Episode 397 Carbonatites

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 396 Turbidity currents

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 395 Connections

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 19^th 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 394 The Mangrullo Formation of Uruguay

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 

Episode 393 The Mountains of the Moon

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 18^th and 19^th 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 18^th 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

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 

Episode 391 Valles Marineris

In today's episode we’re going to space. Specifically, Mars. You didn’t really think that earth science is really limited to the earth, did you? Our topic today will be the Valles Marineris.

The Valles Marineris is a long series of canyons east of Olympus Mons, the largest mountain in the solar system. These canyons are about 4,000 km long, 200 km wide and up to 7 km (23,000 ft) deep. On terrestrial scales, the Valles Marineris is as long as the distance from New York to Los Angeles. That’s about the same as Beijing to Hong Kong or Madrid to Copenhagen for our international listeners. They are as wide as central Florida, central Italy, or the middle of the Korean peninsula. Two and a half times deeper than Death Valley, though only about 60 percent of the depth of the Marianas Trench, the lowest point on earth.

Valles Marineris Image Courtesy NASA/JPL-Caltech

Not to be outdone, our planet, Earth, has even bigger valleys. These occur at the oceanic ridges, where plate spreading takes place. The longest rift valley on earth lies in the middle of the Mid-Atlantic Ridge, and it is more than double the length of the Valles Marineris. But let’s not belittle Mars. After all, while we have a pretty good idea for how oceanic rifts form on earth, there is quite a bit of debate about how Mars’ great valley formed.

The most popular theory suggests that the Valles Marineris are an analog to our oceanic rifts, and formed by the same process. As the volcanoes of the nearby Tharsis region developed, the Martian crust bowed down toward the center of the planet due to the weight of the new volcanic rocks. In time, the crust began to crack. This crack is what we see in the Valles Marineris. Unlike on Earth, this rift valley did not continue expanding, but shut down as the Tharsis Region, and Mars as a whole, cooled. Remember that unlike Earth, Mars does not have plate tectonics. It doesn’t have a continual process of hot material (like lava) rising to the surface, while relatively cold material (like the oceanic crust) is brought down towards the planet’s center.

More recent work has used satellite images, and high resolution elevation data to develop new insight into how the Valles Marineris formed. While images from the 1970’s Mariner 9 orbiter were quite blurry by today’s standards, new missions in the late 90’s to early 2000’s have given us a better view of the Martian surface than we have available for the earth. The Mars Reconnaissance Orbiter can take images where each pixel is about 0.5 m or 20 inches. That is, the color on each image is an average of an area of 0.25 square meters, or 2.5 square feet. It can then use image pairs to estimate the elevation of any point on the Martian surface with a pixel size of 0.25 m, or about 10 inches.

These new satellite images include multispectral data, or images that look at different wavelengths of light. The camera on your phone works in the same way: There are sensors that pick up, red light, green light, and blue light. Your phone records the intensity of each color in each part of the image, and then plays it back on your phone’s screen to create a picture.

Some of the satellites orbiting Mars take this to the next level. They don’t just take different slices of colored light, but also longer wavelength, infrared light. If you’ve ever seen an image from a thermal imaging camera, you know what this is. Parts of you show up as hotter or colder on the screen. It’s the same with the surface of the earth, or Mars. Scientists can compare the intensity of different wavelengths of light from each point on the surface. They can then compare these values, with what would be expected for different rock types. In other words, we’re able to roughly determine the types of rocks on the Martian surface without ever setting a boot, or rover tread, on the red planet.

Data from these images has shown that the Valles Marineris have layered rock formations both on the sides of the canyons, and within them. The great valley has seen many landslides over the last 3.5 Billion years of its existence, as well as new and smaller canyons carved into it. Scientists now speculate that rather than just forming as a big crack in the Martian surface, the Valles Marineris have been sculpted by flowing water, either in its liquid form as rivers, or in its solid form as glaciers.

An alternative hypothesis proposes that the Valles Marineris formed as a crack during a massive, planetary scale landslide. This landslide was about half the size of the US or China. How do you form a landslide that big? Well, you need a large pile of relatively weak rock, and high elevations for the landslide to flow from.

A key player here is salt. Salt is relatively weak as compared to rock, and can deform easier when squeezed. It can also hold water, which can be driven off by heating. On Earth, weak salt layers are partly responsible for undersea landslides in the Gulf of Mexico. The Opportunity rover had found some salt layers during its mission on Mars, so we know salt is present on the red planet.

Some scientists interpret the layers on the sides of the Valles Merinaris to be made of salt, and possibly include pockets of ice. This would imply that those layers are weak, and could potentially move downhill under the right circumstances.

Heating in the Tharsis region helped de-water salts under the future landslide, melted ice pockets, and created high elevations on one side of it. Think of it like putting a can on a wet metal sheet. If you raise one side of the sheet, the can will slide to the lower side. Just like that, the salty Martian crust broke, and slid downhill.

A crack in the side of this landslide allowed massive amounts of underground water to escape. As the water flowed downhill, it eroded the crack to form a massive canyon. This canyon is the Valles Marineris. The flood that helped form the Valles Marineris was probably bigger than any seen on earth. Bigger than the massive glacial outburst floods that formed the channeled scablands of the northwestern United States. Dick Gibson discussed outburst flooding in the December 27, 2014 episode. Unlike the Earth, the Martian surface has been relatively quiet since the Valles Marineris formed 3.5 billion years ago.

—Petr Yakovlev

This episode was recorded at the studios of KBMF-LP 102.5 in beautiful and historic Butte, Montana. KBMF is a local low-power radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.

Episode 390 Mud Volcanoes

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 

Cretaceous and Cenozoic Vertebrates compilation

Smilodon and dire wolves (drawing by Robert Horsfall, 1913)

Running time, 1 hour. File size, 69 megabytes.

This is an assembly of the episodes in the original series from 2014 that are about Cretaceous and Cenozoic 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!

Triassic and Jurassic Vertebrates compilation

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!

Episode 389 Vanadium

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 20^th 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.

Way back in 1971 when I was a teaching assistant for the Indiana University Geologic Field Station, on one mapping project we went to the Mayflower gold mine south of Whitehall, Montana. I collected a bunch of rocks with interesting looking sparkly crystals – some of which I’ve only recently gotten around to really studying. I gave a talk at the 2017 MontanaBureau of Mines and Geology Mineral Symposium on minerals from there that turned out to be vanadium-bearing, including vanadinite, although it’s probably an arsenic-rich variety, and stranger minerals like descloizite, a lead-zinc vanadate, tangeite, calcium-copper vanadate, and some others. I even think there are some tiny bits of roscoelite, the vanadium mica named for the chemist who first prepared vanadium metal.  

Even more exciting for me are some tiny, millimeter-sized red-orange crystals in the specimens I found at the Mayflower Mine. All I knew for a long time was that I couldn’t figure out what they were. By looking at their crystal shapes and properties, I narrowed it down to two very strange and very rare minerals – gottlobite, a calcium-magnesium vanadate, and calderónite, a lead-iron vanadate. Both of these minerals are so obscure I didn’t really seriously imagine I had actually collected one of them. But, thanks to an analysis by Stan Korzeb, the economic geologist at the Montana Bureau of Mines and Geology, it turned out that I did indeed find calderónite, 32 years before it was described as a new mineral in 2003. Stan’s analysis in January 2018 used EDX, or energy-dispersive x-ray spectroscopy, a technique that gives not only the elements present in a mineral, but their relative proportions, which allowed Stan to calculate the chemical formula. The lead-iron vanadate calderónite he found is intergrown with descloizite, a lead-zinc vanadate. This probably indicates changing iron-zinc concentrations in the fluids that precipitated the minerals. This represents just the 11^th documented calderónite occurrence in the United States and the second in Montana. Stan identified the first in Montana in the fall of 2017.

It’s an obscure mineral, and the crystals are tiny, but it made this mineral collector’s day.

—Richard I. Gibson

Link:

USGS Mineral Commodities - Vanadium (PDF) 

Episode 388 Folds in Algeria

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

Episode 387 Geology of Beer

It isn’t true that all geologists drink beer. But many do, and I’m one of them. Today I’m going to talk about the intimate connection between geology and beer.

Beer is mostly water, and water chemistry has everything to do with beer styles. And water chemistry itself depends mostly on the kinds of rocks through which the water flows. You know about hard and soft water – hard water has more dissolved chemicals like calcium and magnesium in it, and while salts of those chemicals can precipitate out of hard water, making a scum on your dishes, they also can be beneficial to development of bones and teeth. In the United States, the Midwest and Great Plains have some of the hardest water because of the abundant limestones there, and in Great Britain, southern and eastern England have harder water than Scotland for similar reasons.

But it wasn’t limestone that made Burton-upon-Trent a center of brewing in the 19^th Century, when it was home to more than 30 breweries. The water there is rich in sulfate which comes from gypsum, calcium sulfate, in the sandstone underlying the region. Those sandstones are Permian and Triassic in age, representing a time when much of the earth was arid. Those dry conditions allowed gypsum to crystallize in the sediments. Gypsum is much more soluble than limestone, and the slightly acidic waters of Burton help with that. Burton water has ten times the calcium, three times the bicarbonate, and 14 times the sulfate of Coors’ “Rocky Mountain Spring water” in Colorado. That certainly makes Coors’ Burton brewery product rather different from that made in Colorado.

In fact, the addition of gypsum to beer is called “Burtonization.” This increases the hops flavor, but more important to history, sulfates act as preservatives in beer, enough so that Burton brews of pale ales could survive the long trip to British India, giving us the India Pale Ale style of beer. Not from India, but brewed with sulfates derived from gypsum in Britain’s rocks.

That slight acidity in Burton’s water depends on the calcium and magnesium content, and also lends itself to extracting sugars from malted barley in the mashing process. Calcium and magnesium also help yeast to work its magic. Today, home brewers can buy “Burton Water Salts” to imitate the product from England.

Truman, Hanbury, Buxton & Co., Black Eagle brewery, Derby Street, Burton-upon-Trent, in 1876,
from University of London

Less hoppy beers often originated in areas where the sulfate content of the water was low. Pilsen in the Czech Republic, home to pilsner beer, has almost no sulfate and only 7 parts per million calcium in its water, compared to around 300 for Burton. Pilsen is in an area of metamorphic rocks that don’t yield the typical hard-water-making elements.

The presence of Carboniferous age limestones in Ireland make waters that are high in calcium and carbonate, but they lack the sulfate of northern England. Together with other differences, that makes the area around Dublin ideal for making a stout porter known today as Guinness.

After water, it’s the soil that makes the most difference to beer. Hops can grow in a wide range of soils, even the decomposed granite we have here in Butte, but the thick, well-drained soils of Washington and Oregon, weathered from volcanic rocks, make those states the source of 70% of the hops grown in the United States.

The surge of craft breweries in the United States has given rise to some interesting geological names for brews. Great Basin Brewing in Reno and Sparks, Nevada, has Ichthyosaur IPA, known as Icky, as well as Orogenesis, a Belgian-style amber ale. Socorro Springs, in New Mexico, brews Isopod Pale Ale and Obsidian Stout is available from Deschutes in Oregon.  You can get Triceratops Double IPA at Ninkasi Brewing in Eugene, Oregon, and Pangaea Ale at Dogfish Head in Delaware. And even though it’s more chemical than geological, we shouldn’t leave out Atomic Ale’s Dysprosium Dunkelweizen, made in Richland, Washington. Dysprosium is a rare-earth element found in the phosphate mineral xenotime and other stranger minerals.

San Andreas Brewing Company, near the fault in California, boasts Oktoberquake and Aftershock Wheat.

And I’m undoubtedly prejudiced, because I’m the House Geologist at Quarry Brewing here in Butte, which probably has the best mineral collection in a brewery in the United States, but I think their collection of geological names for their beers is unexcelled: Shale Pale Ale, Galena Gold, Open Cab Copper, and Gneiss IPA, and seasonals including Albite, Basalt, Bauxite, Calcite, Epidote, Halite, Ironstone, Porphyry, Opal Oktoberfest, Schist Sour, Rhyolite Rye Pale Ale, Pyrite Pilsner, and more. Mia the bartender and I tried to come up with a fitting name for a 50-50 mix of basalt and gneiss. I wanted it to be charnockite, but we ended up calling it Mia’s Mixture.

Next time you enjoy a beer, thank geology!

—Richard I. Gibson

More Geology of Beer 
And another from Lisa Rossbacher

Image: Truman, Hanbury, Buxton & Co., Black Eagle brewery, Derby Street, Burton-upon-Trent, in 1876 from University of London

Paleozoic Vertebrates compilation

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 386 Dynamic Topography

What is dynamic topography? Well, it depends on who you ask. Dynamic topography is similar to other terms, like uplift, that have been used in so many different ways that you really have to look at the document you’re reading to understand what the author is talking about. This term has been applied to places around the world, like the Colorado Plateau in the United States, South Africa, the Aegean, and East Asia, which makes it even more complicated to tease out its meaning.

Most broadly, dynamic topography refers to a change in the elevation of the surface of the earth in response to something going on in the mantle. This “something” can include both the flow of the mantle, as well as differences in mantle temperature or density. For the purposes of this podcast, I will use a more strict definition: Dynamic topography is the change in the elevation of the surface of the earth in response to the upward or downward flow of the mantle.

How much higher or lower can dynamic topography make the earth’s surface? Well, that’s a matter of debate. Earlier studies have suggested that several kilometers, or over 6000 feet of modern elevations can be explained by things going on in the mantle. More recent work instead suggests that dynamic topography creates changes of at most a three hundred meters, or a thousand feet.

A good example of a place where this process is thought to be active is Yellowstone. As Dick Gibson discussed in the December 19th, 2014 episode, Yellowstone is thought to be a hot spot. That is, an area of the earth where hot material moves from deep within the mantle to the base of the crust, causing significant volcanism at the surface of the earth. Other well-known hot spots are located in Hawaii, and Iceland.

So how can a hot spot like Yellowstone cause dynamic topography? Well, you’ve probably seen a similar process at play the last time you played in a pool or a lake. Think of the surface of the pool like the surface of the earth. If you start moving your hands up and down under water, the surface of the pool starts to move up and down. If you ever tried to shoot a water gun upwards underwater when you were a kid, you probably remember it pushing up the surface of the water, and being disappointed that it didn’t shoot out at your friend or sibling. As an adult, you could try holding a hose upwards in a pool. Again, it probably won’t shoot out, but will gently push upwards on the surface of the pool.

Dynamic topography concept. © Commonwealth of Australia (Geoscience Australia)
 2017, used under Creative Commons Attribution 4.0 International Licence 

The principle for a hot spot creating dynamic topography is the same. The flow of the mantle pushes upwards, warping the crust and increasing the elevation of the earth’s surface above the hot spot. Near Yellowstone, this results in an area of high elevation which lies next to the Snake River plain.

But Dynamic Topography doesn’t just cause increases in elevation, it can also pull the earth’s surface downward. In North America, dynamic topography is thought to have been in part responsible for the creation of the Cretaceous interior seaway. 

As a reminder, the Cretaceous interior seaway was a shallow sea that covered parts of western North America, in middle to late Cretaceous time, about 100 to 79 million years ago. Its size varied, but at its greatest extent the seaway stretched through Texas and Wyoming in the US, and Alberta and to the Northwest Territories in Canada. It was widest near the US-Canadian border, where it stretched from Montana to western Minnesota.

Low elevations in western North America that allowed the ocean to flood in and form this shallow sea may have been caused by downwards flow in the mantle. This downwards flow was likely caused by oceanic crust that was subducted at the western margin of North America. That is, oceanic crust that went underneath the North American plate and into the mantle. Because this crust was part of the Farallon oceanic plate, it is often referred to as the Farallon slab.

As oceanic crust associated with the Farallon plate continued to sink into the mantle, it continued to cause changes in the elevation of North America. This drop in elevation likely decreased in size as the Farallon slab moved towards the eastern edge of North America, and deeper into the mantle.

Since Eocene time, or about 55 million years ago, dynamic topography associated with the Farallon slab is thought to have been in part responsible for lower elevations in the eastern United States. A wave cut escarpment called the Orangeburg Scarp is now located 50 to 100 miles inland from the coasts of Virginia, Georgia and the Carolinas. It formed at sea level and now lies up to 50 meters, or about 165 feet above the modern coast line. In fact, a good part of the southeastern US to the east of this escarpment contains marine sediments, and smooth topography as a record of its time underwater.

Differences in the elevation of the Orangeburg Scarp along its length suggest that rather than just going up and down, the Atlantic coast experienced a broad warping caused by mantle flow. The most recent phase of warping brought this area to modern elevations, as warm material moved into the upper mantle beneath the Atlantic coast. This warm material helped push the crust up to higher elevations, creating the southeastern US as we see it today.

This example also highlights an important part of dynamic topography: If you are already at really high or really low elevations, you might not notice it much. If you are near the coast, it can have a big impact as the sea starts to flood in and out due to changes in the mantle. Provided of course, you’re there for the millions to tens of millions of years it takes for the mantle to flow this way and that. That’s why geologists typically rely on the rock record to provide evidence for processes like dynamic topography.

—Petr Yakovlev

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This episode was recorded at the studios of KBMF-LP 102.5 in beautiful Butte, Montana. KBMF is a local low-power community radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.

Links:

What is Dynamic Topography?

Surface expressions of mantle dynamics

Dynamic topography, eastern US