in the centers of porphyrins in crude oil are ideal for trapping metal molecules, and apparently vanadium, in the form of a VO2 ion, is one of the easiest to trap because of its molecular size and electronic valence. The vanadium comes from the original oil source rock, so there’s quite a range in vanadium content around the world. Heavy oils, like the tars in Venezuela, hold more than fluid oils like those in Saudi Arabia. This has more or less been known since at least the 1920s, and today the vanadium and other metal contents of oils are being used to characterize the original source rocks even when those source rocks no longer exist or are no longer what they once were.
The United States has had no mine production of vanadium since 2013 and even then we were 94% dependent on imports. Today 100% of our vanadium is imported, and we also produce some vanadium from imported crude oil and ash. More than 90% of the world’s vanadium is mined in China, Russia, and South Africa, although the US imports much of what it needs from the Czech Republic and Canada as well as Russia. We also imported enough ash and refining residues to account for 9000 tons of vanadium in 2015, mostly going as I said to making steel alloys. A new emerging use is in high-capacity storage batteries, where vanadium compounds make the electrolyte. These batteries have potential uses for renewable energies such as wind and solar power, and although in 2015 and 2016 several companies were working on prototype designs, they’re still pretty expensive batteries.
Episode 397 Carbonatites
History of the Earth
04/30/18 • -1 min
Carbonatites are strange igneous rocks made up mostly of carbonates – common minerals like calcite, calcium carbonate. Igneous rocks that solidify from molten magma usually are high-temperature rocks containing lots of silicon which results in lots of quartz, feldspars, micas, and ferro-magnesian minerals in rocks like granite and basalt. Carbonatites crystallize from essentially molten calcite, and that’s really unusual.
Most carbonatites are intrusive, meaning they solidified within the earth, and it wasn’t until 1960 that the first carbonatite volcano erupted in historic times, proving that they form from cooling magma. The eruption at Ol Doinyo Lengai in Tanzania occurred on a branch of the East African Rift System, and most carbonatites are associated with these breaks in continental crust where eventually a new ocean may form.
Mt Lengai, Tanzania, photo by Clem23
(Creative Commons License - source) Eruptions at Lengai, whose name means “mountain of god” in the Maasai language, are the lowest-temperature magmas known because calcite melts at a much lower temperature than silica-rich compounds, around 510 degrees C versus 1000 degrees or more for most magmas. It isn’t even red-hot like most lava flows.
A simple and early interpretation of carbonatites was that they represented melting of limestone, but geochemical data indicate that they really do come from primary igneous material that probably originated in the mantle. Exactly how they form is debated, in part because they are so rare, but one idea is that they result from special cases of differentiation within more common magmas, or maybe an example of certain chemicals – the carbonates – separating out in an unusual way.
Another unusual aspect of carbonatites is the minerals associated with the dominant calcite. It’s common to get rare-earth compounds, tantalum, thorium, titanium, and many other minerals that are unusual in high concentrations in other settings. The Mountain Pass rare-earth deposit in California, once the largest producer of rare earths in the world, is in a Precambrian carbonatite. Rare earths are used in lots of modern technologies, including turbines for wind energy, batteries in electric car motors, cell phones, solar cells, and eyeglasses.
Rare earths are also produced from the Mt. Weld carbonatite in Western Australia, but it’s more famous for its tantalum, an element that’s vital in capacitors for cell phones, video games, and computers. Australia has by far the greatest reserves of tantalum, but mining didn’t begin until 2011 and production is just now ramping up. The United States, which is 100% dependent on imports for tantalum, imports most of it from Brazil, Rwanda, China, and Kazakhstan.
Magnetite is a common associated mineral in carbonatites, and at Magnet Cove, Arkansas, there’s enough to give the name to the place. It’s also rich in titanium, often in the form of the mineral rutile, titanium dioxide. When I was there on a geology field trip in 1969, I remember walking into the Kimzey Calcite Quarry. It was like walking into a giant calcite crystal, with gigantic cleavage faces the size of a person or bigger. We collected lots of cool rutile and pyrite crystals.
More common economic minerals can be associated with carbonatites as well. At one in South Africa the main products are copper and vermiculite.
While I said earlier that carbonatites are really rare, there are still a few dozen known. It’s possible that their rarity is a reflection of the fact that calcite is much more easily eroded and dissolved than the typical basaltic rocks that derive from most volcanoes, so they may simply be poorly preserved.
—Richard I. Gibson
Episode 394 The Mangrullo Formation of Uruguay
History of the Earth
04/03/18 • -1 min
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
Triassic and Jurassic Vertebrates compilation
History of the Earth
03/04/18 • -1 min
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!
Paleozoic Vertebrates compilation
History of the Earth
02/11/18 • -1 min
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!
Episode 396 Turbidity currents
History of the Earth
04/17/18 • -1 min
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
Episode 393 The Mountains of the Moon
History of the Earth
03/27/18 • -1 min
Today we’re going to the Mountains of the Moon – but not those on the moon itself. We’re going to central Africa.
There isn’t really a mountain range specifically named the Mountains of the Moon. The ancients, from Egyptians to Greeks, imagined or heard rumor of a mountain range in east-central Africa that was the source of the river Nile. In the 18th and 19th centuries, explorations of the upper Nile found the sources of the Blue Nile, White Nile, and Victoria Nile and identified the Mountains of the Moon with peaks in Ethiopia as well as 1500 kilometers away in what is now Uganda. Today, the range most closely identified with the Mountains of the Moon is the Rwenzori Mountains at the common corner of Uganda, the Democratic Republic of Congo, and Rwanda.
This location is within the western branch of the East African Rift system, an 8,000-kilometer-long break in the earth’s crust that’s in the slow process of tearing a long strip of eastern Africa away from the main continent. We talked about it in the episode for December 16, 2014.
The long linear rifts in east Africa are grabens, narrow down-faulted troughs that result from the pulling apart and breaking of the continental crust. The rifts are famously filled in places by long, linear rift lakes including Tanganyika, Malawi, Turkana, and many smaller lakes.
Virunga Mountains (2007 false-color Landsat image, annotated by Per Andersson : Source)
When rifting breaks the continental crust, pressure can be released at depth so that the hot material there can melt and rise to the surface as volcanoes. In the Rwenzori, that’s exactly what has happened. The Virunga volcanoes, a bit redundant since the name Virunga comes from a word meaning volcanoes, dominate the Rwenzori, with at least eight peaks over 10,000 feet high, and two that approach or exceed 4,500 meters, 15,000 feet above sea level. They rise dramatically above the floors of the adjacent valleys and lakes which lie about 1400 meters above sea level.
These are active volcanoes, although several would be classified as dormant, since their last dated eruptions were on the order of 100,000 to a half-million years ago. But two, Nyiragongo and Nyamuragira, have erupted as recently as 2002, when lava from Nyiragongo covered part of the airport runway at the town of Goma, and in 2011 with continuing lava lake activity. Nyiragongo has erupted at least 34 times since 1882. The volcanic rocks of these and the older volcanoes fill the rift enough that the flow of rivers and positions of lakes have changed over geologic time.
Lake Kivu, the rift lake just south of the volcanoes, once drained north to Lake Edward and ultimately to the Nile River, but the volcanism blocked the outlet and now Lake Kivu drains southward into Lake Tanganyika. Local legends, recounted by Dorothy Vitaliano in her book on Geomythology, Legends of the Earth (Indiana University Press, 1973), tell the story of demigods who lived in the various Virunga volcanoes. As demigods do, these guys had frequent arguments and battles, which are probably the folklore equivalent of actual volcanic eruptions. The stories accurately reflect – whether through observation or happenstance – the east to west migration of volcanic activity in the range.
The gases associated with the volcanic activity seep into the waters of Lake Kivu, which has high concentrations of dissolved carbon dioxide and methane. Generally the gases are contained in the deeper water under pressure – Lake Kivu is the world’s 18th deepest lake, at 475 meters, more than 1,500 feet. But sometimes lakes experience overturns, with the deeper waters flipping to the surface. When gases are dissolved in the water and the pressure reduces, they can abruptly come out of solution like opening a carbonated beverage bottle. This happened catastrophically at Lake Nyos in Cameroon in 1986, asphyxiating 1700 people and thousands of cattle and other livestock. The possibility that Lake Kivu could do the same thing is a real threat to about two million people.
The critically endangered mountain gorilla lives in the Virunga Mountains, which also holds the research institute founded by Dian Fossey. —Richard I. Gibson
Episode 392 Ophiolites
History of the Earth
03/20/18 • -1 min
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...
Show more best episodes
Show more best episodes
FAQ
How many episodes does History of the Earth have?
History of the Earth currently has 14 episodes available.
What topics does History of the Earth cover?
The podcast is about Geology, History, Natural Sciences, Earth, Podcasts and Science.
What is the most popular episode on History of the Earth?
The episode title 'Episode 397 Carbonatites' is the most popular.
What is the average episode length on History of the Earth?
The average episode length on History of the Earth is 1 minutes.
How often are episodes of History of the Earth released?
Episodes of History of the Earth are typically released every 7 days.
When was the first episode of History of the Earth?
The first episode of History of the Earth was released on Feb 11, 2018.
Show more FAQ
Show more FAQ