Is Glass a Liquid?

In 1994, a massive earthquake shook the Northridge suburb of Los Angeles, killing 57 people and injuring over 5000. The cost of damages was in excess of $20 billion. It's earthquakes like this one that make us question just how solid is the earth beneath our feet, and what does it mean to be solid anyway?

At first glance, pitch looks like a solid, but it's not. It's actually a liquid at room temperature—just a very viscous one. Viscosity is a measure of resistance to flow—what we often think of as the "thickness" of a liquid. Olive oil is nearly 100 times more viscous than water, and honey is about 100 times more viscous than that. Meanwhile, pitch has a viscosity 2.3×10¹¹ times that of water.

At the University of Queensland in Australia, pitch is the subject of the world's longest running lab experiment, and it's still going to this day. Back in 1927, this glob of pitch was placed into a funnel, and ever since then, in nearly 90 years, it has produced only 9 drips—roughly 1 a decade—and no one has ever been in the room to see a drop fall. Though in 1988, the former custodian of the experiment, John Mainstone, came very close to observing a drip fall, except he stepped out of the room for just a few minutes to get a cup of tea.

Now, you can actually watch this experiment live—there's a link in the description—but since the last drop happened in 2014, I think you'll probably be waiting a while. Another substance that you may have heard is a very viscous liquid is glass. If you look at the stained glass windows of old churches, you'll find the bottom of the pane is decidedly thicker than the top, and that's because the glass has been flowing down over centuries...

Actually, no, it hasn't! You know, we've looked at old telescopes where the optics is very sensitive to slight shifts in the lens glass, and we find they still work perfectly after hundreds of years. Plus, studies of thousand-year-old windows find no real evidence of flow. So the truth is that it's just very difficult to make glass of uniform thickness, and so when the glass was originally installed thousands of years ago, they would install the thickest part towards the bottom.

The lead actually has a lower viscosity than the glass, so if the glass had even thickened a little bit, then the lead should be a puddle on the floor by now. Now, glass is unusual in that it's an amorphous solid, meaning that the silica molecules are not regularly arranged as in a regular crystalline lattice. Instead, they're all in a jumble, and this is because the glass is cooled down so quickly from the liquid state to the solid state that the molecules don't have time to arrange themselves in a nice regular crystal structure—but what makes something a solid rather than a liquid is that all of the atoms or molecules are so strongly bonded together chemically that they can't slide past each other.

So in water, or olive oil, or pitch, the molecules can slide past each other, but in glass, at room temperature, they can't. So what about the interior of the Earth? Beneath the Earth's crust is the mantle, which is responsible for plate tectonics, and therefore earthquakes. Is it a solid or a liquid? We can obviously never observe the mantle directly, but when we do see material come out from underground, it is red hot rock—it's lava.

So you might be imagining that the mantle is very similar, made up of this molten magma—hot liquid rock—and that would make sense, because in order for it to flow, it must be a liquid, right? Actually, wrong! The mantle is a solid. Under all that pressure, even though it's at very high temperature, it remains in a solid state, and we know the mantle is solid because shear waves from earthquakes can actually propagate through the mantle.

These waves cannot propagate through liquid, like the molten iron of Earth's outer core, because liquids flow in response to being sheared—or rubbed sideways—and as a result, we can see the shadow of the liquid outer core by measuring seismic waves from an earthquake on the other side of the world—but how exactly does this solid rock flow? Well, the answer lies in the fact that crystals aren't perfect. There may be a missing atom here or there, and under the high pressures in the mantle, sometimes a neighboring atom will pop in to fill that gap.

Now, from a human perspective, it takes a very long time for this to have a noticeable effect, but from the Earth's perspective, it happens in no time at all. The viscosity of the mantle is similar to that of glass, to several orders of magnitude greater, so it is really only over these geological time scales that the mantle is fluid-like at all.

So pitch—a liquid—can flow so slowly as to seem like a solid, whereas the Earth's mantle—a solid—behaves like a fluid if you just wait long enough. As the famous American geologist Grove Karl Gilbert once said: "To my mind it appears that the difficulty is only imaginary and not real. Rigidity and plasticity are not absolute terms, but relative, and all solids are in fact both rigid and plastic... When great masses and great forces are involved ... the distinction loses value."

Sometimes the rigid definitions we create for ourselves can introduce misconceptions, or viscous rumors, like the idea that the core of the Earth is a giant ball of magma. If only we could think about liquids and solids a little bit more...fluidly.

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