Book 27, Question 31
Why do some rocks have a few big crystals sitting in tiny ones?
Some rocks, called porphyry, look like a speckled cookie.
Big crystals sit in a background of tiny ones. This happens when melted rock starts to cool slowly deep underground, which lets a few crystals grow big. Then the rest of the melt rises or erupts and cools quickly, so the leftover liquid makes tiny crystals around the big ones. The rock keeps a record of two cooling speeds.