Gal Science: On Lava Flows and Glass -The Toast

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glass1As The Toast searches for its one true Gal Scientist, we will be running a ton of wonderful one-off pieces by female scientists of all shapes and sizes and fields and education levels, which we are sure you will enjoy. They’ll live here, so you can always find them. Most recently: In Which We Learn About Endosymbiosis.

For most of my dissertation, I spent easily half of my work hours staring down a microscope with a pair of tweezers in my hand, meticulously sorting through crushed rocks for small chips of pristine, brown glass. Volcanic glass, “obsidian” when it’s found in large quantities: so dark a brown it looks black in thicker pieces, with the same beautiful, arcuate broken surfaces familiar from fabricated glass. Despite my practiced hand (I am a good hand picker), sometimes I sent a little chip flying across the room with my sharp-tipped jeweler’s tweezers, so little bits of broken volcanic glass dotted the desk in my graduate student office. After a couple years, my back began to hurt, and without realizing it I started to limp slightly. All those hours of leaning over the microscope had induced the nerdiest of all repetitive back injuries–the microscope injury–which later necessitated hours of physical therapy and specialized treatment. That office had a big beautiful glass window, and I used to look up from my glass chips and out over the open ocean while unknowingly injuring myself.

Unlike some friends and colleagues in volcanology, I have luckily never suffered a serious injury in the field. Although not exactly safe, it is nonetheless in some ways less dangerous than people typically assume to walk across a recent lava flow, whether it is already solidifying or still quite molten*. Silicate lava behaves as a Bingham plastic, nearly: a non-Newtonian fluid that, if left undisturbed, resists flow in response to stress unless and until a sufficient force is exerted upon it. In other words, you won’t sink into a flow, even one so hot and fresh that it lacks a frozen outer crust, and you can typically avoid being hurt by the intense heat as long as you keep moving. It’s hot through your boots, though, and on those flows without much of a crust your soles will leave flaming-rubber footprints. When there is a thicker, safer crust (highly recommended!), in many places you can actually stand still in relative safety and feel the intense heat on your feet. That and the glowing incandescence through the cracks always makes me want to keep moving, though.

It turns out that one of the more dangerous things about walking on active lava flows, even those safer ones that have relatively stable, unmoving crusts, is the crust itself. The surface is extremely uneven, making it easy to trip and fall or scrape against pieces that jut out; and if you do, that surface is made of broken glass. Broken-glass crust is, unsurprisingly, very scratchy! My pants have a lot of holes. This is also killer for boot soles. (Volcanologists go through a lot of boots.)

Fabricating glass is one of our oldest technologies. Our ancestors figured out that heating a substance until it was completely molten, and then freezing it rapidly, produces the vitreous, brittle, often transparent material that we find useful in jewelry, dishwares, and windows. Volcanic glass forms the same way: melt a rock completely to make magma; erupt it as lava; and freeze it rapidly.

The rapid freezing (“quenching”) is the key ingredient in making a glass. Volcanologists taking fresh lava samples will often scoop some lava off the edge of a flow and then quickly dump it into a bucket of water, to force this to happen. For another part of my dissertation I was making glass myself **. I mixed powdered oxides of different elements in proper weighed proportions, built small capsules and cells to hold them, packed in a few cubic millimeters of the mixture using dentistry tools (dentistry tools are so great), and pressurized the whole thing to 2.5 gigapascals while heating it to ~1400ºC. This simulated upper mantle conditions and made it mostly molten, my own little magma. Then I shut the experiment down by flushing it with coolant, to (hopefully) quench the liquid. You see, the glass would then be as well-mixed as the liquid had been, so I could safely measure what the liquid had been like when it was still hot. Glass is essentially a frozen liquid.

The difference between glass and liquid is, in a way, really a question of timescales. Glasses deform and flow, though so slowly we can’t see them do it in our lifetimes. They are internally amorphous, like liquids. Glasses exhibit brittle behavior when you exert forces on them that are relatively rapid, like smashing them with a hammer, and they do not relax in response to stress quite the way that liquids do. But a glass is considered thermodynamically “metastable”: it occupies a temporarily stable state, but it probably won’t last forever. In old volcanic rocks, glass breaks down to grow other, more stable substances long before everything else in the rock does so.

Most inorganic solids, like the ones making up rocks, are ordered. Their ions (high school chemistry refresher: charged atoms) are arranged in a highly organized way that is as energetically favorable as possible: their charges are balanced, and their radii accommodated as comfortably as possible into that ordered atomic lattice (i.e., the crystal). For the most part, those ordered crystals (minerals; this is, in fact, what the word “mineral” means, when it’s not on a cereal box) originally grew at higher pressures and/or temperatures than typically exist at the surface of the Earth, so they’re no longer entirely stable at the cold, low-pressure surface, either. But the minerals are stable enough that it takes some real work and fair bit of energy to force them to rearrange to lower temperature, lower-pressure ordered forms (like clays).

Liquids, unlike minerals, are disordered. Most of the magmatic ones are silicate, just like most of the minerals in Earth’s crust. [Thought question #1: Is water just a low-temperature magma? Is ice just a rock? Haha, these are trick questions with no answer!] This is mostly because the solar system has a lot of silicon and oxygen, whose ions readily arrange themselves into strongly-bonded, ordered structures where four oxygen surround a silicon in a symmetrical little four-sided, four-cornered shape. [Thought question #2: Do you think the silicon-oxygen bond is covalent or ionic? Haha, also a trick question! It’s both! That’s super strong, you guys.] Those tetrahedra can share corner oxygens with each other and so become linked. They link in different ways to make different mineral arrangements, but in a liquid the linkages are disordered. There can be loose rings of tetrahedra, or long networked strings. This means that magma is essentially highly polymerized, just with silica instead of carbon. What makes it a liquid (what makes anything a liquid) is that those linking bonds can be broken and remade more easily than the bonds in solids: liquids flow because of the constant reworking of bonds.

And if you freeze a liquid before it has a chance to become more ordered, it becomes an amorphous, disordered solid that is too cold to flow anymore, except perhaps very, very slowly. It’s unstable compared to ordered minerals, but it’s cold; it’s stuck. To sum up: glass has no minerals, it has no order, it can deform like a liquid on very long timescales, it breaks on short ones. My office window was less solid than it appeared.

Molten rock that is cooling naturally is rarely pure glass: there is typically sufficient time for some mineral crystals to nucleate and grow. In general, that happens even when lava is cooling quickly in direct contact with air or water, which are very cold compared to a ~800-1200 ºC liquid. An outer glassy crust will form, but the interior of the flow cools just a little more slowly and will produce tiny mineral grains while it is freezing, interspersed with glass (vocab: a mixture of teensy mineral grains in a glassy matrix is called “groundmass”). Underground, where magma is even better insulated by solid rock (a poor heat conductor), the crystals have time to get even larger. Most crustal magma chambers probably contain a slushy mix of liquid plus mineral grains while they cool, not pure liquid. In general, the farther underground a magma solidifies, the slower it cools and the larger its crystals can become.

The slow growth and removal of mineral grains, which have distinct chemical formulas based on their internal atomic structure, preferentially depletes the partly cooled liquid residue in some elements over others. Progressive crystal growth during the cooling process produces a more and more exotic—and, it turns out, viscous and dangerously explosive—residual magma. (Fun party fact: colder lava is the more dangerous kind.) But this doesn’t just happen underground: even rapid, in situ crystal growth in erupted lava flows can create a heterogeneous lava rock.

cowgirlProcesses like cooling down and growing crystals cause the magma’s chemical composition to evolve and change, obscuring the “primary” magma composition that geochemists need to answer questions about magma origins. In my own work, I have attempted to select places in the world where scientists think underground cooling effects are minimal, though it is probably never completely absent. Happily, the extremely thin crust found at the oceanic spreading ridges in the center of Earth’s ocean basins does not permit much pooling of magma in crustal chambers, a property that hopefully reduces crystallization underground as much as possible.

Underground processes are not the only ones to consider, though. To be sure I was measuring the most primary magma composition possible, I have had to avoid the mineral grains that are carried by and crystallized within the erupted lava. I likewise had to guard against accidentally measuring the chemical makeup of any materials that had been altered or deposited by shallow, water-rich processes in the crust and on the seafloor, and erroneously assigning those compositions to the original magma.

Crushing up the glassiest parts of ocean floor volcanic rocks and selecting pure, perfect glass chips under the microscope was slow, painstaking work. Some of the rocks I worked on contained such low concentrations of the elements I needed to measure that I had to collect several grams of those tiny rock chips for each and every sample. (That is seriously a lot.) There were nights when my dreams were populated by a sea of quiet, floating, magnified grains of glass. But without those hours of work, my analyses would have been meaningless, because I could not have been sure what substances I was really measuring. Those little bits of glass were my window into Earth’s melting mantle.

* There are a lot of caveats to this. Please don’t go running around on lava unless you really know what you are doing! I am not encouraging you to do that!

** Elkins, L.J., Gaetani, G.A., Sims, K.W.W. (2008) Partitioning of U and Th during garnet pyroxenite partial melting: Constraints on the source of alkaline ocean island basalts. Earth and Planetary Science Letters, v. 265, p. 270-286.

Lynne explores how magma is made by studying the mineralogical, chemical, and isotopic makeup of igneous rocks. She also teaches geology, plays music, dances, and hangs out with her fabulous fluffy pets.

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