When hot magma rises up near the earth's surface or under water and cools down to become solid, various forms result. The process of cooling and solidification also lead to volume shrinkage resulting in hexagonal fractures and the formation of columnar joints. The exhibited specimen, found in the south of Australia, is 20 million years old.
The Pillow lava on the left was formed after magma gushed out under water and solidified when it cooled. The name comes from its pillow shape. The exhibited specimen, found in the northwestern part of Canada, is approximately 2 billion years old.