Tuesday, April 12, 2011

To the Moho -- or bust

The Earth is rather more like a partly boiled egg than it is like a billiard ball. It has a rigid crust (lithosphere) that is as thin (thinner!) compared to the Earth as the shell of the egg -- 30 to 60 kilometers thick at the continents, but as little as 6 kilometers thick under the oceans. Beneath the crust is the rocky mantle, hot, plastic, in convective motion, extending down to the metallic core -- the white and yolk of the egg. Between the crust and the mantle is a sharp discontinuity in the composition and physical properties of the rock, called the Mohorovicic discontinuity (or simply Moho) after its discoverer Andrija Mohorovicic, who in 1909 detected a significant change in the speed of earthquake waves at that depth.

It has long been a dream, of geologists to drill a hole to the Moho and sample the mantle of the Earth, an achievement comparable to bringing rocks back from the Moon. There was a failed attempt in the early 1960s to drill to the Moho in the ocean off Baja California. The technology and political will were not yet up to the task. So far, no one has drilled deeper than 2 kilometers into the ocean crust.

Now geologists believe the time is almost ripe for a new "journey" to the mantle of the Earth. The Japanese scientific drilling vessel Chikyu may be technologically up to the task. A proposed pace to drill is the Cocos Plate just off the western coast of Costa Rica. The crust was formed superfast at a nearby spreading center and is therefore relatively thin. Maybe within the next dozen years or so.

Beginning April 14, the research vessel JOIDES Resolution will drill there, hoping to extend an existing hole even closer to the Moho and retrieve the deepest rocks ever from the oceanic crust.

This is the expedition that will occupy the JOIDES Resolution before it makes its Panama Canal transit in early June. Mo and I will be joining the ship just as it completes this historic mission.

(On the 150th anniversary of the firing on Fort Sumter, here are some thoughts of my great-grandfather Theodore Raymo.)