Ring of Fire

The Ring of Fire is a relatively narrow band of land composed of parts of the continents and island chains that surround the Pacific Ocean. The region is characterized by extensive volcanic and earthquake activity. The "ring" is the surface expression of a tectonic boundary in which oceanic crust is forced into the upper mantle, producing molten rock that rises to the surface and forms volcanoes. The stresses imparted to the crustal rocks as they are forced into the mantle provide the energy for numerous earthquakes. Geologic investigations of the Ring of Fire have helped scientists refine the theory of plate tectonics.

Plate Tectonics

The “Ring of Fire” is the dramatic name of a narrow band of the earth's surface surrounding the Pacific Ocean. This band extends from the islands of the South Pacific, through Japan, to the Aleutian Islands between Asia and North America, down the West Coast of North America, and through Central America and the western portions of South America. The name was applied to the roughly circular region because of its many active volcanoes. Twenty-two of the twenty-five largest volcanic eruptions of the Holocene epoch (the current geological epoch, starting approximately 11,700 years ago) were volcanoes in the Ring of Fire.

In addition to volcanic activity, this region of the earth's surface exhibits a combination of geologic features that have intrigued earth scientists for years. The region is characterized by mountain chains with many active and dormant volcanoes that parallel coastlines. Another characteristic of the area is its numerous strong earthquakes. In addition, some of the deepest portions of the world's oceans lie just offshore the volcanically active mountain chains of the region's land masses. Geologists had noticed the correlation between the land masses surrounding the Pacific, the volcanic and earthquake activity, and the oceanic depths long before they had an explanation relating these phenomena. The advent of the theory of plate tectonics provided the mechanism to explain the relationships among these geologic features.

The theory of plate tectonics, developed during the 1960s, views the earth's surface as divided into a number of crustal plates. Like the pieces of a jigsaw puzzle, the plates fit together at their edges and cover the surface of the earth. However, these pieces are in motion. Each plate moves slowly over the surface of the earth, riding on currents of molten rock in the upper mantle, the layer within the earth immediately underlying the crustal plates.

Geologists have long known that rock temperatures increase with depth in the earth. For example, temperatures at the bottom of the deepest mines are sometimes uncomfortably hot. Deeper into the earth, in the upper mantle and beyond, temperatures are so high that most rocks begin to melt. However, sources of heat within Earth's mantle are not evenly distributed, and circulation systems exist. In areas of high heat, the molten rocks are less dense because they are hotter and rise. In cooler areas of the mantle, the molten rocks become denser and sink. This circulation system acts as a conveyor belt, propelling the crustal plates across the surface.

Crustal plates are composed of two main types of material, continental crust and oceanic crust. Because the types of rocks making up these crustal types are chemically different, they also differ in density, or the mass per unit volume of the rock. Continental crust is less dense than oceanic crust. Tectonic plates may be composed of oceanic crust (for example, the Pacific plate), continental crust (for example, the Arabian plate), or some combination of the two types.

Plate Boundaries

There are three basic types of plate boundaries. Divergent boundaries, in which two plates are moving away from each other, are probably associated with the areas of rising molten rock within the mantle. A good example of a divergent plate boundary is found in the middle of the Atlantic Ocean, where an underwater mountain chain, the Mid-Atlantic Ridge, runs the length of the sea floor. Transverse or transform boundaries, in which two plates move horizontally against each other, are characteristically marked by a major fault. An example of a transverse boundary is found in the San Andreas Fault zone of Southern California.

Convergent boundaries, the third type of plate boundary, occur where two plates collide. These boundaries are characteristic of the Ring of Fire. When two plates being moved along on mantle currents collide at a convergent plate boundary, something must give. In many cases where the plates are composed of different crustal types, such as continental and oceanic, the denser crustal type (oceanic) will tend to be pushed beneath the lighter, less-dense crust (continental) and into the upper mantle. An example of this type of convergent boundary exists along the West Coast of South America. There, the denser oceanic plate is subducted beneath the lighter continental plate. The subduction zone, where the oceanic plate is pushed beneath the continental plate, runs parallel to the coastline. Immediately offshore, the subduction zone is characterized by extremely deep ocean water. As the oceanic plate is subducted, it is bent down into the mantle, and the bottom of the ocean in the subduction zone becomes very deep. These deep linear features in the ocean, called submarine trenches, are some of the deepest parts of the ocean.

As the oceanic crustal plate sinks deeper into the upper mantle, temperatures rise, and the rocks begin to melt. The molten rock, which is much hotter than the solid oceanic crust it came from, is less dense than the surrounding mantle material, and it tends to rise, like a beach ball held under water. As the molten rock from the oceanic plate rises, it hits the bottom of the overlying continental crust. Working its way through cracks and other openings in the continental crust, the molten rock often forces its way to the surface, where it pours out from a volcano. The volcanic activity tends to occur in a narrow band within the continental plate some distance inland from where the oceanic plate is being subducted. Mountain chains on the continent, composed of many volcanic peaks, are characteristic of this zone.

Another type of convergent plate boundary occurs where two similar crustal types collide. In the case of the Ring of Fire, this type of convergence is found in the South Pacific, where plates of oceanic crust are colliding. The result is that one of the plates again sinks beneath the other. The subducted plate is heated and melts within the upper mantle, and rising melt finds its way through the overlying oceanic crust to the surface of the ocean floor. The molten rock that builds these volcanoes continues to spill onto the ocean floor until the volcano rises above sea level to form a volcanic island. Volcanic island chains, called island arcs, form with a deep submarine trench offshore.

Hot Springs and Geysers

Another feature of volcanic areas in the Ring of Fire is the presence of hot springs and geysers at the earth's surface. The abundance of hot and molten rock at relatively shallow depths above a subduction zone provides a perfect setting for hot-spring formation. Water from surface precipitation enters the earth's surface to become groundwater. Some of this groundwater will penetrate deeply into the earth's crust, where it will be heated by the rocks and melt associated with the Ring of Fire. When the water is heated, like the molten rock rising from the subducted plate, it becomes less dense and rises toward the surface through cracks and fractures in the earth's crust to emerge as a hot spring or geyser.

In addition to absorbing heat from the surrounding rocks and melt, the circulating water may dissolve many chemical elements from the rocks. These dissolved metals are carried along with the water through the cracks in the earth's crust toward the surface. However, as the water rises, it cools, and its ability to keep the elements dissolved decreases. As a result, minerals containing specific elements, especially metals such as lead, zinc, and copper, are deposited along the path of the water. Mineral deposits being mined today are thought to have been deposited from just such hot (hydrothermal) waters.

Earthquakes

Earthquakes are another common feature of the Ring of Fire. Rocks at plate boundaries are subjected to the stresses at that boundary. Rocks are being forced apart, are sliding past each other, or are colliding. Under these pressures, all rocks tend to break along planes called faults. Sometimes the movement along a fault is not smooth. The rocks along the fault get stuck, and the pressures driving the movements build up. Eventually, the built-up pressures become so great that the sticking point along the fault breaks free, and the stored energy is released. This energy travels through the surrounding rocks as vibrations—similar to the vibrations from a bell after it is struck—producing an earthquake. The intensity of the vibrations is related to the amount of energy released along the fault. The vibrations from a strong earthquake can do severe damage to buildings and roads.

Earthquakes are common in the Ring of Fire because areas of converging plates are ideal locations for rocks to break and form faults. In addition, the pressures of collision are constant and can build up if movement along the faults ceases. Deep earthquakes are characteristic of the Ring of Fire. Faults only occur in solid rocks. In a subduction zone, solid rocks are forced deep into the upper mantle, an area usually composed of molten or partially melted rock. The faults in the relatively cold subducted plate are the only places in the upper mantle where pressures can build up and be released as an earthquake.

Ocean Surveys

The explanation for the Ring of Fire has depended, in large part, on investigations into the processes of plate tectonics. Geologists have used a broad range of technologies to understand how plate tectonics operates, and that understanding has helped explain the association of geologic phenomena composing the Ring of Fire.

Geologists have mapped the ocean floor with tools as crude as a weighted rope and with more sophisticated tools such as sonar. Sonar uses sound waves generated within the water to measure the distance to any object that can reflect some of the sound waves back to a receiver. The time that elapses between when the sound is made and when the reflected sound is received can be used to calculate the distance to the reflecting object. A map of the ocean floor can thus be created by towing a sound source and sound receiver behind an oceanic research vessel as it crisscrosses the ocean. Data from many such surveys have been compiled into maps of the ocean floor, which clearly show the mid-ocean ridges and the submarine trenches characteristic of divergent and convergent plate boundaries.

Oceanic surveys also often involve the taking of heat measurements. Scientists use a sensitive heat sensor that is towed near the ocean floor, behind a research vessel, and they are able to map the amount of heat entering the ocean through the oceanic crust. The results of these surveys show that the warmest oceanic crust and the areas where the most heat enters the oceans are associated with the mid-ocean ridges. Conversely, the coolest portions of the ocean floor are those associated with submarine trenches. However, some of the highest heat-flow measurements taken on land are associated with the areas of volcanic activity in island arcs and continental mountain chains within the Ring of Fire.

Seismology

Seismologists are geologists who specialize in the study of earthquakes. Seismologists use seismographs, sensitive instruments that measure vibrations in the earth's crust, to identify where an earthquake has occurred. Seismographs can be used to identify the time earthquake vibrations arrive at a seismic measurement station. Knowing the velocity of earthquake vibrations through rocks, seismologists can calculate the distance from their measuring point to the earthquake source. With only a single measurement point, a seismologist would know only that an earthquake could have been centered anywhere on a circle of that radius. However, with many seismographs around the world, seismologists at each station combine measurements from other stations to locate the point on the earth's surface directly over the source of the earthquake. This point is called the epicenter of the earthquake. Measurements showing the locations of many epicenters over the years were used to identify the Ring of Fire as an area of high earthquake activity.

Another characteristic of the Ring of Fire is the depth of the earthquake source. Earthquakes occur where two bodies of solid rock are moving past each other, separated by a fault plane, when that movement is temporarily halted. The rocks must be solid; within the earth's depths, however, temperatures increase rapidly, and solid rock is not thought to exist below a certain depth. The implication is that earthquakes cannot occur below the depths predicted for melting of continental or oceanic crustal rocks. However, seismologists can use seismograph data to determine the depth of an earthquake source. Examination of seismograph data from many earthquakes in the Ring of Fire clearly indicates that, in addition to relatively shallow earthquakes, there are many very deep earthquakes. The epicenters of the very deep earthquakes are centered inland, away from coastlines and submarine trenches. In the context of plate tectonics theory, the data suggest that deep earthquakes are centered within the relatively cold, and solid, rocks of the subducting plate; researchers have theorized that the plate remains solid and able to generate earthquakes because it is being driven into the earth's depths rapidly enough that the rock does not heat up and melt, as geologists might otherwise predict. Additional analysis of the seismic data has allowed seismologists to differentiate between subduction zones where the subducting slab is entering the upper mantle at a sharp angle, which produces a relatively narrow zone of earthquake activity, with deep earthquakes closer to a submarine trench, and those entering the upper mantle at a shallow angle, which produces a broader earthquake zone and deep earthquakes farther inland.

Principal Terms

continental crust: near-surface rocks primarily composed of low-density, light-colored minerals and constituting the bulk of continental land masses

mantle: the zone of the earth immediately underlying the crust; the uppermost region of the mantle is composed of partially melted and melting rocks

oceanic crust: near-surface rocks primarily composed of dense, dark-colored minerals that underlie the ocean floors

plate: a relatively thin slab of crustal rock, either continental or oceanic, that moves over the face of the globe, driven by currents of circulating molten rock in the underlying mantle

plate tectonics: a theory that explains the distribution of geologic features and phenomena across the face of the earth based on the interactions of crustal plates

subduction: the process occurring at colliding plate boundaries when denser oceanic crust is forced down into the upper mantle by overriding continental crust

trench: a very deep portion of the ocean where oceanic crust is being forced beneath continental crust

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