History of the Earth

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 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

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.

—Richard I. Gibson

Links:

Piñeiro et al. 2012 Environmental conditions

Paleogeography from Ron Blakey

Today we’re going to the Mountains of the
Moon – but not those on the moon itself. We’re going to central Africa.

There isn’t really a mountain range specifically named the
Mountains of the Moon. The ancients, from Egyptians to Greeks, imagined or
heard rumor of a mountain range in east-central Africa that was the source of
the river Nile. In the 18th and 19th centuries,
explorations of the upper Nile found the sources of the Blue Nile, White Nile,
and Victoria Nile and identified the Mountains of the Moon with peaks in
Ethiopia as well as 1500 kilometers away in what is now Uganda. Today, the
range most closely identified with the Mountains of the Moon is the Rwenzori
Mountains at the common corner of Uganda, the Democratic Republic of Congo, and
Rwanda.

This location is within the western branch of the East
African Rift system, an 8,000-kilometer-long break in the earth’s crust that’s
in the slow process of tearing a long strip of eastern Africa away from the
main continent. We talked about it in the episode for December 16, 2014.

The long linear rifts in east Africa are grabens, narrow
down-faulted troughs that result from the pulling apart and breaking of the
continental crust. The rifts are famously filled in places by long, linear rift
lakes including Tanganyika, Malawi, Turkana, and many smaller lakes.

Virunga Mountains (2007 false-color Landsat image, annotated by Per Andersson : Source)

When rifting breaks the continental crust, pressure can be
released at depth so that the hot material there can melt and rise to the
surface as volcanoes. In the Rwenzori, that’s exactly what has happened. The Virunga
volcanoes, a bit redundant since the name Virunga comes from a word meaning
volcanoes, dominate the Rwenzori, with at least eight peaks over 10,000 feet
high, and two that approach or exceed 4,500 meters, 15,000 feet above sea
level. They rise dramatically above the floors of the adjacent valleys and
lakes which lie about 1400 meters above sea level.

These are active volcanoes, although several would be
classified as dormant, since their last dated eruptions were on the order of 100,000
to a half-million years ago. But two, Nyiragongo and Nyamuragira, have erupted
as recently as 2002, when lava from Nyiragongo covered part of the airport
runway at the town of Goma, and in 2011 with continuing lava lake activity.
Nyiragongo has erupted at least 34 times since 1882. The volcanic rocks of
these and the older volcanoes fill the rift enough that the flow of rivers and
positions of lakes have changed over geologic time.

Lake Kivu, the rift lake just south of the volcanoes, once
drained north to Lake Edward and ultimately to the Nile River, but the
volcanism blocked the outlet and now Lake Kivu drains southward into Lake
Tanganyika. Local legends, recounted by Dorothy Vitaliano in her book on
Geomythology, Legends of the Earth (Indiana University Press, 1973), tell the
story of demigods who lived in the various Virunga volcanoes. As demigods do,
these guys had frequent arguments and battles, which are probably the folklore
equivalent of actual volcanic eruptions. The stories accurately reflect –
whether through observation or happenstance – the east to west migration of
volcanic activity in the range.

The gases associated with the volcanic activity seep into
the waters of Lake Kivu, which has high concentrations of dissolved carbon
dioxide and methane. Generally the gases are contained in the deeper water
under pressure – Lake Kivu is the world’s 18th deepest lake, at 475
meters, more than 1,500 feet. But sometimes lakes experience overturns, with
the deeper waters flipping to the surface. When gases are dissolved in the
water and the pressure reduces, they can abruptly come out of solution like
opening a carbonated beverage bottle. This happened catastrophically at Lake
Nyos in Cameroon in 1986, asphyxiating 1700 people and thousands of cattle and
other livestock. The possibility that Lake Kivu could do the same thing is a
real threat to about two million people.

The critically endangered mountain gorilla lives in the
Virunga Mountains, which also holds the research institute founded by Dian
Fossey.

—Richard I. Gibson