In February 2010, China’s deep-sea research vessel, Dayang Yihao, sailed in the Indian Ocean above an ultraslow-spreading segment of the Southwest Indian Ridge. The research vessel collected data that revealed a superthick crust and a source of excess magma: two unexpected characteristics of a shallow stretch of the mid-ocean ridge and possible evidence of volcanic activity.
In regions like this, where tectonic plates move apart at less than 20 millimeters per year, the seafloor is thought to form through a primarily tectonic process. Thin crust and a preponderance of peridotite—a rock derived from Earth’s upper mantle—found at ultraslow-spreading ridges have served as evidence for the tectonic activity.
The crew of the Dayang Yihao deployed ocean bottom seismometers to gather seismic refraction data on a 60-kilometer stretch of the ridge. Li et al. used an array of four air guns to generate sound waves at the surface of the ocean, which travel through the water and rock at varying speeds. Using the time elapsed and the magnitude and phase of the sound waves when they reached the seismometers, the researchers were able to construct a model of Earth’s structure beneath the ridge.
The team found that beneath the central volcano—and extending roughly 20 kilometers out from the axis of the ridge—the crust was approximately 9.5 kilometers thick, which the authors note is on par with the thickest oceanic crusts at mid-ocean ridges around the globe. The average crust thickness along the 60-kilometer stretch of the Southwest Indian Ridge was 6 kilometers, which is closer to the global average for fast-spreading ridges than for ultraslow ones.
Most new seafloor crust at ultraslow-spreading ridges has been observed to be formed by amagmatic processes. Rather than mantle materials rising to the surface as lava, rocks from the inner Earth and lower crust are pushed up to the surface without melting.
Surprisingly, within the thickest crust, the team found evidence for a highly localized reservoir of melted rock from the upper mantle beneath this region. Since the crust near the melt supply is thicker than average well beyond the axis of the ridge, the authors believe the melt reservoir has likely been feeding the budding seafloor here for the past 3 to 10 million years. (Geophysical Research Letters, doi:10.1002/2014GL062521, 2015)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2015), A magmatic seafloor source at an ultraslow-spreading ridge, Eos, 96, doi:10.1029/2015EO036307. Published on 28 September 2015.