Magma Merry-Go-Round

Sunday, December 01, 1996
While mapping currents and the movement of deep-sea sediments off the eastern coast of New Zealand, geologist Lionel Carter came upon something unexpected: evidence of a unique volcanic recycling system in which seafloor sediments plunge into Earth’s interior, erupt out of volcanoes in the New Zealand Alps, and then settle onto the ocean floor to begin the cycle anew. The entire process, says Carter, probably takes some 3 million years to complete.

Carter, who works at the New Zealand National Institute of Water and Air in Wellington, knew that the seafloor sediments he was studying originated in the New Zealand Alps. Both the sediments and the rocks and soil on the slopes of the Alps contain identical amounts of a specific type of mica, a mineral consisting of aluminum, potassium, and other elements. This mica signature told Carter that the sediments had eroded from alpine slopes, washed into rivers, and settled on New Zealand’s coastal shelf.

Carter traced the current-driven movement of the sediments from the coastal shelf to the edge of a steep submarine canyon. There, in a slurry resembling an avalanche, the sediments plunge about three miles down through channels in the canyon sides and fan out onto the ocean floor, where deep currents drag them to the Kermadec Trench, a four-and-a-half- mile-deep gash in the seafloor near New Zealand’s North Island. At the trench the Pacific floor dips under the Australian continental plate, plunging into Earth’s interior at about 1.5 inches per year.

At this point the trail of the sediments would appear to be lost. But almost on a whim, Carter wondered if the same sediments, after melting miles below the seafloor, might resurface in New Zealand’s volcanoes in the form of lava and ash. The mica content would no longer be of any use as a geologic fingerprint--the mineral would melt and reform with different concentrations of elements. But Carter knew that one characteristic of the sediments wouldn’t change during their fiery passage--the ratio of two forms of lead contained in them. John Gamble, a colleague of Carter’s at Victoria University in Wellington, analyzed the isotope ratios and found that the sediments from the very bottom of the trench had the same lead fingerprint as the volcanic rocks in New Zealand’s Alps.

It’s a remarkable recycling system and probably the only one of its kind, says Carter. First you need some system to take the sediments from land to deep ocean. Then you must have plate boundaries colliding and a system to grab sediments and bring them into the mantle and up through the volcanoes.
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