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Volcano Overview

Volcanoes occur when magma (melted rock) from within the Earth's crust reaches the Earth's surface, and are generally located along the boundaries of tectonic plates (large segments of the Earth's crust). Volcanic activity is a function of the plate boundary type; therefore, different types of volcanoes form at different plate boundaries. Three main groups of volcanoes exist:

  1. Those that form volcanic mountain chains along convergent plate boundaries, where one plate is being pushed (subducted) below another, such as the Cascade Range in the Pacific Northwest,
  2. Those that form at "hot spots" in the middle of plates, such as the Hawaiian Islands, and
  3. Those that form where two plates are being pulled apart and, as a result, the crust is thinning and new crust is being formed, such as in Iceland the along the Mid-Atlantic Ridge.
The Ring of Fire is a narrow zone of frequent volcanic eruptions and earthquakes that partially encircles the Pacific Ocean. Volcanic activity within the Ring of Fire is associated with subduction-type plate boundaries. In general, subduction-zone volcanoes produce more explosive eruptions than the other two groups. 
Map of volcanoes around the world. Note the Ring of Fire indicated by the series of volcanoes encircling the Pacific Ocean. Image courtesy of Smithsonian Institute Global Volcano Program

Composite or Strato Volcano - steep-sided, cone-shaped volcanoes composed of alternating layers of high viscosity lava flows and tephra (fragments of volcanic rock and lava that are blasted into the air by explosions or carried upward by hot gases during eruptions). They occur along convergent plate boundaries and are known for violent eruptions. Mount Saint Helen's is an example of a composite or strato volcano. Hazards associated with composite volcanoes include: ejected gas and rock, lahars (fast-moving flows of melted snow and ice mixed with volcanic debris), earthquakes, pyroclastic flows (fluidized bodies of hot gas and tephra), and collapse. 

Photograph of Mount Saint Helens. For more information on the 1980 eruption, see Historic Volcanic Events

Photograph courtesy of Worldbook Encyclopedia

Shield Volcano - broad, gently sloping volcanoes with bases as large as 100 kilometers in diameter and slope inclinations less than 10 degrees. The shield volcano is composed of basaltic rock formed by low-viscosity lava flows and generates little to no ash during eruptions. They occur along divergent plate boundaries and hot spots. The Hawaiian Islands are examples of shield volcanoes. Hazards associated with shield volcanoes include: lava flows, airborne ash particles, corrosive volcanic gases, volcanic glass, ground cracks and settling, earthquakes, and explosions caused by the interaction of lava with seawater. Of these hazards, lava flows are the most common.

Photograph courtesy of USGS

View of the northwest flank of Mauna Loa Volcano in Hawaii. For more information on the eruption of Hawaiian volcanoes, see Historic Volcanic Events 

Fissure Volcanoes
 - volcanoes where basaltic lava is ejected through a vertical crack in the ground. Rather than forming a cone, the lava spreads out over a large area. These volcanoes are typical of oceanic ridges where the plates are spreading apart and new crust is being formed. Hazards associated with fissure volcanoes are similar to those associated with shield volcanoes.

Photograph courtesy of Michael Ryan of the USGS

Photograph of Krafla in Iceland. Lava is erupting through a fissure related to the Mid-Atlantic Ridge. 

Vulcanologists study and understand volcanoes and the mechanisms that produce them. They monitor seismic activity and gas emissions associated with volcanoes, and note any changes from the norm as possible precursers to eruptions. Engineering geologists evaluate hazards associated with different volcano types and their potential impact on humans, buildings and infrastructure. For example, shield volcanoes typically produce slow moving lava flows but not explosive eruptions. In contrast, composite volcanoes are known to produce explosive eruptions of gas and ash capable of traveling in the atmosphere extensive distances and lahars capable of moving debris great distances from the volcano. 

Numerous volcanoes throughout the world have been recorded in human history. Some notable recent volcanic events include:Historical Volcanic Events Back to top
  • Mount Saint Helens Eruption of May 18, 1980: Mount Saint Helens is a composite volcano located within the Cascade Mountains and associated with subduction of the Pacific Plate under the North American Plate. On March 20, 1980 a cluster of small seismic events near the volcano was recorded. A week later, steam and ash began ejecting from the dome and the USGS issued warnings that an eruption was probable. The following week, geologists arrived to monitor a large bulge forming on the mountain's north side. By the end of April, the bulge was expanding at a rate of 1.5 meters per day and scientists began to monitor gas emissions for sudden increases, often indicative of an eruption. Between May 15 and May 17, the number of seismic events decreased from 39 to 18. The following day at approximately 8:32 am, a magnitude 5 earthquake caused a debris flow on the north slope. The loss of overlying rock released gas and magma from the volcano, resulting in a lateral blast of rock, ash, and gas (Hamblin and Christiansen, 1995). For more details on the Mount Saint Helens eruption visit USGS

  • Mount Pinatubo, Philippines Eruption of June 1991: Mount Pinatubo is one of 22 composite volcanoes located on the Philippine Island chain. Prior to June 9, 1991, Mount Pinatubo had remained dormant for approximately 400 years. However, on this date, the volcano began ejecting ash, steam and pyroclastic materials. By June 12, the area was experiencing intense seismic activity and explosions of ash and pyroclastics (rock debris thrown from a volcano) as high as 25 kilometers above the volcano's summit. These explosions continued for the next three days, increasing in intensity. The largest explosion occurred on June 15, 1991, ejecting debris approximately 40 kilometers above the summit. A total of 19 eruptions were recorded that day. This eruption of Mount Pinatubo released 5 million cubic meters of ash and pyroclastic debris, and resulted in the deaths of 847 people and the displacement of 1 million. Additionally, the ejected material remained suspended in the atmosphere for months, and reduced the global temperature by 1 degree Celsius. (Pinatubo Volcano).

  • Mount Kilauea, Hawaii Eruption of January 3, 1983 to Present: Mount Kilauea, the youngest shield volcano on the big island of Hawaii, has been erupting continuously since 1983. Currently, lava is being erupted from the cinder cone vent Pu'u 'O'o in its 55th eruptive episode. In January 2005 it was determined that 2.7 cubic kilometers of lava had added 230 hectares to the volcano's southern shore. During its lifetime Kilauea has destroyed 189 structures and covered approximately 14 kilometers of highway. While shield volcanoes are not typically explosive, Kilauea's continuous supply of lava produces flows that threaten property but move slow enough for people to avoid (HVO-USGS). 

Links to More Information Back to top

Technical References Back to top

Hamblin, W.K. and E.H. Christiansen, 1995, Earth's Dynamic Systems: Prentice Hall, New Jersey, 7th Ed., p. 241-242.

For more in-depth information about volcanoes, check out AEG's Technical References page.

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