Why the Ring of Fire exists

Look at a map of the Pacific Ocean.

Around its edges is a long line of volcanoes. It goes from South America, up past North America, over to Japan and down to New Zealand. People call it the Ring of Fire.

The Ring of Fire is a path of volcanoes and earthquake zones around the edges of the Pacific Ocean. NOAA explains that it is shaped more like a horseshoe than a ring. It runs from the southern tip of South America, up the west coast of North America, across the Bering Strait, down through Japan and on to New Zealand.

It is huge: about 40,000 kilometers long. National Geographic says more than 450 volcanoes lie along it, about three out of every four volcanoes on Earth. Scientists also call it the Circum-Pacific Belt, which means the belt around the Pacific.

The Ring of Fire, or Circum-Pacific Belt, is a path of active volcanoes and frequent earthquakes around the Pacific basin. NOAA gives its length as nearly 40,250 km (25,000 miles) and describes it as a horseshoe, running from the southern tip of South America along western North America, across the Bering Strait and down through Japan into New Zealand. National Geographic gives about 40,000 km.

National Geographic counts more than 450 volcanoes, about 75 percent of Earth's volcanoes, along it, and says 90 percent of Earth's earthquakes occur along its path. The USGS, measuring a different thing, says about 81 percent of the planet's largest earthquakes occur in this circum-Pacific seismic belt and calls it the most seismically and volcanically active zone in the world. The figures differ because they count different things: all earthquakes versus the largest ones.

The Ring of Fire is where giant plates meet.

Heavy ocean plates slide down under other plates. Deep down, rock melts. The hot, melted rock rises and makes volcanoes.

The Ring of Fire follows the edges of several tectonic plates, including the Pacific Plate. Along most of it, plates are moving toward each other. When a heavy ocean plate meets another plate, the ocean plate is pushed down underneath it into the mantle. This is called subduction, and the place where it happens is a subduction zone.

As the sinking plate goes deeper, it heats up and water is squeezed out of it. The British Geological Survey (BGS) explains that this water lets the hot mantle rock above melt more easily, and that makes magma. Magma is lighter than the solid rock around it, so it rises up through the plate above and erupts as volcanoes. That is why the volcanoes form long lines beside the edges of the plates.

National Geographic lists the plates whose boundaries the Ring traces: the Pacific, Juan de Fuca, Cocos, Indian-Australian, Nazca, North American and Philippine Plates. Along much of it, plates converge at subduction zones: the denser oceanic plate is forced beneath the other, and NOAA explains that melting produces magma that rises through the overriding plate and erupts.

BGS explains the melting: subduction carries water-bearing sediments and minerals into the mantle, and as the slab heats up the water is released, lowering the melting temperature of the mantle above by about 60 to 100 °C. That generates magma at depth. Older USGS text records a long debate over how much of the melt comes from the slab itself and how much from the overlying plate. Either way, the volcanoes form chains parallel to the plate boundary, which is why the Ring appears as a narrow belt on a map rather than a scatter across the ocean.

Where one plate slides under another, the sea floor dips down very deep.

These deep valleys are called trenches. Next to them, volcanoes grow into islands and mountains.

Where an ocean plate bends down to sink, it makes a long, deep valley in the sea floor called a trench. The USGS says that if you could drain the Pacific, you would see trenches thousands of kilometers long and 8 to 10 kilometers deep. The Mariana Trench in the western Pacific, part of the Ring of Fire, holds the deepest known point in the ocean, about 10,935 meters down (NOAA).

Beside the trenches, volcanoes build up. Where an ocean plate sinks under another ocean plate, the volcanoes grow into curved chains of islands, such as the Aleutian Islands. Where it sinks under a continent, it builds volcanic mountains like those of the Andes in South America and the Cascade Range in the Pacific Northwest of North America.

Subduction bends the downgoing plate, forming trenches, which NOAA notes are Earth's deepest ocean trenches; the USGS describes Pacific trenches thousands of kilometers long and 8 to 10 km deep. The Mariana Trench, where the fast-moving Pacific Plate converges on the slower Philippine Plate, reaches about 10,935 m at the Challenger Deep, the deepest known point in the ocean (NOAA).

Ocean-ocean subduction builds volcanic island arcs, curved chains that parallel their trenches, such as the Marianas and the Aleutians, where magma comes from partial melting of the descending plate and/or the overlying oceanic lithosphere (USGS). Ocean-continent subduction builds continental volcanic arcs: the Nazca Plate sinking under South America has lifted the Andes, and subduction also sustains the volcanoes of the Cascade Range. Earthquakes occur as the plates scrape past each other and as the sinking plate bends.

The ground shakes often around the Ring of Fire.

Plates push and rub against each other. Sometimes they slip. That makes an earthquake. Scientists watch these places very carefully.

The USGS calls the Ring of Fire the world's greatest earthquake belt. Where one plate sinks under another, the plates scrape against each other. They can get stuck for a long time and then suddenly slip, shaking the ground.

Not every part of the Ring has a sinking plate. Along the coast of California, the Pacific Plate and the North American Plate slide sideways past each other instead. That kind of edge is called a transform boundary. BGS says volcanoes do not usually form at this kind of edge, because there is little or no magma there. But it does make many earthquakes as stress builds up and is released.

About 81 percent of the world's largest earthquakes occur in the circum-Pacific seismic belt (USGS). In its subduction zones, earthquakes are caused by slip between the plates and by rupture within the plates; BGS notes that subduction zones produce most earthquakes larger than magnitude 6 and the deepest earthquakes on Earth. Earthquakes the USGS lists for this belt include the M9.5 Chile (Valdivia) earthquake of 1960 and the M9.2 Alaska earthquake of 1964.

The boundary between the Pacific and North American Plates along California is a significant exception (National Geographic): it is a transform boundary, where the plates move sideways past one another, generating many earthquakes; BGS notes that volcanoes do not typically form at transform boundaries, because little or no magma is available there. Around the Pacific basin, the USGS measures the distances between GPS stations to learn more about the events building up to earthquakes and eruptions.

Many volcanoes in the Ring of Fire are under the sea.

We cannot see them from the beach. Scientists send cameras deep down to watch them.

Much of the Ring of Fire is hidden under the ocean. NOAA says that most of the active volcanoes on Earth are underwater, along the Ring of Fire.

Because they are so deep, underwater eruptions are very hard to see. NOAA reports that the first underwater eruption ever seen in action was filmed in 2006, at a vent called Brimstone on an undersea volcano named NW Rota-1. It was seen by a NOAA expedition called Submarine Ring of Fire.

NOAA notes that most of Earth's active volcanoes are underwater, many of them along the Ring of Fire. Submarine volcanoes in island arcs grow from the sea floor over countless eruptions; the USGS explains that, over millions of years, erupted lava and debris pile up until a submarine volcano breaks the surface as an island.

Watching them is difficult. NOAA's 2006 Submarine Ring of Fire expedition filmed an eruption at the Brimstone vent on NW Rota-1, a submarine volcano, which NOAA describes as the first underwater eruption ever witnessed in action. NOAA's ocean-depth page also shows active hydrothermal vent chimneys about 9 m tall deep in the Mariana Arc region, seen by an earlier Ring of Fire expedition in 2004.

Not every volcano is in the Ring of Fire.

The islands of Hawaii are volcanoes in the middle of the Pacific. Iceland has volcanoes in the Atlantic Ocean.

Most volcanoes and earthquakes happen near the edges of plates, but not all of them are in the Ring of Fire. Iceland sits on the Mid-Atlantic Ridge, where two plates pull apart, and it has volcanoes and earthquakes too.

The Hawaiian Islands are a different puzzle. They are volcanoes in the middle of the Pacific Plate, more than 3,200 kilometers from the nearest plate edge. Scientists explain them with a hotspot: a very hot place deep in the mantle that partly melts the plate above it as the plate moves slowly over it, building one volcano after another.

The USGS describes three main earthquake belts. Besides the circum-Pacific belt, the Alpide belt, from Java and Sumatra through the Himalayas and the Mediterranean to the Atlantic, accounts for about 17 percent of the largest earthquakes, and the Mid-Atlantic Ridge, a divergent boundary, forms a third; Iceland, astride the ridge, has had earthquakes of at least M6.9.

Volcanoes in plate interiors need another explanation. The Hawaiian Islands formed more than 3,200 km from the nearest plate boundary. In 1963, J. Tuzo Wilson proposed the hotspot theory: a long-lived, exceptionally hot region in the mantle supplies magma to the plate moving over it, so as the plate moves, a chain of volcanoes forms; the theory predicts that each volcano is older and more eroded the farther it has been carried from the hotspot (USGS). BGS adds that hotspot volcanoes typically erupt runny lava and form flatter shield volcanoes.