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Lava Rocks

Hi all,
Thought we need to clear the air on "lava" rocks.  There are basiczlly two
kinds of lava, (which is simply magma that has reached the surface) MAFIC
and FELSIC.  The kind you see most often is the mafic.  This lava is high in
iron and therefore dark in color. If the magma had cooled underground there
would be no holes in it and it would appear black --> basaltic rock.  As far
as lava goes it is very thin and flows easily.  Depending on the rate of
cooling as the lava hardens, you can end up with different looking rock all
with the same chemical composition.  This is the kind of rock found in
Hawaii and lots of other places.
    Felsic magma is much thicker as it has less iron and therefore more
silicon.  It holds tremendous amounts of gas and can erupt very
violently ---see Mt. St Helens.  The rocks produced by felsic magma are
light in color and depending on the rate of cooling (undergound or on the
surface) you may or may not have trapped bubles.  Pumice is the volcanic
glass produced with the trapped bubbles and as you know it floats and
doesn't make a very good addition to the aquarium.  However, the undergound
version of this rock is quite nice in appearance and is used in monuments
all over the world --- granite!
    Any of the volcanic rocks that have been thouroghly washed and collected
at the surface will do fine in the aquarium.  Some just look better than

tneill at zbzoom_net