A new kind of hydrothermal vent
"The vents are very special," said Cann. "It's a very unusual day when you find a totally new kind of hydrothermal vent. There are only three kinds of hydrothermal vents in the world that have been discovered, bit by bit, in the last 25 years. It's a very special day when you find a fourth kind."

Fig. 5. What will the researchers have to say about the Lost City when they return? We'll keep an eye out and lead you to info on earthguide.ucsd.edu.

Karson, a Duke University structural geologist and another co-principal investigator on this expedition, said hydrothermal vent systems have three different components: a heat source, a "permeable" natural plumbing system for fluids to flow through, and a source for the fluids. With the Lost City,"it looks like all three are going to be different," Karson said. "That's one of the reasons it is so exciting."

The discovery elated John for another reason. She said the presence of hot water corroborates data she's collected along the Southwest Indian Ridge showing that "crusts stay warm for a significant amount of time off axis." Scientific "dogma" for places like the Mid-Atlantic Ridge stipulate that recently formed crust should be cool by the time it migrates that far from where it was made, she said.

Karson is also intrigued by other types of carbonates that Argo runs and Alvin dives found arranged in relatively flat, broken up layers above mantle rocks at the top of the dome. That suggests rocks that may have undergone deposition and later weathering there. That can happen if they were exposed at the surface, perhaps through uplifting at one stage in the massif's evolution. "That's one of the most exciting observations you can make in complicated terrain," he said.

But the presence of carbonate deposits have also hindered the search for a major detachment fault that, according to hypothesis, would separate the hanging wall from the footwall along the north part of the massif's dome.

"I still think everything points to a detachment fault," said Cann. "It's frustrating that we couldn't get at it because of a carbonate crust only a few feet thick. It's frustrating because that's why I came out here. You can see it on sonar so well," he added.

"I think it's a surprise that we didn't find really strong, definitive evidence of a detachment fault," weighed in Karson. "That opens the door to a lot of other interpretations. The possibility of seeing active faults and mapping them out is going to be a lot harder than we thought. It's still an open question."

During her Dec. 7 dive to the another targeted location for possible detachment fault exposures, John did find "increasing deformation as we went up toward the top of the massif," she recalled later. "But it was increasingly brittle deformation. If there were a detachment fault there it would be very different from ones on land." She called it "unfortunate" that there weren't more Alvin dives to the same vicinity.

Followup geochemical analysis may eventually say more, Kelley added. "There are fault scarps near active hydrothermal systems. I know there are faults here. It is hard to put your finger on where the fault starts. But from looking at the gabbros and peridotites, I know there was a fault a some point."

Today's pages: Wrapping up | Where we stand | A new kind of hydrothermal vent | One thing leads to the next