On Disasters

7.19.2010: Volcano Basics

A volcano is a vent in which magma rises from beneath the Earth’s surface to erupt as lava, create seismic activity, or release gas.¹ There are three mechanisms for forming a volcano.

When two continental plates spread apart, magma rises up between them.¹ Such spreading zone volcanoes form long ridges with almost continuous low-grade activity. Most spreading zones are under water, but Iceland and the African Spreading Zone are examples of spreading zones that cross land.²

When two continental plates collide and one sinks beneath the other, the friction causes small columns of magma to form and rise through the upper plate.¹ The resulting volcanoes tend to be on land, though they can be mid-oceanic. They initially have a conical shape, although chamber collapse, explosive eruption, and erosion can change their shape. Numerous examples of this type of volcano dot the west cost of the U.S., including Mount St. Helens.²

Finally, a plume of hot magma can rise from deep in the mantle, breaking through the crust somewhere in the middle of a continental plate. When this happens in the ocean, the magma tends to be a fast-flowing liquid that forms wide, flat volcanos, such as the Hawaiian volcanoes.³

Three Types of Volcanoes

Three types of volcanoes are discussed below. Although these do not cover all the different types, the information will show how volcanoes can be formed and erupt differently.

Shield volcanoes: Shield volcanoes are made primarily of basalt. These types of volcanoes have a broad, flat dome-like shape due to the layers of lava that have crossed its surface and then cooled. Eruptions tend to be relatively gentle, in which a large amount of lava overflows from the volcano.¹

Composite volcanoes: Composite volcanoes (strato-volcanoes) are made of a composite of both rock fragments and lava. There are a large number of shapes these volcanoes can take (e.g., concave, helmet-shaped, nested, and pyramidal). Due to the composite layers of these volcanoes, gas and lava can become backed up within the vent. The build-up of pressure can result in an explosive eruption.¹

Caldera volcanoes: These volcanoes form as large amounts of lava erupt from underground lava cavities. These eruptions form a void underground, causing the earth above to collapse into the cavity. The result is the formation of a caldera, a basin-shaped depression several miles in diameter. These volcanoes are considered the most dangers because tsunamis can result, along with large amounts of widespread ash.¹

Active or Dormant?

If a volcano is erupting lava, creating seismic activity, or releasing gas, it is considered active. A dormant volcano is one that has not erupted for any length of time (up to 10,000 years ago); however, it could erupt sometime in the future. An extinct volcano is one that has not erupted for more than 10,000 years.¹

1 Pillai, P. (2010). How are Volcanoes Formed? Buzzle.com. Found May 29, 2010, from http://www.buzzle.com/articles/how-are-volcanoes-formed.html

2 United States Geological Service. Understanding Plate Motions. Updated May 5th, 1999. http://pubs.usgs.gov/gip/dynamic/understanding.html

3 Geologic Resources Division, "Hawai‘i Volcanoes National Park," Natural Resource Report NPS/NRPC/GRD/NRR—2009/163 http://www.nature.nps.gov/geology/inventory/publications/reports/ havo_gri_rpt_view.pdf