"A vent in the surface of the earth through which magma and associated gases and ash erupt; also, the form or structure, usually conical, that is produced by the ejected material."


A. Mid-Ocean Ridges: total of 30,000 miles long - spreading rate averages a few centimeters per year

B. Fissure Eruptions: (includes mid-ocean ridge eruptions) - on the continent, called flood basalts; examples: (1) Columbia Plateau (20,000 sq. miles) covering Washington, Oregon, and Idaho (2) Deccan Plateau (India)

C. Individual Volcanoes - Locations

Circum-Pacific "Ring of Fire" (Japan, Philippines, U.S. Cascade Range, Andes), Java Trench (Indonesia, Krakatoa), Mediterranean (Mt. Vesuvius, Mt. Aetna).

D. Hot Spots: Volcanoes not related to plate boundaries - stationary mantle plumes over which the crust passes over (Example: the Hawaiian islands).

3. CLASSIFICATION OF VOLCANOES by shape and composition

A. Shield volcanoes: broad, flat dome of basaltic lava - low viscosity, low silica content, relatively low gas content, fairly quiet eruption (no explosions) Example: Kilauea, Hawaii.

B. Composite volcanoes: symmetrical cone composed of alternating layers of lava and pyroclastics (bits of violently erupted volcanic material, including ash, cinders, and blocks). Lava is usually more viscous (higher silica content) and gas-charged, with the composition of rhyolite(Mt. Pelee, Martinique), dacite (Mt. St. Helens), or andesite (Mt. Fuji, Japan).

C. Cinder cones: symmetrical cone of mostly erupted pyroclasts. Example: Paricutin, Mexico.


1. Lava: very hot (950 F. to 2,500 F.), but its direction of flow is usually predictable.

2. Pyroclastics: more dangerous than lava because they occur by explosive eruptions. Large blocks usually fall close to the vent. Fine ash and dust is dangerous to breathe; microscopic glass particles damage lung tissue, a condition known as "pneumonoultramicroscopicsilicovolcanoconiosis"

3. Lahars: volcanic mudflow, mixture of ash and melted snow or ice. Examples: Mt. St. Helens (1980), Herculaneum (79 A.D.).

4. Nuees Ardentes: glowing ash clouds of incandescent gas, moves at nearly 100 M.P.H; very dangerous because of their heat (2000 oF.). Examples: Mt. Pelee (1902), Mt. St. Helens (1980).

5. Toxic gases: include carbon monoxide, sulfides, hydrochloric acid. Examples: Cameroon, East Africa (1987)

6. Steam (Phreatic) Explosions: seepage of water into a magma vent, instantaneous conversion of water to steam causes explosion. Example: Krakatoa (1883).

Copyright  © 1988 by William K. Tong