Ice spikes grow as the water in an ice cube tray turns to ice. The water first freezes on the top surface, around the edges of what will become the ice cube. The ice slowly freezes in from the edges, until just a small hole is left unfrozen in the surface. At the same time, while the surface is freezing, more ice starts to form around the sides of the cube.
Since ice expands as it freezes, the ice freezing below the surface starts to push water up through the hole in the surface ice (see diagram). If the conditions are just right, then water will be forced out of the hole in the ice and it will freeze into an ice spike, a bit like lava pouring out of a hole in the ground to makes a volcano. But water does not flow down the sides of a thin spike, so in that way it is different from a volcano. Rather, the water freezes around the rim of the tube, and thus adds to its length. The spike can continue growing taller until all the water freezes, cutting off the supply, or until the tube freezes shut. The tallest spike we've seen growing in an ordinary ice cube tray was 56mm (2.2in) long.
