Water and lava, but--curiously--no explosion.

• Position: Iceland

Rocky pillars dotting Iceland's Skaelingar Valley were projectiles tossed into the fields by warring trolls. That, at least, is the tale that University at Buffalo (N.Y.) geologist Tracy Gregg heard from a tour guide and focal hiker when she visited the site on two occasions, but Gregg and a colleague have a new explanation for the presence of the lava formations--this one also unexpected.

She and former master's student Kenneth Christie report that the pillars, hollow and made from basalt, likely formed in a surprising reaction where lava met water without any explosion occurring. "Usually, when lava and water meet in aerial environments, the water instantly flashes to steam," explains Gregg, associate professor of geology. "That's a volume increase of eight times--boom.

"Formations like the ones we see in Iceland are common in the ocean under two miles of water, where there's so much pressure that there's no explosion. They've never been described on land before, and it's important because it tells us that water and lava can come together on land and not explode. This has implications for the way we view volcanic risk."

Deep-sea basalt pillars form when columns of super-heated water rise between pillows of lava on the ocean floor, cooling the molten rock into hollow, pipe-like minarets. The structures grow taller as lava levels rise, and remain standing even after volcanic eruptions end and lava levels fall again.

Gregg and Christie propose that the same phenomenon sculpted the land-based lava pillars in Iceland. It happened in the 1780s, when lava from a nearby eruption entered the...