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 December 5, 2000   1   2   3   4 

The first surprise
One was a section of the massif's south wall Karson and Gee visited in Alvin on Nov. 28. At a briefing following their return, Karson reported seeing "fantastically steep" bare rock slopes with undulating shear zones. He also found "bizarre surfaces" at the top, with sedimentary deposits directly over seawater-altered rocks of magmatic origin - called serpentinites. And there were features showing possible signs of erosion. Photo by Monte Basgall. Nicholas School of the Environment, Duke University.

The following night Argo revisited that terrain, which left Karson in an a late-night adrenaline rush. As the fish reached the cliff face's pinnacle, Karson saw successive layers of meters-thick deposits of cemented broken rock - called breccia - that strongly resemble gravelly deposits found on some beaches.

That suggests the possibility that at least part of the mountain was once pushed up above sea level, only to move back down again later, he said. "That's one of many ideas we are playing with."

In geological terms, such interfaces between serpentenites and breccias are examples of an "unconformity." After seeing more of them during a subsequent Argo run, Karson called these unconformities "a fundamental kind of geological contact."

Another surprise
Photo by Monte Basgall. Nicholas School of the Environment, Duke University. Figs. 9 & 10. Scientists thinking what to make of the evidence, in groups and in their own minds.

The greatest surprise came the night of Nov. 30-Dec. 1, when Argo moved up another south-facing cliff face along the proposed footwall. "It's surprising because there are pillow lavas," watch-stander Ross said in the control room. "They are not in an area of the massif where they were expected to be."

Cann sat staring at the video monitors in sheer amazement. "We are in a state of shock," he said. "There are not just pillow lavas, but undeformed, happy pillow lavas and a couple of dikes. We didn't expect to see pillow lavas in the footwall. There is no obvious simple answer."

The implication was clear: If these lava deposits are "happy" and undeformed, that would mean they may have not have been violently relocated there from somewhere else. That would place the lava-based plumbing of a typical Mid-Atlantic Ridge spreading center squarely where a central hypothesis of this mountain's formation says it shouldn't be.

Karson said a key question is the age of the pillows. "It raises a lot of new questions. But it may not be that difficult to explain with a different model," he added.

"Somewhere between here and here," John said while pointing to a map of the southeastern scarp, "we go from almost all pillows to almost all peridotites. We thought it was a simple model: Here's the hanging wall. Here's the footwall. The corrugations, originally noted by Cann and Blackman, were grooves from the fault movement... Now we're not so sure."

Blackman had the last word: "We haven't disproven anything at this stage," she said.

Today's pages: The Hidden Mantle | Rocky clues | Testing hypotheses, & Imagery | Surprises!



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