Lava Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Rhyolitic Lava

This type of lava is the most felsic of the different types. It is a light colored lava and erupts at temperatures of 800–1000°C. It has a higher silica content and is much thicker than basaltic lavas. Think of the flow differences between milk and molasses. Rhyolitic lava moves about ten times slower than basalt and tends to pile up in thick globular deposits.

Andesitic Lava

This type of lava is a middle-of-the-road lava between basaltic and rhyolitic. It has a median silica content and its characteristics fall in between basalts and rhyolites.

In 1943, the postmaster of the island of Hokkaido, Japan and an amateur geologist noticed some unusual geological activity in a nearby potato field. What first began as escaping steam, and then turned into spewing ash and lava became the beginnings of a new volcano. The postmaster, a geologist’s assistant at one time, watched the volcano’s progress through his office window. (That was before color TV and DVDs.) He noticed that the window screen surrounded his view like a piece of graph paper, so he made notes of the volcano’s growth month after month based on the squares of the screen. When geologists later studied his notes, they liked the graphing idea and found it to be a great way to measure a volcano’s progress. The postmaster’s volcano, Showa Shinsan (Japanese for “new volcano”) was formed over an active subduction zone along the Pacific Rim. It produced andesite lava. Andesite is a basalt containing a high silica content.

Lava Lakes

Lava lakes are often a product of Hawaiian eruptions where fluid basalt will pond in vents, craters, and wide low spots. Sometimes lava pours from a vent within a crater or wide depression and fills it. Today, active lava lakes are found in only a few places: Mount Erebus in Antarctica, Erta Ale in Ethiopia, and Nyiragongo in the Congo. Hawaii’s Kilauea volcano has had an active past with lava lakes in several of its craters at different times.

As a lava lake cools, a silvery crust of only a few centimeters thick forms on the surface of the lake. This crust is constantly disturbed and reformed. The movement of the molten lava beneath the crust causes it to crack into slabs that sink. The newly exposed lava cools to form another crust and the process begins all over again.

Practice problems of this concept can be found at: Volcano Practice Test

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