Subduction zones circle the Pacific Ocean, forming the Ring of Fire.
Credit: USGS.
Uma zona de subducção é a maior cena de colisão na Terra. Essas fronteiras marcam a colisão entre duas placas tectônicas do planeta. As placas são pedaços de crosta que se movem lentamente pela superfície do planeta ao longo de milhões de anos.
Onde duas placas tectônicas se encontram em uma zona de subducção, uma se dobra e desliza embaixo da outra, curvando-se no manto. (O manto é a camada mais quente sob a crosta.)
As placas tectônicas podem transportar tanto a crosta continental quanto a crosta oceânica, ou podem ser feitas de apenas um tipo de crosta. A crosta oceânica é mais densa que a crosta continental. Em uma zona de subducção, a crosta oceânica geralmente afunda no manto abaixo da crosta continental mais leve. (Às vezes, a crosta oceânica pode ficar tão velha e tão densa que desmorona e forma espontaneamente uma zona de subducção, pensam os cientistas.)
Se o mesmo tipo de crosta colidir, como continente-continente, as placas podem colidir sem se subdividirem e se dobrarem como carros em colisão. A enorme cadeia montanhosa do Himalaia foi criada dessa maneira, quando a Índia bateu na Ásia.
Os cientistas identificaram pela primeira vez zonas de subducção na década de 1960, localizando terremotos na crosta descendente. Agora, novos instrumentos podem rastrear com precisão as placas tectônicas móveis.
"Podemos ver imagens muito claras de como as placas se movem, principalmente devido aos dados do GPS", disse Vasily Titov, diretor do Centro para Pesquisa de Tsunamis da Administração Nacional Oceânica e Atmosférica, em Seattle, Washington.
Zonas de subducção ocorrem ao redor da borda do Oceano Pacífico, no mar de Washington, Canadá, Alasca, Rússia, Japão e Indonésia. Chamados de "Anel de Fogo", essas zonas de subducção são responsáveis pelos maiores terremotos do mundo, pelos tsunamis mais terríveis e por algumas das piores erupções vulcânicas.
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. Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook & Google+. Additional resources
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