Stories tagged plate tectonics

New seafloor map: the red dots represents earthquake occurrences
New seafloor map: the red dots represents earthquake occurrencesCourtesy David Sandwell, Scripps Institution of Oceanography, UC San Diego
Researchers at the Scripps Institution of Oceanography at UC San Diego have used archived satellite data to develop new, detailed maps of the ocean floor, giving scientists new insights into earthquakes, spreading seafloors, and plate tectonics. The maps are the latest to be compiled in over two decades and provide a higher resolution picture of the ocean bottom revealing features not seen in previous versions. The study appears in the journal Science.

Scripps Institution of Oceanography report

Stromatolites at Shark Bay: the living fossils are featured in NOVA's "Australia: First 4 Billion Years".
Stromatolites at Shark Bay: the living fossils are featured in NOVA's "Australia: First 4 Billion Years".Courtesy Paul J. Morris
NOVA's excellent 4-part documentary series "Australia: First 4 Billion Years" is scheduled to re-broadcast on July 16th, 23rd, 30th and August 6th. Check your local PBS schedule for times. But if the dates don't work for you, the entire series is (or at least was when I watched it) on YouTube. Here are the links:

Part 1: Awakening
Part 2: Life Explodes
Part 3: Monsters
Part 4: Strange Creatures

The series is beautifully put together with gorgeous high definition video shot in locations all over Australia. The host, biologist Richard Smith, explains the science in a thoughtful and comprehensible manner while introducing viewers to many of the continent's stunning topographical features, and the strange and wonderful lifeforms - both past and present - found there. It's well worth your time.

The future me?: Becoming a fossil isn't as easy as it sounds.
The future me?: Becoming a fossil isn't as easy as it sounds.Courtesy Mark Ryan
My plan following my inevitable demise has long been to be cremated and have my ashes dumped into Amity Creek from the seventh stone bridge along Seven Bridges Road in Duluth, Minnesota so they eventually end up in Lake Superior some two miles downstream. But now, after watching this nifty and informative video detailing how to become a fossil, I may reconsider and just have my intact carcass dumped into the creek so it ends up at the bottom of the Great Lake and gets covered by sediment that eventually turns my bones to stone. Who knows? - some future reader of this post may be able to view my fossilized remains at some museum exhibit.

A brief and nifty refresher video on plate tectonics and plate boundaries to help finish up the year.

Location of Cameron's deep dive: Challenger Deep is about 300 miles southwest of Guam.
Location of Cameron's deep dive: Challenger Deep is about 300 miles southwest of Guam.Courtesy Kmusser via Wikipedia Creative Commons
Hollywood director James Cameron returned safely from a dive that took him nearly seven miles to the bottom of the Mariana Trench. Encased in a narrow submersible of his own design, Cameron reached the bottom in an area of the trench known as Challenger Deep after a 2.5 hour descent. He spent three hours exploring the sea bottom using the well-outfitted submarine's cameras and sampling equipment to collect images, fauna, and other data from the silty seabed. The single-person capsule - built to withstand up to 1000 atmospheres of pressure - held up well under the eight tons(!) per square inch that six and a half miles of ocean water exerted upon it. As today goes on, I'm sure more information will come out about this remarkable feat. In the meantime, I'm really anxious to see what images he captured down there, and we'll all get that chance when the National Geographic Society - one of the expedition's sponsors - comes out with a planned future program about the dive.

National Geographic website
BBC story
Mother Nature Network

New Madrid earthquakes: Earthquake fissure filled with intruded sand in Mississippi County, Missouri, formed at the time of the New Madrid earthquake. 1904 photograph by M. L. Fuller.
New Madrid earthquakes: Earthquake fissure filled with intruded sand in Mississippi County, Missouri, formed at the time of the New Madrid earthquake. 1904 photograph by M. L. Fuller.Courtesy US Geological Survey Photographic Library
Today marks the bicentennial of the start of the historic New Madrid earthquake series, which began at 2am on December 16, in 1811. The quakes were so powerful, large areas of land uplifted and sank creating new lakes and swamps, and causing islands to disappear. Large waves spawned by the tremors raked across the banks of the Mississippi causing massive landslides, and even briefly changing the course of the mighty river.

Named after the nearby river village of New Madrid in the then Louisiana Territory (now Missouri), the quake and its many aftershocks affected an area 10 times larger than the famous 1906 San Francisco earthquake. Luckily, the New Madrid area was sparsely populated when the line of strong earthquakes took place, as they were the strongest recorded earthquakes ever to take place east of the Rocky Mountains.

Earthquakes of such magnitude as those that struck New Madrid (~ 7.0) typically occur along plate boundaries - areas where one tectonic plate is colliding with another, such as along the West Coast's San Andreas Fault. The mid-section of the country sets on only one plate - the normally stable North American plate. Faults do run through it, such as the Cottonwood Grove and the Reelfoot faults which some scientists hypotheisze were responsible for the New Madrid series.

But researchers don't agree on what caused the strong intraplate earthquakes. They could have been triggered by other distant earthquakes or by the release of energy built up by the heating of the crust from an upper mantle magma plume or from isostatic rebound - that is the release of stresses caused by the retreat of glaciers that once covered the region.

Whatever the cause and despite new data being gathered by present day geologists, the New Madrid earthquakes were an historic anomaly that remain wrapped in mystery.

Earth magazine story
More about the New Madrid earthquakes


Sonar study location along the Tonga Trench
Sonar study location along the Tonga TrenchCourtesy NOAA (with adaptation by author)
Here’s something you don’t see everyday: some very amazing images of a chain of mountains heading toward a subduction zone in the South Pacific. (Make sure you watch the video at the top of this story link - it seems to take a few seconds to load). The pictures were unveiled this week at the annual American Geophysical Union meeting held in San Francisco, California.

Researchers from Oxford and Durham universities took sonar readings along the bottom of the South Pacific northeast of New Zealand that show a chain of underwater mountains being dragged westward on the Pacific plate and subducted into theTonga Trench . This chasm is second only to the Marianas Trench in seabed depth – nearly 11 kilometers (6.6 miles) deep. The computer model created from the data shows one giant volcano at the very edge of the trench breaking into huge blocks and beginning to collapse into the abyss. It’s actually pretty cool to see. Earthquakes occur less frequently near where the volcanoes are being gobbled up, and scientists differ on whether the giant broken chunks of the volcano help or hinder the subduction process, but the images clearly show the mechanism at work.

Illustration of plate boundaries and subduction
Illustration of plate boundaries and subductionCourtesy USGS
According to the theory of plate tectonics both oceanic crust and continental crust ride atop rigid plates that migrate slowly across the globe, colliding with and pulling away from each other. There are three main types of boundary zones created by this movement: convergent (moving toward each other), divergent (moving away from each other) and transform (moving side by side). In the first example, which is the type this article deals with, the lighter oceanic plate (Pacific Plate) is subducting under the heavier continental plate (Indo-Australian Plate). The process is part of the creation and recycling of the Earth’s lithosphere – that is it’s rocky crust along with the uppermost part of the mantle. Some mantle material is forced upward in the process, and the land near these subduction zones – like that in Japan and along the coast of Chile in South America - is often populated with volcanoes. This collision of plates causes tremendous tensions to build up along the contact zone. The extreme pressure can continue building over hundreds or even thousands of years until it's too much, and the plates start to shift. All the pent-up energy is suddenly released in fits and starts in the form of earthquakes and aftershocks, as happened this year (and is still happening) in Sendai, Japan and Christchurch, New Zealand.

The underwater volcanic chain spreads across the ocean bottom in a southeasterly direction for several thousands kilometers as each mountain makes it way westward toward the trench at the rate of about 6cm per year. That's about as fast as your fingernails grow in two months. The sonar images were taken at a depth of six kilometers below the ocean surface as part of a project funded by Australia’s Natural Environment Research Council (NERC) to help determine if the massive debris from the crumbling volcanoes have any effect on the frequency of earthquakes and tsunamis in the area.

BBC report


Was a bolide the cause for plate tectonics?
Was a bolide the cause for plate tectonics?Courtesy Navicore via Wikipedia Creative Commons
Recently, geologist Vicki Hansen, a professor of earth and planetary sciences at the University of Minnesota-Duluth, proposed a hypothesis that plate tectonics were triggered by ancient bolides crashing into Earth.

Plate tectonics arose from Alfred Wegener’s observations that some continents appear to fit together like puzzle pieces and at one time probably made up a single land-mass he named Pangaea that broke up and drifted apart. It was a theory dismissed by most geologists at the time, and Wegener himself was unsure how the process took place (he proposed magnetism or centrifugal force). It wasn’t until the 1960s, more than 30 years after Wegener’s death, that the theory gained wide acceptance. Today, scientists point to convective heat in the Earth’s mantle as the driving force that causes continental drift and sea-floor spreading. As new material is being added along mid-oceanic ridges, older crust is being pushed into other plates in a process called subduction, where one crust sinks beneath another and is remelted back into the mantle. It’s along these boundary zones where the plates collide that most of the world’s earthquakes and volcanoes occur, and where mountain ranges rise up. But what started the process? Why would Pangaea suddenly break up into pieces and begin drifting apart?

Hansen theorized that early in Earth’s history - perhaps as much as 2.5 billion years ago - impacts from large extra-terrestrial objects could have been the catalyst for two prime elements of plate tectonics: the spreading out of new crust, and particularly subduction. Since numerous impact craters can be found on Mars and on the Moon, it’s a good bet that Earth suffered a similar steady barrage of meteor impacts in its formative years. According to Hansen, the Earth’s crust at the time was more uniform in thickness, except in certain zones where mantle heat rising up from below would have caused it to thin.

A meteor or asteroid (one large enough to create a 600 mile-in-diameter crater) slamming into one of those weakened zones could have caused magma to erupt to the surface as flood basalts that would spread out and eventually push against the sides of the crater where they would begin subducting back down into the mantle. Such impacts could have happened several times around the world, enough to put the process of plate tectonics into motion.

Professor Hansen’s theory was first published in Geology magazine, but the study has reached the popular press. I came across it in the most recent issue of Science Illustrated, an interesting and jammed-packed-with-science publication new to me that I found at Barnes & Noble.

Professor Vicki Hansen webpage
UMD Planetary Processes Lab
More about plate tectonics

Alfred Wegener: Greenland, 1930.
Alfred Wegener: Greenland, 1930.Courtesy Public domain
Today is the birthday of Alfred Lothar Wegener, the scientist who first developed the theory of continental drift. Wegener was born in 1880, schooled as an astronomer, and became interested in climatology and meteorology. When he noticed how the shapes of some continents fit nicely into the forms of others, (such as how South America fit into Africa), he proposed in 1915 that they had once all made up a supercontinent he called Pangaea, and later drifted apart. Similar rock strata and fossils found in coastlines of distant continents seemed to corroborate his theory, but Wegener was unable to come up with a mechanism that would cause such movement, so his theory lay dormant, mostly spurned and unaccepted until the 1950's when new geological evidence regarding plate subduction and sea-floor spreading came to light. Wegener's theory of continental drift is the basis for present-day theory of plate tectonics. Unfortunately, Wegener didn't live to see his theory gain acceptance. He died tragically sometime in late 1930 while on a meteorological expedition to Greenland.

More about Alfred Wegener


Boundary of North American and Eurasian plates seen along the Mid-Atlantic Ridge in Iceland: Westward view over the top of the east edge of the North American tectonic plate.
Boundary of North American and Eurasian plates seen along the Mid-Atlantic Ridge in Iceland: Westward view over the top of the east edge of the North American tectonic plate.Courtesy Joe Hatfield
Geochemists at UCLA have determined plate tectonics – the theory involving the movement and collision of crustal plates – began much sooner after the Earth’s formation 4.5 billion years ago than thought previously.

Until now, plate tectonics were thought to have begun around 350 million years ago - or even later – but this new UCLA data points to much earlier beginnings.

"We are proposing that there was plate-tectonic activity in the first 500 million years of Earth's history," said professor Mark Harrison, director of UCLA's Institute of Geophysics and Planetary Physics and co-author of the paper. "We are reporting the first evidence of this phenomenon."

Their report appears in the science journal Nature.

The theory of moving crustal plates floating on molten rock was first championed in Alfred Wegener’s 1912 work, The Origins of Continents and Oceans. Wegener suggested all the continents existing today originated in a super-continent he called Pangaea that spread apart over time due to “continental drift”. Most scientists were skeptical of the theory until the 1960s when it was bolstered by the discovery of sea floor spreading.

Harrison and his colleagues analyzed zircon crystals found in 3 billion year old rocks from Western Australia. The rocks were formed from ancient magmas that had cooled and froze the mineral crystals in place. Using an ion microprobe, they bombarded the zircon with a beam of charged atoms (ions). The bombardment caused the zircon crystals to release their own ions and these were then analyzed using a mass spectrometer. The analysis showed the zircon crystals were more than 4 billion years old. It also showed the zircons had formed in an area where the heat flow was much lower than expected.

"The global average heat flow in the Earth's first 500 million years was thought to be about 200 to 300 milliwatts per meter squared," said Michelle Hopkins, a UCLA graduate student in Earth and space sciences, and the study's lead author. "Our zircons are indicating a heat flow of just 75 milliwatts per meter squared — the figure one would expect to find in subduction zones, where two plates converge, with one moving underneath the other."

The only places on Earth today where the average heat flow is one third that of the rest of the planet are in convergent plate-tectonic boundaries where magmas are forming.

Harrison published an earlier study in 2001 proving water was present early on the surface of the Earth during its formative years, and this current data strengthens his claim because plate tectonics can’t occur on a dry planet.

All this new information forces scientists to reevaluate their conception of how Earth appeared early in its formation.

"Unlike the longstanding myth of a hellish, dry, desolate early Earth with no continents, it looks like as soon as the Earth formed, it fell into the same dynamic regime that continues today," Harrison said. "Plate tectonics was inevitable, life was inevitable. In the early Earth, there appear to have been oceans; there could have been life — completely contradictory to the cartoonish story we had been telling ourselves."


Story in Science Daily
What is a geochemist?
More about heat flow
More about plate tectonics