New data obtained during a recent Integrated Ocean Drilling Program (IODP) expedition in the Pacific Ocean may help explain the sea floor giants' origins.
In fall 2009, an international team of scientists participating in IODP Expedition 324 drilled five sites in the ocean floor. They studied the origin of the 145 million-year-old Shatsky Rise volcanic mountain chain. Located 1,500 kilometers (930 miles) east of Japan, Shatsky Rise measures roughly the size of California.
This underwater mountain chain is one of the largest supervolcanoes in the world: the top of Shatsky Rise lies three and a half kilometers (about two miles) below the sea's surface, while its base plunges to nearly six kilometers (four miles) beneath the surface.
Shatsky Rise is composed of layers of hardened lava, with individual lava flows that are up to 23 meters (75 feet) thick.
"Seafloor supervolcanoes are characterized by the eruption of enormous volumes of lava," says William Sager of Texas A&M University, who led the expedition with co-chief scientist Takashi Sano of Japan's National Museum of Nature and Science in Tokyo. "Studying their formation is critical to understanding the processes of volcanism, and the movement of material from Earth's interior to its surface."
Since the 1960s, geologists have debated the formation and origin of these large oceanic plateaus. The mystery lies in the origin of the magma, molten rock that forms within the Earth. Current scientific thinking suggests that these supervolcanoes were caused by eruptions over a period of a few million years or less--a rapid pace in geologic time.
A magma source rising from deep within the Earth has a different chemical composition than magma that forms just below Earth's crust. Some large oceanic plateaus show signs of a deep-mantle origin. Others exhibit chemical signatures indicative of magma from a much shallower depth.
The IODP Shatsky Rise expedition focused on deciphering the relationship between supervolcano formation and the boundaries of tectonic plates, crucial to understanding what triggers supervolcano formation.
A widely-accepted explanation for oceanic plateaus is that they form when magma in the form of a "plume head" rises from deep within the Earth to the surface. An alternative theory suggests that large oceanic plateaus can originate at the intersection of three tectonic plates, known as a "triple junction."
Shatsky Rise could play a key role in this debate, because it formed at a triple junction. However, it also displays characteristics that could be explained by the plume head model.
"Shatsky Rise is one of the best places in the world to study the origin of supervolcanoes," says Sager. "What makes Shatsky Rise unique is that it's the only supervolcano to have formed during a time when Earth's magnetic field reversed frequently."
This process creates "magnetic stripe" patterns in the seafloor. "We can use these magnetic stripes to decipher the timing of the eruption," says Sager, "and the spatial relationship of Shatsky Rise to the surrounding tectonic plates and triple junctions."
Sediments and microfossils collected during the expedition indicate that parts of the Shatsky Rise plateau were at one time at or above sea level, and formed an archipelago during the early Cretaceous period (about 145 million years ago).
Shipboard lab studies show that much of the lava erupted rapidly, and that Shatsky Rise formed at or near the equator.
As analyses continue, data collected during this expedition will help scientists resolve the 50 year-old debate about the origin and nature of large oceanic plateaus.