Friday, August 30, 2013

Volcanoes Be Complicated: Silicon Rocks!

Like any good science nerd, I made baking soda and vinegar volcanoes as a kid. This seems to have persuaded me that nothing much was going on in volcanic eruptions. Donald Dingwell’s Coursera course on the subject has certainly disabused me of that notion. It turns out that my chemistry is more than a bit short of what’s needed to properly understand the subject, but what I can understand is really interesting.

I signed up for the course because the promised workload was small and explosions are cool, thinking that I might watch a bunch of videos of lava, pyroclastic flows, etc. Actually it turns out to be perusing a lot of complicated graphs with multiple axes plotting such things as “speciazation of water in haplogranitic glasses and melts” or “glass transition temperature at a specified sheer viscosity as a function of water weight content”. Unfortunately, I’m not entirely clear why sheer viscosity is an important thing to specify when measuring glass transition temperature, but it has something to do with a notion of equivalence. If I understand that, it in turn means that a whole bunch of properties, including sheer viscosity at the glass transition temperature, are good proxies for the structural content of the silica melt.

Silicates turn out to impressively complex. Somewhat like carbon, silicon likes to form four tetrahedrally directed chemical bonds, with oxygen in the case of silicates, and these silicon-oxygen tetrahedra are the basis of the polysilicates. Of course oxygen is not very happy with just one chemical bond, so something else is needed to glom onto, and the most obvious possibility in a silicate melt is another silicon atom, resulting in two silicon oxygen tetrahedra sharing a common oxygen. Chains of such tetrahedra can be formed, but linking of ends is thermodynamically favored, so usually triangles and hexagons of linked tetrahedra are formed. Higher degrees of linking produce sheets and solids of tetrahedra. When all the oxygen atoms are shared, four oxygen per Silicon, two Silicon per oxygen, we have SiO2.

In a silicate melt, with a good mixture of other chemical components present, the various configurations of silicon tetrahedra will be present in some sort of equilibrium mixture of structural species characterized just by temperature, pressure, and chemistry. Cooling abruptly can produce glassy states far from equilibrium, and the exact type of non-equilibrium depends on the history of the heating and cooling. Glasses are hysterical* – they exhibit hysteresis.

So why is this stuff important as well as cool? Because silicates exhibit an enormous range of viscosities (depending mainly on the relative content of the various arrangements of the tetrahedra), and viscosity is the key to understanding whether the volcano will be a mild mannered, flowing lava will obliterate your town in days or weeks, type volcano, or a hot tempered, explosive volcano that will blow your whole island to hell in seconds, type.

*Alternate title for present note