The Sentry, an autonomous underwater vehicle, was used for mapping the magnetic stripes of Pito Deep. This large chasm is located in the Pacific Ocean and provides a cross-sectional view of samples of the lower oceanic crust. Barbara John and Michael Cheadle, both UW professors in geophysics and geology, contributed to the paper Three-Dimensional Magnet Stripes Require Slow cooling in Fast-Spread Low Ocean Crust, which was published in Nature on Sept. 23rd. Credit: Michael Cheadle
Two University of Wyoming researchers were part of the first ever mapping of magnetic stripes, one of the foundations of platetectonics within the lower gabbroic section of rapidly-spreading oceanic crumb.
The group may have solved a 30-year-old scientific question: How fast does oceanic crust form in fast-spreading oceanic regions?
This has never been done before. "Magnetic stripes are a record of the Earth's magnet field changing through time and, in particular how the Earth’s magnetic field flips/reverses when the magnetic North Pole becomes magnetic South Pole and vice-versa," Michael Cheadle, a professor at the UW Department of Geology and Geophysics. This mapping in the third dimension is fascinating in its own right because the magnetic stripes were first discovered in early 1960s. They provide key evidence for plate tectonics theory, which explains how and why mountain ranges, ocean basins, and volcanoes occur.
Cheadle is the third author in a new study. This is highlighted in a paper entitled "Three Dimensional Magnetic Stripes Requires Slow Cooling In Fast-Spread Low Ocean Crust", published in Nature, an international weekly journal.
Cheadle and Barbara John (a UW professor of geophysics and geology), were the fourth authors of the paper. Jeff Gee, a professor of Geosciences at the University of California, San Diego's Scripps Institute of Oceanography, also designed and carried out data and sample collection. Sarah Maher is a Scripps Institution of Oceanography Ph.D. student. Gee and she performed the data processing, analysis and reporting.
This manuscript addresses the question of how fast ocean crust, which accounts for 40% of oceanic crust, and therefore 25 percent of Earth's surface, cools and forms by novel applications of crustal magneticization. The third dimension magnetic stripes show that the ocean crust cools very slowly.
Cheadle said that the study's findings had placed a significant constraint on how half of Earth's crust forms.
John, Cheadle and Gee were the principal investigators during the 2017 cruise to Pito Deep. Pito Deep, located near Easter Island is a large chasm that allows sampling of the lower oceanic crust. Pito Deep measures approximately 3.5 km deep, which is twice as deep as the Grand Canyon.
Jason II, a remote-operated robot submarine connected to the ship by cable, was used for collecting rock samples from the seafloor. Credit: Lucas Kavanagh
Midocean ridges are where ocean crust forms. It is formed by the freezing and crystallization magma. This is made by melting of Earth's mantle. The temperature of the magma is 1,200 degrees Celsius when it is first released from the mantle. It then cools to form rock. It becomes magnetic as it cools to below 580 degrees Celsius. This makes it trap a record of Earth's current orientation. It records periodic flips and reversals in the polarity Earth's magnet field. This is when the magnetic North Pole becomes magnetic South Pole and vice versa. These polarity reverses lead to the normal magnetized and reversely magnetized stripes of ocean crust.
Cheadle states that the magnetic stripes could be considered a tape recording of Earth's history in terms of its magnetic field. Cheadle says that the tape recording's pattern shows that the fast-spread oceanic crust had cooled very slowly.
The subhorizontal magnetic stripe, or polarity border, was found by the research team and extends more than 8 km from the paleospreading axis. Sentry, an autonomous submarine used by the research team, was used to map the subsurface magnetization of gabbroic rocks over two 8-to-10-kilometer-long regions. Jason II, a remote-operated submarine, took direct measurements of the magnetic pole of more than 200 samples. Gabbroic rock refers to the magma below volcanoes that forms a magma cavity, which then erupts into lava on the ocean floor.
As it cools, the ocean crust retains changes in the magnetic field polarity or intensity. This can happen instantly in the lava section which cools immediately upon eruption, or slower in the deeper gabbro sections. This is because the crustal cross-section records the boundaries between normal magnetized and reversely magnetized rocks. It reflects the ocean crust's past cooling history.
Cheadle states that the research has led to two important and testable predictions based on the study's findings.
Cheadle states, "First, our cooling model is consistent avec 100- to 200-meter off-axis faults that occur 8-10 kilometers from axis allowing for deep hydrothermal circulatory," If correct, this would indicate that there is significant hydrothermal circulation, probably diffuse and relatively unexplored, approximately 10 km off the axis at fast spreading ridges.
He continues, "Secondly, our results indicate only shallow earthquakes will occur within 8-10 kilometers of the spreading-axis." "Our results have wide implications across many fields of geoscience including formation of the Earth’s crust, fluid flow within oceans, geochemistry, and seismology."
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Additional information: Sarah M. Maher and colleagues, Three-dimensional magnetic strips require slow cooling in fast–spread lower ocean crust. Nature (2021). Information from Nature Sarah M. Maher and colleagues, Three-dimensional magnetic strips require slow cooling in fast–spread lower ocean crust (2021). DOI: 10.1038/s41586-021-03831-6
