Getting to the bottom of the crust.

Getting to the bottom of the crust

An international team of scientists has successfully drilled and sampled rocks from the deepest layer of the crust that forms the floor of the world's oceans. In the past this layer, normally covered by 2 kilometers of upper crust, had escaped the reach of even the longest drillhole in the ocean. But during a three-month cruise in the Indian Ocean that ended in December, researchers on Leg 118 of the Ocean Drilling Project (ODP) found a way of bypassing the upper crust.

The scientists found a site along what is called the Atlantis II Fracture Zone, where the lower crust is uncovered. They cored 500 meters of the basement rock and recovered more than 87 percent of the rock, one of the highest rates ever achieved in hard-rock drilling, says Richard P. Von Herzen from the Woods Hole (Mass.) Oceanographic Institute and co-chief of this leg of the project.

The Atlantis II Fracture Zone is part of the midocean ridge that is the boundary between the African and Antarctic plates. Running down the center of this ridge is a feature called a spreading center, so named because it periodically breaks apart, leaving a gap that becomes filled with molten rock from the earth's mantle. As it hardens in place, this rock forms new oceanic crust that joins to the ends of the plates. In this fashion, the plates slowly move away from each other at a rate of a few centimeters a year.

Resembling the steps in a staircase, spreading centers are all short, straight segments roughly parallel to each other. Fracture zones are valleys that connect the adjacent segments, and are comparable to the vertical jumps between the steps.

When the ODP scientists drilled into a level spot on the wall of the fracture zone valley, they found coarsely crystalline rocks called gabbros, which are known to make up the lower segment of the crust. By analyzing the transitions between different minerals in the gabbros and the changes in the crystalline structure, geologists will be able to reconstruct the evolution of the lower crust, says Henry J. B. Dick of Woods Hole, a member of Leg 118. Moreover, the samples will help scientists understand how fracture zones develop and will allow them to take a closer look at the fine-scale processes involved in seafloor spreading.