|
Sci Tech
Current thinking on Earth's crust formation questioned
THE DISCOVERY that an ocean ridge under the Arctic ice cap is unexpectedly volcanically active and contains multiple hydrothermal vents may cause scientists to modify a decades-long understanding of how ocean ridges work to produce the Earth's crust.
The new results, which come from a study of the Gakkel Ridge, one of the slowest spreading ridges on Earth, have broad implications for the understanding of the globe-encircling mid-ocean ridge system where melting of the underlying mantle creates the ocean floor.
According to a report in the journal Nature, scientists present striking results obtained during a research cruise.
In general, fast-spreading ocean ridges, where the Earth's crust is produced, are very much volcanically active. So scientists on the Arctic Mid-Ocean Ridge Expedition (AMORE) expected the Gakkel, where the spreading rate is one cm (.39 inches) per year, to exhibit little, if any, volcanic activity. The spreading rate on the Gakkel is about 20 times slower than that of more-frequently studied ocean ridges, such as the East Pacific Rise.
The Gakkel extends 1770 km (1100 miles) from north of Greenland to Siberia. It is the deepest and most remote portion of the global mid-ocean ridge system. Because the spreading rate decreases progressively towards Siberia, "we expected that the amount of melting and magma production would decrease as we worked our way from Greenland towards the east," said Peter Michael, the AMORE chief scientist from the University of Tulsa.
Instead, the very first sampling station brought up fresh volcanic rock, and the new map published in Nature shows large young volcanoes dominating the part of the ridge nearest Greenland.
"By contrast, the central portions of the ridge showed virtually no volcanism and large faults as pieces of the Earth's mantle were emplaced directly onto the sea floor," noted Henry Dick, who specialises in mantle materials. Even larger volcanic edifices appeared much farther to the east.
The researchers obtained high-resolution, well-navigated maps of the entire portion of the ridge, collected thousands of samples by dredging the sea floor, explored for regional anomalies in the water column that would indicate the amount and location of deep hydrothermal vents surrounded by ecosystems that thrive in the absence of sunlight.
Michael noted that the results obtained at sea continually surprised the research team, which was co-lead by Henry Dick, of the Woods Hole Oceanographic Institution, and Charles Langmuir of Harvard University.
Based on the picture the Gakkel data painted, factors other than spreading rate must be taken into account when characterising the likelihood of a given area's volcanic activity.
"It's an interconnected multi-variate system," said Michael. "The level of volcanic activity was higher than that predicted from the spreading rate and did not vary continuously as the spreading rate decreased.
The chemical composition and temperature of the mantle that melts to form the magma must also be of substantial importance to the process of ridge formation."
"At the slowest spreading rates, small changes in the other factors have more dramatic effects, making their importance more visible," noted Langmuir.
Printer friendly
page
Send this article to Friends by
E-Mail
Sci Tech
|
