The oases of living things confirmed Lizarralde’s hunch that magma was pushing up toward the seafloor far from the spreading center. But why does that happen in the Guaymas Basin? Does it occur elsewhere? And what does it reveal about how new ocean crust is formed?
Lizarralde said the conventional wisdom, that magma gets focused deep below the seafloor and emerges only at or near a spreading center, was based on the fact that there are very few observations of magma below a spreading center not being focused. But the mechanism—how melt gets focused—has never been adequately explained.
He thinks the situation in the Guaymas Basin suggests that the key to focusing is the lack of sediment found at most spreading centers. With little sediment to cover and plug up cracks in the igneous crust, frigid seawater circulates through the crust and removes heat in much the same way that a car’s radiator cools a hot engine. This type of advective heat loss is so efficient that magma either freezes in the mantle or stops rising at the bottom of the cold crust and flows laterally along the freezing front toward the hottest spot in the region—the graben at the plate boundary.
At Guaymas, the situation is different, he said. The very thick sediment acts like sand in a radiator, preventing cold water from circulating efficiently and from cooling the magma via advection. Instead, cooling takes place via conduction, which is much less efficient. As a result, everything is hotter, and magma isn’t channeled along a freezing front toward the plate boundary, but simply keeps moving straight up—and sometimes breaks through the crust into sediments far from the graben.
Lizarralde and Soule said the large number of these so-called “warm seeps” they found is significant.
“The fact that we saw so many means that the phenomenon is happening fairly frequently,” said Soule. “To see a hundred of them is amazing. Given their short lifespan, it means that in the last 500 years, 50 or 60 of them could have been created.”
And that’s just in one relatively small ocean basin. The scientists think the same thing could be happening at many sites on the ocean floor. Similar warm seeps seen elsewhere, such as in the Okinawa Trough, had been attributed to a form of volcanic activity, but Lizarralde now suspects they may be like those in the Guaymas Basin.
“I suspect that any time you have a spreading system that has a bunch of sediments on it, the mechanisms that focus magma get turned off,” he said. “So you get a wide region where magma gets into the sediments and cooks off carbon.”