Synoptic Description of Volcanoes

Volcanoes are openings in the earth's crust created when molten material under the crust is propelled upward though the surface. The magma chamber collects the magma that is expelled to the surface in an eruption. A volcanic event could be:
  • Destructive, with voluminous lava flows or explosive activity. This usually occurs when magma is sticky and contains a lot of gas. Hot debris particles called pyroclastic are expelled during violent explosions. Heavier pieces land near the crater and lighter pieces can be carried by the wind for hundreds of miles.
  • Non-destructive, with little release of solids or magmatic liquid. These eruptions occur when the magma is more fluid and contains less gas. The solids or magma rocks and lava cools on it's slope.

Cross Section of a Volcano

The material expelled during violent and quite eruptions build a cone shaped mountain around the central vent. Usually there is a bulge shaped crater at the top of the central vent. Volcanologist use the VEI or Volcanic Explosively Index to rate eruptions.

Types of lava

There are three types of lava and lava flows: pillow, pahoehoe, and aa.
  • Pillow lava are volumetrically the most abundant type because they are erupted at mid-ocean ridges and because they make up the submarine portion of seamounts and large intraplate volcanoes, like the Hawaii- Emperor seamount chain.
  • Pahoehoe is the second most abundant type of lava flow. Pahoehoe lava is characterized by a smooth, billowy, or ropy surface. A ropy surface develops when a thin skin of cooler lava at the surface of the flow is pushed into folds by the faster moving, fluid lava just below the surface.

Pahoehoe near the coast of Kilauea. Photo by Steve Mattox, 1989.

  • Aa(right) is characterized by a rough, jagged, spinose, and generally clinkery surface. Aa flows advance much like the tread of a bulldozer.

Pahoehoe lava(above-left), Kilauea, Hawaii. Aa lava(right), this photo is looking across an aa channel. Photograph by J.D. Griggs, U.S. Geological Survey, June 15, 1989.

Causes of Volcanoes

The most basic requisite for volcanoes to form is the presence of a molten rock reservoir, or magma chamber, which is under sufficiently high pressure to force various forms of volcanic materials to the surface. The passage of this ejecta can take place through zones of structural eakness, or fissures, or through a more central conduit generally referred to as volcanic vent (Ebert, 26).

Mantle Picture

Occurrences of Volcanoes

A volcanic event occurs when there is a sudden or continuing release of energy caused by near-surface or surface magma movement. The energy can be in the form of earthquakes, gas-emission at the surface, release of heat (geothermal activity), explosive release of gases (including steam with the interaction of magma and surface of ground water), and the non-explosive extrusion or intrusion of magma. A volcano may erupt many times during it's lifetime of 1,000 of years

This map of the Earth shows occurrences of volcanoes around the world. The red triangles show locations of the volcanoes. For up to date information on volcanic ativity, visit

A volcanic event can include:
  1. An eruptive pulse (essentially an explosion with an eruption plume, but also non-explosive surges of lava. A pulse may last a few seconds to minutes.
  2. An eruptive phase that may last a few hours to days and consist of numerous eruptive pulses that may alternate between explosions and lava surges.
  3. A single eruption or eruptive episode, composed of several phases,that may last a few days, months or years (Fisher and Schmincke, 1984). Paricutin, Mexico was in eruption for nine years. Stromboli, Italy has been in eruption for over 2000 years.
Mauna Loa is the largest volcano on Earth with an estimated volume of 9,600 cubic miles (40,000 cubic kilometers). It makes half of the area of the Island of Hawaii. Mauna Loa began to form nearly a million years ago. There is a caldera, Mokuaweoweo, at the summit and rift zones extend to the northeast and southwest.

Mauna Loa:(19.5N, 155.6W)
Elevation: 13,681 feet (4,170 m)
As seen from sea level and space

Mauna Loa is in the shield-building stage and is one of the most active volcanoes on Earth, erupting 15 times since 1900. The last eruption was in 1984 and sent lavas within 4 miles (6.5 km) of Hilo. This photograph looks to the southwest to the northeast rift of Mauna Loa. Volcano Village is in the foreground. The summit of Kilauea volcano is left of center. Photograph by J.D. Griggs, U.S. Geological Survey, January 10, 1985.

Social and Economical Impact of Volcanoes

The social and economical impact that volcanoes posses is indeed a serious one. Throughout history volcanoes have shed their destruction on humankind, an their legacy will continue. One of the most destructive eruptions are Pelean eruptions which are named after the eruption of Mt. Pelee on May 8, 1902 that destroyed the town of St. Pierre.

Island of Martinique in St. Pierre, Pelee.
Left: Eruptive cloud of Pele, typical of Pelean eruptions.
Right: Destruction in St. Pierre during 1902 eruption.

Of 30,000 inhabitants in 1902, only two survived the avalanches of incandescent ash that leveled the city in only two minutes. Victims recounted a burning sensation on all over their bodies and throat. Instantaneous death is usually the result of inhaling the highly heated gases or from burns. All the houses were unroofed or demolished. The trees were stripped of leaves and branches, down to the bare trunks.

The eruption of Vesuvius on August, AD 79 was even yet more terrifing. It produced major pumice-and-ash falls, followed by pyroclastic flows(nuees). Pompeii, a nearby city, was blanketed by thick pumice-fall deposits, which were in turn covered by pyroclastic flows, 4 to 5 meters thick. Herculaneum, another nearby city, was upwind from the early falling debris but was inundated by multiple pyroclastic flows, which total 10 to 20 meters in thickness.

The Shore of the Bay of Naples in central Italy, Mt. Vesivuis
Left:Painters interpretation of Vesivuis AD 79 Eruption.
Right:A child overcome by pyroclastic-flows, body encased.

Among the causes of death are fractured skulls, overwhelmed by ashes or noxious gases, or overcome by pyroclastic-flows. Excavation reveled 2,000 body-shaped cavities in Pompeii alone.

Current And Recent Volcanic Activity
Volcano World!
Recent Eruptions
Long Valley Caldera
Recent Seismicity
Cascade Range Volcanoes
Images of Volacanoes
Smithsonian Institution - Global Volcanism Program
Erupt-a-Word Java Style Crossword Puzzle
GLOSSARY - Volcanic Eruptions
Stratospheric Perturbations caused by Volcanic Eruptions
MSHPPF - COMPARISONS WITH OTHER ERUPTIONSEruptions of Mount Spurr Volcano, Alaska, Photographs of the 1992 Eruptions
Latin American Volcanoes
Eye on the World Violent Planet Page
Absolutely Volcanic - Contents