Follow the blog as scientists investigate one of the world's largest supervolcanoes
Map right: the ship traveled nearly the width of the Unites States from Honolulu to reach Shatsky Rise.
Many scientists think that plateaus are formed from massive volcanic eruptions caused by the arrival at the surface of a huge blob of hot material—called a plume head—that has risen from deep in the mantle. Other scientists propose that such mountains result from shallower-source eruptions caused by processes at plate boundaries. Shatsky Rise is a unique place to test these hypotheses because ocean floor magnetic anomalies show that it formed at a confluence of three spreading ridge plate boundaries (a triple junction) during the late Jurassic (160 to 145 million years ago).
Will Sager on the flying bridge of the R/V Marcus G. Langseth with Honolulu and Diamond Head in the background Shatsky Rise has many features that support the theory of a plume head eruption, including apparently massive eruptions that built a supervolcano, Tamu Massif. This huge volcano has an area similar to that of the state of Arkansas or that of Olympus Mons (on Mars), the purported largest volcano in the solar system.
Although Shatsky Rise has been crossed by many oceanographic ship tracks in the past four decades, its geology is poorly known because it is so large and because most transiting ships did not have equipment to probe its interior. Scientists only know for the most part the sediment thickness, and little is known of the interior structure. Last fall, Integrated Ocean Drilling Program Expedition 324 cored Shatsky Rise, showing that Tamu Massif is characterized by massive lava flows, similar to those of flood basalt provinces (the greatest eruptions on Earth). The drilling results further imply that volcanism waned after the Tamu Massif eruption as Shatsky Rise formed other, smaller volcanoes.
This expedition is gathering geophysical data, especially seismic wave data, to show the interior structure of Shatsky Rise. Sailing on the U.S. multichannel seismic ship, R/V Marcus G. Langseth (Lamont-Doherty Earth Observatory), scientists will use seismic equipment similar to that used by industry to explore deep in the Earth for hydrocarbons. An array of 28 ocean bottom seismometers (OBS) will sit on the ocean bottom and record the seismic waves as air guns are shot at the surface. The resulting seismic refraction will probe the thickness, composition and density of the layers of Shatsky Rise. "With this technique, we believe we can trace the Moho discontinuity—the boundary between ocean crust and mantle—revealing the base of the crust," said Will Sager, co-principal investigator from Texas A&M University
Right: track of the ship from the first days of the cruise
Sager expects that the crust beneath Shatsky Rise to be roughly 30 km thick. "We will gather seismic images of the upper portions of the crust of Shatsky Rise, which will allow us to see structures such as layering, lava flow packages, and faults and will help us understand how this huge volcano formed."
Results from this cruise will add to the growing body of information that will help scientists better understand the relationship between supervolcano formations and the boundaries of tectonic plates, and the origin and nature of large oceanic plateaus.
Jinchang (Sam) Zhang working with seismic equipment
Funding agency NSF
Project Title Collaborative Research: Geophysical constraints on mechanisms of ocean plateau formation from Shatsky Rise, northwest Pacific
Principal Investigators Jun Korenaga (Yale Univ.), William Sager (Texas A&M Univ.), John Diebold (Lamont-Doherty Earth Observatory, Columbia Univ.)
Students Texas A&M University: Christopher Paul, Jinchang Zhang, Kai Gao, Danl Lewis, Caroline Brooks; Yale University: Duwayne Reiger, Tolu Olugboji
Professor, Chair in Ocean Drilling Science, Technology and Education