Lava balloons

Imagine glowing, hissing, steaming balls of floating rock up to three metres across emerging from the depths of the ocean – these are lava balloons.

I talk with quite a few engineers, and one of the things they like to impress upon me is that every material has its breaking point. We like and trust concrete, stone, and steel to stay safe under our feet and over our heads. They carry the illusion of permanence. But, of course, under the right conditions these materials can still flex, shrink, warp, break – and so remind us that nothing is truly and fully permanent.

Rock is solid, unchanging, reliable. It’s the metaphorical epitome of those properties: steady as a rock, rock-solid, “the Lord is my rock.” You can imagine, then, my visceral discomfort with the idea with giant inflated balls of molten rock popping up out of the sea. Rocks aren’t supposed to float, and they’re not supposed to burst, but the lava balloons do both.

The name “lava balloon” is surprisingly accurate. This phenomenon begins as a large pocket of gas or steam emerging from within a submarine lava flow or volcanic vent. A crust of rock solidifies around the hot gas, but the balloon is still buoyant enough to rise to the water’s surface. The result: a blackened or glowing sphere of lava floating on the surface of the sea.

It hisses as it cools, hot enough to produce a plume of water vapour (as you can see in the picture above). Lava balloons as large as three metres across have been observed off the coast of Terceira Island in the Azores – but those are outliers; most are one tenth that size. Usually lava balloons cool, crack, let in water, and sink back into the depths in a few minutes. But sometimes, they pop instead, and a popping lava balloon might spray bits of rock dozens of metres high.

• Lava balloon