Reference: Torsvik, T. H. et al. Nature Geosci. 2013, 223. http://dx.doi.org/10.1038/NGEO1736

Further reading: Sid Perkins in Nature News

Image credit: Nature Geoscience

“What we may be detecting is the start of one of these large eruptive events that — if it ever happens — could cause very massive destruction on Earth,” says seismologist Michael Thorne, the study’s principal author and an assistant professor of geology and geophysics at the University of Utah.

But disaster is “not imminent,” he adds, “This is the type of mechanism that may generate massive plume eruptions, but on the timescale of 100 million to 200 million years from now. So don’t cancel your cruises.”

The new study, set for publication this week in the journal Earth and Planetary Science Letters, indicates that two or more continent-sized “piles” of rock are colliding as they move at the bottom of Earth’s thick mantle and atop the thicker core some 1,800 miles beneath the Pacific. That is creating a Florida-sized zone of partly molten rock that may be the root of either of two kinds of massive eruptions far in the future:

1. Hotspot plume supervolcano eruptions like those during the past 2 million years at Wyoming’s Yellowstone caldera, which covered North America with volcanic ash.

2. Gargantuan flood basalt eruptions that created “large igneous provinces” like the Pacific Northwest’s Columbia River basalts 17 million to 15 million years ago, India’s Deccan Traps some 65 million years ago and the Pacific’s huge Ontong Java Plateau basalts, which buried an Alaska-sized area 125 million to 199 million years ago.

“These very large, massive eruptions may be tied to some extinction events,” Thorne says. The Ontong eruptions have been blamed for oxygen loss in the oceans and a mass die-off of sea life.

Since the early 1990s, scientists have known of the existence of two continent-sized “thermochemical piles” sitting atop Earth’s core and beneath most of Earth’s volcanic hotspots — one under much of the South Pacific and extending up to 20 degrees north latitude, and the other under volcanically active Africa.

Using the highest-resolution method yet to make seismic images of the core-mantle boundary, Thorne and colleagues found evidence the pile under the Pacific actually is the result of an ongoing collision between two or more piles. Where they are merging is a spongy blob of partly molten rock the size of Florida, Wisconsin or Missouri beneath the volcanically active Samoan hotspot.

This map shows Earth’s surface superimposed on a depiction of what a new University of Utah study indicates is happening 1,800 miles deep at the boundary between Earth’s warm, rocky mantle and its liquid outer core. Using seismic waves the probe Earth’s deep interior, seismologist Michael Thorne found evidence that two continent-sized piles of rock are colliding as they move atop the core. The merger process isn’t yet complete, so there is a depression or hole between the merging piles. But in that hole, a Florida-sized blob of partly molten rock — called a “mega ultra low velocity zone” — is forming from the collision of smaller blobs on the edges of the continent-sized piles. Thorne believe this process is the beginning stage of massive volcanic eruptions that won’t occur for another 100 million to 2100 million years – Credit: Michael S. Thorne, University of Utah

The study’s computer simulations “show that when these piles merge together, they may trigger the earliest stages of a massive plume eruption,” Thorne says.

Thorne conducted the new study with Allen McNamara and Edward Garnero of Arizona State University, and Gunnar Jahnke and Heiner Igel of the University of Munich. The National Science Foundation funded the research.

Probing the Deep Earth with Seismic Waves

Seismic imaging uses earthquake waves to make images of Earth’s interior somewhat like X-rays make CT scan pictures of the inside of the human body.

The new study assembled the largest set of data ever used to map the lower mantle in the Pacific region by using 4,221seismograms from hundreds of seismometers around the world that detected 51 deep earthquakes originating more than 60 miles under the surface.

Thorne and colleagues looked for secondary earthquake shear waves known as S-waves that travel through much of Earth, hitting the core, and then convert to primary compressional waves or P-waves as they travel across the top of the core. Then they convert back to S-waves as they re-enter the mantle and then reach seismometers. Thorne says the short bursts of P-wave energy are very sensitive to detecting variations in the rock at the core-mantle boundary.

Thorne performed 200 days of supercomputer simulations at the University of Utah’s Center for High Performance Computing. He simulated hundreds of possible shapes of the continent-sized piles and state-sized blobs until he found the shapes that could best explain the seismic wave patterns that were observed.

A Look at the Core-Mantle Boundary

The new study provided an unusual look at one of the most remote parts of Earth, located about 1,800 miles beneath the surface: the boundary between the planet’s molten outer core and its warm mantle rock, which has convection movement that has been compared with a conveyor belt or slowly boiling tomato soup. (Tectonic plates of Earth’s crust and uppermost mantle drift atop the warmer, convecting lower mantle.)

“We did hundreds of simulations for lots of different variations of what Earth might look like at the core-mantle boundary — the most simulations anybody has ever done to look at the core-mantle boundary structure,” Thorne says

At some places where oceanic and continental tectonic plates collide — such as offshore from the Pacific Northwest to Alaska — the seafloor plate dives or “subducts” beneath the continent and plunges slowly into the mantle. Thorne suspects subducting plates ultimately fall deep enough to help push the piles around on Earth’s core.

Whether hotspots originate at the core-mantle boundary or at shallower depths has been debated for decades.

But in the 1990s, geophysicists found evidence for the continent-size thermochemical piles beneath Africa and the Pacific. These are known technically as LLSVPs, or “large low shear velocity provinces,” because seismic shear waves passing through them move 5 percent slower that through surrounding mantle rock. That suggests they have a different composition and-or temperature than the surrounding mantle.

Previous studies also have observed smaller blobs of rock, measuring perhaps 60-by-60 miles on the edges of the continent-sized masses. Seismic shear waves move as much as 45 percent slower through these blobs — known technically as ULVZs or “ultra low velocity zones” — indicating they may be spongy and partly molten.

Thorne says his analysis of seismic waves passing through the core-mantle boundary reveals the Pacific pile really represents two or more continent-sized piles slowly sliding atop the core and colliding so that partly molten blobs on their edges are merging into the largest such blob or ULVZ ever observed — roughly the size of Florida.

“My study might be the first to show actual seismic evidence that the piles are moving,” he says. “People who have done previous simulations have suggested this. They are sitting atop the core and getting pushed around by overlying mantle forces like subduction. They move around on the core somewhat like continental plates drift at Earth’s surface.”

Thorne says the merging LLSVP piles are each about 1,800 miles diameter, forming a single pile some 3,600 miles wide from east to west and stretching across Earth’s core beneath an area from Australia almost to South America. Two blobs, or ULVZs, on the piles’ edges merged to form a new blob that is perhaps 6 to 10 miles thick and covers an area about 500 miles long and 150 miles wide, about the area of Florida or “eight to 10 times larger than any ULVZs we observed before,” Thorne says.

Because the larger piles haven’t fully merged, seismic imaging shows there is a depression or “hole” between them, and the Florida-sized blob is forming there as smaller UVLZs merge in the hole.

“We are actually seeing that these piles are being shoved around,” Thorne says. “If hotspots actually are generated near the core-mantle boundary, where they are being generated seems related to where these piles and ULVZs are. So if we are pushing these piles around, we also are pushing around where hotspot volcanism may occur.”

Warmer rock is less dense than cooler rock. Thorne says that where the ULVZ blobs form seems to be related to where the hot rock starts convecting upward to begin the long, slow process of forming a plume that eventually causes massive eruptions.

Source: University of Utah [February 06, 2013]

Elevated levels of iridium, magnetic spherules, and titanomagnetite grains, collectively called “impact markers,” form the bulk of the evidence for the Younger Dryas impact hypothesis, a hotly contested idea that links climate change, extinctions, and the demise of the Clovis culture.

Scientists found high levels of the reported markers in deposits called black mats, the organic-rich remains of old marshes and swamps, at several sites in the southwestern U.S. and the Atacama Desert of northern Chile. Markers were found in black mats ranging in age from 6,000 to more than 40,000 years in areas far removed from the purported impact location. These findings indicate the markers accumulated naturally in wetlands and are not the result of a catastrophic impact event. The full report is available online.

Sediments spanning the Younger Dryas boundary in Arizons. The 'black mat" is the dark horizon at mid-height - Credit: Web

“Luis and Walter Alvarez’s proposal that an extraterrestrial impact was responsible for extinctions at the Cretaceous-Tertiary boundary eventually moved from unlikely hypothesis to accepted theory, and with its acceptance came the temptation to apply this explanation to any rapid change in Earth’s conditions,” said USGS Director Marcia McNutt. “The results of this study demonstrate the importance of maintaining a healthy skepticism and multiple working hypotheses.”

The controversial Younger Dryas impact hypothesis contends that an extraterrestrial object, possibly a comet, exploded over North America about 12,900 years ago, resulting in dramatic climate change, massive wildfires, and the extinction of many large herbivores and their predators. If true, the recency of such a large impact might have implied a greater risk to humanity than previously imagined.

“When the idea was first promoted in 2007, those of us familiar with black mats suspected that normal depositional processes in wetlands might be responsible,” said Dr. Jeff Pigati, a USGS geologist and lead investigator of the new study. Indeed, this is what Pigati and coauthors now report in this week’s issue of Proceedings of the National Academy of Sciences, USA.

“This is a great object lesson for how scientific hypotheses are done and undone,” said Paul Baker, Professor of Earth and Ocean Sciences at Duke University and a member of National Geographic Society’s Committee for Research and Exploration.

Source: U.S. Geological Survey [April 23, 2012]

The volcano is close to the ash-spewing Eyjafjoell, which last year caused the world’s biggest airspace shut down since World War II, affecting more than 100,000 flights and eight million passengers.

The Iceland Civil Protection Authority said it was closely monitoring the situation.

“The movements around Hekla have been unusual in the last two to three days,” University of Iceland expert Pall Einarsson said.

While this might not necessarily mean an immediate blast, “the volcano is ready to erupt,” he stressed.

“The mountain has been slowly expanding in the last few years because of magma buildup,” he explained.

The volcano, dubbed by Icelanders in the Middle Ages as the “Gateway to Hell,” is one of Iceland’s most active, having erupted some 20 times over the past millennium, most recently on Feb. 26, 2000.

It measures 4,891-feet (1,491-meters) and is located about 70 miles (110 kilometers) east of Reykjavik, not far from Eyjafjoell.

The news comes just over a month after this year’s violent eruption at the Grimsvotn volcano, in the southeast of the country.

That eruption subsided after less than a week, having spit out far more ash than Eyjafjoell, but due to more favorable winds for Europe caused far less air traffic disruption.

Both of Iceland’s latest eruptions provided warning signs several hours before, but Hekla is known for having a very short fuse.

“Hekla never gives you much of a warning,” Einarsson said, pointing out that in 2000, it began rumbling an hour and a half before the outbreak of magma, which “was actually an unusually long warning. In 1970 we only got 25 minutes notice.”

Rongvaldur Olafsson, a project manager at the Icelandic Civil Protection Authority, said no immediate safety precautions were being taken but, “We will watch the mountain and developments very closely.”

Source: Fox News [Jul 6th, 2011]

In almost every way, the “Garden of the Gods at Colorado Springs” excursion at the annual meeting of the Geological Society of America (GSA) last year was a normal — even enjoyable — field trip. Standard geologic terminology was used in the accompanying field trip guide, and throughout the trip itself. The trip leaders discussed past events in terms of millions and billions of years. At each stop along the trip, the guides relied on orthodox geologic thinking, including a standard examination of sedimentary features and the nature of contacts between units.

But in reality, the trip was anything but a normal geology field trip. Instead, it was an example of a new strategy from creationists to interject their ideas into mainstream geology: They lead field trips and present posters and talks at scientific meetings. They also avoid overtly stating anything truly contrary to mainstream science.

But when the meeting is over, the creationist participants go home and proudly proclaim that mainstream science has accepted their ideas.
It’s a crafty way of giving credence to creationism. But is there anything mainstream scientists, or the conveners of meetings and field trips, can or should do about it?

CXontinue reading: | Source: Earth Magazine [June 10, 2011]

Throughout the world there exist regions of ancient crust, referred to as cratons, which have resisted being recycled into the interior of our tectonically dynamic planet. These geologic anomalies appear to have withstood major deformation thanks to the presence of mantle roots. A mantle root is a portion of Earth’s mantle that lies beneath the craton, extending like the root of a tooth into the rest of the underlying mantle.

Just like a tooth, the mantle root of a craton is compositionally different from the normal mantle into which it protrudes. It is also colder, causing it to be more rigid. These roots were formed in ancient melting events and are intrinsically more buoyant than the surrounding mantle. The melting removed much of the calcium, aluminum, and iron that would normally form dense minerals. Thus, these roots act as rafts bobbing on a vigorously convecting mantle, on which old fragments of continental crust may bask in comparative safety.

However, geophysical calculations have suggested that this buoyancy is not enough to stop destruction of the mantle roots. According to these calculations, the hotter temperatures that are widely thought to have existed in Earth’s mantle about 2.5 to 3 billion years ago should have warmed and softened up the base of these roots sufficiently to allow them to be gradually eroded from below, leading to their eventual destruction as they were entrained, piece by piece, into the convecting mantle. A stronger viscosity contrast between the root and the underlying mantle is required to ensure preservation.

In the Sept. 2 issue of Nature, Anne Peslier, an ESCG-Jacobs Technology scientist working at NASA-Johnson Space Center and her colleagues David Bell from Arizona State University and Alan Woodland and Marina Lazarov from the University of Frankfurt, published measurements of the trace water content of rocks from the deepest part of a mantle root that offer an explanation for this mystery.

“It has long been suspected, but not proven, that cratonic mantle roots are dryer than convecting upper mantle,” explains Bell, an associate research scientist in the School of Earth and Space Exploration and the department of chemistry and biochemistry in ASU’s College of Liberal Arts and Sciences. “The presence of very small quantities of water is known to weaken rocks and minerals. During partial melting, such as that experienced by the mantle roots, water – like calcium, aluminum and iron – is also removed.” Continue reading, physorg.com