Subduction Zones

Introduction

Where tectonic plates collide, enormous geologic forces are unleashed. Earthquakes strike, volcanoes erupt, and mountains rise. Although scientists are well aware of the power of collision, they are still seeking to understand the processes involved. Because of their grand scale and great depth, plate movements are often difficult to study, especially when they occur under the sea.

One important tectonic process that occurs only beneath the ocean is called subduction. The results of subduction are dramatic—the most visible example is the “Ring of Fire”, a bright arc of active volcanoes that outlines the Pacific Ocean.

Lesson

Subduction occurs when two plates collide at a convergent boundary, and one plate is driven beneath the other, back into the Earth’s interior. Not all convergence leads to subduction. Continental rocks are too buoyant to be forced downward, so when continents collide, they crumple but stay at the surface. Only oceanic plates, which are topped with basalt, are dense enough to sink into the mantle. As a result, only oceanic plates are subducted.

When an oceanic plate collides with a continental plate, the denser oceanic plate is bent downward and slides under the edge of the continent. A trench forms on the seafloor above the bend. Trenches are the deepest places on the surface of the earth, dropping off as much as 10 kilometers below the rest of the seafloor.

The leading edge of the continent is buckled and compressed by the force of continuing impact. A folded mountain belt rises parallel to the plate boundary, a belt with deep roots that extend down into the upper mantle. As the mountains rise, they shed enormous amounts of rocky debris, which rolls off the continent and into the trench. This is joined by seafloor mud and ooze scraped off the descending plate creating a thick organic-rich sediment wedge.

Far beneath the mountains, the descending plate has carried seawater down with it. As the plate heats up from friction and from exposure to the hot mantle, the water is expelled. The presence of water lowers the melting point of the surrounding rocks and magma begins to form. The magma rises up into the plate above melting and incorporating bits of the continent and becoming more and more silica-rich. Most of the magma cools and solidifies within the continent forming huge granite masses called plutons.

Some magma eventually reaches the surface and erupts. A volcanic mountain chain grows inland of the plate boundary—a string of tall conical peaks running parallel with the trench offshore. As the continental margin crumples as magma pushes into volcanoes, and as the seafloor is driven back into the mantle, both plates are shaken by earthquakes again and again. The eventual fate of the oceanic plate is unclear. Some studies suggest it melts away into the upper mantle, while others indicate it sinks nearly whole all the way down to the core.

Subduction also occurs where two oceanic plates converge, with many of the same results. When collision begins, the denser plate is forced downward. Because oceanic crust becomes denser with age, the plate that has the oldest rock on its leading edge is the one that is subducted.

Above the rim of the descending slab, a deep trench forms, and sediments begin to accumulate within it. Magma is generated above the sinking plate and rises. Because the over-riding oceanic plate is relatively thin, the magma burns through it quickly and lavas soon pour out onto the sea bottom. A string of undersea volcanoes begins to rise and some eventually grows above sea level to form a volcanic island arc. Frequent earthquakes rumble through the islands and all along the subducting plate as well.

Animals—such as mussels, crabs and shrimp—flock to undersea hot springs that dot the feet of the volcanoes. Life thrives in the warm mineral-rich water rising from the hot crust.

Global Impact

The geologic activity at subduction zones is enormously beneficial to all mankind. Dry land on Earth exists only because continents are born and kept above sea level by the volcanism and mountain building that occurs at subduction zones.

Many important natural resources are derived from subduction processes. Oil and natural gas reserves, fresh, highly fertile soils, and gold, silver, uranium, and diamonds are all formed at convergent plate boundaries.

However, the beauty and abundance created by subduction comes at a high price. Powerful earthquakes and violent, unpredictable volcanic eruptions cause great destruction and death near convergent boundaries. Underwater earth movements and explosions trigger enormous sea waves, called tsunamis, which travel across entire oceans to crash upon the distant shores of unsuspecting lands.

Occasionally, volcanoes and volcanic islands erupt cataclysmically, essentially blowing themselves sky high and pumping great volumes of gas and ash into the upper layers of the atmosphere. Swept around the planet by the jet stream, the volcanic debris blocks incoming sunlight, causes global cooling, and can lead to widespread crop failure and famine.

Scientific knowledge of subduction is only a few decades old and still incomplete. Geologists continue to explore subduction zones, hoping that greater understanding of their complex processes will lead to methods of predicting and minimizing the dangers they pose.

Additional Information not included in the Presentation

The geologic activity at subduction zones is enormously beneficial to all mankind. Dry land exists only because of subduction. Continents are born from the destruction of oceanic crust. The magma produced at subduction zones hardens into granite, the bedrock of all the continents.

Once created, continents keep above sea level through further subduction. Without the volcanism and mountain building that occurs at convergent boundaries, weathering would soon wear the Earth’s surface as flat as a smooth blue marble.

Important natural resources are derived from subduction processes. Oil and natural gas develops in the thick, highly organic sediments that accumulate around the plate boundary. Volcanic rocks release nutrients as they weather forming some of the most fertile soils on Earth. The hydrothermal fluids that accompany rising magma inject valuable minerals into surface rocks, including gold, silver, and diamonds.

The beauty and abundance created by subduction, however, comes at a high price. Powerful earthquakes cause great destruction and death near convergent boundaries. The strongest measured earthquake ever to strike the United States, the 1964 Alaska quake, was caused by subduction. With a Richter magnitude of 8.3, shaking lasted 4 minutes killing hundreds and leveling towns even in a very sparsely populated state. The volcanoes fueled by subduction magmas are unpredictable and prone to explode with great violence and little warning. The Pacific Ocean basin is outlined with such volcanoes—a “Ring of Fire” that threatens millions.

Those who live near a subducting plate generally bear the brunt of its power with some effects having a very long reach. When earthquakes, eruptions, or the landslides they can trigger occur near or under the ocean, enormous waves called tsunamis are set off. Sometimes known as tidal waves, tsunamis travel across entire oceans to crash upon the distant shores of unsuspecting lands. In 1883 the eruption of the Indonesian island of Krakatoa generated a 135-foot wave that killed over 30,000 people on the islands of Java and Sumatra.

Occasionally, volcanoes and volcanic islands erupt cataclysmically, essentially blowing themselves sky high and pumping great volumes of gas and ash into the upper layers of the atmosphere. Swept around the planet by the jet stream, the volcanic debris blocks incoming sunlight, and causes global cooling.

The island arc volcano of Tambora exploded in 1815 in the largest eruption in recorded human history. As a result, 1816 was known as “the year without a summer” in Europe and North America. Snow fell throughout the summer, crops failed, and 80,000 people starved.