History of the Earth Richard I. Gibson

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

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

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 

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 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

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 a