STRANGE STRUCTURES

Stalagmites and stalactites aren’t always found in caves. You can see them under concrete buildings, in lava tubes and even hanging off the side of your garage in the winter. This is because stalagmites and stalactites aren’t always made from limestone. Icicles are probably the most common type of stalactite. They form when it’s cold enough for water to freeze, but sunny enough for it to melt again. The melted water trickles towards the ground and refreezes before it hits the floor.

Scientists from the University of Cambridge and the University of Arizona used a combination of photographs, maths and physics to work out why icicles are pointy. They noticed that as the water trickles down, it blends together to form a sheet. That sheet gives off heat, making a warm pocket of air around the icicle. The warm air rises, which means that water freezes faster at the bottom of the icicle, making them grow long and thin at the tip.

Another kind of stalactite you might see in your everyday life is a concrete stalactite. You can find them in car parks and even on the pipes in your home. Concrete contains calcium oxide, which dissolves when alkaline liquid passes through it. When that liquid hits the air, the calcium comes out of the solution, forming a hard substance called calthemite. If the drips come fast enough, calthemite stalagmites can start to form.

Other strange stalactites are a bit harder to find. Lava stalactites form inside tunnels called lava tubes, which carry molten rock beneath the Earth. When the roof of a lava tube starts to cool, it gets a skin, a bit like custard. Underneath, hot gases keep expanding, pushing on the skin and stretching it out to form hollow tubes that harden into solid rock.