Stratovolcanoes

A volcano that erupts both cinder and lava is a composite volcano, or stratovolcano. These volcanoes are found at subducting tectonic plate margins and are the most abundant of the large volcanoes. The tallest and most famous stratovolcanoes of the world, such as Vesuvius, Fuji, and Mount St. Helens, have produced the most devastating and violent eruptions.

Characteristics of Stratovolcanoes

The most abundant type of large volcano on the earth is the stratovolcano, also referred to as a composite volcano. These classic “poster volcanoes” constitute 80 percent of all active volcanoes, including Mount Fuji, Mount Vesuvius, Mount St. Helens, and Krakatau. Stratovolcanoes vary in elevation from a few hundred to 4,000 meters (13,123 feet) or more above base levels, with diameters approaching 40 kilometers (25 miles). Their slopes vary from 10 to 35 degrees, with an increase toward the summit because of larger amounts of volcanic material deposition there. In humid regions, much of this volcanic material, in the form of tephra, is washed downslope in mudflows, but when transported aerially, very little tephra actually reaches the base.

The shapes of most of these volcanoes are symmetrical, with eruptions from a single pipe vent at the center, although some build up around long fissure vents. Mount Hekla in Iceland is an example of the latter, with a 5-by-10-kilometer (3-by-6-mile) oval shape. In the former case, a central crater, usually a funnel-shaped hole, is found on the summit. The central vent tends to remain in the same position despite the explosive events. Often, the crater may be enlarged by wall collapse because of magma withdrawal. A flat central floor in the crater may be more prevalent in humid areas, where heavy rains wash fine sediments downward.

Stratovolcanoes may reach great heights, but these large, symmetrical cones require a riblike structure of lava for support. Pure cinder cones, in contrast, rarely exceed 500 meters (1,640 feet) in elevation because their symmetry tends to be destroyed by slumping. One eruptive cycle may be an effusive flow of lava, followed in some later period by an explosive event that produces cinder and ash layers. The relative proportions of tephra and lava in stratovolcanoes' structures vary considerably, from pure ash or cinder to pure lava flows. Rarely do volcanoes exhibit classic “layer-cake” geology—that is, alternating layers of lava and ash. Some mountains (for example, Taal in the Philippines) consist of 70 to 80 percent pyroclastics, but others are dominated by lava eruptions.

Eruption Patterns

Eruption patterns vary from one stratovolcano to another; three basic types are described here. The Vulcanian type of eruption, characteristic of many active volcanoes, produces a solid crust that lasts until the next eruption. Gas pressure builds up in the magma column, eventually blowing out the solidified obstruction. A great explosion may follow, accompanied by a large, dark, cauliflower-shaped eruption cloud. With the reduction of pressure, gas-charged magma is replaced by pumice and ash. After the vent has cleared, lava flows may issue forth from the crater. Vesuvius, the stratovolcano east of Naples in Italy, falls within this group of eruptions.

The Strombolian eruption is distinguished by glowing fragments of lava accompanied by a white eruption cloud. In contrast to the Vulcanian eruption, which contains much ash, the Strombolian cloud contains very little ash. The crust that forms over the lava column is very thin, allowing for frequent, mild eruptions. Stromboli, a stratovolcano located on an island west of Italy, is the classic example, but some volcanoes may exhibit Strombolian activity during some portion of their history.

The Peléan eruption gives rise to the expulsion of nuées ardentes. Pumice and ash are characteristic, and no liquid lava develops. The magma is under such intense pressure that it is shattered into a fine dust and mixed with superheated steam. This mixture rolls over the top of the crater as an avalanche of red-hot dust. In the final stages of eruption, the gas content of the magma is greatly reduced, no longer breaking the surface but instead pushing upward to form a dome with spinelike projections, as in the case of Mount Pelée on the island of Martinique.

Stratovolcanoes tend to be positioned at the margins of descending or subducting masses of ocean floor or tectonic plates beneath the continents. The western United States has thirty-five volcanoes that have erupted and may erupt again in the future. One region of volcanic activity is found in the Cascade Range, extending from Washington and Oregon to Northern California, with a continuation into Nevada and through Idaho to Yellowstone National Park. Another volcanic zone winds through southeastern Utah into Arizona and Mexico.

In terms of frequency of eruption, volcanoes are divided into two groups. The first comprises those that have experienced eruptions on average every two hundred years and have last erupted within that period of time. Examples in this group are Mount St. Helens, Lassen Peak, and Mounts Shasta, Rainier, Baker, and Hood, all in the Cascade Range. The other group includes those whose last eruption was more than one thousand years ago; their frequency of eruption is greater than one thousand years. Examples are Crater Lake Volcano and Mounts Adams and Jefferson, also located in the northern Cascade Range.

The stratovolcano gains its heat from subducting-plate friction. The magma tends to have a lower temperature and a higher viscosity than does the basaltic magma of shield volcanoes (found at rift zones and located over hot spots). The eruptions are explosive because of the high silica and gas content and high viscosity. Rhyolite and andesite are often present in the flows, along with volcanic products called pyroclastics. Not all stratovolcanoes, however, erupt lava containing such high amounts of silica; Mount Etna in Sicily and Mount Fuji in Japan tend to have a silica composition of 50 to 60 percent in a basaltic lava.

Stratovolcanic Eruptions

Several stratovolcanic eruptions have had significant consequences for people living nearby. Located in the southeastern state of Chiapas, Mexico, El Chichón experienced a series of explosive eruptions on March 28, 1982, killing 187 people and leaving another 60,000 homeless. A large dust cloud, which was monitored by satellite, rose to an altitude of 25 kilometers (16 miles) and lasted for one month after the eruption. Scientists recorded 50 centimeters (20 inches) of ashfall 16 kilometers (10 miles) away and 20 centimeters (8 inches) of ashfall 80 kilometers (50 miles) away. In Palenque, 120 kilometers (75 miles) east of the volcano, ashfall was measured at 40 centimeters (16 inches).

On the eastern shore of Sicily, Mount Etna, the largest and highest of the European volcanoes, rises 3,320 meters (10,892 feet) above sea level. Small subsidiary cones around its flanks, up to 1,000 meters (3281 feet) high, erupt rather frequently, but only rarely does sufficient energy accumulate to cause an eruption from the summit crater. Early eruptions, several centuries B.C.E., were recorded by the Greeks. Thousands of lives have been lost and several towns destroyed during Etna's history. In 1669, 20,000 died in an eruption. The town of Mascati was destroyed in 1853, and in 1928 the village of Nunziata was nearly leveled. Explosive eruptions near the summit in 1979 claimed nine lives. Mount Etna is classified as a transitional volcano, transforming from shield to stratovolcano, and has had more than 150 eruptions since the first recorded one in 1500 B.C.E.

On June 6, 1912, one of the greatest eruptions of the twentieth century occurred on the Alaska Peninsula in the Valley of Ten Thousand Smokes. Mount Katmai is a complex stratovolcano, with both a caldera and a lake on its summit. The explosion was heard in Juneau, 1,200 kilometers away. The valley was flooded with 25 cubic kilometers (6 cubic miles) of pumice, ash, and gas and was filled to a depth of nearly 200 meters (656 feet). In Kodiak, some 160 kilometers (99 miles) away, pumice and ash nearly blocked out the sunlight, reducing visibility to about 2 meters (6 1/2 feet). The upper 1,900 meters (6,234 feet) of the mountain collapsed, forming a giant caldera more than 2 kilometers (1 1/4 mile) wide, which filled with water and became a crater lake. A new vent, Mount Trident, on the west flank of Katmai, became active in 1949 and has since experienced several thick lava flows, the last erupting in 1974.

Stromboli, one of the most active volcanoes, lies on a Liparian island 60 kilometers (37 miles) north of Sicily. It has been in a continuous state of eruption its entire history, although the intensity varies considerably. It was known for centuries to sailors as the Lighthouse of the Mediterranean because eruptive flashes from its summit were visible far out to sea. Even during World War II, it was used by Allied bombers as a navigational aid. The more explosive eruptions occur at about fifteen-minute intervals, with moderate activity in between. The principal crater, located 600 meters (1,969 feet) up the 900-meter (2,953 feet) mountain, is always full of semifluid lava and thus does not greatly resist magma pressure from beneath.

Mount Tambora, on the island of Sumbawa, Indonesia, began a series of eruptions on April 5, 1815, that did not end until July of that year. The early explosions sounded like cannon fire and could be heard 720 kilometers (447 miles) away. The greatest eruptive activity occurred on April 11 and 12, when explosions were heard in Sumatra, 1,600 kilometers (994 miles) to the west. There were only a few survivors of a population of 12,000 on the island, and 45,000 other lives were lost on surrounding islands. Volcanic ash was so heavy that there was darkness at noon in Java, 480 kilometers (298 miles) distant. The dust sent into the atmosphere obscured sunlight more than had any other volcanic dust within the previous four hundred years. The year 1816 was known as the “year without summer.”

Vesuvius is the only active volcano on mainland Europe. The famous eruption of August 24, 79 C.E., was described by Pliny the Younger from a distance of 30 kilometers (19 miles). His account describes a strange cloud that rose up and away from the mountain as well as roars and explosions coming from deep within the mountain. The residents of Pompeii were overcome with searing ash and toxic gases. Some 16,000 lives were lost, mainly by suffocation; the residents of nearby Herculaneum, for example, were overcome by boiling mudflows. Another violent eruption took place in 1631, causing the loss of 18,000 lives. During World War II, on March 18, 1944, an eruption forced the evacuation of the city of Naples, and lava flows caused extensive damage at an air base close to the mountain. Mount Vesuvius is a complex stratovolcano, with lava flows generally following explosive eruptions.

Volcanology

A major goal in volcanology is the prediction of the time, place, and nature of an eruption to minimize loss of lives and property. A variety of methods are employed, for no single factor is sufficient to warn of impending eruption.

Geophysicists monitor the earthquakes and tremors that accompany volcanic eruptions with seismographs that measure the intensity of earthquake activity. Almost every eruption or increase in volcanic activity is preceded by earthquakes—often very small earthquakes, or microearthquakes, that occur in swarms of hundreds or thousands. A volcanic tremor is almost always present during an eruption and often begins before the surface outbreak. This motion has a natural frequency from 0.5 to 1.0 hertz and produces a very low hum. The source of this noise is not clearly understood, but it may be related to the formation of gas bubbles in the lava.

Small changes in the slopes, shown by distances between markers on the flanks and summits of active volcanoes, are another precursor of volcanic activity. Techniques for detecting such changes include conventional leveling, reflected light beams, and instruments called tiltmeters capable of measuring changes in slope of less than one part in one million. One type of tiltmeter uses brass pots filled with water and fastened to support posts connected by hoses. The posts are placed a few meters apart. Micrometers that measure fractional water-level change are placed inside the pots.

With the rise of magma toward the surface, the earth's magnetic field is disturbed by molten lava, which loses its magnetic properties at high temperatures. Such changes may be detected by a magnetometer carried aerially over the volcano. Moving magma within the volcanic chambers may cause changes in the electrical currents within the ground. Sensitive resistivity meters placed under the surface are used to measure electrical conductivity. An increase in temperature of hot springs, fumaroles, and groundwater may signal an eruption; several months before the 1965 eruption of Taal in the Philippines, the temperature of the water inside the crater increased by 12 degrees Celsius (22 degrees Fahrenheit). The chemical composition of waters and volcanic emissions may also change during eruptions. An increase in sulfur dioxide or hydrochloric acid in groundwater or surface emissions can precede an eruption.

Direct visual observations are helpful in monitoring volcanoes. Ice and snow on a volcano may melt from the intense heat. A volcano may bulge from movement of magma along one of its flanks, as was observed prior to the 1980 eruption of Mount St. Helens. Active volcanoes are observed from nearby observatories or sentry outposts. Valuable contributions are made by volcanologists in these facilities, but close observation can be very dangerous.

Significance

The majority of the world's six hundred active volcanoes lie within the Pacific Ocean Basin, with about one-half in the western Pacific region. They form an almost continuous pattern known as the Ring of Fire along the edges of the Pacific. Stratovolcanoes that form along the ocean margins are termed marine stratovolcanoes. Their initial structure resembles basaltic seamounts, with pillow basalts building up on the sea floor, followed by pyroclastics and less effusive eruptions.

Deep marine volcanoes are less explosive than volcanoes on land, because the pressure of the seawater retards the expansion of steam within the magma. Subduction-zone and island-arc volcanoes, in contrast, contain a high concentration of gases in the upper parts of their magma chambers, making them more explosive than are volcanoes in other locations. The lava tends to have a greater amount of silica, so it is more viscous and resistant to flow. The lava that solidifies within the vent, or throat, forms a hardened plug that is blasted into pieces, or pyroclastics, by the pressure of the gas trapped below. Lava flows that are rough and blocky are termed aa, while those with a smooth, ropy texture are called pahoehoe.

Island-arc stratovolcanoes have had an explosive and dangerous history. Volcanoes such as Krakatau and Tambora in Indonesia as well as Rabaul in Papua New Guinea are examples. It is believed that these volcanoes are economically important for the deposition of metallic sulfides. The magma chambers beneath may be regions where copper, molybdenum, and gold are concentrated during the final consolidation.

It is important to study volcanoes to minimize volcano damage, including loss of lives. Various methods have been devised to reduce volcano damage; these include the digging of channels and construction of levees to divert lava flows. Lava streams have been doused with water in attempts to slow down and solidify the lava. In Java, dams have been constructed to direct volcanic mudflows away from cities and agricultural lands. In Hawaii, the U.S. Air Force has bombed lava flows emerging from Mauna Loa, with limited success.

Some volcanoes eject considerable amounts of ash and dust into the atmosphere. Certain massive volcanic eruptions had a significant effect on global temperatures for months afterward. The 1963 eruption of Mount Agung, a stratovolcano in Bali, Indonesia, ejected clouds of gas and dust 10 kilometers into the stratosphere. The average worldwide temperature dropped 0.5 degrees Celsius (about 1 degree Farenheit) for three years after the eruption. Whether volcanic eruptions do effect significant changes in the weather and climate cannot be satisfactorily answered until scientists can obtain accurate measurements of the volcanic dust and debris entering the stratosphere and the heating and cooling of the earth's atmosphere that result from such dust.

Principal Terms

andesite: a gray volcanic rock with a silica content of about 60 percent

ash: fine volcanic ejecta less than 2 millimeters (less than 1/10 an inch) in diameter

caldera: a large circular basin with steep walls that resulted from the collapse of a summit or of an earlier volcanic cone

cinder cone: a small volcano composed of cinder or lumps of lava containing many gas bubbles, or vesicles; often the early stage of a stratovolcano

pumice: a solidified volcanic froth of a glassy texture light enough to float on water

pyroclastics: fragmented ejecta released from a volcanic vent

rhyolite: a light-colored volcanic rock composed of a viscous lava containing about 70 percent silica

tephra: all pyroclastic materials blown out of a volcanic vent, from dust to large chunks

viscosity: the resistance of lava to flow; it depends upon the chemical composition, temperature, and crystalline nature of the magma

Bibliography

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“Stratovolcanoes.” Volcano World, Oregon State University, volcano.oregonstate.edu/stratovolcanoes. Accessed 31 July 2024.