CM – Mapping magnetic stripes to find out how quickly sea crust forms


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September 23, 2021

from University of Wyoming

Two University of Wyoming researchers are involved in the very first mapping of magnetic stripes – one of the foundations of plate tectonics – in the lower Gabbroic section of the rapidly expanding oceanic crust.

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The group may have solved a 30-year-old question in the scientific debate: At what speed does the rapidly expanding oceanic crust form?

“That has never happened before. The magnetic strips document how the earth’s magnetic field changes over time and, in particular, how the earth’s magnetic field flips or reverses when the magnetic north pole becomes the magnetic south pole and vice versa. “Says Michael Cheadle, professor in the UW Department of Geology and Geophysics. “This mapping in the third dimension is exciting in itself because the magnetic stripes, first discovered in the early 1960s, provided some of the most important pieces of evidence for plate tectonics theory – the theory that explains how and why we mountain ranges, Preserve ocean basins, volcanoes and earthquakes. « 

Cheadle is the third author of a new study highlighted in an article titled » Three-Dimensional Magnetic Stripes Require Slow Cooling in Fast-Spread Lower Ocean Crust, « which is published today in Nature, an international weekly science journal.

Cheadle and Barbara John, a UW professor of geology and geophysics, the fourth author of the article, and Jeff Gee, a professor of earth sciences at the Scripps Institution of Oceanography University of California-San Diego, designed the experiment and performed samples and data collection. Sarah Maher, Ph.D. student at the Scripps Institution of Oce anography, is the main author of the work. You and Gee have completed the data processing and analysis.

The manuscript addresses the question of how the rapidly expanding ocean crust – which makes up 40 percent of the oceanic crust and thus 25 percent of the earth’s surface – is changing through novel applications of the Crust magnetization cools and forms. The shape of the magnetic stripes in the third dimension shows that the ocean crust is actually cooling very slowly.

« So we just severely restricted the shape of the earth’s crust, » says Cheadle of the results of the study.

Cheadle, John and Gee were the three lead investigators on the 2017 cruise to Pito Deep. Located near Easter Island, Pito Deep is a major divide that is one of the few places in the Pacific Ocean that can be cross-sectioned of the lower oceanic one Crust. Pito Deep is about 3.5 kilometers deep, which is about twice the depth of the Grand Canyon.

Ocean crust occurs on mid-ocean ridges and is formed by the freezing and crystallization of magma that is created by melting the earth’s mantle. This magma has a temperature of 1,200 degrees Celsius when it first escapes from the mantle before it cools down and solidifies into rock. When it cools below 580 degrees Celsius, it becomes magnetized and captures a record of the orientation of the Earth’s magnetic field at that point in time. As a result, it records the periodic reversals or reversals of polarity of the Earth’s magnetic field as the north magnetic pole becomes the south magnetic pole and vice versa. These polarity reversals lead to the normally magnetized and reverse magnetic stripes of the sea crust.

« The magnetic stripes can be viewed as a tape recording of the history of the Earth’s magnetic field, » says Cheadle. « And the pattern of this tape recording shows that the rapidly expanding oceanic crust must have cooled very slowly. » To do this, the group used Sentry, an autonomous submarine, to map the seabed magnetization of gabbro rocks over two 8 to 10 kilometer long regions and took direct measurements of the magnetic polarity of more than 200 oriented samples, that of Jason II remote controlled submarine. Gabbroic rock is the frozen magma that forms a magma chamber beneath volcanoes that erupts lava on the ocean floor.

The ocean crust preserves changes in magnetic field polarity and intensity as it cools due to its lock-in or blocking temperature. This happens either instantaneously, as in the lava section, which cools down like a flash when it erupts; or slower in the lower gabbro section. The geometry of the boundaries recorded between normally and inversely magnetized rocks in the crustal cross-section thus reflect the past cooling history of the ocean crust.

“First, we propose that our cooling model corresponds to faults that are 100 to 200 meters apart, 8 to 10 kilometers outside the axis, which allows for deep hydrothermal circulation, ”says Cheadle. « If this is correct, it means that there is a significant area of ​​relatively unexplored, likely diffuse hydrothermal circulation occurring about 10 kilometers off-axis on rapidly expanding ridges.

 » Second, our results suggest that only shallow earthquakes would occur within 8 to 10 kilometers of the axis of propagation, ”he continues. « Our results have far-reaching implications for several areas of geosciences, including the formation of the earth’s crust, ocean fluid flow, geochemistry and seismology. »

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Similar title :
researchers play an important role in studying the mapping of magnetic stripes, how quickly ocean crust forms
mapping of magnetic stripes, to find out how quickly the sea crust is formed


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