Did you ever play “the floor is lava” as a child? Jumping from piece of furniture to piece of furniture, trying never to set foot on the floor because—obviously—the floor is lava, and stepping on lava would be bad.
Unfortunately, I didn’t, because in Italy, where I grew up, this game doesn’t exist. However, in many other countries lava is a part of everyday playtime for virtually every child, regardless of whether they have ever experienced it in real life.
Lava (which as you undoubtedly know, is partially molten rock erupted by volcanoes) typically comes from the mantle—the Earth’s middle layer, sandwiched between the crust and the core. Once it reaches the surface, lava quickly cools down and solidifies completely, creating new land.
As adults (and even as kids) we know that the living room floor is not actually lava; we’re just playing pretend. But most of us probably still believe that the Earth’s mantle is lava, or something very similar. It might disappoint you to learn that this is not the case either.
That belief likely comes from the textbook illustrations describing the phenomenon of plate tectonics that we’ve all seen in school. As geologists discovered during the middle of the 20th century, the Earth’s crust is broken into a number of wide, relatively thin “plates” of solid rock, hundreds or thousands of miles across but only tens of miles thick. These tectonic plates slowly move around and rearrange themselves over millions of years (so if the east coast of South America looks on a map like it could nestle easily into the west coast of Africa, that’s because it once did).
It’s quite tempting to believe that they do so by floating on a massive underground magma ocean. When magma gets squeezed up through the cracks—the boundaries between plates—volcanic eruptions happen. This is a very satisfying explanation: all pieces appear to fit together nicely. The textbook illustrations just reinforce that impression: the crust is often depicted in brown (much like soil), while the mantle is red. That’s an unfortunate color choice, because the association between red, hot and molten is immediate and hard to shake.
But a red mantle is very misleading. The mantle should be represented in green instead. That’s partly so as not to suggest that the mantle is molten. But mainly, it’s because the mantle really is green. The green mineral olivine, one of the main components that make up the Earth’s mantle, is responsible for that. We know because as magma rises, it sometimes snatches a piece of the mantle and brings it all the way to the surface (we call that a xenolith). And it’s green.
It isn’t molten, though. It hasn’t been for several billion years. You might assume otherwise: the temperature of the Earth’s interior increases by about 30 degrees C for every kilometer of depth, which means mantle rocks should melt between 27 and 40 kilometers below the surface. But temperature is not the only thing that increases with depth—pressure does as well, and pressure hinders melting. There’s a constant competition between temperature and pressure, and pressure almost always wins. Hence the proper conditions for melting simply do not exist in Earth’s mantle.
They don’t, that is, except at very special places, marked at the surface by active volcanoes. There, things are a bit more complicated. For example, where two plates move away from each other (a rift zone, like Africa’s Great Rift Valley) some pressure is relieved, prompting the underlying mantle to melt. Where one plate sinks beneath another (a subduction zone, like the U.S. Cascades), water comes into play, lowering the melting point of the mantle, much like salt lowers the melting point of ice (which is why we put salt on the roads in the winter). The mantle can melt—just not from heating nowadays.
Now you know that there is not a magma ocean a few kilometers beneath your feet, but you might still picture sparse “magma pools” lying underneath volcanoes. We tend to imagine that magma gathers in those underground pools, waiting to erupt. But—you guessed it—that’s not quite an accurate description of a magma chamber either. Picture a wet sponge, and swap magma for water. The majority of the sponge is still solid rock. That’s because the conditions for melting exist (in some places), but barely—just enough to melt a small percentage of the rock.
So the floor is not lava, because it would quickly solidify once exposed to air. The basement—that is, the mantle—isn’t lava either, because the Earth’s mantle can’t really melt in most places. And there is no lava-filled pool either, because even when the mantle melts, it does so very sparingly.
Only at very few special places on Earth, a little bit of the mantle does melt and turns into magma. It then rises through the Earth’s crust, and some of it eventually makes it to the surface, through a volcano, as lava. Lava then cools down and solidifies into rock pretty fast. But before it does, for a few precious moments, the floor really is lava.
You don’t want to play any games with it, though: while field volcanologists have long known this from experience, undergraduate students at Leicester did research that quantified the fact that the air a few meters above the lava floor would be too hot for you to survive—let alone play in.
As a volcanologist, I can definitely vouch for that! Not that I have actually attempted to play “the floor is lava” while doing fieldwork, but I have stood really close to lava, and contrary to what my smiling selfies would suggest, it was painfully hot to do so.
Nevertheless, observing a lava flow up close as an adult professional volcanologist is just as exciting as playing “the floor is lava” was as a kid. And knowing the incredible obstacles that lava had to overcome to form and make it all the way to the Earth’s surface makes it all the more fascinating.