|Why is volcanic rock so much lighter than other
rock and what happens when you put it in on water?
|Question Date: 2001-12-04|
Many volcanic rocks are not light at all. But
certain ones, especially those that come from
magma rich in water are low density because they
are samples of frozen foam! That is, when the
magma gets close to the Earth's surface, the
pressure is low enough so that H2O
initially dissolved in the molten stuff comes out
of solution. This is analogous to what happens
when you pop open a bottle of soda! In this case
the CO2 that is dissolved in the liquid
comes bubbling out. Now, in the case of magma,
since it solidifies at 900 degree C, the bubbles
become frozen when the temp drops and we have
volcanic foam. Some volcanic rocks that are
very vesicular (called PUMICE) actually float on
Great question. You probably remember that
volcanic rock forms when a volcano erupts, spewing
hot lava and ashes everywhere. When that lava and
ash cools down, it turns into solid rock. But --
this is the key point -- lava and ash often has
a lot of air mixed into it during the eruption,
resulting in pockets of air trapped in the
cooled-down rock. It's those air pockets
which make many volcanic rocks lighter than other
rocks. (But the amount of air trapped can vary a
lot among volcanic rocks, so some volcanic rocks
can be quite dense.) If there's enough air
trapped, the volcanic rock *could* float on water
-- but the vast majority will sink, just like any
other rock. Hope this helps!
Not all volcanic rock is 'light' or low density
-- basalt is relatively heavy and is organic,
granite is lighter, but still volcanic in origin.
I think you are thinking of gabro and volcanic
bombs -- these are rocks with dissolved gasses
which caused bubbles to form when the pressure was
released (by the eruption). Due to the bubbles,
these rock can indeed be very light and many will
float on water.
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