Tag Archives: subduction

Scientists resurrect mysterious missing tectonic plate beneath Canada

Plate tectonic reconstruction of western North America 60 million years ago showing subduction of three key tectonic plates, Kula, Farallon and Resurrection. Credit: University of Houston.

The Pacific Plate is the largest of Earth’s 17 currently known tectonic plates. However, during the Cenozoic Era about 60 million years ago, the Pacific Basin consisted of some other tectonic plates that have since subducted into Earth’s mantle. Geologists have always known about two plates in the Pacific Ocean during this era, known as Kula and Farallon. Now, in a new study, researchers have confirmed the existence of a previously proposed third ancient Pacific plate called Resurrection.

Reverse engineering tectonic movements

Tectonic plates glide over Earth’s mantle, in the outermost shell known as the lithosphere. These plates can be thought of like pieces of a cracked shell that rest on the hot, molten rock of Earth’s mantle and fit snugly against one another.

Across the eons, tectonic plates have changed significantly. When two such plates come together, one of them has to give in. Kula and Farallon, both oceanic plates under the northern Pacific Ocean, sunk under the North American plate millions of years ago.

The fate of Ressurection, which is believed to have formed a special type of volcanic belt along the coast of Alaska and Washington State, had until now been a matter of contention.

“We believe we have direct evidence that the Resurrection plate existed. We are also trying to solve a debate and advocate for which side our data supports,” Spencer Fuston, a third-year geology doctoral student at the University of Houston, said in a statement.

Jonny Wu (left), assistant professor of geology in the UH Department of Earth and Atmospheric Sciences and Spencer Fuston, a third-year geology doctoral student, applied slab unfolding to reconstruct what tectonic plates below the Pacific Ocean looked like during the Cenozoic Era. Credit: Houston University.

The geologists at the University of Houston analyzed existing mantle tomography images, which are like a CT scan of the Earth’s interior. They employed a technique called slab unfolding that essentially turns back the clock, enabling the researchers to reconstruct what tectonic plates in the Pacific Ocean looked like millions of years ago.

This 3D mapping technique pulled out subducted plates, stretching them to their original shapes. Volcanoes are known to form right at the boundaries of tectonic plates, and the boundaries of the reconstructed Resurrection plate match well with the location of ancient volcanic belts in northern Canada.

In the animation below, you can see how the Kula, Farallon, and Resurrection tectonic plates sunk beneath the North American Plate 60 million years ago to the present day.

Since volcanoes heavily impact the climate, this method could prove useful in modeling the earth and its past climate.

The findings appeared in the journal GSA Bulletin.

Evidence of granite found on Mars – Red Planet geology more complex than previously thought

Geologists have now found the most compelling evidence of granites on Mars – something which prompts more complex theories about the geology and tectonic activity on the Red Planet.

Granites and basalts

Basalt and Granite. Credits: Rice University.

Granites are igneous rocks, pretty common on the surface of Earth. It is often called a ‘felsic’ (white rock) – because it is very rich in so-called white minerals, such as quartz or feldspar. It is contrasted with mafic rocks (for example basalt), which are relatively richer in magnesium and iron. Now, large amounts of feldspar have been found in a Martian volcano. Interestingly enough, minerals commonly found in basalts are completely absent from that area; considering how basalts are almost ubiquitous on Mars, this initially came as a shock, but now, geologists have come up with a theory to explain this.

Granite, or its eruptive equivalent, rhyolite, is often found on Earth in tectonically active regions such as subduction zones. However, since Mars isn’t tectonically active, there are no subduction zones there, so there has to be a different cause. The team studying the case concluded that prolonged magmatic activity on Mars can also produce these granitic compositions on very large scales.

“We’re providing the most compelling evidence to date that Mars has granitic rocks,” said James Wray, an assistant professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology and the study’s lead author.

Red Planet geology

A 'spectral window' into the Martian geology - bright magenta outcrops have a distinctive feldspar-rich composition. (Credit: NASA/JPL/JHUAPL/MSSS)

A ‘spectral window’ into the Martian geology – bright magenta outcrops have a distinctive feldspar-rich composition. (Credit: NASA/JPL/JHUAPL/MSSS)

For many years, the geology of Mars has been considered to be very simplistic, consisting of mostly one single type of rock: basalt – a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface. The dark rock can also be found on Earth in many volcanically active areas, such as Hawaii or Iceland for example.

But earlier this year, the Mars Curiosity started to cast some doubt on those beliefs, when it reported finding soils with a composition similar to granite. No one really knew what to make of this discovery, but since it appeared to be very localized, it was just considered a local anomaly. However, this new research analyzed things at a much larger scale, using remote sensing techniques with infrared spectroscopy to survey a large volcano on Mars that was active for billions of years. The volcano is perfect for this type of study, because it is dust free (a true rarity on Mars) – some of the fastest-moving sand dunes on Mars sweep away any would-be dust particles on this volcano.

Much to the delight of researchers, the limitations of the remote sensing technology were an advantage in this case:

“Using the kind of infrared spectroscopic technique we were using, you shouldn’t really be able to detect feldspar minerals, unless there’s really, really a lot of feldspar and very little of the dark minerals that you get in basalt,” Wray said.

Separating the white and the black

So we have an island of white feldspar amidst an ocean of black basalt – how did it form?

When you have magma in the subsurface, it cools off very, very slowly. In a tectonically inactive planet like Mars, this process can be very stable. While the magma slowly cools off, low density melt separates from high density crystals, and if the conditions are just right, this process can take place for billions of years, leading to the creation of granitic rocks, as computer simulations showed.

“We think some of the volcanoes on Mars were sporadically active for billions of years,” Wray said. “It seems plausible that in a volcano you could get enough iterations of that reprocessing that you could form something like granite.”

While we are trying to figure out the existence (or lack of it) of life on Mars, this is another wake-up call, showing just how little we understand about the geologic processes on the Red Planet – which ultimately govern the appearance of life. Anyway, the geology of Mars just got a lot more interesting.

Journal Reference:

  1. James J. Wray, Sarah T. Hansen, Josef Dufek, Gregg A. Swayze, Scott L. Murchie, Frank P. Seelos, John R. Skok, Rossman P. Irwin, Mark S. Ghiorso. Prolonged magmatic activity on Mars inferred from the detection of felsic rocksNature Geoscience, 2013; DOI: 10.1038/NGEO1994

Reunderstanding how West America was formed

For decades, the accepted theory was that the mountain chains running from Alaska to Mexico were created from fragments of land scraped off a huge tectonic plate moving eastwards, called the Farallon, that converged with North America and sunk below it over the past 200 million years or so. But alas, geologists and geophysicists aren’t always in accord, and a new seismologic study suggests that island arcs, like those in today’s western Pacific, may have piled atop one another, sinking and forming the verical buried slabs we see today. As north American moved to the west and Farallon moved to the east, it scraped off the tops of these slabs, raising mountains in the process.

Farallon plate, 3-D computer model

“This is an important, even momentous, paper,” says Eldridge Moores, a geologist at the University of California in Davis, who proposed a similar westward-diving crustal motion, or subduction, for western North America in 1970 (Moores, E. Nature 228, 837–842 (1970)). “I’m really glad to see it.”

The study was conducted by Karin Sigloch, a seismologist at Ludwig Maximilians University in Munich, Germany, and Mitchell Mihalynuk, a geologist at the British Columbia Geological Survey in Victoria, Canada. Using a method called seismog tomography, they created 3D images of the subducted slabs deep below the continent. Their conclusions, while certainly out of the box, make a lot of sense.

farallon3

Judging from the images they created, the slabs now look like massive, nearly vertical walls extending from 800 to 2,000 kilometres below the surface of western North America. The first red flag was when they observed that the biggest of these slabs was not connected to the plate now subducting beneath the US Pacific Northwest. Geologists previously believed that the piece was a remnant of the Farallon, but the new data showed the biggest buried slab wasn’t connected to the Farallon at all.

But Sigloch and Mihalynuk took the results from their image and tried to reconstruct where the crustal pieces fit together.

“It was geometrically just not possible,” says Sigloch. “The more we saw, the less sense it made.”

Instead, they came up with a different theory. Island arcs to the west of North America began crashing into the continent, piling atop one another. As North America was pushed to the West, it overrode the islands, some of which piled up and some of which sank beneath the slabs. Finally, oceanic crust began subducting eastward under the continent, in the configuration we see today.

farallon2

This also solves the longstanding question as to why the buried slabs are piled vertically rather than at an angle.

“A lot of things are just simpler now,” she says.

Via Nature

(c) Brown University

Ancient tectonic plate re-discovered beneath California

(c) Brown University

(c) Brown University

Millions of years ago, an ancient tectonic plate called the Farallon oceanic plate used to sit between the Pacific and North American plates. In time, the plate “disappeared” beneath the North American one, however geologists at Brown University have now found physical surface remnants of the plate under sections of central California and Mexico. The Farallon surface fragments may now explain a seismic anomaly in the region that has eluded scientists for some time.

Though they used to be spaced apart by the Farallon plate, currently the North American and Pacific tectonic plates are joined through a continental transform fault known as the San Andreas fault. In the process the Farallon plate was forced underneath the North American one through subduction, leaving a few small remnants at the surface that became part of the Pacific plate.

Research carried out by Brown University geologists has revealed however that bits of the Farallon plate have remained attached at the surface. Surprisingly part of Mexico’s Baja region as well as a considerable landmass in central California rest upon slabs of this ancient plate.

Oddly enough, close to the region a seismic anomaly has been registered in the Sierra Nevada mountains in the Golden State. Dubbed the  Isabella anomaly, it suggests that a sizable mass of relatively cool and dehydrated material is present at a depth of 100 to kilometers below the Earth’s surface. This was found after many years ago geologists mapped out the region many miles beneath the earth’s surface by transforming seismic waves that can be fast or slow into actual images. These seismic waves travel at velocities varying according to the materials they encounter.

For some time, geologists have been trying to explain this peculiarity and many theories have been proposed to justify it, like delamination – the breaking of  the lithospheric plate under the mountains. High-magnesium andesite deposits on the surface near the eastern edge of the anomaly found by the Brown researchers, often linked to the melting of the oceanic crust, provided evidence, however, that in fact the Farallon plate broke off and melted into the mantle.

This evidence convinced Brown geophysicist Donald Forsyth and his collaborators that the Isabella anomaly might also be part of a slab connected to an unsubducted fragment of the Farallon plate. The findings, published in the journal Proceedings of the National Academy of Sciences, might warrant a new investigation into the ancient plate tectonics of western North America, especially considering the region is extremely seismically active.

“The geometry was the kicker,” Forsyth said. “The way they line up just makes sense.”

“However the Sierra Nevada was delaminated,” Forsyth said, “it’s probably not in the way that many people had been thinking.”

Massive 8.6 quake strikes Indonesia – but didn’t create a monster tsunami

An 8.6-magnitude earthquake and powerful aftershocks struck the coast of Indonesia today, resurrecting fears of a big tsunami like the one responsible for one of the biggest modern disasters ever. However, this earthquake, which struck at 2:38 p.m. local time (4:38 a.m. ET), about 270 miles (435 kilometers) off the coast of the Indonesian island was drastically different than the 9.1 earthquake that hit Indonesia in 2004.

Strike slip, subduction, and the earthquake in Indonesia

Earthquakes come in different sizes and “flavors”. The can occur as a result of subduction zones, when one tectonic plate is moving beneath another one, causing a massive amount of friction and stress, they can be strike-slip earthquakes – when plates are moving laterally one in respect to the other, or they can occur even due to mineral phase changes in the depths of the Earth. While the one in 2004 was a subduction, this one was a strike-slip earthquake, and typically, these kind of events aren’t associated with big tsunami risks.

Diagram of a subduction zone; the earthquake in 2004 was a result of such a subduction zone

“With a strike-slip event you don’t have the same potential hazard for a tsunami as you do with a subduction event because the plates are moving adjacent to each other,” said Julie Dutton, a geophysicist with the U.S. Geological Survey (USGS).

The 8.6 earthquake

The epicenter was pretty much in the same place as the 9.1 one in 2004, which created a tsunami that killed over 220.000 people. However, even though the magnitudes may seem similar, the difference is quite big: the magnitude is measured in a logarithmic scale, which means that in this case, a magnitude of 9 is 10 times bigger than than a magnitude of 8. In the same way, an 8.6 one is over 3 times less powerful than a 9.1 one, so the magnitude difference is quite significant.

The earthquake was followed by an 8.2-magnitude aftershock hit at 6:43 a.m. ET, the USGS said, raising the tsunami alerts, but as I already said, it’s usually subduction earthquakes that are associated with major tsunamis. This happens because when one tectonic plate is diving beneath another, a huge portion of the seafloor is shoved beneath, and that displacement of sea floor also displaces ocean water. Basically, the more seafloor you shove beneath the other plate, the bigger tsunami you get. At a strike slip earthquake, faults in the crust essentially moved from side to side instead of up and down, and as a result, the largest tsunami recorded was smaller than 2 meters, so all alerts have been lowered.

An estimation of the damage caused by the earthquake is still not available, but we’ll keep you posted on the developments as they occur.

Spectacular underwater volcano eruption shows geologists a land forgotten by time

Scientists who observed an underwater eruption in 2009 concluded they observed a tear in the planetary crust that mimicks the birth of a subduction zone.

Subduction takes place at convergent tectonic plate boundaries; basically, one plate slides below the other – an extremely complicated and complex process that drives along a series of other processes, including continental drift.

The researchers in case published their conclusions in Nature, depicting the collection of boninite, an extremely rare and distinct kind of lava that can only be found in subduction areas; even when it is found in these areas, it is collected from volcanoes dead of over a million years – so this find is extremely important and unique. To put it simply, nobody has ever before collected fresh boninite, nor observed its eruption.

“Everything about the eruption itself – how fast, how intense, the ratio of lava to explosive fragments, the amount and composition of gas released – is new to us,” said co-author Kenneth Rubin, University of Hawaii geologist. “Plus, having a young, fresh occurrence of this very rare rock type to study gives us the opportunity to examine subtle chemical and mineralogical variations in a pristine specimen.”

The researchers writing in Nature Geoscience believe, and for good reason, that the reason why the eruption was so explosive is the combination of water, carbon dioxide and sulphur dioxide. They are also interested in returning to the site once more, and study it even further.
Source: Planetsave