Re: What is the chemical/molecular composition and density of Lava?

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