Possibly the largest lava rock ever recovered from the bottom of the ocean is on display at the Clark Laboratory on the Quissett Campus of the Woods Hole Oceanographic Institution.
The pillow lava sample is three feet wide by four feet long and weighs 800 pounds. It was recovered from a depth of 2,095 meters off the Galápagos Islands in May 2010. The sample is 40,000 years old, according to carbon dating.
“Normally the samples are orange-size to grapefruit-size, with some as big as a bowling ball,” said Senior Scientist Daniel Fornari, who is studying the sample. “An intact sample is very rare. There are others who have gotten pillows, but they haven’t been nearly this big.”
Scientists in Germany use a different technique to collect lava rocks, but to his knowledge none are as big as the sample his team collected, Dr. Fornari said.
The technique for collecting lava rock samples is to drag a square steel bucket along the bottom of the ocean. The dredge bucket is dragged up a slope so there is a reasonable chance of encountering some rocks, Dr. Fornari said. “After that it’s only luck that gets any rocks in the bag,” Dr. Fornari said.
Normally the samples are orange-size to grapefruit-size, with some as big as a bowling ball. An intact sample is very rare. There are others who have gotten pillows, but they haven’t been nearly this big.
Scientist Daniel Fornari
It took luck and balance to capture the giant pillow. On the first attempt to collect samples on the cruise, the scientists got a reading that there were 6,000 to 8,000 pounds of drag at the end of the line. “We thought we had some rocks in the bag,” Dr. Fornari said.
But when they pulled the bucket to the surface, they saw that the wire attached to the dredge had slipped through a crook in an arm of the pillow and it was balanced on the steel wire. “When we brought it up, we realized that the sample is not in the bag, it’s on the bag,” he said.
The scientists wrestled the rock onto the deck and then Dr. Fornari had it shipped back to Woods Hole. Last winter a quarry in Rhode Island cut the rock into sections. Dr. Fornari and his colleagues were surprised to find small cavities inside that contained sediment.
The sediment is calcium carbonate made up from the shells of tiny organisms that are at the low end of the food chain in the oceans, with some volcanic glass shards mixed in, he said. A hard, gray sediment near the pillow’s center was baked, presumably from the heat of the cooling lava. Dr. Fornari said that the unusual pockets were either material that was captured as the lava flowed over the sediment-covered seafloor, or sediment that infiltrated cracks in the pillow after it formed.
After the rock was divided, Dr. Fornari asked the carpentry shop at WHOI to turn a piece into a coffee table. The carpenters made a custom base for the lava to sit on. Now the pillow-lava coffee table and another piece of the sample are on display in the Clark Laboratory.
Pillow lava forms as 1,200-degree centigrade lava erupts into 2-degree centigrade sea water on the ocean floor. Within a fraction of a second, a frozen, glassy skin forms on the lava surface, over a blob of lava. The lava continues to inject into the blob, which causes the pillow to stretch and expand like a water balloon. New pillows form when hot lava bursts through the chilled skin of a previous pillow.
Scientists are studying the seamount province between the Galápagos Islands and the Galápagos Spreading Center to investigate interactions between the mantle plume and mid-ocean ridge.