A Groundbreaking Expidition
An international geoscience mission to drill into the lower oceanic crust has succeeded (http://www.sciencedaily.com/releases/2013/12/131203191159.htm), redefining our understanding of processes occurring beneath the seafloor. The 10 million dollar mission, referred to as Ocean Drilling Program Expedition 345, involved collaboration among 30 researchers from all over the world.
The primary aim of the mission was to drill into the seafloor beneath the Hess Deep, an area off the coast of South America. Previous attempts to drill into the lower oceanic crust failed because the overriding layers of dikes and lava flows were too thick. The Hess Deep provides a unique opportunity because these layers have been pulled back by tectonic forces, revealing deeper rocks, which are known as gabbro.
Gabbro (http://geology.com/rocks/gabbro.shtml) is a coarse grained mafic rock that underlies most of the earth’s oceanic crust. Since the ocean basins are so large, gabbro may be the most common type of crystalline rock on Earth. Despite this, little is known about exactly gabbro forms. The drilling mission was a logical next step towards an understanding of the physical and chemical processes at work beneath the seafloor.
The expedition led to three major new findings, which were published in the journal Nature in December 2013 (see the full text at http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12778.html).
The first new finding is that the gabbros formed at mid-ocean ridges crystallized in many different horizontal layers. Prior to this finding, some scientists hypothesized that the gabbros only formed in one massive layer at the top of the lower crust.
The second finding was that orthopyroxene, a mineral long thought to be absent in the lower crust, is actually somewhat common. This means that scientists will need to rethink their chemical models for the lower crust.
Finally, scientists found large and delicate crystals of the mineral olivine. Although olivine has long been known to be present in the lower crust, the existence of large crystals was a surprise because scientists had predicted that the lower crust flowed in a molten state, which would break up any delicate crystals that didn’t melt.
The Future of Lower Oceanic Crust Drilling
These findings are exciting and unusual enough that scientists are returning to the area in order to collect samples from a slightly different location. The International Ocean Discovery Program (http://www.iodp.org/new-program), which began in October, will allow scientists to determine whether the rocks found on the first expedition are unusual or representative of most of the lower oceanic crust.
(If you’re more interested in attempts to drill into the continental crust, check out http://www.damninteresting.com/the-deepest-hole/ )