Mysteries of the formation of a new lithosphere in the ocean

Scientists fr om the German center for polar research in Bremerhaven in cooperation with Novosibirsk geophysicist Ivan Kulakov have received results that allowed a fresh look at the process of origin of the lithosphere in the ocean.

The results of the study are published in the prestigious journal Scientific Reports from the Nature group. This was told by the co-author of the article, the head of the Laboratory of Seismic Imagery of the Earth of the NSU and the Laboratory of Seismic Tomography of INGG SB RAS, Corresponding Member of the Russian Academy of Sciences Ivan Kulakov.

To analyze the results, Ph.D. student Florian Schmidt went to Novosibirsk. For several months he mastered the seismic data processing techniques developed in the laboratories of NSU and INGG SB RAS. As a result, together with Ivan Kulakov, a seismic model of the lithosphere was constructed in the area of network operation based on the obtained data.

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The mid-oceanic ridges, mining-enc.ru

— In the mid-oceanic ridges, the thickness of the lithosphere is tiny. Therefore, it is believed that earthquake foci are located in the uppermost layers at depths not exceeding several kilometers. Imagine our surprise when, after processing new material from the ultra-slow ridge in the Indian Ocean, it became apparent that just in the upper few kilometers of earthquakes are practically absent! The main seismicity is observed below and spread to a depth of 15-20 km.

— This indicates that after the eruption, the cooling of matter in the region of the ultra-slow ridge occurs much faster than it previously thought, and the thickness of the new oceanic lithosphere on the axis of the ridge may exceed 20 km, says the scientist.

The second unexpected conclusion was related to the nature of the volcanic activity on the ridge. Seismic stations were installed around a large volcano, which experts considered active. However, not a single earthquake under the volcano was recorded. The authors of the article explained this paradox by the fact that the lithosphere in the volcano area is very warm and saturated with water, which makes it plastic. As a result, it deforms like soap; it does not produce earthquakes, which usually accompany the volcanic activity. However, at another point on the ridge line, located 30 km from this volcano, the instruments recorded a characteristic «sound», which was very similar to the signal observed under active volcanoes, when magma breaks its way through cracks.

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(a) Location of the survey area (red star). (b) Bathymetry with dashed lines outlining the extent of the axial rift valley and the Segment 8 volcano. Black dots are epicentres and  triangles are stations deployed for the microearthquake study in 2012–2013. Scientific Reports

Based on these observations, scientists hypothesized that in the region of the ultra-slow ridge magma could migrate horizontally a considerable distance. This migration is accompanied by some earthquakes and forms new centers for the expansion of the ocean floor, wh ere the formation of a new oceanic lithosphere takes place.

—The results of the observations broke some existing stereotypes and required the revision of some fundamental aspects related to the formation of a new oceanic lithosphere, says Ivan Kulakov.