How and where do volcanoes form?

Some volcanoes crown island areas lying near the continents, and others form chains of islands in the deep ocean basins. Volcanoes tend to cluster along narrow mountainous belts where folding and fracturing of the rocks provide channelways to the surface for the escape of magma. Significantly, major earthquakes also occur along these belts, indicating that volcanism and seismic activity are often closely related, responding to the same dynamic Earth forces.

An important clue to understanding volcanoes is knowing the location of the volcanic bands. Many of the world's active volcanoes are located around the edges of the Pacific Ocean (so called the "Ring of Fire"): the west coast of the Americas, the east coast of Siberia, Japan, the Philippines, Indonesia, and in island chains from New Guinea to New Zealand. Recently, active volcanoes were also found in Iceland, the Kenya Rift Valley in eastern Africa, Italy, and Hawaii. Looking at the locations of these volcanoes through the glasses of plate tectonics, we also notice that most volcanoes occur near the edges of the large "plates" that comprise the solid surface of Earth. Looking even more closely, we also notice that the dangerous explosive volcanoes, such as Mount St. Helens and Mount Pinatubo, that make the evening news are located where plates are crunching together. The quieter, "effusive" volcanoes, like Iceland and Hawaii, are found mostly where plates are coming apart or in the middle of a plate.

Plate-Tectonics Theory

According to the now generally accepted "plate-tectonics" theory, scientists believe that the Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called "plate-boundary" volcanoes. However, some active volcanoes are not associated with plate boundaries, and many of these so-called "intra-plate" volcanoes form roughly linear chains in the interior of some oceanic plates. The Hawaiian Islands provide perhaps the best example of an "intra-plate" volcanic chain, developed by the northwest-moving Pacific plate passing over an inferred "hot spot" that initiates the magma-generation and volcano formation process. The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates, are dotted by many active volcanoes that form the so-called "Ring of Fire." The "Ring" provides excellent examples of "plate boundary" volcanoes, including Mount St. Helens.

Let's carefully examine the edges of colliding plates to see why volcanoes might be found there. Remember that the plates move because heat flowing up from Earth's core causes mantle rock to slowly flow in giant convection currents. The rigid plates are carried on the currents, crunching into each other. Plate edges may be either oceanic crust or continental crust. So when plates collide, we have only three possibilities: oceanic-oceanic, oceanic-continental, or continental-continental collisions.  

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There are 3 main places where volcanoes originate: convergent plate boundaries (subduction zones), divergent plate boundaries (such as rifts and mid-ocean ridges) and hot spots.

Convergent plate boundaries

The final major place where volcanism originates is at convergent margins (subduction zones)--where an oceanic plate dives under either another oceanic plate or perhaps a continental plate. As the plate gets pushed further and further it starts to give off its volatiles (mostly water), and these migrate upwards into the mantle just under the overriding plate. The addition of these volatiles to this overriding mantle probably lowers the melting point of that mantle so that magma is generated. Part of the magma may also be generated by the downgoing plate actually starting to melt as it gets into the hotter and hotter interior.

Most of the Earth's volcanoes are located around the Pacific Ring of Fire because that the location of most of the Earth's subduction zones. A subduction zone is a place where one plate of oceanic lithosphere (= the crust + uppermost mantle) is shoved under another plate. The down-going plate is always the oceanic one. All while it was oceanic plate it collected water-saturated sediments and its uppermost few hundred meters got water saturated also. As it is shoved into the hotter mantle the plate heats up and all this water and other volatiles boil off and migrate upwards through the overlying plate. The addition of volatiles such as water to the hot overlying mantle causes partial melting and the production of magma. This magma rises up through the over-lying plate to erupt at the surface. If the overlying plate is a continent, you get a chain of volcanoes such as the Andes or Cascades. If the overlying plate is ocean you get a chain of volcanic islands such as the Marianas or Aleutians. This is also where the Earth's deep ocean trenches are and where the Earth's deep earthquakes are. The trenches form because the down-going plate is bent downward as it subducts. The earthquakes form as the two plates scrape against each other (earthquakes down to about 150 km) and then as the down-going plate bends (earthquakes down to about 700 km). The earthquakes do a very good job of tracing the position of the down-going plate. These zones of earthquakes are called Wadati-Benioff zones, after the two seismologists who first recognized them.

Oceanic-oceanic collisions-- In oceanic-oceanic collisions, one of the edges is forced down into the upper mantle. Oceanic crust is able to sink into the mantle because its main component, cold basalt, is about the same density as the mantle rocks. The edge that goes down is called the "descending slab." Although the descending slab is mostly basalt, it also contains layers of clay, limestone, and serpentine that formed on the ocean floor. The clay and serpentine contain water. Limestone is made of minerals combined with carbon dioxide (CO2). As the slab containing all these different kinds of rocks descends into the mantle, it heats up. The heat causes the rocks containing water and CO2 to decompose, releasing these gases. Dry basalt will not melt at mantle temperatures, but when it comes in contact with water and CO2 its melting point is lowered. The effect is like that which occurs when salt or anti-freeze is used to lower the melting point of water ice to clear roads in winter or to keep the car's radiator from freezing in cold weather. Consequently, some of the basalt melts and absorbs the water and CO2. The liquid basalt is less dense than the surrounding solid rocks, so buoyancy forces cause the liquid rock to rise through the overlying mantle and oceanic crust to form a volcano on the ocean floor. As the lavas pile up, they form groups of volcanic islands like Japan and Indonesia.

The most likely intersection of a flat descending slab with the curved surface of the earth is a long, gentle curve or arc. Since the volcanic islands tend to form a few hundred miles "downslab" from the collision site (marked by a deep trench on the ocean floor), they usually form an arc-shaped group. The arc-shape is easy to see in many examples: the islands of Japan, the Aleutian Arc extending from the SW coast of Alaska almost to Siberia, and the Timor-Java-Sumatra chain in Indonesia. Consequently these types of volcanoes are called "island-arc volcanoes." What kind of eruptions do we get with island arc volcanoes? Remember the lava forms because it contains a great deal of dissolved water and CO2. Thus when the lava reaches the surface, the gases are released, making a very explosive eruptions. Some of the most violent historical eruptions are of the island arc type, such as Tambora (1815), Krakatoa (1883), and Mount Pinatubo (1992).

Oceanic-continental collisions-- In these cases, the oceanic crust always forms a descending slab beneath the continent because the granite in the continental crust is too light to sink into the mantle. In the descending oceanic slab, the same conditions for melting occur as before; except this time, as the liquid rock ascends through the continental crust, some of the granite is melted and mixed with the liquid lava. Granite contains lots of silica, so the lava becomes very viscous. Consequently, when these lavas reach the surface, the resulting eruptions tend to be even more explosive than the island arc eruptions. The Cascade volcanoes in the American Northwest are the best studied examples of this type and include Mount Rainier and Mount St. Helens.

Digram showing continental environmentContinental-continental collision-- The third type of plate collision is continental-continental. The rocks on both sides of this type of collision are too light to sink into the mantle, so the edges simply crumble and fold into giant mountain ranges. The best example of this type of collision is the Himalayan Mountains in central Asia, which are the result of India crashing into Asia. Very little rock in these collisions is forced to great depths, so little melting occurs, and few volcanoes form. (Just in case you are wondering, there are only a few volcanoes in the Himalayas.)

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Divergent plate boundaries

At a divergent margin, two tectonic plates are moving apart, and magma that is generated in the upper mantle flows upward to fill in the space. This magma is originally under great pressure, but as it rises into the gap between plates, the pressure drops faster than the magma can cool.  There is little or no dissolved gas in this lava, so the eruptions are not explosive. The amounts of lava erupting at any given time are small, but the eruptions are continuous and occur along all the oceanic ridges, making this type of volcano the most common in the world. The magma is probably generated at depths that are shallower than those for hotspot magmas. People argue about whether the magma forcing its way to the surface causes the plates to move apart or whether the plates move apart and the magma just reacts to that and fills in the space. Perhaps it is a combination of these two. The most extensive example of this type of volcanism is the system of mid-ocean ridges. Continental examples include the East African Rift, the West Antarctic Rift, and the Basin and Range Province in the southwestern US.

The lava in plate collisions is formed from cold basaltic rocks being carried down to great depths where they are heated at high pressure. Even then, melting only occurs because the dissolved water and CO2 carried down with the basalt lowers its melting temperature. The lava in plate separations is formed by carrying already hot rock up to low pressures near the surface. This type of melting is called "depressurization melting" and works for nearly all solids. This type of melting is not observed in everyday life because the necessary pressure changes are very large.

Hot spots

We have now accounted for most of the volcanoes in the world, but there are few other types not associated with plate boundaries. The lavas in these volcanoes are basaltic and contain little dissolved gas. This type of volcano is usually found at one end of a long chain of extinct volcanoes. For example, the active volcanoes in Hawaii--Mauna Loa and  Kilauea, form a small cluster of active volcanoes at one end of a linked chain of hundreds of extinct volcanoes nearly three thousand miles long. Curiously, the ages of the volcanoes in these chains increase systematically along the chain from one end to the other. In the Hawaiian chain, their ages range from zero for the active volcanoes on Hawaii to several million years for those on Niihau, the westernmost part of the major Hawaiian islands, to over 100 million years for those at the far end of the chain. At the "old" end of such chains, one often finds a huge flood basalt type volcano. These gigantic formations include the largest deposits of volcanic rock in the world. One of these deposits, the Columbia River Basalts (CRB), covers more than one third of the state of Washington. A chain of dead volcanoes leads eastward from the Columbia Basalts to the "live" one now located at Yellowstone. Volcanic chains and flood basalt deposits are scattered randomly in time and place around the world and range in size from single volcanoes to giant groups of volcanoes and flood deposits half the size of the United States.

The long chains of extinct volcanoes with a "live" one at one end motivated the idea of "hotspot" volcanism: a small, fixed source of lavas from deep in the mantle that continuously "burns through" the overlying lithospheric plate as it passes over. We currently think that hot spots are caused by plumes that rise from the lower mantle. Remember that not all of the earth's internal heat is transported by mantle convection, but that part is transported by plumes of very hot rock. Experimental and computer models suggest that when a plume reaches the lithosphere, the plume flattens out against the bottom of the lithosphere, heats it, and causes it to bulge and fracture. Decompression melting converts some of the plume material to lava that rises through cracks to the surface to form a huge flood basalt volcano. Later, as the plate continues to move over the hot spot, the "head" of the plume is sheared off, leaving the narrower hot "tail." Lava rising up the tail and through the plate over a long period of time leaves a trail of shield volcanoes leading away from the huge flood basalt deposit. Since the lavas come from the mantle, the plumes are basaltic in composition and low in dissolved gases. Plumes come in all different sizes and can occur anywhere.

An interesting consequence of hotspot volcanism is that a hotspot volcano can occur anywhere, even in your own back yard! In fact, a volcano did occur recently in one farmer's back yard. In 1943, Dionisio Pulido found a crack spewing out red hot rocks and lava in his cornfield. Within a year, the crack grew into a volcano--Paricutin--that covered his field, his farm, his local village, and everything else within five miles. Paricutin is still active in southwestern Mexico. Of course, a glance at the map will show that Paricutin is in a zone of many active volcanoes caused by plate subduction under Mexico; therefore, its location was not exactly a complete surprise. On the other hand, it is estimated that hotspot volcanism will affect every part of the earth's surface on an average of every 500 million to 800 million years. So even if you live in Lafayette, Indiana, you might wake up some morning to find a brand new hotspot volcano in your own back yard! Photo is by Courtesy of NGDC/NOAA.

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