Why does gas and water make lava thick? (Like in a cinder cone)
Mrs. DeHaan's 4th grade class Liberty, MO
Hi Mrs. DeHaan's class,
I'm not sure what you mean by "thick". Do you mean thick as in the
distance from bottom to top? Or do you mean thick as in really gooey? (in
which case you really mean "viscous"
Anyway, there are a number of effects of gas and water on lava. Magma
beneath the earth always has at least a little bit of gas dissolved in it.
This means that H2O, CO2, SO2, and other gas molecules are just wandering
around in the magma. They are not little bubbles of gas because the high
pressure keeps them from forming bubbles. These dissolved gas molecules
like to attach themselves to silica molecules. To do this they first have
to break the silica molecules, which can be really big. Big molecules in
a fluid means that that fluid is going to be viscous (it makes
sense--trying to move a whole bunch of really big molecules around is
harder than moving a whole bunch of little ones). Perhaps you could
imagine making soup and putting in a whole bunch of tinker-toys. If
you put in great big tinker-toy constructions the soup is going to be
hard to stir (it will be viscous). If you put in only little pieces
then the soup won't be too hard to stir (it will have a lower
viscosity). Anyway, the effect of these gas molecules is to break the
big silica molecules into little ones and therefore the viscosity drops.
The more gas you have in the melt, the lower the viscosity.
As the magma gets closer and closer to the surface the pressure on it gets
lower (there is less and less rock overhead to press down). As the
pressure drops, some of the gas molecules can start to glom together to
form actual bubbles of gas. This means that there are now fewer molecules
around to break up the big tinker-toy silica molecules so the viscosity
starts to go up. Another effect is that as soon as bubbles form, they
want to expand. As they expand they force the magma upward even faster,
and this means that more gas molecules start to form more bubbles, and on
and on and on. Eventually the magma has so many bubbles trying to expand
that it just explodes. This is an eruption! The more gas you started
with at the beginning, the more violent the explosions or the higher the
lava fountains. Now, the gas has another effect on the viscosity of the
lava flows. Lava flows form from the material that gets thrown up in the
air in a lava fountain. If it accumulates fast enough it can flow away.
But, if it has been thrown high into the air, it will cool a lot on the
way down and it will have a high viscosity. This means that the more gas
you have in your magma, the higher the fountains, the more the lava blobs
in the fountain cool during fountaining, and the more viscous the flows
will be. Lava with only a little gas, that is able to just well quietly
out of the ground, will have a low viscosity because it hasn't undergone
all the cooling while flying through the air in a fountain.
There is a very long-winded answer to your question. If it doesn't make
any sense, please ask me again.
Sincerely,
Scott Rowland
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