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.
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.
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.
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.