By Gail Gallessich

Answers to important questions about the formation of the Earth’s crust may be close at hand as a result of recent findings by an international team of scientists. For the first time, researchers have recovered black rocks—known as gabbros—from intact sea floor, according to the April 20th online Science Express, the Internet edition of the journal Science.

First author Douglas S. Wilson, an associate research geophysicist with UCSB’s Marine Science Institute, chose the location for drilling into the Earth’s crust. He originated the idea for these missions by studying the crust’s magnetic properties, and is co-chief scientist, with four others, on this project under the United States’ Integrated Ocean Drilling Program.

Geophysical theories suggest that oceanic magma chambers freeze to form the coarse-grained, black rocks called gabbros, which are commonly used for facing stones on buildings and kitchen countertops. Although gabbros have been sampled in shallower parts of the oceans, this is the first time that gabbros have been recovered from intact crust.

“Finding the right place to drill was probably key to our success,” said Wilson. His research identified a 15-million-year-old region of the Pacific Ocean that formed when the East Pacific Rise was spreading at a rate of more than 200 millimeters per year, much faster than any mid-ocean ridge on Earth today.

“We planned to exploit a partially tested observation that magma chambers should be closest to the Earth’s surface in crust formed at the fastest spreading rate,” said Wilson, who has participated in prior expeditions. “If that theory was correct, then we should only need to drill a relatively shallow hole, compared to anywhere else on the planet, to reach gabbros.”

The drill site was approximately 800 kilometers west of Costa Rica, and the vessel used was the JOIDES Resolution. “This particular place has a normal spread where we could sample an entire sequence where the crust hasn’t been broken up by earthquake faulting,” Wilson explained.

After three years of research and three trips to the site, the borehole into the magma chamber more than 1,500 meters below the sea floor took nearly six months to drill. Twenty-five hardened steel and tungsten carbide drill bits were worn out in the process. The rocks directly above the frozen magma chamber were extremely hard because of repeated baking by the underlying magma.

They will help explain whether ocean crust, which is about six to seven kilometers thick, formed from one high-level magma chamber or from stacked chambers.

“By sampling a complete section of the upper oceanic crust, we’ve accomplished a major goal scientists have pursued for over 40 years,” said Damon Teagle of Britain’s National Oceanography Centre.