Some rocks can tell you of their strange evolution.
The picture to the right shows "pillow lava" forming. The
water cools the crust of the lava quickly giving it a crust that curls
into itself. It looks like many pillows stacked on one another. When you
see pillow lava, a geologist knows that the eruption occurred under
Scoria is a vesicular rock due to the escape of
volcanic gases during an eruption. Scoria is usually black with a high
iron content, but also has a distinct reddish color due to a chemical
reaction with oxygen in our atmosphere.