William Schenck, a scientist at the Delaware Geological Survey, says much of the blue rock beneath Wilmington is metamorphosed volcanic rock. It's a byproduct of a chain of volcanic islands that flanked North America 430 million years ago and collided with the continent. The Appalachians also were born of that tectonic activity.
Beneath the sea near Iceland, a rising supply of magma spreads the Eurasian and North American plates farther apart every year.
"It's actually quite expected that it would do that," Schenck says. "That is the spreading center. It just happens to be exposed in Iceland."