|MadSci Network: Earth Sciences|
Dear Keith, I can give you some indication of what lava contains and the range of densities it can have - unfortunately it is not a simple answer. This is because lava does not have just one composition. In fact, the physics and chemistry of molten rock is pretty complex due to the wide variation in its composition. (Keeps a lot of geologists employed, too!) Lava does contain many silica-based compounds and the types and numbers of silica-containing compounds can vary. So, as you might expect, the density of a lava changes with its composition. The densities of four types of lava at two different temperatures are given below. Density is given in grams per cubic centimeter and temperatures are in degrees celcius. The lavas are completely liquid at 1250 degrees Celcius and completely solid at room temperature (25 degrees C). Lava Type Density at 1250 deg. Density at Room Temp. Rhyolite 2.17 2.28 Andesite 2.41 2.59 Basalt 2.60 2.76 (Hall 1996) The increase in density from rhyolite (rye-oh-lite) to andesite to basalt is generally due to increasing amounts of magnesium (Mg) and iron (Fe). All of these rocks contain silicon (Si) or SiO2 (quartz) but compounds with Mg and Fe are denser than compounds with aluminum or calcium, for instance. The quartz content of these 3 lava types are given below as a percentage of their entire composition: Rhyolite 63 to 80% Andesite 52 to 63% Basalt 45 to 52% [This information is from a diagram of igneous rocks by the University of British Columbia: http:// www.science.ubc.ca/~geol202/rock_cycle/igneousrx.html ] The list of minerals that these lavas can contain is extensive so I have not listed them all here. Basalt (the lava that flows out of the vents in Hawaii) commonly contains the following minerals given in order of decreasing abundance: plagioclase feldspars: labradorite, bytownite, anorthite pyroxenes: augite, hypersthene olivine biotite hornblende quartz To find out more about lava and the minerals it contains I recommend a geology field guide such as The Audubon Society Field Guide to North American Rocks and Minerals. You can find more information on how and why lava composition changes in general Igneous Petrology textbooks in a college library. To order a book, try the Oxford University Press website or any of the online retail bookstores. References: Hall, Anthony, 1996. Igneous Petrology, 2nd Edition. Longman Group Limited, England. Chesterman, Charles W. 1978. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A
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