Lead author Henry Dick and WHOI colleagues Jian Lin and Hans Schouten of WHOI's Geology and Geophysics Department studied two of the most remote locations on the planet -- the Southwest Indian Ridge in the Atlantic and Indian Oceans thousands of miles south of the African continent, and the Gakkel Ridge beneath the Arctic ice cap.
Ocean ridges form a chain of mountains and valleys that encircle the globe like seams on a baseball. Dick says he first noticed the ultraslow spreading ridges -- a name for the ridges used by WHOI scientists – because they were much thinner than ridges found elsewhere in the ocean. Initially he thought they were just a very slow variety of slow-spreading ridge, creaking apart at only 0.15 to 0.8 inches per year. Slow and fast ridges – the other types of ocean ridges -- spread at a rate of one to 7 inches annually.
Dick says that the ultraslow ridges account for 12,000 miles of the total 30,000-mile mid-ocean ridge system. Among other reasons, this is important as hot springs at these ridges are now known to be far more abundant than anyone had suspected. These springs produce deposits rich in copper, zinc and other metals at ocean ridges, and the ultraslow ridges may contain the largest future ore deposits to be found in the oceans.
The discovery of this new class of ocean ridge may also solve a major puzzle in plate tectonics: what the plate boundary looks like in the early stages of continental breakup and continental drift. It has been difficult to fit the deep structure seen in seismic images of the North America and European continental margins where these continents broke apart millions of years ago with models based on slow and fast spreading ridges. Scientists are now seeing similarities between the structure of ultraslow spreading ridges and the deep structure of those continental margins.
Dick says the findings alter the commonly held view that sea floor spreading creates a uniform layer of volcanic rock about four-miles thick. The study means that up to 40% of the ocean floor forms in a different manner than geologists thought, with the mantle being emplaced as blocks over large regions of the seafloor where there is little or no crust.
Since the acceptance of continental drift and plate tectonics in the early 1960's, scientists have recognized only two major classes of ridge – slow and fast spreading ridges -- each with distinct characteristics that seemed to depend largely on how fast the seafloor was spreading.
Slow spreading ridges, like the Mid-Atlantic Ridge, are spreading about 1 to 2 inches per year, forming rift valleys with canyons over a mile deep bordered by rugged mountains. Fast spreading ridges, like the East Pacific Rise, are pulling apart or spreading at a rate of about 3 to 7 inches per year. The plates are moving apart so quickly that magma rising from deep within the earth to the ocean floor fills in the gap and forms an axial ridge a half mile or so high.
Typical ocean ridges, like the Mid-Atlantic Ridge and the East Pacific Rise, have regularly spaced volcanoes that create a nearly continuous layer of crust on the seafloor. Mantle rocks are only rarely found at fracture zones that locally offset these volcanic ridges. At ultraslow ridges, the WHOI scientists found that volcanoes occur only at widely spaced intervals, and their lavas are richer in sodium, potassium and other chemical elements than typical mid-ocean ridge lavas.
The ultraslow ridges are spreading less than one inch per year, so slowly that the earth's mantle itself is exposed on the ocean floor in great slabs of rock between volcanoes. Where fresh, mantle rock is grass green and is known as peridotite, named for the gemstone peridot for which it is largely composed.
More typically, where the scientists sampled it, the mantle peridotite was highly altered to the rock serpentinite or was deeply weathered asperidotite is highly unstable at seafloor conditions. Nonetheless, these large regions of mantle rock with little evidence of significant volcanic activity provide earth scientists with their first direct look into the Earth's mantle, the deep layer of the earth that is elsewhere covered by the planet's crust.
Fast and slow spreading ridges consist of volcanic segments linked by transform faults where the earth's tectonic or crustal plates slide past each other. California's San Andreas Fault is one example of a transform fault on land. It is a major geologic hazard since earthquakes often occur along the fault. Transform faults are normally abundant on ocean ridges but have weaker earthquakes because the oceanic plates are thinner than the continental plates.
At ultraslow spreading ridges, marine geologist Dick and marine geophysicists Lin and Schouten found that transform faults don't form at all. Instead, the earth cracks apart between the volcanoes, pulling solid rock up onto the seafloor in any direction needed to fit the plate boundary. The three types of spreading ridges - fast, slow and ultraslow - can be described in terms of the mantle temperature beneath as hot, cool and cold ridges. The scientists believe the extremely slow rate at which the mantle rises beneath ultraslow spreading ridges allows the mantle to cool and become more rigid than at other locations.
Journal
Nature