Mind the gap

In 1869, explorer John Wesley Powell did the first systematic study of the geology of the Grand Canyon.  As impressive as it is, the Grand Canyon's not that complicated geologically; it's made of layers of sedimentary rock, most of them relatively undeformed, one on top of the other from the oldest at the bottom to the newest at the top.  A layer cake of billions of years of Earth history, and a wonderful example of the principle of superposition -- that strata form from the bottom up.

However, Powell also noted something rather peculiar.  It's called the Great Unconformity.  In geologic parlance, an unconformity is a break in the rock record, where the layer below is separated from the layer above by a gap in time when either no rocks were deposited (in that location, at least), or the rocks that were laid down were later removed by some natural process.  At that stage in the science, Powell didn't know when exactly the Great Unconformity occurred, but it was obvious that it was huge.  Something had taken away almost a billion years' worth of rocks -- and, it was later found out, that same chunk of rock was missing not only at the future site of the Grand Canyon, but across most of North America.

It was an open question as to why this happened, but one leading hypothesis was that it was massive glaciation.  Glaciers are extraordinarily good at breaking up rocks and moving them around, as I find out every time I dig in my garden and my shovel runs into the remnants of the late Pleistocene continental glaciation.  At that point, where my house is would have been under about thirty meters of ice; the southern extent is the Elmira moraine, a line of low hills fifty kilometers south of here, left behind when the glaciers, pushing piles of crushed rock and soil ahead of them like a backhoe, began to melt back and left all that debris for us gardeners to contend with ten thousand years later.

There was a time in which the Earth was -- as far as we can tell -- completely covered by ice. The Cryogenian Period, during the late Precambrian, is sometimes nicknamed the "Snowball Earth" -- and the thawing might have been one contributing factor to the development of complex animal life, an event called the "Cambrian explosion," about which I've written before.

The problem was, the better the data got, the more implausible this sounded as the cause of the Great Unconformity.  The rocks missing in the Great Unconformity seem to have preceded the beginning of the Cryogenian Period by a good three hundred million years.  And while there were probably earlier periods of worldwide glaciation -- perhaps several of them -- the fact that the Cryogenian came and went and didn't leave a second unconformity above the first led scientists away from this as an explanation.

However, a paper in Proceedings of the National Academy of Sciences, written by a team led by Francis Macdonald of the University of Colorado - Boulder, has come up with evidence supporting a different explanation.  Using samples of rock from Pike's Peak in Colorado, Macdonald's team used a clever technique called thermochronology to estimate how much rock had been removed.  Thermochronology uses the fact that some radioactive elements release helium-4 as a breakdown product, and helium (being a gas) diffuses out of the rock -- and the warmer it is, the faster it leaves.  So the amount of helium retained in the rock gives you a good idea of the temperature it experienced -- and thus, how deeply buried it was, as the temperature goes up the deeper down you dig.

What this told Macdonald's team is that the Pike's Peak granite, from right below the Great Unconformity, had once been buried under several kilometers of rock that then had been eroded away.  And from the timing of the removal -- on the order of a billion years ago -- it seems like what was responsible wasn't glaciation, but the formation of a supercontinent.

But not Pangaea, which is what most people think of when they hear "supercontinent."  Pangaea formed much later, something like 330 million years ago, and is probably one of the factors that contributed to the massive Permian-Triassic extinction.  This was two supercontinents earlier, specifically one called Rodinia.  What Macdonald's team proposes is that when Rodinia formed from prior separate plates colliding, this caused a huge amount of uplift, not only of the rocks of the continental chunks, but of the seafloor between them.  A similar process is what formed the Himalayas, as the Indian Plate collided with the Eurasian Plate -- and is why you can find marine fossils at the top of Mount Everest.

[Image is in the Public Domain]

When uplift occurs, erosion increases, as water and wind take those uplifted bits, grind them down, and attempt to return them to sea level.  And massive scale uplift results in a lot of rock being eroded.

Thus the missing layers in the Great Unconformity.

"These rocks have been buried and eroded multiple times through their history," study lead author Macdonald said, in an interview with Science Daily.  "These unconformities are forming again and again through tectonic processes.  What's really new is we can now access this much older history...  The basic hypothesis is that this large-scale erosion was driven by the formation and separation of supercontinents.  There are differences, and now we have the ability to perhaps resolve those differences and pull that record out."

What I find most amazing about this is how the subtle chemistry of rock layers can give us a lens into the conditions on the Earth a billion years ago.  Our capacity for discovery has expanded our view of the universe in ways that would have been unimaginable only thirty years ago.

And now, we have a theory that accounts for one of the great geological mysteries -- what happened to kilometer-thick layers of rock missing from sedimentary strata all over North America.

John Wesley Powell, I think, would have been thrilled.

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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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Building the Rockies

I recently re-read John McPhee's wonderful quartet of books on geology, Basin and Range, Rising from the Plains, In Suspect Terrain, and Assembling California.  His lucid prose and capacity for focusing on the human stories connected with the subject while teaching us some fascinating science brought me back to these books, which I first read perhaps twenty-five years ago.

The first two, in particular, describe something that is quite surprising -- or at least was to me when I first learned about it.  The biggest mountain range in the United States, the Rockies, is actually quite poorly understood, and contains some features that are still yet to be satisfactorily explained.  A good part of the Rocky Mountain range is non-volcanic, and although there are some areas that have igneous rocks the vast majority is made up of sedimentary and metamorphosed sedimentary rock -- sandstone, limestone, shale, slate, quartzite, and marble.  Even some of the igneous rocks only show at the surface because the overlayment of sedimentary rock that once was present has eroded away.

[Image licensed under the Creative Commons Self, Rocky Mountain National Park, CC BY-SA 2.5]

As McPhee describes it, the current thought is that most of what is west of Colorado and Wyoming is probably the result of accretion -- the huge North American Plate overriding smaller plates to the west and gathering up microcontinents and island arcs they carried, cementing them onto the coastline.  It's certain that this is how California formed -- the boundaries between the different "suspect terranes" (the alternate spelling is used when referring to these chunks of land that end up in a very different place from where they were formed) are pretty well established.  Also, the subduction process that brought them to North America is still ongoing, as the small Explorer, Juan de Fuca, and Gorda Plates (in order from north to south) are pulled underneath -- giving rise to the Cascade Volcanoes such as Mount Lassen, Mount Hood, Mount Rainier, and Mount Saint Helens.

We got another piece added to the puzzle with a paper in Nature, out of the University of Alberta, by Yunfeng Chen, Yu Jeffrey Gu, Claire A. Currie, Stephen T. Johnston, Shu-Huei Hung, Andrew J. Schaeffer, and Pascal Audet.  Entitled, "Seismic Evidence for a Mantle Suture and Implications for the Origin of the Canadian Cordillera," the paper describes research that found a sharp boundary in the mantle of the Earth between the "craton" -- the central, oldest piece of the North American continent, encompassing what is now the Midwest -- and a long, narrow microcontinent that slammed into the North American Plate as a primordial sea closed -- moving the coastline hundreds of miles further west.

"This research provides new evidence that the Canadian section of this mountain range was formed by two continents colliding," said Jeffrey Gu, professor in the Department of Physics and co-author on the study, in an interview with Science Daily.  "The proposed mechanism for mountain building may not apply to other parts of the Rocky Mountains due to highly variable boundary geometries and characteristics from north to south."

The cool part is that the research was done by looking deep into the Earth's mantle -- not just by studying the surface features.  And this collision, which is estimated to have occurred a hundred million years ago, has left a scar that is still detectable.  "This study highlights how deep Earth images from geophysical methods can help us to understand the evolution of mountains, one of the most magnificent processes of plate tectonics observed at the Earth's surface," said study co-author Yunfeng Chen.

And this technique could be applied elsewhere, as the Rockies are far from the only mountain range in the world that were created by accretion rather than volcanism.  (The obvious examples are the Alps and the Himalayas -- the latter of which are still rising as the Indian Plate continues to plow into the Eurasian Plate.)  "There are other mountain belts around the world where a similar model may apply," said Claire Currie, associate professor of physics and co-author on the study.  "Our data could be important for understanding mountain belts elsewhere, as well as building our understanding of the evolution of western North America."

So we're piecing together the picture of how the Rockies formed -- ironic, as they seem to have been assembled from pieces themselves.  In the process, we're learning more about the processes that move the tectonic plates, and create the landscape we see around us.  It reminds me of the haunting lines from Alfred, Lord Tennyson, which seem like a fitting way to end:

There rolls the deep where grew the tree.
O Earth, what changes hast thou seen?
There where the long road roars has been
The stillness of the central sea.
The hills are shadows, and they flow
From form to form, and nothing stands,
They melt like mists, the solid lands,
Like clouds, they shape themselves, and go.

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The bellringer

Between December 16, 1811 and February 7, 1812, a series of four earthquakes -- each estimated to be above magnitude 7, with the first and last perhaps at magnitude 8 -- hit what you might think is one of the most unlikely places on Earth; southeastern Missouri.

The centers of continents are ordinarily thought to be tectonically stable, as they are generally far from any of the three typical sorts of faults -- divergences, or rifts, where plates are moving apart (e.g. the East African Rift Zone); convergences, or thrust faults, where plates are moving together (e.g. the Cascadia Subduction Zone); and strike-slip faults, where plates are moving in opposite directions parallel to the fault (e.g. the San Andreas Fault).  The Midwest is located in the middle of the North American Craton, an enormous block of what should, according to the conventional wisdom, be old, stable, geologically inactive rock. 

But the 1811-1812 earthquake series happened anyhow.  If they'd occurred today, it would likely have flattened the nearby city of Memphis, Tennessee.

So much for conventional wisdom.

The fault responsible was named the New Madrid Seismic Zone for the county right in the center of it, and its capacity for huge temblors is staggering.  The biggest (and final) earthquake of the four was powerful enough that it was felt thousands of kilometers away, and rang church bells in Charleston, South Carolina.  The shift in terrain changed the course of the Mississippi River, cutting off a meander and creating horseshoe-shaped Reelfoot Lake.

So what created a seismic zone where one shouldn't be?

[Image is in the Public Domain courtesy of the USGS]

The topic comes up because I just finished reading seismologist Susan Elizabeth Hough's excellent book Earthshaking Science: What We Know (and Don't Know) About Earthquakes, which is one of the best laypersons' introductions to plate tectonics and seismicity I've come across.  She devotes a good bit of space to the New Madrid earthquakes, and -- ultimately -- admits that the answer to this particular question is, "We're still not sure."  The problem is, the fault is deeply buried under layers of sediments; current estimates are that the hypocenter (the point directly underneath the epicenter where the fault rupture occurred) is between fifteen and thirty kilometers beneath the surface.  And since the quakes in question happened before seismometers were invented, we're going off inferences from written records, and such traces that were left on the surface (such as "sand blows," where compression forces subsurface sand upward through cracks in the stratum, and it explodes through the surface).

As far as the cause, Hough has a plausible explanation; the New Madrid Seismic Zone is an example of a failed rift, where a mantle plume (or hotspot) tried to crack the continent in half, but didn't succeed.  This stretched the plate and created a weak point -- called the Reelfoot Rift -- where any subsequent stresses were likely to trigger a rupture.  Since that time, the North American Plate has been continuously pushed by convection at the Mid Atlantic Rift, which is compressing the entire plate from east to west; those stresses cause buckling at vulnerable points, and may well have been the origin of the New Madrid earthquakes.

One puzzle, though, is what happened to the hotspot since then.  This is still a matter of speculation.  Some geologists think that friction with the rigid and (relatively) cold underside of the plate damped down the mantle plume and ultimately shut down convection.  Others think that as the North American Plate moved, it simply slid off the hotspot, making the plume appear to move eastward (when in actuality, the plate itself was moving westward).  This may be why another anomalous mid-plate earthquake zone is in coastal South Carolina, and it might also be the cause of the Bermuda Rise.

That point is still being debated.

Another open question is the current risk of the fault failing again.  There's paleoseismic data suggesting major earthquake sequences from the Reelfoot Rift/New Madrid Seismic Zone in around 900 and 1400 C.E., suggesting a timing between events of about four to five hundred years.  But these are estimates themselves, and I probably don't need to tell you that earthquake prediction is still far from precise.  Faults don't fail on a schedule -- which is why it annoys me every time I see someone say that an area is "overdue for an earthquake," as if they were on some kind of calendar.

Still, I can say with at least moderate confidence that it's unlikely to generate another big earthquake soon, which is kind of a relief.

So that's our geological curiosity of the day.  I have a curious family connection to the area; my wandering ancestor Sarah (Handsberry) Overby-Biles-Rulong (she married three times, had nine children, and outlived all three husbands) lived in the town of New Madrid in 1800, after traveling there from her home near Philadelphia as a single woman in the last decade of the eighteenth century.  I've never been able to discover what impelled her to leave her home and, with a group of relative strangers, cross what was then trackless wilderness to a remote outpost, and I've often wondered if she might have been either running away from something, or perhaps might have been a prostitute.  I'm not trying to malign her memory; it bears mention that a good eighty percent of my forebears were rogues, ne'er-do-wells, miscreants, and petty criminals, so it would hardly be a surprise to add prostitution to the mix.  And whatever else you can say about my family members, they were interesting.  I've often wished I could magically get a hold of Sarah's diary.

In any case, Sarah was in Lafayette, Louisiana by 1801, so she missed the New Madrid earthquakes by ten years.  But kind of interesting that she lived for a time in the little village that was about to be the epicenter of one of the biggest earthquakes ever to hit the continental United States, one that rang bells thousands of kilometers away, and which created a geological mystery the scientists are still trying to work out.

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Out of sight

Seismologists and volcanologists are unusual amongst scientists in that for the most part, what they're studying are things that are permanently unavailable for direct observation.

Oh, sure, they can access the results on the Earth's surface; fault lines, lava flows, uplift or subsidence from magma movement, and so on.  But the actual processes -- the stuff down there that is causing it all -- is inaccessible.

The deepest hole ever dug is the Kola Superdeep Borehole, on the Kola Peninsula near the Russian border with Norway, which is an impressive twelve kilometers deep; but when you realize that's only one-thousandth of the diameter of the Earth, it puts things into perspective.  Even so, it was deep enough that the bottom had a measured temperature of 180 C -- hot enough to boil water, but far from hot enough to melt rock.  (It bears mention that a claim circulating last year that they'd gone down fourteen kilometers, hit temperatures of 1000 C, and could hear the screams of the damned -- because, apparently, they'd punctured a hole into hell -- was unfounded.)

So the fact remains that much of geological science is based upon inference -- not only using surface processes to infer what's happening in Earth's interior, but using data such as earthquake wave traveling speed to figure out what the mantle and core are made of, whether they're liquid or solid or somewhere in between, and how all that stuff in there is moving around.  And being inferential, our understanding of deep geologic processes is constantly subject to revision.

Which brings us to a study out of Utrecht University that appeared in the journal Nature last week, about a discovery showing that deep in the Earth's mantle there are two continent-sized subterranean "islands" at least a half a billion years old -- showing that the stuff down there isn't mixing around quite the way we thought it was.

The upper mantle has been thought of as basically a big recycler.  As pieces of the Earth's crust get forced down into subduction zones (marked by the oceanic trenches that neighbor some of the most tectonically-active regions on Earth), it melts and gets mixed into what's already down there.  Being colder than the surrounding rock, everyone thought the process was slow; other than the bits that get hot enough to melt and then rise to the surface, causing volcanoes like the ones in the North American Cascades, Andes, Caribbean, Italy, Japan, and Indonesia, the rest just has to sit down there till it blends into the material surrounding it.

Apparently some of this will need to be rethought, because these "islands" in the mantle are still holding together despite being so old that they "should have" completely melted away by now.

One of the chunks is under Africa and the other under the Pacific Ocean, and they were located by using the paths and speeds of seismic waves, giving them the moniker of LLSVPs (Large Low Seismic Velocity Provinces).  "Nobody knew what they are, and whether they are only a temporary phenomenon, or if they have been sitting there for millions or perhaps even billions of years," said Arwen Deuss, who co-authored the study.  "These two large islands are surrounded by a graveyard of tectonic plates which have been transported there by subduction, where one tectonic plate dives below another plate and sinks all the way from the Earth’s surface down to a depth of almost three thousand kilometers."

You might be wondering how they figured out that they are a half a billion years old, given that they're way out of reach of direct study.  That, in fact, is the most fascinating part of the study, and has to do with the fact that rocks which cool quickly (such as obsidian and basalt) have much smaller crystals than ones that cool more slowly (like granite and gabbro).  The molecular reassembly that results in crystal formation takes time, especially in thick, viscous liquids like magma, so if lava is rapidly cooled on the surface it doesn't have time to form crystals.

"Grain size is much more important," Deuss said.  "Subducting tectonic plates that end up in the slab graveyard consist of small grains because they recrystallize on their journey deep into the Earth.  A small grain size means a larger number of grains and therefore also a larger number of boundaries between the grains.  Due to the large number of grain boundaries between the grains in the slab graveyard, we find more damping, because waves lose energy at each boundary they cross.  The fact that the LLSVPs show very little damping, means that they must consist of much larger grains."

Large grain size = a long time spent underground.  Mineralogist Laura Cobden, who specializes in mineral crystallization rates in igneous rock, estimated that based on the inferred crystal size in the two "islands," they've been down there, relatively undisturbed, for around five hundred million years.

[Image from Deuss et al.]

So that's our cool science news from the geologists for today.  Two islands in the mantle that are stubbornly resisting melting away.  Why these structures have been so persistent is beyond the scope of this study; but as with all science, finding out something's there is the first step.  After that, the theorists can figure out how to explain it all.

Even if they never have a chance to see it.

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Tearing down the roadblocks

I wonder if you've heard of Marie Tharp.  I hope you have, but suspect you haven't.  Even in scientific circles, her name is not exactly a household word.

It should be.

Back in 1912, a German geologist and climatologist named Alfred Wegener noticed correspondences that seemed too great to be coincidences.  First, there was the thing that just about everyone wonders about in grade school -- the puzzle-piece contours of Europe and Africa with North and South America.  Then there was the fact that the fossil record of those two regions are similar until about two hundred million years ago, and afterward gradually diverge.  And last, he observed that the Appalachian, Pennine, and Scandinavian Mountains are geologically similar and seem to have formed at around the same time.  As you undoubtedly know, Wegener put all that together and proposed that they were all explained by continental drift -- that the land masses were all united at one point, then broke up and drifted apart, splitting what had been a single continent with a contiguous mountain range into widely-separated pieces.

The main reason this wasn't well-received was not only, or even mainly, because of hidebound scientists clinging to old models; it was that Wegener couldn't explain how, or why, it had occurred.  He proposed no mechanism to account for continents "drifting" in what appeared to be solid rock.  So while it's a pity for poor Wegener that he'd landed on the correct answer and got no recognition for it (he died at age fifty in 1930 on an expedition to Greenland, thirty years before plate tectonics was proposed), his theory's poor reception is honestly understandable.

What happened to Marie Tharp in the 1950s is less forgivable.

Tharp was an oceanographer who fell into the profession almost by accident.  She was fascinated with science, but women back then were actively discouraged from pursuing careers in scientific fields; they were frequently given helpful advice like "it's extremely difficult for women to compete as scientists," with few of the (male) advisors and supervisors asking themselves the question of why that was, and more importantly, if maybe, just maybe, it was a problem they should work on fixing.  During World War II, though, when a lot of college-age men were overseas fighting, colleges started actively recruiting -- well, just about anyone, even those from groups that had been previously excluded.  Tharp took a geology class and was fascinated by the subject, so she enrolled in graduate school at the University of Michigan at Ann Arbor, completing a master's degree in petroleum geology in 1944.

After that, though, she ran into the difficulty that geology and related sciences rely on field work, and nearly all of the companies that hired geologists didn't allow women to work in the field.  So Tharp was relegated to analyzing data -- especially mapping data -- that had been collected and brought back by her male colleagues.

Tharp in 1968 [Image is in the Public Domain]

It was when she was working on a project to map the deep parts of the Atlantic she noticed something odd.  For a decade, ships had been crisscrossing the Atlantic Ocean using sounding devices to map the topography of the ocean floor, initially as a way of locating downed aircraft and ships.  But as she was creating contour maps, Tharp found that there was a huge mountain range running all the way down the center, from north to south -- and that mountain range had a narrow, deep, v-shaped valley right down the middle.  Then she started plotting the epicenters of submarine earthquakes onto the map, and found they coincided almost perfectly with the ridge and valley.

As soon as she saw this, she knew Wegener had been right.

The rift, she claimed, was where the motive force arose that was forcing the continents apart.  It was seismically active, and (she rightly predicted) should be characterized by newly-formed igneous rock, as the split between the continents widened and lava from the mantle bubbled up and froze on contact with cold seawater.  She told her supervisor, geologist Bruce Heezen, who promptly laughed at her, characterizing her explanation as "girls' talk."

Tharp, fortunately, was not so easily dissuaded.  She kept at it, and after several years had enough data amassed that the evidence was absolutely incontrovertible.  Even Heezen finally gave in.  Those ridges and valleys were eventually found to be a network of rifts encircling the globe like the stitching on a baseball, and her idea that they were responsible for plate tectonics was absolutely spot-on.  But it's significant that of the many papers about the Mid-Atlantic Ridge and plate tectonics that Heezen and others published in the 1960s and 1970s, Tharp's contributions were acknowledged on exactly zero of them.  The person who was credited with discovering the Mid-Atlantic Rift Zone, and proposing its role in continental drift, was...

... you guessed it...

... Bruce Heezen.

She was eventually recognized for her brilliance and hard work, but like a lot of women scientists, didn't receive it until quite late in her career.  She was awarded the National Geographic Society's Hubbard Medal in 1978, Woods Hole Oceanographic Institute's Mary Sears Woman Pioneer in Oceanography Award in 1999, and the Lamont-Doherty Earth Observatory Heritage Award in 2001, five years before her death at the age of 86.

It's certainly easier for women in science now, in part due to indomitable women like Marie Tharp.  But the fact that it's not equally easy for men and women -- which it still very much isn't -- illustrates that we have a long way to go in welcoming women, minorities, and LGBTQ+ people into every career avenue.  If you're one of those people who has ridiculed DEI (diversity, equity, and inclusion) drives in education, business, and industry, then maybe you should be working harder to create a world where we don't need them any more.

Odd how those who are most vocally against DEI seldom have any cogent arguments why they think it's appropriate or fair to set up roadblocks that result in wasting over half of the potential talent, drive, passion, and genius we have at our fingertips.

Most people who are interested in geology have heard of Wegener, and pioneers like Drummond Matthews, Frederick Vine, and Harry Hess.  Far fewer have heard of Marie Tharp, who overcame tremendous personal and professional hurdles to revolutionize our understanding of how the Earth's geological systems work.

Hearing about her struggles won't undo the unfairness and misogyny she dealt with during her entire professional life, but maybe it will assure that this generation of women scientists don't have to endure the same thing.

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Piercing the clouds

One of the most unusual stories that H. P. Lovecraft ever wrote is "In the Walls of Eryx."  It isn't his usual fare of soul-sucking eldritch nightmares from the bubbling chaos at the center of the universe; in fact, it's his only real, honest-to-Asimov science fiction story.  It centers around a human colony on Venus, devoted to mining a kind of crystal that can be used for propulsion.  There's an intelligent native species -- reptilian in appearance -- who was content to let the humans bump around in their space suits (Lovecraft at least got right that the atmosphere would be toxic to humans) until the humans started killing them.  At that point, they started fighting back -- and setting traps.

The story centers around a crystal hunter who is out on an expedition and sees a huge crystal in the hands of a (human) skeleton.  He goes toward it, and runs face-first into an unseen obstacle -- completely transparent walls, slick (and therefore unclimbable) and four meters tall (so unjumpable).  The problem is, when he tries to back out, he's already moved around a bit, and doesn't retrace his steps perfectly.

Then he runs into another wall.

What's happened is that he's stumbled into an invisible labyrinth.  And how do you find your way out of a maze if you can't see it?  You'll just have to read it.  It's only a dozen or so pages long, and is one of the neatest (and darkest) puzzle-box stories you'll ever pick up.

It's been known since Lovecraft's time ("In the Walls of Eryx" was written in 1936) that Venus was covered by clouds, and its surface was invisible from Earth.  Of course, a solid mantle of clouds creates a mystery about what's underneath, and speculation ran wild.  We have Lovecraft's partially-correct solution -- a dense, toxic atmosphere.  Carl Sagan amusingly summed up some of the early thinking on Venus in the episode "Heaven and Hell" from his groundbreaking series Cosmos: "I can't see a thing on the surface of Venus.  Why not?  Because it's covered with a dense layer of clouds.  Well, what are clouds made of?  Water, of course.  Therefore, Venus must have an awful lot of water on it.  Therefore, the surface must be wet.  Well, if the surface is wet, it's probably a swamp.  If there's a swamp, there's ferns.  If there's ferns, maybe there's even dinosaurs...  Observation: I can't see anything.  Conclusion: dinosaurs."

Of course, reputable scientists didn't jump to these kinds of crazy pseudo-inferences.  As Neil deGrasse Tyson points out, "If you don't know, then that's where your conversation should stop.  You don't then say that it must be anything."  (It's not a coincidence that Tyson was the host of the reboot of Cosmos that appeared a few years ago.)

The first hint that Venus was not some lush tropical rain forest came in the late 1950s, when it was discovered that there was electromagnetic radiation coming from Venus that only made sense if the surface was extremely hot -- far higher than the boiling point of water.  This was confirmed when the Soviet probe Venera 9 landed on the surface, and survived for 127 minutes before its internal circuitry fried.

In fact, saying it's "hot" is an understatement of significant proportions.  The average surface temperature is 450 C -- 350 degrees higher than the boiling point of water, and hot enough to melt lead.  The atmosphere is 96.5% carbon dioxide (compared to 0.04% in the Earth's atmosphere), causing a runaway greenhouse effect.  Most of the other 3.5% is nitrogen, water vapor, and sulfur dioxide -- the latter being the rotten-egg chemical that, when mixed with water, creates sulfuric acid.

Yeah.  Not such a hospitable place.  Even for crystal-loving intelligent reptiles.

Photograph from the surface of Venus [Image is in the Public Domain, courtesy of NASA/JPL]

But there's still a lot we don't know about it, which is why at the meeting of the American Geophysical Union, there was a proposal to send a probe to our nearest neighbor.  But this was a probe with a difference; it would be attached to a balloon, which would keep it aloft, perhaps indefinitely given the planet's horrific convection currents.  From there, we could not only get photographs, but more accurate data on the atmospheric chemistry, and possibly another thing as well.

One of the things we don't know much about is the tectonics of the planet's surface.  There are clearly a lot of volcanoes -- unsurprising given how hot it is from other causes -- but whether the crust is shifting around the way it does on Earth is not known.  One way to find out would be looking for "venusquakes" -- signs that the crust was unstable.  But how to find that out when probes on the surface either melt or get dissolved by the superheated sulfuric acid?

The cool suggestion was that because of the atmosphere's density, it might be "coupled" to the surface.  So if something shook the surface -- a venusquake or volcanic eruption -- those waves might be transferred to the atmosphere.  (This effect is insignificant on Earth because our atmosphere is far, far less dense.)  Think of a plate with a slab of jello on it -- if you shake the plate, the vibrations are transferred into the jello because the whole thing is more or less stuck together, so the surface of the jello wobbles in resonance.

An airborne probe might be able to tell us something about Venus's geology, which is pretty awesome.  It appeals not only to my fascination with astronomy, but my love of a good mystery, which the second planet definitely is.

So I hope this project gets off the ground, both literally and figuratively.  Even if it's unlikely to detect anything living -- reptilian or not -- we could learn a great deal about what happens when the carbon dioxide levels start undergoing a positive feedback loop.

A scenario we all would like very much not to repeat here at home.

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The microcontinent

One of the nice things about science is that it allows us to understand the parts of the universe that are beyond common sense.  

Don't get me wrong, common sense is often a decent guide to figuring things out, and there's some truth to the lament that it'd be nice if it were more common.  The problem is, our intuitive grasp of how stuff works evolved in the context we live in -- moderate sizes and masses, moderate speeds, and moderate time durations.  Get very far out of that context, and common sense can give you the wrong answer.  One of the first times I ran into this was in high school physics, where I learned the startling fact that an object's vertical and horizontal velocity are entirely independent of each other.  This is illustrated by the oft-quoted example that if you fire a bullet horizontally, and at the same time drop a bullet from the height of the gun's barrel, the two bullets will hit the ground at precisely the same time (assuming level terrain).  It may seem counterintuitive, but it's true -- and it took Isaac Newton to show why that was.

We run into problems not only when we deal with things moving quickly, but when they're moving slowly -- so slowly they appear not to be moving at all.  I got to thinking about this when I was sent a link by my friend, the awesome author Andrew Butters (you should follow him at the link provided, and also immediately order his phenomenal new novel Known Order Girls, which is one of the most poignant books I've ever read).  Andrew is, like me, a science nerd -- we were both drastically unsuccessful physics majors in college, who despite that experience maintained a deep fascination with how the universe works.  (Interestingly, our comeuppance as incipient scientists came in different classes.  His nemesis was Electromagnetic Theory, and mine was Classical Mechanics.  In both cases we passed the class largely because the professor didn't ever want to see our names on his roster again, and afterward we both decided that maybe a career as a physicist was not in the cards.)

In any case, this time the topic he sent me was geology -- in particular, plate tectonics, a particular interest of mine.  Researchers have just found that a part of Nunavut, Canada is actually a microcontinent -- a geologically-anomalous piece of continental crust that came loose from Greenland and welded itself to North America on the other side of the Davis Strait.  

The Davis Strait and the west coast of Greenland [Image licensed under the Creative Commons brewbooks via Flickr (CC BY-SA 2.0)]

What's curious about this is that up until about 45 million years ago, Canada and Greenland had been moving apart.  The evidence is that there was a rift zone -- that's what formed the Davis Strait in the first place -- and that some time in the Mid-Eocene Epoch, the rift failed.  (This is not that uncommon; there's a good possibility that the Cameroon Line and the New Madrid Fault are both failed rift zones.)  In any case, after the Davis Strait Rift sealed back up, Greenland started moving in tandem with the North American Plate -- except for a piece of it that sheared off and stuck to what is now Canada.

"The reinterpretation of seismic reflection data offshore West Greenland, along with a newly compiled crustal thickness model, identifies an isolated terrane of relatively thick (19–24 km [12-15 miles]) continental crust that was separated from Greenland during a newly recognised phase of E-W extension along West Greenland’s margin," the team wrote.  "We interpret this continental block as an incompletely rifted microcontinent, which we term the Davis Strait proto-microcontinent...  As our seismic reflection interpretations indicate an extensional event in the eastern Davis Strait between 58 and 49 Myr, spatially coincident with the zone of thinnest continental crust between the continental fragment and Greenland, we infer this extensional event [rift] led to the separation of this fragment from Greenland."

When you think about it, it's unsurprising that it took so long for geologists to figure plate tectonics out.  Despite such broad hints as the puzzle-piece outlines of South America and Africa, a process this slow is not obvious.  Add to that the fact that this particular plate is in one of the most inhospitable places on Earth, accessible to researchers for maybe two months a year (that's being generous.)  The entire picture is still being pieced together.  Our tectonic map is pretty good, but the new research shows us that we don't have it all parsed quite yet.

Which is the way it should be.  As Neil deGrasse Tyson put it, the more we learn, the more we extend the perimeter of our ignorance.  And this, after all, is what drives science -- the fact that every question we answer brings up a dozen more.

I think we'll be working at this for quite some time to come.

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The sleeping dragon

When most people think of seismically-active regions, Bangladesh is not ordinarily near the top of the list.

Cyclones, sure.  They come roaring up the Bay of Bengal with a horrifying regularity, and most of the country is low enough in elevation that the storms barely even slow down.  The worst was the 1970 Bhola cyclone, which still holds a record as the deadliest storm in recorded history.  The official death toll was five hundred thousand, but is likely higher than that, mostly people who lived in the lowlands near the city of Chittagong.

Unfortunately for the citizens of Bangladesh, though, they're also at high risk for earthquakes -- something that has only been recognized recently.  A 2021 study led by Muhammad Qumrul Hassan of the University of Dhaka found that the region is right on top of the junction of three different tectonic plates, the Eurasian Plate, the Indian Plate, and the small Burma Plate ("small" here means geographic area, not capacity for damage -- the devastating 2004 earthquake and tsunami was caused by a slippage of the Burma Plate relative to the Indian Plate).  But the compression and twisting of the land near the junction has created enough stresses that the entire country is crisscrossed with faults, most notably the Dauki Fault and the Haflong Thrust (which crosses into the Indian states of Meghalaya and Assam to the north).

The whole thing is exceedingly complex, and still poorly understood.  Imagine laying a sheet of pie crust on a table, and you and two friends each stand around it and push, pull, or twist it from the edge.  The sheet will wrinkle, tear, and hump up in places, but exactly where those deformations will end up isn't easily predictable because it depends on where there was weakness in the dough before you started messing with it.  This is the situation with the chunk of the Earth's crust that underlies Bangladesh.  Add to that the fact that the region is poor, and much of it is jungle- or swamp-covered and pretty inaccessible to study, and you have a picture of the extent to which we don't understand the situation.

However -- alarmingly -- a 2016 study found that the entire region has been building up stress for at least four hundred years, meaning when the some piece of fault slips, it's likely to be catastrophic.

The whole topic comes up because of a rather terrifying discovery that was the subject of a paper this week in Nature Communications.  Geoscientists Elizabeth Chamberlain (of Wageningen University). Michael Steckler (of Columbia Univeristy), and colleagues were studying a puzzling historical shift in the channel of the Ganges River, and quite by accident -- it was in an area some locals were digging in to create a pond -- they saw the unmistakable signs of seismites.  These are features in rock layers created by massive earthquakes, in this case a column of sand that had erupted through pre-existing strata during a colossal temblor.  Upon analysis, they found that the river had changed course because of a massive earthquake about 2,500 years ago.

Imagine an event big enough to shift the path of a river that size.

A change in the course of a river is called an avulsion, and it normally takes decades or centuries.  (It's an avulsion of the Mississippi River that the levee system in southern Louisiana is attempting to prevent -- something I wrote about a couple of weeks ago.)  Seismic avulsions are much less common, but when they happen it's sudden and spectacular.  The only other one I've ever heard of is the shift in the Mississippi caused by the 1812 New Madrid earthquake, which dropped the land so much it cut off a meander and created Reelfoot Lake.

The seismic record in Bangladesh indicates that they're dangerously at risk for another earthquake -- and because of the complexity and our lack of comprehension of the fault system underlying the country, the geologists aren't certain where is likeliest to rupture.  There's a sleeping dragon underneath one of the poorest countries in Asia -- and we're only beginning to understand when and how it might suddenly awaken.

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The migrants

Most people know of at least two reasons that organisms can evolve.  The first, of course, is natural selection; members of the same species with inheritable differences can have different survival rates or reproductive rates, leading to overall shifts in the genetic makeup of the population.  The second is catastrophe; a major external event, such as the eruption of the Siberian Traps or the collision of the Chicxulub Meteorite, can completely destabilize what had been a thriving ecosystem, and cause the selective pressures to go off in a completely different direction.  (The two I mentioned were the dominant factors in the Permian-Triassic and Cretaceous-Tertiary extinctions, respectively.)

Less well-known is the role that plate tectonics can play.  When two land masses split apart, the organisms then go their separate ways evolutionarily, especially once the two pieces drift far enough away from each other to experience significantly different climates.  This is what happened to Australia, which most recently was connected to Antarctica; once they diverged, Australia moved northward and Antarctica southward, resulting in just about everything in Antarctica becoming extinct as the temperatures dropped, and leaving Australia with its unique assemblage of species.

The opposite can happen when two continents run into each other.  This occurred when India separated from Africa and collided with Asia, about fifty million years ago, carrying with it species from the southern supercontinent (Gondwana) and introducing them to the northern one (Laurasia).  But an even more striking example occurred when North and South America got close enough that a bit of the seafloor was pushed above water, creating the Isthmus of Panama.

When this happened, on the order of three million years ago, it opened up an easy avenue of two-way migration between the two continents.  This reconnected land masses that had been separated since the breakup of Pangaea in the early Triassic Period, on the order of two hundred million years ago.  That's a long time for species assemblages to evolve in their own directions, and the result was two entirely different floras and faunas.  Those began to move back and forth across the gap as soon as the isthmus formed.

What is curious -- and still largely unexplained -- is why the survival rates of the northward and southward migrants were so drastically different.  Species went both directions; that much is clear from the fossil record.  But just looking at mammals, South America gained (and still has) various species of cats, wolves, foxes, peccaries, deer, skunks, bears, and mice that it gained from North America, to name only a few of the groups that moved in and thrived.  But going the other direction?

There were only three survivors.  The opossum, the armadillo, and the porcupine are the only mammalian South American imports we still have around today.  Others that attempted the northward trek, including ground sloths, glyptodonts, "terror birds," sparassodonts, notungulates, and litopterns, struggled along for a while but eventually became extinct.

[Image is in the Public Domain]

The surmise is that moving from wet forests where it's warm year-round into cooler, drier temperate deciduous forests or grasslands is harder than the reverse, just from the perspective of resources.  Whatever the reason, though, it altered the ecosystems of South America forever, as the North American species proved to be better competitors (and predators), driving entire families of South American mammals extinct.  Some groups continued to thrive and diversify, of course.  Hummingbirds come to mind; they're a distinctly South American group. increasing in diversity as you head south.  Where I live, there's a grand total of one species of hummingbird (the Ruby-throated Hummingbird).

The little country of Ecuador has 132.

The reason all this comes up is the discovery of the complete skeleton of an extinct species of porcupine in Florida, dating to 2.5 million years ago -- and therefore, one of those early migrants northward from its ancestral homeland.  It's related to the modern North American species, but definitely not the same; the extinct species, for example, had a prehensile tail, similar to modern South American species (and which our North American porcupines lack).  It's still unknown, however, if the Florida species is ancestral to our current North American porcupines, or if they're cousins; further study of the skeleton may help to resolve that question.

It's fascinating, though, to see the fingerprints of this mass migration that was to change so radically two different continents.  The process of plate movement continues; Australia will eventually collide with Asia, for example, with similar results, mixing together two sets of species that have been isolated for millions of years.  Change is inevitable in the natural world; it can happen quickly or slowly, and sometimes occurs in ways we're just beginning to understand.

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