The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Monday, April 30, 2018

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

Tuesday, April 17, 2018

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

Tuesday, April 10, 2018

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 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