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Viscosity is a measurement of a
fluid’s resistance to flow. Taffy is more viscous than syrup. Syrup is more viscous than water. How
can we make syrup less viscous (more runny)? Heat it up, remove sugar from it, or add water. To make
it thicker or more viscous, we do the opposite: cool it, add more sugar, or remove water.
How does viscosity relate to magmas? Magmas formed at depth are hot and low in SiO2. These
make them low in viscosity. As they rise upward through the crust they cool, which
makes them more viscous. In addition, as they cool, crystals start to form. As these crystals
are removed from the magma, the SiO2 content of the magma increases, which also makes viscosity
increase. Why? The higher the SiO2 content in a magma, the more silicon-oxygen tetrahedra,
which as we know from the Inside Minerals video tutorial, when found in high abundance,
will attract each other and start bonding together in chains. This bonding will happen
even in the liquid phase and linked tetrahedra will make a magma more resistant to flow.
So what? Magma viscosity impacts igneous rocks in a few ways.
At the surface, when magma viscosity is high, magmas have a hard time migrating
out of vents. They can get stuck in these vents. In addition, gases have a hard time escaping from
viscous magmas. Together: magmas rich in gas plugging up vents can allow pressure to build
behind them heightening the risk of explosive eruptions. In contrast, when viscosity is low,
magmas erupt more easily, and gases are released more easily. Hazard level is lower.
Erupted lavas that are high in viscosity will generally have smaller crystals because the lavas
cool faster than ions can travel through the magma to nucleation sites. In extremely viscous lavas,
there can be no time for any crystals to grow, and we end up with glass, or obsidian.
Magmas that stay underground and have unusually low viscosities will do the opposite: the ions
can migrate rapidly through the low-viscosity magma to nucleation sites, and instead of many
small minerals forming, we see fewer crystals, but each much larger. The texture we get with these
unusually large crystals forming in low-viscosity magmas is called a pegmatite. Pegmatites form when
highly evolved magmas, rich in SiO2, mix with a water phase that had separated and sat at the top
of the magma chamber absorbing incompatible elements from the magma (elements that have
no place in the main crystals forming from the magma). I like to call this phase the “spit phase”
as it’s filled with all the unwanted “garbage” elements (such as gold, silver, and platinum) much
like the spit that’s left behind in a soda can after you take a swig. As we know the more sips
you take of the soda, the more of your spit is left behind until finally the last sip is mostly
spit, or so they say. This now-mixed magma + water or “spit” phase has unusually low viscosity but
also higher pressures, which can crack the rock and inject the magma into veins around
the magma chamber. In these pegmatite intrusions, because of the high concentration of incompatible
elements in the “spit”, we also see rare minerals forming like tourmalines, beryls, and emeralds.
What happens to the “spit phase” if the magma chamber is opened up and the pressure
released to the surface? An explosive volcanic eruption, in which the liquid is carried upward
by the force of the escaping gas. The liquid flash freezes to form ash, and it’s thrown
up high into the atmosphere where winds will carry it potentially thousands of miles away.
This process is similar to what happens when soda is shaken up and then opened. Everything
in that spit phase will migrate upwards with the gas and be distributed along with the ash.
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