There IS a hole in the bottom of the sea

An update from Suzanne O'Connell

Scientific Ocean drilling began almost forty years ago with project Mohole, an effort to drill a deep hole through the Earth's crust and into the mantle. The boundary between these two layers had been identified in 1909 by a Croatian seismologist, Andrija Mohorovicic, because of an abrupt increase in seismic velocity of the upper layers of the Earth by about 1 km/s, to 8 km/s. This boundary, the MOHO, is recognized around the world and is now known to define the boundary between the crust and the mantle, both parts of the lithosphere. This boundary is not uniform. In the continents the boundary can be 20 to 90 km deep. In the ocean, it is 5-7 km deep, so when these scientists decided to core through the MOHO it made sense to try to recover it in the ocean. This boundary still has not been recovered.

Most ocean crust is created at spreading centers such as the Mid-Atlantic Ridge or the East Pacific Rise. Seismic measurements in the 1960's and 1970's defined ocean crust as having a relatively simple layered structure, with some differences between crust created at faster and slower spreading centers. But what were the rocks? Rocks of different compositions may have the same seismic velocity, so it became important to identify which types of rocks provided this layered structure. Some information about the ocean floor came from ocean bottom photographs, submersible dives and dredge samples. (Dredge samples are collected with a metal basket that is dragged from a surface ship over the rocky areas of the ocean floor and pieces that break off tumble into the basket and are brought aboard.) But what lay beneath the surface crust?

Geologists were helped in their interpretation of this sequence of velocities by a few unique rock assemblages on land, ophiolites. These odd assemblages are found in mountain belts and always have the same types of rocks, although not always all in one place. The association consists of marine sedimentary rocks, lava flows (particularly pillow basalts), fine grained diabase, usually as dykes, coarser grained gabbro, and then an olivine-rich type of rock, peridotite. The names basalt, diabase, and gabbro indicate how quickly they cooled from magma, as well as their composition. Basalt extruded onto the seafloor cools the fastest, gabbro the slowest. When the seismic velocities of these rocks were measured, they fit the velocities observed in ocean crust and the layers were roughly the same thickness. Peridotites have the velocity of mantle rocks.

By the early 1970's geologists realized that ophiolites were slivers of ocean lithosphere that broke free from the ocean and mantle and where shoved onto land in collision zones. But the fact that they were on land and rare was suspicious — was there something unusual about this rock assemblage that differed from "normal" ocean crust? Most geologists would say yes.

Today ocean crust covers about seventy percent of the earth. Despite years of trying, no ocean hole has penetrated through the extrusive layers of basalt and diabase into the gabbro, to recover this important contact. When recovered, this contact will provide us with information about how ocean crust is formed, how deep hydrothermal circulation penetrates, conditions in the magma chamber, magnetism of ocean floor, and quite possibly information we have not even begun to ask about. Let's hope that Expedition 312 is finally able to reach the diabase/gabbro contact, a step closer to the MOHO.