News from Afar

I've written here before about the fact that the continents are in motion, something that is only not staggering because we've all known about it since ninth grade Earth Science class.  You can easily see why it took so long to accept.  First, the motion is so slow that it was, for most of human history, beyond the limitations of the technology available at the time to measure directly.  Second, it's just hard to imagine.

Continents?  Moving in solid rock?  What?

But move they do, and it's because if you go down far enough, the rock isn't solid.  Get into the upper mantle, and it's the consistency of taffy, so it flows, pushed by subterranean convection currents.  Those currents create drag forces on the undersides of the tectonic plates, shifting them around.  Although this is an oversimplification, in general, there are three ways that plates can move relative to each other:

• Convergent zones, where plates come together.  When thin, brittle oceanic plates are pushed toward each other, one usually bends and slides under the other at a thrust fault or subduction zone; the subducted plate and the sediment riding on it eventually melt, and the hot, water-rich magma rises to form chains of volcanoes parallel to the fault.  Examples are the Japan Trench and the Sumatra Trench.  When an oceanic plate collides with a thick, cold continental plate, you still get volcanoes boring their way up through the continent -- this is the origin of the Cascade Range.  If it's two continental plates colliding, the rock simply crumples up to form mountains -- such as what is happening in the Alps and Himalayas,
• Divergent zones, where plates move apart.  This is what's happening along the Mid-Atlantic Ridge, and is why the island of Iceland is volcanic -- the eastern and western halves of the island are moving apart, and new basaltic lave bubbling up to fill the gap.

A photograph I took at Meradalir Volcano in Iceland, August 2022

• Strike-slip faults, or transform faults, which occur when plates slide in opposite directions parallel to the fault.  Examples are the San Andreas, Hayward, and Elsinore Faults in California, and the Alpine Fault in New Zealand.

All of these movements can significantly transform the shapes and positions of the continents -- you probably know that 250 million years ago, most of the Earth's land masses were assembled into a giant supercontinent (Pangaea), and the seas into a massive superocean (Panthalassa), with huge consequences to the climate.  Fascinating to realize, though, that Pangaea was only the most recent of the supercontinents; geologists believe that the same lumping-it-all-together occurred at least three or four times before then.

And the reverse can happen, too, when a divergent zone forms underneath a continent, and it tears the land mass in two.  In fact, this is the reason the topic comes up today; a paper last week in Nature Geoscience about the Afar Triple Junction, the point where three faults meet at one point (the Red Sea Rift, the Aden Ridge, and the East African Rift).  Geologists have found that underneath this region, there's a mantle plume -- an upwelling of very hot magma -- that is pulsing like a giant beating heart, driving convection that will eventually tear Africa in two, shearing off a chunk from Ethiopia to Mozambique and driving it east into the Indian Ocean.

[Image licensed under the Creative Commons Val Rime, Tectonic African-Arabian Rift System, CC BY-SA 4.0]

"We have found that the evolution of deep mantle upwellings is intimately tied to the motion of the plates above," said Derek Keir, of the University of Southampton, who co-authored the study.  "This has profound implications for how we interpret surface volcanism, earthquake activity, and the process of continental breakup...  The work shows that deep mantle upwellings can flow beneath the base of tectonic plates and help to focus volcanic activity to where the tectonic plate is thinnest.  Follow on research includes understanding how and at what rate mantle flow occurs beneath plates."

The formation of a new sea -- and the consequent turning of much of east Africa into an island -- isn't exactly what I'd call "imminent;" it's predicted that the Red Sea will breach the Afar Highlands and flood the lowest points of the rift (much of which is already below sea level) in something like five million years.  The region will be highly tectonically active throughout the process, however, and there'll be enough volcanoes and earthquakes in the meantime to keep us interested.

It's a good reminder that although mountains and oceans have been a symbol of something eternal and unchanging, in reality everything is in flux.  It recalls to mind the lines from Percy Shelley's evocative poem "Mont Blanc," which seems a fitting way to end:

Yet not a city, but a flood of ruin
Is there, that from the boundaries of the sky
Rolls its perpetual stream; vast pines are strewing
Its destin’d path, or in the mangled soil
Branchless and shatter’d stand; the rocks, drawn down
From yon remotest waste, have overthrown
The limits of the dead and living world,
Never to be reclaim’d.  The dwelling-place
Of insects, beasts, and birds, becomes its spoil;
Their food and their retreat for ever gone,
So much of life and joy is lost.  The race
Of man flies far in dread; his work and dwelling
Vanish, like smoke before the tempest’s stream,
And their place is not known.  Below, vast caves
Shine in the rushing torrents’ restless gleam,
Which from those secret chasms in tumult welling
Meet in the vale, and one majestic River,
The breath and blood of distant lands, for ever
Rolls its loud waters to the ocean-waves,
Breathes its swift vapours to the circling air.

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The oddest volcano on Earth

As far as we've progressed in our understanding of science, we still have a long way to go.

This is true in every scientific endeavor, but I was thinking about it apropos of geology while reading volcanologist Tamsin Mather's wonderful recent book Adventures in Volcanoland: What Volcanoes Tell Us About the World and Ourselves.  Mather's fascinating and often lyrical narrative takes us all over the world, describing her studies of volcanoes in Nicaragua, Greece, Hawaii, Iceland, Japan, Indonesia, Wyoming, Sicily, Ethiopia, and Tanzania.  It was this last-mentioned that got me pondering the gaps in our understanding, because she described seeing the bizarre Ol Doinyo Lengai volcano in the northern part of the country.

Ol Doinyo Lengai is strange by any standards.  It sits in the middle of the East African Rift Zone, which extends from Ethiopia to Mozambique, and is one of the Earth's only above-water divergent zones, places where two pieces of crust are moving apart.  The Rift being on land won't last forever, of course; ultimately a crescent-shaped chunk of East Africa will cleave off from the rest of the continent, the ocean will flood in, and afterward the rift will (like most of the others) lie at the bottom of the sea floor.  The separated piece will then creep off to the east, becoming an island -- or, depending on how you define it, a new continent.

The violent geology of the region has created a topography that in a post last year I referred to as "a beautiful hellscape."  The Dallol Depression is already 48 meters below sea level; the only thing keeping the water out is the Afar Highlands to the east acting as a barrier.  It's not only filled with bubbling mud pots and hot springs, but its position near the equator and the surrounding mountains creating a rain shadow make it blisteringly hot -- think Death Valley in midsummer -- so despite the otherworldly beauty of the brilliantly-colored rocks, it's not a place most people would ever think of going.

But even by comparison to the strange landscape that surrounds it, Ol Doinyo Lengai is peculiar.  It is the only active volcano on Earth that produces carbonatite lava, which (as you might surmise) is rich in carbonate minerals like calcite, dolomite, sodalite, apatite, and ancylite.  The magma is cool by volcanic standards, at only around 500-600 C, and yet produces some of the most fluid, low-viscosity lava flows known, moving at around five meters per second -- so about a typical human running speed.

Staying out of the way when Ol Doinyo Lengai erupts is a really good idea.

The lava comes out dark brown or black, but once it freezes and is exposed to the air for a few days, carbonate minerals crystallize on the surface and turn it a frosty white.

[Image licensed under the Creative Commons Thomas Kraft, Kufstein, Lava lengai, CC BY-SA 3.0]

So the entire cone has a snow-covered appearance, but there's no snow there -- it's all mineral deposits.

The weirdest thing is that we don't really understand where all this carbonatite magma is coming from.  My first guess was that somehow the plumbing of the volcano was moving up through a limestone or marble deposit, and picking up the carbonates as it went, but geochemical analysis of the rocks produced from it seems to have ruled that out.  Right now, it's thought to be some kind of weird fractionation -- the source magma deep underground is separating into high-carbon and low-carbon bits, and the high-carbon bits are the ones feeding this particular volcano -- but the fact is, this is a guess, and the word you see most often attached to its mineralogy and chemistry is "peculiar."

So what's happening here in northern Tanzania -- and even more apposite, why it isn't happening anywhere else on Earth -- is a mystery.  The strangeness, though, only increases its fascination, and there are geologists who are devoting a lot of time to figuring out what is going on in this odd and inhospitable place.  Strange, too, that the East African Rift Valley is where humanity got its start; Oluduvai Gorge, where some of the best-preserved hominin fossils were found, is part of the Rift complex. 

Perhaps there's a reason we're drawn to this mysterious spot.  Our roots are here -- in one of the most tectonically-active places on the planet.  That it still leaves us with unanswered questions only makes the draw stronger, bringing us back to the place our distant kin left a hundred thousand years ago, to use the tools of science to finally understand our own ancestral home.

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A beautiful hellscape

One of the strangest places on Earth is in eastern Ethiopia.  Not that many people have even heard of it, for the very good reason that if you go there, there are about a million and one ways you could die.

It's called Dallol, which comes from a word in the Afar language meaning "disintegration."  The name comes from the fact that this is what would happen to you if you went for a swim there.  It lies in the Danakil Depression, and is a maze of hot springs, filled with water that gets up to 95 C and can have a pH of less than 1.  It's surrounded by evaporite plains covered with layers of magnesium, calcium, and iron oxide, crystalline salt, and elemental sulfur.

The place doesn't even look real:

[Image licensed under the Creative Commons Kotopoulou Electra, The hydrothermal system of Dallol, CC BY-SA 4.0]

The highest elevation in Dallol is 48 meters below sea level.  The region gets ridiculously hot -- think Death Valley in midsummer -- so tourism, even if you were so inclined, is pretty much out of the question.

Where it gets even more interesting is why, if the place is entirely below sea level, it's not under water.  And this has to do with the geology of the region, and how it was created in the first place.

Dallol and the Danakil Depression are part of the East African Rift System, which formed in the Miocene Epoch on the order of fourteen million years ago.  Basaltic magma upwelling from the mantle created a crack in the Earth's crust and began to fracture the African Plate.  This generated a long rift valley running more-or-less northeast to southwest, from the shore of the Red Sea in Ethiopia, under Lake Victoria, then southward through Tanzania and all the way to Malawi.  The entire thing is seismically active, but the north end especially so, experiencing nearly constant earthquakes and volcanic eruptions -- not to mention a huge amount of hydrothermal activity, such as you see at Dallol.

The water of the Red Sea is currently being held back by the barrier of the Erta Ale Range, which blocks the East African Rift Valley on its northeastern end.  Eventually, though, the barrier will be breached as the rift continues to open up, and the water will come pouring in.  At that point, all of Dallol and the Danakil Depression -- and a large part of the rest of the valley -- will be an inlet of the Indian Ocean.

That won't stop the rift from continuing to spread, though.  The entire "Horn of Africa" will separate from the rest of the continent and go sliding off to the east.  As I've pointed out before, it's only our short life spans that make us think the current configuration of continents is permanent.

For now, though, the Erta Ale Range is holding the ocean back, allowing us to take a look at one of the most inhospitable places on Earth.  What I find most curious is that a part of this same system of rifts -- farther south, in Kenya and Tanzania -- is thought to be the cradle of humanity.  Much of the history of our earliest ancestors, species like Paranthropus and Australopithecus and Ardipithecus, took place here.  Somehow they dealt with the heat and drought and seismic activity (as well as the predators), surviving long enough to evolve into Homo sapiens, who then pretty much rushed out and took over the whole planet.

Odd to think that a beautiful hellscape was where humanity first got its start.

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Moving mountains

I live in a place that doesn't change very much, and I mean that not only in the human sense -- rural upstate New York is not exactly a center of urban development -- but even on the geological time scale.  The bedrock here is Devonian shale, slate, and limestone, on the order of three hundred million years old.  At the point this rock was forming, where I'm sitting right now would have been at the bottom of a shallow subtropical ocean.  Since then, things have dried out a tad, and it's no longer anywhere near subtropical.  There have been a few glaciers in the last few million years; the most recent one started to melt back about 75,000 years ago.  This left behind the Elmira Moraine, only thirty miles south of where I live -- a rubble pile that marks the southern edge of where the glacier pushed rocks and debris ahead of it like a plow.  (The gazillions of rocks of various descriptions that I curse every time I try to dig in my yard are gifts from that last glacier -- glacial erratics -- some of them carried hundreds of miles away from where they formed.)

Other than that, I live in a pretty calm part of the world.  It'd be easy to look around and think the world is static, that the way things are now is the way they'll always be.

Not so in some places.  There are areas of the world where people are well aware that the topography can change in an instant from earthquakes or volcanoes.  Unfortunately, geologically-active areas tend to be heavily populated -- the temblor-prone regions because the scenery is often beautiful (think coastal California) and the volcanic regions because the soil is so fertile (like the land near Naples, Italy, in the shadow of the infamous Mount Vesuvius).  But the fraction of the world's population that lives in an area where the land is changing shape quickly is honestly very small, so most of us figure the mountains and lakes and rivers and whatnot aren't going anywhere.

If you were under any illusions about the fact that the Earth is an active place, consider the paper that came out in Nature last week describing the largest underwater volcanic eruption ever recorded -- one that literally caused a mountain to appear where none had been five years earlier.  Check out the before-and-after photos of the ocean floor, from 2014 and 2019:

Even a non-geologist like myself hardly needs the giant red arrow to see the new mountain where there wasn't one before.

The volcano, fifty kilometers off the coast of the island of Mayotte in the Comoros Archipelago (which lie between the northern tip of Madagascar and the coast of northern Mozambique), is thought to be part of the extremely active East African Rift Zone, an incipient divergent fault system that will ultimately tear off the "Horn of Africa" and create a new microcontinent containing all of Somalia and pieces of Ethiopia, Kenya, and Tanzania.  The eruption seems to have begun in May of 2018, when an earthquake of magnitude 5.8 hit Mayotte.  A team of geologists from France was dispatched to see what was going on, and they installed a monitoring system in February of 2019.  They recorded more than 17,000 seismic events in the next three months, as the mountain grew.

A map showing the chronology of the eruption

Ultimately, the series of eruptions added five cubic kilometers of hardened magma to the seafloor -- a new undersea mountain.

"The volumes and flux of emitted lava during the Mayotte magmatic event are comparable to those observed during eruptions at Earth's largest hotspots," the researchers wrote.  "Future scenarios could include a new caldera collapse, submarine eruptions on the upper slope or onshore eruptions.  Large lava flows and cones on the upper slope and onshore Mayotte indicate that this has occurred in the past."

So if you have a beach home in Mayotte, you might want to consider moving.

All of which makes me once again thankful to live in a place as geologically quiet as we do.  The St. Lawrence Valley, about two hundred kilometers north of us, has earthquakes sometimes, and I recently found out that New York City has fault lines that could potentially generate earthquakes, and in fact have done (none, thus far, severe, at least not in recorded history).  

But here, all is tranquil.  Which is fine by me.  Given that an exciting day in upstate New York is when the farmer across the road bales his hay, I always hope for something unexpected, but if it comes to major earthquakes and volcanic eruptions, I'd just as soon not be anywhere near.

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Mathematics tends to sort people into two categories -- those who revel in it and those who detest it.  I lucked out in college to have a phenomenal calculus teacher who instilled in me a love for math that I still have today, and even though I'm far from an expert mathematician, I truly enjoy considering some of the abstruse corners of the theory of numbers.

One of the weirdest of all of the mathematical discoveries is Euler's Equation, which links five of the most important and well-known numbers -- π (the ratio between a circle's circumference and its diameter), e (the root of the natural logarithms), i (the square root of -1, and the foundation of the theory of imaginary and complex numbers), 1, and 0.  

They're related as follows:

Figuring this out took a genius like Leonhard Euler to figure out, and its implications are profound.  Nobel-Prize-winning physicist Richard Feynman called it "the most remarkable formula in mathematics;" nineteenth-century Harvard University professor of mathematics Benjamin Peirce said about Euler's Equation, "it is absolutely paradoxical; we cannot understand it, and we don't know what it means, but we have proved it, and therefore we know it must be the truth."

Since Peirce's time mathematicians have gone a long way into probing the depths of this bizarre equation, and that voyage is the subject of David Stipp's wonderful book A Most Elegant Equation: Euler's Formula and the Beauty of Mathematics.  It's fascinating reading for anyone who, like me, is intrigued by the odd properties of numbers, and Stipp has made the intricacies of Euler's Equation accessible to the layperson.  When I first learned about this strange relationship between five well-known numbers when I was in calculus class, my first reaction was, "How the hell can that be true?"  If you'd like the answer to that question -- and a lot of others along the way -- you'll love Stipp's book.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]