Remnants of lost oceans

A few days ago I ran across some new research out of Utrecht University about the Pontus Plate, which because (1) I was raised Catholic, (2) I have a thing for Roman history, and (3) apparently I need new glasses, I keep misreading as "Pontius Pilate."

The Pontus Plate has nothing to do with New Testament Bad Guys.  It's a remnant of a (very) old tectonic plate that mostly vanished on the order of 120 million years ago, during a time when the Pacific Ocean was a great deal bigger than it is now, and the Atlantic Ocean had just started to form, rifting apart North and South America from Europe and Asia.

To see what's going on here, first a bit of background.

In general, there are three kinds of boundaries you find on the edges of plates.  A real geologist (not just a dilettante layperson like myself) would tell you it's way more complex than this, which is certainly true, but this is at least a broad-brush categorization:

1. Divergent zones, also called rifts, which are where magma is upwelling from the mantle, creating drag that moves plates apart.  Examples are the Mid-Atlantic Rift Zone and the East African Rift Valley, the latter of which was the subject of a post here at Skeptophilia only a few days ago.
2. Convergent zones, where plates move together.  When one or both of the plates is an oceanic plate -- which are thinner and more brittle -- one will dive underneath the other, causing a trench or a thrust fault.  The plate that dives down (subducts) eventually melts, giving rise to volcanoes, such as the ones in Japan, Indonesia, the Caribbean, and the Cascade Range.  When both of the colliding plates are continental plates -- thick, stiff, and cold, kind of like Ron DeSantis -- the two simply pile up against each other until friction slows them down.  This is the process that formed -- and is still forming -- the Himalayas and the Alps.
3. Transform faults, also called strike-slip faults, where two plates slide more-or-less parallel to each other.  An example is the San Andreas Fault in California, amongst many others.

New oceanic plate is constantly being formed at divergent zones and destroyed at convergent zones, so the entire tectonic map of the Earth is always shifting, the pieces breaking up and reassembling a bit like sheets of ice on a flowing river in March.

Sometimes, when the process of destroying a particular plate exceeds the process of forming it, the plate is doomed to disappear eventually.  This is happening right now to the Juan de Fuca Plate, off the northwest coast of North America:

The Juan de Fuca Plate, sandwiched between the much larger Pacific and North American Plates. The blue line is a convergent zone, the red lines are divergent zones, and the green lines are transform faults. [Image licensed under the Creative Commons Alataristarion, JuanDeFucaPlate, CC BY-SA 4.0]

The Juan de Fuca Plate is one of five small chunks that are all that are left of the enormous Farallon Plate, which once extended under much of the eastern half of the Panthalassa Ocean, the enormous mega-ocean covering seventy percent of the Earth's surface when the continents were locked up as Pangaea.  (The other four pieces are the Explorer, Gorda, Nazca, and Cocos Plates.)

The rather roundabout point I'm trying to make here is that the plates don't last forever, and there are some of them that have undoubtedly disappeared entirely.  Which makes what geologist Suzanna van de Lagemaat and her team did pretty astonishing.

Using data on remnants of oceanic rock in  Japan, Borneo, the Philippines, New Guinea, and New Zealand, van de Lagemaat was able to reconstruct one of the huge oceanic plates that was on the opposite side of Panthalassa from the aforementioned Farallon Plate, a now mostly-vanished plate she christened Pontus.  The biggest hints came from the northern region of the island of Borneo and from the highly active plate margin near the Philippines (which is responsible for the earthquakes and volcanic eruptions that strike the island chain with clocklike regularity).

"We... conducted field work on northern Borneo, where we found the most important piece of the puzzle," van de Lagemaat said.  "We thought we were dealing with relicts of a lost plate that we already knew about.  But our magnetic lab research on those rocks indicated that our finds were originally from much farther north, and had to be remnants of a different, previously unknown plate...  The Philippines is located at a complex junction of different plate systems.  The region almost entirely consists of oceanic crust, but some pieces are raised above sea level, and show rocks of very different ages."

The research is pretty impressive.  "Eleven years ago, we thought that the remnants of Pontus might lie in northern Japan, but we’d since refuted that theory," said Douwe van Hinsbergen, Van de Lagemaat’s Ph.D. supervisor, and senior author of the study.  "It was only after Suzanna had systematically reconstructed half of the 'Ring of Fire' mountain belts from Japan, through New Guinea, to New Zealand that the proposed Pontus Plate revealed itself, and it included the rocks we studied on Borneo."

[Image from van de Lagemaat et al., Nature, October 2023]

The whole thing is fascinating.  Geologists studying what are now widely-separated rock formations are able to reconstruct the remnants of a lost oceanic plate from over a hundred million years ago, a time when our ancestors were still small, scurrying furry creatures, and the asteroid collision that would end the dinosaurs' hegemony was still a good sixty million years in the future.  Reconstructing a puzzle of that magnitude is an amazing feat -- making me wonder what pieces of the past still lie undiscovered, waiting for some brilliant researcher like Suzanna van de Lagemaat to reassemble.

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Diamonds in the rough

Sure, diamonds are pretty and sparkly and rare and valuable, but do you know how they form?  Because that's honestly the coolest thing about them.

Diamonds are found in geological formations called kimberlite pipes.  This is a structure shaped like a long, narrow ice cream cone, extending downward into the Earth (how far downward we'll get to in a moment), and characterized by some rocks and minerals you usually don't find lying around -- chromium-rich pyrope garnets, forsterite, and various types of ultramafic (low-silica igneous) rocks that break down to a very specific kind of clay.  Jewel hunters long ago figured out that diamonds were likely to be found in association with these rocks and minerals, and used those as indicators of where to look -- such as the diamond-rich Kimberly region of South Africa (which gave its name to kimberlite), a couple of spots in Greene and Indiana Counties, Pennsylvania, and the Udachnaya area of Siberia.

[Image licensed under the Creative Commons Rob Lavinsky, iRocks.com – CC-BY-SA-3.0]

All of that's just background, though.  Here's the cool part, if (like me) you like things that are big and powerful and scary and can kill you.

Geologists discovered more or less simultaneously that the composition of kimberlite pipes is consistent with magma found in the (very) deep mantle, and that known kimberlite pipes extend a (very) long way down.  The best models indicate that the eruption that forms them starts on the order of four hundred kilometers below the surface of the Earth, making it the deepest known volcanic feature.

No one knows what triggers the eruption to begin.  It seems to be a rare occurrence, whatever it is.  Fortunately.  Because once it starts, and the magma moves upward through the mantle, the drop in pressure makes dissolved gases bubble out, rather like popping the cork off a bottle of champagne.  This speeds up the movement, which lowers the pressure more, so more gas bubbles out, and so on and so forth.  Also -- gases expand as the pressure drops, so the higher it rises, the more volume it displaces.

The result is what's called a diatreme.  What seems to happen is that with no warning, there's a Plinian eruption -- the same sort that destroyed Pompeii and Herculaneum -- but moving at supersonic speeds.  Imagine what it must look like -- from a distance, preferably -- everything is calm, then suddenly a several-kilometer-wide chunk of land gets blown up into the stratosphere.  The conical hole left behind fills with material from the deep mantle (thus its odd composition by comparison to other igneous rocks).  Give it a few million years, and weathering results in the characteristic clay found in a typical kimberlite.

So what's all this got to do with diamonds?

Well, in the intense heat and pressure of the eruption, some of the carbonate ions in minerals in the magma are reduced to elemental carbon, and that carbon is compressed to the point that its crystalline structure changes to a hexoctahedral lattice.  The result is a transparent crystal that looks nothing like the soft, black, powdery stuff we picture when we think of carbon.  (Further illustrating that bonding pattern is everything when it comes to physical properties.)

The reason all this comes up is a discovery described in a press release from the University of British Columbia that I found out because of a friend and loyal reader of Skeptophilia.  Kimberlite pipes are not only unusual, they differ from each other, so the composition of each acts as a geological fingerprint.  So when a UBC geologist named Maya Kopylova tested samples of a kimberlite on Baffin Island, she found that its composition was inconsistent with the rocks of the nearby geological province -- the nearest rocks it matched were in Labrador, almost two thousand kilometers away.

This was sufficient to identify it as part of the North American craton, a (relatively) stable piece of continental crust that currently extends from eastern Canada, through southern Greenland, and over to Scotland.  (It was torn into chunks when the Mid-Atlantic Rift Zone formed on the order of two hundred million years ago, breaking up what was the supercontinent of Pangaea.)

How a chunk of a billion-year-old craton ended up two thousand kilometers away is uncertain, but it does give us a lens into how the continents have shifted during geologic history.  "The mineral composition of other portions of the North Atlantic craton is so unique there was no mistaking it,"  Kopylova said.  "It was easy to tie the pieces together.  Adjacent ancient cratons in Northern Canada—in Northern Quebec, Northern Ontario and in Nunavut—have completely different mineralogies...  Finding these 'lost' pieces is like finding a missing piece of a puzzle.  The scientific puzzle of the ancient Earth can’t be complete without all of the pieces."

Cool, too, that the discovery was made using remnants of what is very likely to be one of the most unpredictable and violent geological events on Earth.  (Okay, the formation of igneous traps is worse.  But still, kimberlites should surely come in second.)  The universe never ceases to fascinate me, and I'm always struck by the fact that no matter how much you know, there's always more to find out.

More, too, to worry about.  Although considering the current state of affairs, a supersonic volcanic eruption might actually lighten everyone's mood.

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Any guesses as to what was the deadliest natural disaster in United States history?

I'd speculate that if a poll was taken on the street, the odds-on favorites would be Hurricane Katrina, Hurricane Camille, and the Great San Francisco Earthquake.  None of these are correct, though -- the answer is the 1900 Galveston hurricane, that killed an estimated nine thousand people and basically wiped the city of Galveston off the map.  (Galveston was on its way to becoming the busiest and fastest-growing city in Texas; the hurricane was instrumental in switching this hub to Houston, a move that was never undone.)

In the wonderful book Isaac's Storm, we read about Galveston Weather Bureau director Isaac Cline, who tried unsuccessfully to warn people about the approaching hurricane -- a failure which led to a massive overhaul of how weather information was distributed around the United States, and also spurred an effort toward more accurate forecasting.  But author Erik Larson doesn't make this simply about meteorology; it's a story about people, and brings into sharp focus how personalities can play a huge role in determining the outcome of natural events.

It's a gripping read, about a catastrophe that remarkably few people know about.  If you have any interest in weather, climate, or history, read Isaac's Storm -- you won't be able to put it down.

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

The turning of the tides

It's tempting to think that conditions on Earth have always been like they are now.

On one level, we know they weren't.  When people picture the time of the dinosaurs, it usually comes along with images of swamps and ferns and rain forests.  (And volcanoes.  Most of the kids' books about dinosaurs illustrate them as living near erupting volcanoes, which seems like a poor choice of habitat.)

But the basics -- the air, water, soil, and so on -- we picture as static.  It's been the basis of hundreds of science fiction stories; people go back into the distant past, and although there are (depending on when exactly they went to) often giant animals who want to eat you, you have no problem breathing or finding food.

I had a neat hole punched in that perception last week when I read Peter D. Ward's book Out of Thin Air.

 

Ward is a paleontologist at the University of Washington, and his contention -- which is well-argued and supported by a wealth of evidence -- is that the oxygen content of the atmosphere has varied.  A lot.  It's at about 21% at sea level now, but hit a staggering low of 13% immediately after the Permian-Triassic extinction, comparable on today's Earth to being at an altitude of 12,500 feet (think the High Andes).  Humans time-traveling back then would have a seriously difficult time breathing, and life was probably confined to areas that were near sea level -- and those areas would be completely isolated from each other by higher ground in between where there was not enough oxygen to survive.

There were times when it was much higher, too.  Ward says in the late Carboniferous Era, the oxygen content suddenly spiked to around 30%, which explains why coal formation stopped; at 30% oxygen, dead plant matter will combust with little encouragement, resulting in little left behind to form coal seams.

If you'd like to find out more, I highly recommend Ward's book, which is not only an argument for the fluctuating-atmosphere model, but is a good overview of the major events in Earth's history.

Parasaurolophus skeleton [image courtesy of the Wikimedia Commons]

I had another blow delivered to the static-Earth perception from a study that was published last week in Geophysical Research Letters, called "Is There a Tectonically Driven Super‐Tidal Cycle?", by Mattias Green, J. L. Molloy, H. S. Davies, and J. C. Duarte, which considered the possibility that even the tides haven't always been as they are today.

What their study did was to look at a model of the dispersal of tidal energy, and they found that when all the continents were joined into a single land mass (Pangaea), which last happened at the end of the Permian Era a little over 250 million years ago, it represented a tidal energy minimum.  This meant that the tides were smaller than today, and that the majority of the (single) ocean was effectively a stagnant pool of water, with little vertical mixing of nutrients.  Stagnant, low-nutrient, low-oxygen water generally has little biodiversity -- a few species that can tolerate such conditions do exceptionally well, but the rest die out.  So this could be yet another reason that the cataclysmic Permian-Triassic Extinction happened, in which (by some estimates) 90% of the species on Earth became extinct.

What the Green et al. study suggests is that we're near a tidal maximum.  As the press release about the study put it:

In the new study, scientists simulated the movement of Earth’s tectonic plates and changes in the resonance of ocean basins over millions of years. 

The new research suggests the Atlantic Ocean is currently resonant, causing the ocean’s tides to approach maximum energy levels.  Over the next 50 million years, tides in the North Atlantic and Pacific oceans will come closer to resonance and grow stronger.  In that time, Asia will split, creating a new ocean basin... 

In 100 million years, the Indian Ocean, Pacific Ocean and a newly formed Pan-Asian Ocean will see higher resonance and stronger tides as well.  Australia will move north to join the lower half of Asia, as all the continents slowly begin to coalesce into a single landmass in the northern hemisphere... 

After 150 million years, tidal energy begins to decline as Earth’s landmasses form the next supercontinent and resonance declines.  In 250 million years, the new supercontinent will have formed, bringing in an age of low resonance, leading to low tidal energy and a largely quiet sea, according to the new research.

It's a little humbling to think about, isn't it?  The processes that shape the continents, drive the tides, control the chemistry of the atmosphere, will keep chugging along long after we're a paleontological footnote in the textbooks of our far distant descendants.  It's not that what we're doing now isn't critical; in the short term, the out-of-control fossil fuel burning is doing things to our atmosphere that will certainly cause us grievous harm, not to mention the short-sighted pollution of the very resources we depend on.

But if we do succeed in wiping ourselves out, which lately has seemed increasingly likely, the processes that govern the Earth will keep on going without us.  So will natural selection; the survivors of the current mass extinction will evolve into other "forms most beautiful and most wonderful," as Darwin put it in The Origin of Species.

Not that this will be much consolation to us, of course.  But I do find it comforting, in a strange way.  However important we think we are, on the scale of the natural world, we're pretty tiny.  Whatever damage we do, eventually the Earth will recover, with or without us.  And the atmospheric, geological, and tidal ups and downs will continue -- world without end, amen.