Is lava wet? And other questions

Hello readers! I’ve been playing more Tears of the Kingdom recently, and some of the “lava physics” in that game reminded me of various questions I have been asked over the years about lava in films and video games. This blog has hosted plenty of “cool rocks” since its inception, but we haven’t yet talked about rocks in their hot, molten form. Lava flows and volcanoes evoke fascination and fear, so it’s no surprise how often they crop up in games and blockbusters – and I think we all suspect that most representations aren’t entirely accurate. But how far off are they?

Lava is an intriguing fluid. There are scientists out there who devote their entire careers to understanding specific behaviours within molten rock, because understanding lava and magma is the key to understanding a huge range of geological processes. If you go back through the family tree of any rock, you’ll find that it is descended from something molten – either magma or lava, depending on whether it was below or above the surface. Understanding the way that rocks melt, flow and solidify is critical for understanding the formation of mineral resources, the behaviour of volcanoes, and the speeds and extents of lava flows. So, with that in mind, here are some answers to some questions about lava.

What is lava?

This isn’t a straightforward question. The simplest answer is that lava is molten rock erupted onto the Earth’s surface. Magma is the term used for molten rock that still resides below the ground – but physically speaking, they’re the same thing.

However, the picture grows complicated when we consider the various scenarios in which the term “lava” gets used. For example, a geologist may well hand you a lump of rock and tell you that it is a “piece of lava”, by which they mean that it is a piece of solidified lava. Similarly, if you happen to be on the flanks of a volcano, a geologist may identify a “lava flow” which is very much a solidified lava flow.

Molten lava vs. solid lava

The lack of distinction between molten lava and solidified lava even extends to rocks that have been cold for millions of years. A geologist might look at a band of igneous rock, scratch their head, and ask whether you think it is more likely to be a lava flow or a sill (at which point you can respond with a politely bemused expression). Evidently, the rock band in question isn’t hot or flowing – but the geologist doesn’t have the words to distinguish the molten process from the solid product. It is perfectly acceptable for them to call solid rock “lava”.

(Sidenote: sills have the opposite problem. These were named centuries ago, before anyone knew how they got there, and the term “sill” merely described flat, rocky surfaces protruding from the landscape. These days, we know that sills were originally molten intrusions, and so the term “sill” is now used to describe the molten propagation phase, too. We have ended up with the same word for the active process and the solid product!)

The term “magma” has escaped all this molten vs. solid confusion. A geologist won’t hand you a lump of granite and tell you that you’re holding a piece of magma, because they’ll just call it granite. Or gabbro. Or whatever it happens to be. So why does lava have multiple definitions when magma doesn’t?

This is my best guess: a geologist will use the term “piece of lava” rather than “piece of basalt” because the process that formed the rock is more important than its composition. A piece of basalt could come from a lava flow, or it could come from a sill, or it could have been launched as a projectile. Therefore, it makes sense to introduce it in the context of its formation. A granite, meanwhile, can only ever have formed from magma, so there is never any doubt surrounding its origin.

So… What is lava?

Our definition might seem foggier after that discussion. But in short, lava is something that was erupted out of a volcano. The word alone gives us no indication on how long the lava has been out in the open. When a geologist says “lava”, it could be fresh out of the vent, several months sober, or crumbling under the pressure after several million years of failing to keep on top of things. It all depends on the context.

What do you call molten rock in caves?

Ooo, the cave conundrum. Caves containing molten rock appear fairly frequently in video games (especially Minecraft), but if the molten rock is beneath the surface, is it lava or magma? I think the answer is probably lava. This is because it has reached the open air, meaning that it will be radiating heat and forming a crust. As such, it will behave more like a lava flow than a confined magma, even if it hasn’t seen daylight.

We can also consider the scenario where a surface lava flow intersects a deep crack and ends up underground again. Just because the molten rock has gone underground doesn’t make it magma; indeed, plenty of lava ends up travelling within tubes beneath thick crusts, which are essentially caves (just very hot ones). If anything, this question has only served to demonstrate that having separate words for the same substance adds needless complexity. Thankfully, “cave lava” as you are imagining it occurs more often in video games than it does in reality, so this isn’t a common geological conundrum.

Is lava wet?

I was asked this a few years ago, and the question haunts me to this day. It might sound simple, but there are numerous interpretations and complexities to be unravelled. Firstly, we need to establish what “wet” means in this context. A geologist might use the terms “wet” and “dry” to refer to the amount of water dissolved in the magma, which influences its viscosity and melting point, and the type of volcano that it produces at the surface. However, these “wet” and “dry” definitions are broad terms applied at a regional scale, rather than to individual lava flows.

I suspect that the questioner really wanted to know how “wet” lava would feel if they were to stick their hand into it. Would it feel like water? Would it splash? Of course, it wouldn’t feel like anything, because their nerve endings would be destroyed before they even made contact. But, for the sake of argument, let’s imagine that we could stick a long, titanium spoon into the lava. Would it feel more like water, or more like marmalade?

Using this definition, we can take “wetness” to mean the runniness of lava, also known as its viscosity. A basalt lava at 1200 °C has a viscosity that is fairly similar to golden syrup at room temperature, so it doesn’t have a watery consistency at all. You probably wouldn’t call it “wet”. It also gets much more viscous the cooler it gets – and this is where things get interesting.

Lava has some strange properties that make it a much more complicated fluid than water. It contains all three physical phases: solid crystals, molten liquid, and gaseous bubbles. Suspended crystals increase its viscosity, but their interactions can lead to very strange behaviours depending on their shape, degree of alignment, size distribution, or migrations through the fluid. Bubbles are even more complicated: small bubbles are hard to deform, so they act like solid particles, but big bubbles are highly deformable and cause an overall reduction in lava viscosity. Particles and bubbles give the lava non-Newtonian properties – making it very different to water or golden syrup, which are both Newtonian fluids. When we consider that crystals and bubbles are continuously forming, growing and interacting within lava, we can see that the behaviour will be incredibly complicated. Is lava wet? Not really.

To me, the only way a lava can be considered “wet” is if we’re referring to solid lava, as mentioned earlier. An old lava flow in the rain is definitely wet. Similarly, the chunk of basalt that you accidentally dropped into your cup of tea whilst pondering this conundrum is now, almost certainly, wet. And what a waste of tea.

In summary…

Molten rock and lava flows are fascinating fluids. The lava presented in films and video games rarely behaves like lava in real life – in fact, it usually acts in a far more simplistic and suspiciously watery manner. Part of me wonders how much of this comes from ignorance, and how much of it comes from deliberate artistic choice. There is no denying that lava is at its prettiest when it is at its hottest and runniest, but you couldn’t get close to it at that temperature without giving yourself severe burns. I wonder if directors realise quite how hot this stuff is. In any case, I hope you enjoyed this discussion, and if you have any lava questions to add to the list, let me know! Happy reading, and have a lovely week.