What Is a Subduction Zone?
Subduction zones circle the Pacific Ocean, forming the Ring of Fire.
Credit: USGS.
A subduction zone is the biggest crash scene on Earth. These boundaries
mark the collision between two of the planet's tectonic plates. The
plates are pieces of crust that slowly move across the planet's surface
over millions of years.
Where two tectonic plates meet
at a subduction zone, one bends and slides underneath the other,
curving down into the mantle. (The mantle is the hotter layer under the
crust.)
Tectonic plates can transport both continental crust and oceanic crust,
or they may be made of only one kind of crust. Oceanic crust is denser
than continental crust. At a subduction zone, the oceanic crust usually
sinks into the mantle beneath lighter continental crust. (Sometimes,
oceanic crust may grow so old and that dense that it collapses and spontaneously forms a subduction zone, scientists think.)
If the same kind of crust collides, such as continent-continent, the
plates may crash together without subducting and crumple together like
crashing cars. The massive Himalaya mountain chain was created this way, when India slammed into Asia.
Scientists first identified subduction zones in the 1960s, by locating
earthquakes in the descending crust. Now, new instruments can precisely
track the shifting tectonic plates.
"We can see very clear pictures of how the plates move, mostly due to
GPS data," said Vasily Titov, director of National Oceanic and
Atmospheric Administration's Center for Tsunami Research in Seattle,
Washington.
Subduction zones occur all around the edge of the Pacific Ocean,
offshore of Washington, Canada, Alaska, Russia, Japan and Indonesia.
Called the "Ring of Fire," these subduction zones are responsible for
the world's biggest earthquakes, the most terrible tsunamis and some of
the worst volcanic eruptions.
Earthquakes
Shoving two massive slices of Earth's crust together is like rubbing
two pieces of sandpaper against each other. The crust sticks in some
places, storing up energy that is released in earthquakes. The massive
scale of subduction zones means they can cause enormous earthquakes. The
largest earthquakes ever recorded were on subduction zones, such as a
magnitude 9.5 in Chile in 1960 and a magnitude 9.2 in Alaska in 1964.
"Subduction zones are huge boundaries, so they generate very large earthquakes," Titov told Live Science.
Why are subduction zone earthquakes the biggest in the world? The main
reason is size. The size of an earthquake is related to the size of the fault that
causes it, and subduction zone faults are the longest and widest in the
world. The Cascadia subduction zone offshore of Washington is about 620
miles (1,000 kilometers) long and about 62 miles (100 km) wide.
Smaller earthquakes also strike all along the descending plate, also
called a slab. Seismic waves from these temblors and tremors help scientists "see" inside the Earth,
similar to a medical CT scan. The quakes reveal that the sinking slab
tends to bend at an angle between 25 to 45 degrees from Earth's surface,
though some are flatter or steeper than this.
Sometimes, the slabs may tear, like a gash in wrinkled paper. Pieces of
the sinking plate can also break off and fall into the mantle, or get
stuck and founder.
A 3D model of a subduction zone off the coast of Washington and Oregon.
Credit: USGS.
Tsunamis
Subduction zones are usually along coastlines, so tsunamis will always
be generated close to where people live, Titov said. "There's a silver
lining there," he said. "If these earthquakes happened underneath a
city, the city would have no chance. But the bad news is sometime a
tsunami is generated."
When subduction zone earthquakes hit, Earth's crust flexes and snaps
like a freed spring. For earthquakes larger than a magnitude 7.5, this
can cause a tsunami,
a giant sea wave, by suddenly moving the seafloor. However, not all
subduction zone earthquakes will cause tsunamis. Also, some earthquakes
trigger tsunamis by sparking underwater landslides.
Whatever their cause, the tsunami threat from subduction zones is monitored by
government agencies such as NOAA in countries around the Pacific Ocean.
Tsunamis may strike in minutes for coastal areas near an earthquake, or
hours later, after the waves travel across the sea.
Volcanoes
As a tectonic plate slides into the mantle, the hotter layer beneath
Earth's crust, the heating releases fluids trapped in the plate. These
fluids, such as seawater and carbon dioxide, rise into the upper plate
and can partially melt the overlying crust, forming magma. And magma
(molten rock) often means volcanoes.
Looking at the Pacific Ring of Fire reveals the link between subduction
zones and volcanoes. Inland of each subduction zone is a chain of
spouting volcanoes called a volcanic arc,
such as Alaska's Aleutian Islands. The Toba volcanic eruption in
Indonesia, the largest volcanic eruption in the past 25 million years,
was from a subduction zone volcano.
Epic Shoving Match Takes Place Far Below Tibet
What's going on under there?
The whole of the Tibetan Plateau, in true color, captured by satellite.
The Himalayas can be seen along the bottom edge of the image.
Credit: Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC.
The Tibetan Plateau, with an average elevation of roughly 16,000 feet
(4,900 meters), is one of the highest, flattest places on earth. It
lies at the intersection of the most vigorous collision of continental
plates on the planet, where the Indian continental plate smashes into
the Eurasian plate and dives beneath it. The slow-motion crash helped
create the massive Himalayas yet for all its violence, scientists aren't exactly sure what processes are at work in the region.
A new study, published in the April 7 edition of the journal Nature,
suggests that two widely accepted theories about the mechanics and
materials at work in the region are wrong.
Fluid rocks?
Around three decades ago, many scientists began to believe that the
friction of the two continental plates created such heat that the deep
underground rocks caught up in the high temperatures took on a fluid-like quality , said Brian Wernicke, a professor of geology at the California Institute of Technology who is an author of the paper.
Wernicke said that in the accepted scenario, the rock is fluid in the
same way that glaciers are fluid still hard enough that you could hit
them with a hammer, but able to flow and move over a long time scale
and that there isn't much of an interaction between the Indian and
Eurasian plates.
If that were the case, it would help explain why the Tibetan Plateau
has stayed so flat, since a weaker material beneath the massive plain
would have less effect on the Earth's surface, as if the plateau were
resting on a water bed.
However, a new model reveals a very different situation.
Wrestling plates
The new model indicates there is no layer of lubricating, fluid-like
rock at the intersection of the two plates and that instead, the
subsurface materials are rigid and strong, with the two plates locked in
a subterranean wrestler's embrace with the Indian Plate pushing hard
against the Eurasian Plate above.
To illustrate, Wernicke suggested placing your right hand over the
left. If you push the knuckles of your left hand against your right
palm, you can feel the effect. Drizzle your hands with oil, and the
effect will change.
"Your hands slide really easily, and the motion of your lower hand
isn't able to affect what the upper hand is doing," Wernicke said. "You
can't transmit the forces across really weak material."
This satellite radar image
reveals the rugged nature of the mountains on a southeastern portion of
the Tibetan Plateau. Scientists don't agree on what geological
mechanisms are at work beneath the plateau, and how they helped form
these mountains.
Credit: NASA JPL.
The authors of the study used a complex computer model for their
research, plugging in some data points that are well established such
as the velocity with which the tectonic plates are moving and an
accepted range for other variables, such as the strength of the rocks
and their temperature.
It turned out that the scenario that best matched observed conditions
didn't allow for the weak, fluid layer between the massive slabs of
crust, and the resulting weak relationship between the plates.
Wernicke said the model that indicated rigid Indian crust grinding up
against the overlapping Eurasian plate best matched the situation on
the surface.
"It successfully reproduces the tectonics of the surface geology of
southern Tibet for the present," Wernicke told OurAmazingPlanet.
Unraveling earthquakes
The region is one of the most tectonically active on the surface of the Earth , and Wernicke said that any improved understanding of the materials and mechanics lying at the heart of the action is helpful.
"These are all fundamental questions that bear on the physics of how earthquakes happen," Wernicke said.
However, Wernicke said he and co-authors Jean-Philippe Avouac, also
of Caltech, and Alex Copley of the University of Cambridge in England
stress that their study provides a look at the current geophysical
situation, and that conditions in the region could have changed over the
50 million years since the Indian Plate first slammed into the Eurasian
continent.
"Our model doesn't bear on what was going on 15 million years ago,"
Wernicke said, "because we don't have all the information we have
today."
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