The Last Word: Roger Searle

Roger Searle is going down under—way under. Last week the Durham University geophysics professor and a group of fellow scientists set sail from Tenerife aboard the £40 million research ship RSS James Cook. They are investigating a hypothesis that part of the earth's crust is missing. Halfway between the Canary Islands and the Caribbean lies what one of Searle's colleagues called "an open wound on the surface of the earth." Nearly 100 kilometers of seafloor seems to be, in effect, missing. The scientists will spend several weeks investigating why this area did not develop a normal crust and how it appears to challenge current tectonic-plate theories. With the Atlantic waves crashing around him, Searle spoke to NEWSWEEK's Ginanne Brownell by satellite phone from Tenerife. Excerpts:

BROWNELL: A huge hole in the bottom of the ocean sounds pretty dire.
SEARLE: It's not just a big hole—it's much more subtle than that. Tectonic plates are thin, rigid shelves that slide around on the surface of the earth. The interior of the earth is ductile, a little bit like hard toffee. These plates move apart from each other at mid-ocean ridges—such as the mid-Atlantic ridge—leaving a gap, which has to be filled somehow. The earth's mantel moves up very slowly to fill up that gap. As it does, some of it can melt, and produce magma, or molten rock. Magma can then be erupted from volcanoes, making its way to the seafloor. Most of the earth's crust is essentially a buildup of volcanoes and volcanic products. The ocean floor has a six-kilometer stack of volcanoes underneath it. What people have found in the last decade in the area we are going to is that there are 10 to 100 kilometers where that volcanic crust is just missing. The question: why is it missing?

How was the gap discovered?
A lot of science comes from random exploration, and over the years people have taken samples of seafloor rocks. There are clear characteristic patterns that we can recognize about the normal volcanic crust—long, thin volcanic ridges, which run parallel to the plate boundary and are close together. That pattern is missing where we are, and instead of these long, thin parallel ridges, you see a much more blocky structure. If you throw a pile of spaghetti down on the kitchen floor, it will tend to line up in parallel ridges, whereas if you throw down a pile of walnuts, you get a much more blocky surface. It is a matter of the shape of the seafloor.

So there's a different kind of crust, rather than no crust at all?
Well, our hypothesis is that the crust never formed. Maybe there was less melting going on than in other places, so that as the gooey mantel rises up it just never melts and it comes up to the surface and is just rafted away to either side. So that would make a different kind of crust or plate. There is also another process that goes on. If you bring mantel rocks into contact with water, they will undergo a chemical reaction and produce a mineral called serpentine, which has a low density. It is very hard to distinguish that from volcanic rocks. So it may be that there is a crust made from serpentine instead of volcanic rocks. The other possibility is that it got erupted on the seafloor but has been removed. But we are thinking that one is less likely.

How will you find out what happened?
Exploring the ocean floor is as technologically difficult as exploring outer space. We will map the sample rocks. We have two specialist instruments that we will use to do our work. One tows geophysical instruments that drag along the seafloor and carry two main sensors. One is a side-cam sonar [which] takes images of the seafloor [using] sound. Then we will use the drill robot. It is unique because as it takes rock out, it marks a geographical direction on it, so we can get it back to the ship and check out the crystals and see what direction this gooey material was growing.

Were it possible to walk along the sea-floor, would the difference be visible to the naked eye?
If you were right on the ridge axis where plates were separating. On the normal parts of the seafloor you can recognize a lava flow. And if you went to a place where the crust is missing, then those rocks would look different. It would be like walking from volcanic to sedentary rocks, but the surface is still there.

No pun intended, but is what we currently know about the interior of the earth just a drop in the ocean?
This unusual kind of crust that we are studying was unknown 15 years ago. Only a small part of the seafloor has been mapped and there are enormous areas of the earth that have not been studied. It was only about five years ago that we got the first decent geological expedition to the arctic ridge near the North Pole. So yes, there is still a lot to be learned.

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