The site of extensive volcanic activity and sea-floor spreading, the Galapagos Rise in the eastern equatorial Pacific Ocean has yielded groundbreaking research results for the field of plate tectonics. Jacqueline Floyd and her colleagues, all of Columbia University's Lamont-Doherty Earth Observatory, are introducing a new model for the process of mid-ocean ridge propagation (lengthening), which is responsible for seafloor spreading and the consequent formation of ocean basins. Their study is featured as this week's cover story in the scientific journal Science.
Using recently recorded earthquake data that had not been available to previous models, the researchers show that mid-ocean ridge propagation is preceded by a complex breakdown process and earthquake activity that allows the ridge to lengthen stably in the brittle crust of the ocean's lithosphere, independent of the enormous resisting forces proposed by previous models.
"Previous investigators idealized mid-ocean ridges as perfect cracks in the oceanic crust, but our results show that the seismicity and crustal structure around the ridge tip is more complex. This has critical implications for our ideas of how the crust rifts apart to form a new seafloor spreading center," said Floyd. "The hydroacoustic seismicity data were critical for making these observations since the magnitudes of the earthquakes in Hess Deep lie below the magnitude threshold of global teleseismic networks. The earthquake data show a concentration of earthquake activity at the tip of the Galapagos rise in Hess Deep that we almost immediately recognized as being similar to acoustic emission patterns observed at the tips of propagating cracks in the laboratory. The strikingly similar seismicity and faulting patterns allowed us to apply principles from fracture mechanics studies in the lab, at scales of centimeters or less, to the problem of mid-ocean ridge propagation in the oceanic crust, at the scales of 10s of kilometers and more."
By examining earthquake data and the topography of the Hess Deep rift, an enormous underwater canyon at the western tip of the Galapagos Rise, the researchers found that what makes an underwater ridge lengthen is more complex in comparison to previous theories. The researchers observe that small-magnitude earthquakes and micro cracking in a region called the process zone precede propagation, and are followed by nucleation of the rift axis and upwelling of magma, which leads to seafloor spreading.
Computer modeling of the stress field at the Hess Deep rift supports the authors' interpretation of the seismicity data as resulting from rifting at the tip of a crack-like mid-ocean ridge in the oceanic plate.
Hess Deep was an excellent place to test prevailing models of ridge propagation because it is a relatively simple structure, its tectonic history is well understood, it has minimal sediment cover, and a long-term record of seismicity was available.
This new understanding of mid-ocean ridge propagation in Hess Deep can now be applied to more complex rift settings in both the oceans and the continents.
Jacqueline Floyd is a Faculty Fellow and Ph.D. Candidate with the Department of Earth and Environmental Sciences and the Lamont-Doherty Earth Observatory, one of the world's leading research centers examining the planet from its core to its atmosphere, across every continent and every ocean. From global climate change to earthquakes, volcanoes, environmental hazards and beyond, Observatory scientists continue to provide the basic knowledge of Earth systems that must inform the future health and habitability of our planet. Her co-authors on the study are Maya Tolstoy, John Mutter, and Christopher Scholz, all of Columbia's Lamont-Doherty Earth Observatory.
The Lamont-Doherty Earth Observatory is a research unit of the Earth Institute at Columbia University, the world's pioneer academic center for mobilizing the sciences and public policy in pursuit of a sustainable future, especially for the world's poor. Its director is international economist Jeffrey D. Sachs. More than 800 scientists with strength in Earth science, ecology, health, social science or engineering are working together to reduce poverty, hunger, disease and environmental degradation. The Institute brings their creative knowledge to bear through teaching, research and outreach in dozens of countries around the world. In all it does, the Earth Institute remains mindful of the staggering disparities between rich and poor nations and the tremendous impact that global-scale problems – from the AIDS pandemic to climate change to extreme poverty in much of the developing world – will have on all nations. For more information, visit www.earth.columbia.edu.
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