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Volcanoes

The Pacific Ring of Fire and Volcanoes

Geologists coined this rather poetic and graphic phrase to illustrate that 75% of the earth’s active volcanoes and a great number of its earthquakes are encountered along the circumference of the Pacific Ocean. In its most simple expression, the explanation lies in the fact that the Pacific Ocean basin is comprised of active tectonic plate margins, or boundaries.

In a “subduction zone”, where an oceanic plate slides under a continental or another oceanic plate, volcanism occurs. Such is the case in the Andes, for example, where the Nazca oceanic plate is sliding under the South American continental plate; or the Marianas where the fast-moving Pacific plate converges with the slower moving Philippines oceanic plate.

In a “transform fault”, where a plate slips past another, tremors are frequently triggered. Just think of the fabled San Andreas fault…the Pacific Ring of Fire is also “speckled” with a few “hotspots”, particularly intense and consistent fixed sources of heat underneath these mobile tectonic plates, in the earth’s mantle. Where the thermal plumes manage to make their way though the planet’s crust, volcanoes rise from the sea floor: Hawaii and Galapagos archipelagos, mysterious Easter Island, Samoan Islands…

Volcanoes
The word “volcano” comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan – the blacksmith of the Roman gods. They thought that the hot lava fragments and clouds of dust erupting form Vulcano came from Vulcan’s forge as he beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia the people attributed eruptive activity to the beautiful but wrathful Pele, Goddess of Volcanoes, whenever she was angry or spiteful. Today we know that volcanic eruptions are not super-natural but can be studiedand interpreted by scientists.

The Nature of Volcanoes
Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products — lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A volcano is mostcommonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.

Driven by buoyancy and gas pressure, the molten rock, which is lighter than the surrounding solid rock, forces its way upward and may ultimately break through zones of weaknesses in the Earth’s crust. If so, an eruption begins, and the molten rock may pour from the vent as nonexplosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and accumulations of fallback fragments may move downslope as ash flows under the force of gravity. Some of the finer ejected materials may be carried by the wind only to fall to the ground many miles away. The finest ash particles may be injected miles into the atmosphere and carried many times around the world by stratospheric winds before settling out.

* Excerpts from: Tilling, 1985, Volcanoes: USGS General Interest Publication

Links to volcanology websites:

- Smithsonian Institute Global Volcanism Program here.
- USGS Cascades Volcanoes Observatory here.
- Volcanoworld here.
- Canadian volcanoes here.
- The Electronic Volcano here.
- International Association of Volcanology and Chemistry of the Earth’s Interior here.
- Michigan Technological Institute here.

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