As 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.
Processes 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.
Tags: gal science, glass, lava, lynne elkins, science, volcanos
I AM FREAKING OUT, it was my childhood dream to be a Volcanologist! KEEP ON LIVIN THE DREAM.
You can do it! Volcanoes are the best! Isotopes, too, but also volcanoes!
I appreciate your encouragement! 🙂 Regrettably, my interest in Vulcanology was fleeting and fueled only by episodes of the Magic Schoolbus. I’m an English major now, working my way to being a librarian!
Toasties, Lynne is one of my besties! I can confirm the fluffiness of her pets. I cannot confirm any of the science, though, because I am not hardcore like she is.
Oh! Hi The Toast!
Oh! You get to see my cowgirl picture! In that picture, you guys, I was AT A RODEO. For science. Well, sort of. not really. not directly.
Anyway: I could be wrong, but believe I have written the first Toast article to include a photomicrograph with a scalebar. Nerd points?
Scalebars on everything!! Missing scalebars are one of my biggest pet peeves.
A thousand nerd points.
OMG Lynne Elkins! Big ups from a once-upon-a-time Smith orchestra cello colleague!
Hey! Exciting! I am seriously loving the BMC and Smithie shoutouts.
hey! she was my geo professor at BMC! girls study and rule the planet!!!
Hi! I don’t know who you are from your username, but thanks so much for reading! Girls do, indeed, study and rule.
High five from the large Smithie contingent reading this today!
I’m really rethinking not putting my double-ladyschool cred in my bio.
I like all of the Gal Science articles; I liked this one A LOT! Maybe I just love glass and fear volcanoes?
This was a great read, and I wish I got to call myself a volcanologist!
This was a fantastic, interesting, and engaging piece. I love Gal Science.
Related: I suddenly want a volcanology-themed parody of Sir Mix-a-Lot’s “Put ‘Em On The Glass.”
There is a weird font issue that I want to clarify, for the nerds/pedants who might be extremely concerned about this sort of thing: the experiments I ran were at around 1400ºC. That’s a tilde, not a minus sign (my browser is not displaying the difference properly in this font, so you might also be confused!). -1400ºC would be (very very far) below absolute zero, and thus require no heating (and different laws of physics, so those would be quite the extradimensional magma experiments).
This is the most awesome font correction I have ever seen. “Extradimensional magma experiments” is my new favorite phrase.
It’s my new dubstep album.
UPDATE: Also the other temperature. Silicate magma is approximately 800-1200ºC, not -800ºC. No tildes on The Toast, folks!
It auto-translated to that, I will try to get you a tilde later today!
Thanks! But it might actually be one that just doesn’t draw properly–when I copied it into another font, it was indeed still a tilde. But it’s worth a try!
Yes! I just did the same thing. GUYS, it’s a fuckin’ tilde. Listen to Lynne.
This was actually bothering me a little, so I”m glad to know that it’s a font issue!
I do not wish to walk on lava, solid or flowing, but I would like to see photos of a flaming footprint trail across some lava. Though I suppose one is too busy staying alive and moving across the lava to take pictures–
Confession: I’ve actually never done/seen it. I’ve only walked on hardened crusts (with the hot glow through the cracks. and some tumulus outbursts all around). Important tip: only walk on completely filled lava tubes, folks! No half-empty ones. Roof cave-ins are not your friend.
So glad it wasn’t just me who wants to see the flaming footprints.
This makes me miss igneous petrology so bad. Partial melt partitioning is the most beautiful ordered thing EVARRR.
It makes me so happy!
I know nothing whatsoever about volcanology but this was a fascinating read anyway. Thankyou! Also, I really want to go somewhere and leave flaming rubber footprints! (But I will bear your caveats in mind and will not.)
Thank you! That is for mega-experts only. Even I have not done it (because I have not had to, and it’s not worth it if you don’t have to).
Also known as DRAGONGLASS. (Sorry, GRR Martin science is taking over my brain.)
This is super cool. I mean, hot? I’m sorry (I’m not sorry).
This was a very lovely article! Volcanoes are neat! Can you tell us a little more about what it is you are finding out about with your tiny flecks of glass?
I can! It’s pretty involved and probably properly deserves a much longer explanation. But basically, I use the chemical and isotopic makeup of magmas to investigate how they formed in the first place, which tells a lot about how the mantle of the Earth works and how new crust is created.
Neat! Thanks.
You guys! I was just reminded of the popular perception (which I, too, was taught, by professors, no less) that glass is thickest at the base of old windows because the glass flowed in response to gravity.
This is, almost certainly, a myth! Not because the glass doesn’t flow, but because it can’t flow anywhere near that quickly. Apparently the best explanation for this is actually imperfect manufacturing methods that made the glass thickness uneven; and then it was more stable to mount the uneven panes thick-side-down.
This one is a particularly excellent party fact, because you can first tell the first part as a clever trick (“DID YOU KNOW that old class windows are thicker at the bottom because GLASS FLOWS LIKE A LIQUID??”), and then reveal the truth (“HA I tricked you! That’s not true! You are dumb!”). …Also I use only the best pedagogical methods.
Urg, I feel the need to clarify this to grumpy internet pedants: In case anyone misunderood, I did not say glass is a liquid. It isn’t a liquid! It just shares some properties with liquids that ordered solids do not. And, the ability of glasses to deform under very slow strain rates is not unique to glasses, and it is VERY very slow (it is, however, a lot like _how_ liquids flow!).
Urg, I feel the need to clarify this to grumpy internet pedants: In case anyone misunderood, I did not say glass is a liquid. It isn't a liquid! It just shares some properties with liquids that ordered solids do not. And, the ability of glasses to deform under very slow strain rates is not unique to glasses, and it is VERY very slow (it is, however, a lot like _how_ liquids flow!).
I want to hear more from this scientist!
I am confused about the cooling magma part and the explosions. Is it just because the magma is contained in the crystal growths? Are there videos with diagrams and voice overs and explosions?
Nope! It’s actually because the chemical makeup of the magma has changed so much. It has to do with the polymerization part: the residual magma is much more silica-rich, so it’s much more viscous than the original, lower-silica magma, and can barely flow. Between that and the fact that magmas are full of dissolved gases (which also can be more concentrated in the more evolved, silica-rich magmas), and how those gases tend to exsolve as bubbles right before or upon eruption, the result is often pretty explosive. Like an exploding shaken-up soda bottle. It’s especially explosive because the ability of an expanding gas bubble to migrate through such a viscous liquid is pretty limited: the bubbles have no room to expand, so all that outward-pushing force builds up and then explodes (the bubbles can undergo fragmentation; i.e. the bubbles come into contact with each other and their walls burst). The result is volcanic ash: little flying tiny bits of magma. (Nothing like fireplace ash. Magma’s not on fire. Volcanic ash is really mostly little glass shards.)
Every time I read the Gal Scientist posts, it makes me wish that I hadn’t had to drop out of college. I was part way through a geology degree, with an emphasis in coastal geology, but ultimately had to make the decision to either go in to bone-crushing debt, or be able to eat and have a roof *now*.
This one kinda hit close to home for me. I COULD BE DOING COOL ROCK THINGS ON AN OCEAN, INSTEAD OF WORKING IN A CUBE FAR TOO FAR INLAND!
Of course, if I had my real science dream job, I’d be an Indiana Jones-style paleontologist, but with less stealing from other countries, and more running through the Great Raptor Temples of the Utah salt flats.
I hope at some point you can go back, even part time, even just to enjoy the classes! It’s totally worth it! Though for what it’s worth, most of us geologists don’t go to our daily jobs in our field areas. The field trips are special trips once or twice a year. I don’t live anywhere near any modern volcanoes. Most days I go to my office and teach classes and use my microscopes and my chemistry lab.
Hey! Exciting! I am seriously loving the BMC and Smithie shoutouts.
Nope! It's actually because the chemical makeup of the magma has changed so much. It has to do with the polymerization part: the residual magma is much more silica-rich, so it's much more viscous than the original, lower-silica magma, and can barely flow. Between that and the fact that magmas are full of dissolved gases (which also can be more concentrated in the more evolved, silica-rich magmas), and how those gases tend to exsolve as bubbles right before or upon eruption, the result is often pretty explosive. Like an exploding shaken-up soda bottle. It's especially explosive because the ability of an expanding gas bubble to migrate through such a viscous liquid is pretty limited: the bubbles have no room to expand, so all that outward-pushing force builds up and then explodes (the bubbles can undergo fragmentation; i.e. the bubbles come into contact with each other and their walls burst). The result is volcanic ash: little flying tiny bits of magma. (Nothing like fireplace ash. Magma's not on fire. Volcanic ash is really mostly little glass shards.)
I hope at some point you can go back, even part time, even just to enjoy the classes! It's totally worth it! Though for what it's worth, most of us geologists don't go to our daily jobs in our field areas. The field trips are special trips once or twice a year. I don't live anywhere near any modern volcanoes. Most days I go to my office and teach classes and use my microscopes and my chemistry lab.