Subduction zones circle the Pacific Ocean, forming the Ring of Fire.
A subduction zone is the biggest smash-up on Earth, marking the collision between two of the planet's tectonic plates, the pieces of crust that slowly move across the surface over millions of years.
When two tectonic plates meet, one may slide underneath the other, curving down into the mantle. (The mantle is the hotter layer under the crust.) When this happens, geologists call the boundary between the plates a subduction zone.
Tectonic plates can transport both continental crust and oceanic crust, or they may carry only one kind of crust. If two colliding plates carry the same kind of crust, such as continent-continent, they may crash together without subducting and crumple like cars in a front-end collision. The massive Himalaya mountain chain was created this way, when India slammed into Asia.
But if the plates are different densities, a subduction zone usually forms. Oceanic crust is denser than continental crust, and will typically sink into the mantle when these two kinds of crust meet at a subduction zone. (Sometimes, oceanic crust may grow so old and that dense that it collapses and spontaneously forms a subduction zone, scientists think.)
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 biggest earthquakes, the most terrible tsunamis, and some of the worst volcanic eruptions on Earth.
The massive scale of subduction zones means they can cause enormous earthquakes. The largest earthquakes ever recorded in history were on subduction zones, such as a magnitude-9.5 in Chile in 1960 and a magnitude-9.2 in Alaska in 1964.
Why are subduction zone earthquakes the biggest in the world? The primary 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 a wrinkled carpet. Pieces of the descending slab can also break off and fall into the mantle, or get stuck and founder.
When subduction zone earthquakes hit, the Earth's crust flexes and snaps like a freed spring. 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 via underwater landslides, instead of seafloor motion.
Whatever their cause, the tsunami threat from subduction zones is monitored by government agencies 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. [Infographic: How Japan's 2011 Earthquake Happened]
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 water and carbon dioxide, rise into the upper plate and can partially melt the crust, forming magma. And magma 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.