Volcanoes Vicki M. Harder

Introduction A volcano is a vent through which magma, solid rock debris, and gases are erupted. The compositions of magma have a significant influence on the style and explosiveness of volcanic eruptions. The most common types of magma are basaltic, andesitic, and rhyolitic. Basaltic magma accounts for about 80 percent of all magma erupted by volcanoes. Whereas rhyolitic and andesitic magma accounts for 10 percent each. Magma Characteristics and Volcanic Landforms The viscosity, or the ability to flow, of a magma is dependent upon its temperature and silica and dissolved gas content. Basaltic magmas are the least viscous and rhyolitic magmas the most viscous. A volcanic landforms' shape is determined by the composition and characteristics of the volcanic materials and the types of eruptions that have occurred. Shield volcanoes are formed from the build-up of basaltic lava, forming a broad formation that resembles a shield lying horizontally, with an average slope of only a few degrees. Stratovolcanoes, also know as composite volcanoes, are built up of interlayers of andesitic lava and pyroclastic deposits, with very steep slopes.

Eruptive Styles Dissolved gases come out of solution during the upward movement of magma, forming bubbles. The ease or difficulty of the escape of the bubbles (magma viscosity) determines the eruptive style and explosiveness of a volcano. The melting temperature of a basaltic magma is approximately 1400 degrees C with a silica content in the 50 percent range, giving it a relatively low viscosity. The combination of high temperature and low viscosity allows the dissolved gas bubbles to escapee easily, creating a rather nonexplosive volcanic eruption. Temperature decreases and the silica content rises with andesitic and rhyolitic magmas with a corresponding increase in magma viscosity causing explosive eruptions.

The Electronic Volcano

Once you are at the Electronic Volcano Web site scroll down the screen until you come to "Catalogs". Under Catalogs you will need to scroll down further until you come to the heading "Update on Current Volcanic Activity" where you can view photographs and obtain additional information on the latest volcanic activity. You have two choices in viewing the photographs, you can view the available map and click on a triangle or go the the listing of the volcanoes. Create a list of at least five (5) volcanoes from the current activity list being sure to give the name, location, and classification as either a shield or stratovolcano. From the map of current volcanic activity and your interpretation of the photographs determine where it would be safest to live.