Once a week.
Subscribe to our weekly newsletter.
The ‘Atlas Of The Underworld’ And The Ghosts Of Past Geography
An upcoming, massive “atlas of the underworld” will allow geologists to rewind geologic time.
When two tectonic plates meet, they don’t meet cute. In fact, it’s a battle for supremacy, with one plate being subducted down under the other into the earth’s mantle. Inching downward they go, toward the earth’s molten core, 2,900 kilometers underground. The only thing left at the surface is the battle-scarred winner. “Every day, we’re losing geologic information from the face of the Earth,” according to Jonny Wu, a geologist at the University of Houston in Texas, speaking to Science. “It’s like losing pieces of broken glass as you’re trying to put it together again.”
This is, of course, a source of frustration for geologists trying to reconstruct the early history of the earth, and they’ve been increasingly looking to the earth’s mantle for answers. Using “mantle tomography,” they’ve been tracking earthquake waves as they travel through rock and using computer models to generate images similar to computerized tomography (CT) scans.
At first, the scans showed underground masses that looked like globs in a lava lamp. As the models have evolved, though, they’ve revealed what Science calls the “ghosts of past geography,” stiff, straight 500-kilometer-thick cold slabs. It’s already been an eye-opening enterprise: “Almost everywhere we’ve looked at this,” says John Suppe of the University of Houston, “What we find in the mantle isn’t exactly what would be predicted.”
Most interestingly, the tomography has shown that the slabs flex but don’t crumple, so it’s fairly easy to extrapolate backwards their paths to the surface, rewinding time and putting geologists on track to solving countless mysteries of ancient formations and current geologic features.
Now scientists Douwe van Hinsbergen, Douwe van der Meer, and Wim Spakman of Utrecht University in the Netherlands, are about to publish a catalog of 100 subducted plates. “Step by step we went deeper and deeper, older and older,” according to Van der Meer. The team describes their publication, due in December 2016, as the “atlas of the underworld.” Using their own tomographic model cross-checked with other models, it will describe each slab’s age, size, and related surface rock records.
“It’s a pretty exciting time to be able to pull all of these pieces together,” enthuses Mathew Domeier, who models tectonics at f the University of Oslo.
Even with this new atlas in hand, though, the science will continue to be refined, since mantle tomography sometimes produces questionable results, finding slabs that may not really be there. This is due to a few factors. The seismic data the models depend on is picked up through a scattershot collection of sensors that don’t cover every area, and gaps affect the integrity of the data since waves become less distinct when they’re picked up from far away. (This is also a factor as they travel away down closer to the core.) Also, not every area has earthquakes that generate waves, and scientists lack data from those locations.
An additional issue is that scientists use over 20 different models to analyze their data, sometimes leading to conflicting conclusions — one model may pinpoint a slab whose existence another model disputes. A postdoc at University of Oslo, Grace Shephard, intends to publish a report soon on the slabs that 14 models using the same data concluded are most likely to actually exist.
There’s one other thing even the atlas of the underworld can’t resolve: A slab doesn’t last forever. In about 250 million years it reaches the core, where it’s broken down for reuse. This means that the earth’s history from between 250 million years ago to its birth is likely to remain an unanswerable mystery.
An open letter predicts that a massive wall of rock is about to plunge into Barry Arm Fjord in Alaska.
- A remote area visited by tourists and cruises, and home to fishing villages, is about to be visited by a devastating tsunami.
- A wall of rock exposed by a receding glacier is about crash into the waters below.
- Glaciers hold such areas together — and when they're gone, bad stuff can be left behind.
The Barry Glacier gives its name to Alaska's Barry Arm Fjord, and a new open letter forecasts trouble ahead.
Thanks to global warming, the glacier has been retreating, so far removing two-thirds of its support for a steep mile-long slope, or scarp, containing perhaps 500 million cubic meters of material. (Think the Hoover Dam times several hundred.) The slope has been moving slowly since 1957, but scientists say it's become an avalanche waiting to happen, maybe within the next year, and likely within 20. When it does come crashing down into the fjord, it could set in motion a frightening tsunami overwhelming the fjord's normally peaceful waters .
The Barry Arm Fjord
Camping on the fjord's Black Sand Beach
Image source: Matt Zimmerman
The Barry Arm Fjord is a stretch of water between the Harriman Fjord and the Port Wills Fjord, located at the northwest corner of the well-known Prince William Sound. It's a beautiful area, home to a few hundred people supporting the local fishing industry, and it's also a popular destination for tourists — its Black Sand Beach is one of Alaska's most scenic — and cruise ships.
Not Alaska’s first watery rodeo, but likely the biggest
Image source: whrc.org
There have been at least two similar events in the state's recent history, though not on such a massive scale. On July 9, 1958, an earthquake nearby caused 40 million cubic yards of rock to suddenly slide 2,000 feet down into Lituya Bay, producing a tsunami whose peak waves reportedly reached 1,720 feet in height. By the time the wall of water reached the mouth of the bay, it was still 75 feet high. At Taan Fjord in 2015, a landslide caused a tsunami that crested at 600 feet. Both of these events thankfully occurred in sparsely populated areas, so few fatalities occurred.
The Barry Arm event will be larger than either of these by far.
"This is an enormous slope — the mass that could fail weighs over a billion tonnes," said geologist Dave Petley, speaking to Earther. "The internal structure of that rock mass, which will determine whether it collapses, is very complex. At the moment we don't know enough about it to be able to forecast its future behavior."
Outside of Alaska, on the west coast of Greenland, a landslide-produced tsunami towered 300 feet high, obliterating a fishing village in its path.
What the letter predicts for Barry Arm Fjord
Moving slowly at first...
Image source: whrc.org
"The effects would be especially severe near where the landslide enters the water at the head of Barry Arm. Additionally, areas of shallow water, or low-lying land near the shore, would be in danger even further from the source. A minor failure may not produce significant impacts beyond the inner parts of the fiord, while a complete failure could be destructive throughout Barry Arm, Harriman Fiord, and parts of Port Wells. Our initial results show complex impacts further from the landslide than Barry Arm, with over 30 foot waves in some distant bays, including Whittier."
The discovery of the impeding landslide began with an observation by the sister of geologist Hig Higman of Ground Truth, an organization in Seldovia, Alaska. Artist Valisa Higman was vacationing in the area and sent her brother some photos of worrying fractures she noticed in the slope, taken while she was on a boat cruising the fjord.
Higman confirmed his sister's hunch via available satellite imagery and, digging deeper, found that between 2009 and 2015 the slope had moved 600 feet downhill, leaving a prominent scar.
Ohio State's Chunli Dai unearthed a connection between the movement and the receding of the Barry Glacier. Comparison of the Barry Arm slope with other similar areas, combined with computer modeling of the possible resulting tsunamis, led to the publication of the group's letter.
While the full group of signatories from 14 organizations and institutions has only been working on the situation for a month, the implications were immediately clear. The signers include experts from Ohio State University, the University of Southern California, and the Anchorage and Fairbanks campuses of the University of Alaska.
Once informed of the open letter's contents, the Alaska's Department of Natural Resources immediately released a warning that "an increasingly likely landslide could generate a wave with devastating effects on fishermen and recreationalists."
How do you prepare for something like this?
Image source: whrc.org
The obvious question is what can be done to prepare for the landslide and tsunami? For one thing, there's more to understand about the upcoming event, and the researchers lay out their plan in the letter:
"To inform and refine hazard mitigation efforts, we would like to pursue several lines of investigation: Detect changes in the slope that might forewarn of a landslide, better understand what could trigger a landslide, and refine tsunami model projections. By mapping the landslide and nearby terrain, both above and below sea level, we can more accurately determine the basic physical dimensions of the landslide. This can be paired with GPS and seismic measurements made over time to see how the slope responds to changes in the glacier and to events like rainstorms and earthquakes. Field and satellite data can support near-real time hazard monitoring, while computer models of landslide and tsunami scenarios can help identify specific places that are most at risk."
In the letter, the authors reached out to those living in and visiting the area, asking, "What specific questions are most important to you?" and "What could be done to reduce the danger to people who want to visit or work in Barry Arm?" They also invited locals to let them know about any changes, including even small rock-falls and landslides.
What makes some people more likely to shiver than others?
Some people just aren't bothered by the cold, no matter how low the temperature dips. And the reason for this may be in a person's genes.
Eating veggies is good for you. Now we can stop debating how much we should eat.
- A massive new study confirms that five servings of fruit and veggies a day can lower the risk of death.
- The maximum benefit is found at two servings of fruit and three of veggies—anything more offers no extra benefit according to the researchers.
- Not all fruits and veggies are equal. Leafy greens are better for you than starchy corn and potatoes.