Credit: A.R. Philpotts

This 1-cm cube of rock provides a persuasive explanation for a common feature of basalts, a type of igneous rock. When basalts form from magma, horizontal sheets of coarser grain segregate out as the magma cools and crystallizes.

As they describe elsewhere in this issue (Nature 395, 343-346; 1998), Philpotts et al. have examined the process by heating samples of basalt to temperatures of around 1,100 °C, and then quenching and examining serial sections of them. As shown here, the cube retains its shape even when largely melted. The authors think that in the reverse process, crystallization, a three-dimensional crystal network in the basalt starts to maintain its integrity when the surprisingly low level of 25% crystallization is reached. At 35% crystallization, that network becomes strong enough to resist compaction from overlying material. But in between the two values, it is weak and permeable (‘mushy’), and in this interval liquid can be expelled — as is evident from the drop in the photograph. It is this liquid that forms the coarser-grained sheets characteristic of many occurrences of basalt.