Spreading Centers

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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In the 1950's, before plate tectonics won wide acceptance, geologists had a plan, called Project Moho, to drill through the oceanic crust into the earth's mantle. Why "Moho?" The boundary between the crust and mantle was first detected by a Serbian seismologist named Mohorovicic, and the proper name, Mohorovicic Discontinuity, is a mouthful, so it's known as the Moho for short.

The hidden agenda was that since everyone knew the continents and oceans had always been in the same place, drilling through the ocean crust would also drill through all the sediment on top, yielding a complete history of life in the oceans.

Long before the project got going, cracks began to appear in its premises. Nowhere in the oceans did we find rock older than about 200 million years. Also, the 10+ kilometers of sediment that were supposed to be there weren't - in fact, on the mid-ocean ridges there was sometimes none at all.

It turns out the project wasn't necessary because complete chunks of ocean crust are sometimes found on land, in Cyprus, Oman, Greece, Newfoundland, and many other places. A cross section through such a chunk, called an ophiolite, looks like the figure at left.

Project Moho did develop the technology for deep drilling in the oceans. It evolved into the Ocean Drilling Program, a tremendously productive source of information on the geology of the ocean floors.

In a typical ophiolite, starting at the top, we find:

The diagram above shows the probable anatomy of a mid-ocean ridge. A magma chamber, in yellow, feeds dikes and lava flows, but the vast majority of the magma hardens deep in the crust. That takes time, however, allowing the light feldspars to rise and the heavy olivine and pyroxenes to sink.

Above, the process is animated. Among the things to observe:

Project FAMOUS

If you've seen The Hunt for Red October, much of it involves submarines playing hide and seek along the Mid-Atlantic Ridge. The Navy feels that if people are going to wage submarine warfare against them, they can jolly well go get their own data, and highly detailed submarine topographic data is classified (but scientists who've seen it agree it's fabulous.)

In the 1970's, a portion of the Mid-Atlantic Ridge was studied using the best available methods and the results were released. The survey was called Project FAMOUS (French-American Mid-Ocean Undersea Survey). As usual, the acronym drove the name, I mean, what other kind of Mid-Ocean Survey could there be except Undersea? A name where the initials come first and the meaning is chosen later is sometimes called a backronym.

Below is the bathymetry of the FAMOUS area. Even on a scale of a few kilometers, the pattern of rift valleys connected by cross-cutting transform faults is clear. Submersible studies in the rifts shows pillow lava and small submarine cones. This area straddles the boundary of the North American Plate, which extends all the way to Alaska, and the African Plate, which extends all the way to Madagascar, yet almost all the crustal activity on the boundary is confined to a zone a few hundred meters wide.

Below is a magnetic map of the FAMOUS area. Yipes! Stripes! Colored areas are magnetically high. Even on this scale, the symmetrical stripes are evident. Gray areas are places where stripes overlap or cannot be clearly identified.


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Created 24 April 2002, Last Update 14 December 2009

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