CATANIA, SICILY, ITALY–These are Dr. Meagen Pollock’s favorite kind of rocks: pillow basalts. Above we have a spectacular example of pillow basalts exposed in cross section below a castle ruin in Aci Castello a few kilometers north of Catania. The pillows (more are shown below) are in the middle of this natural outcrop carved by the sea.
Pillow basalts are formed when basaltic lava is erupted underwater. The surface of the flow quickly cools and begins to solidify as the interior fills with lava. The result is a flattened spheroid of basalt with chilled margins. The castle, by the way, was built in 1076 by conquering Normans.
The light was not great for this shot, but you should be able to make out in the lower right a large body of basalt with columnar joints radiating from the center. This is, I was told, a “megapillow’ of basalt from a large flow.
This is a view of the wave-eroded platform below the castle showing the pillows form the top. I left the roasting Europeans in the frame for scale. Note that while these pillows appear with almost circular outlines in cross-section, they are actually serpentine in shape.
These pillow lavas were formed with the beginning of volcanic activity roughly 600,000 years ago that led to the present Mount Etna complex. They show the submarine phase of eruption before the eruptive center was uplifted above sea level. They are the most spectacular pillows I’ve ever seen.