Deciphering the mysteries of an ancient seafloor goliath

The eruptions of 'supervolcanoes' on Earth's surface have been blamed for causing mass extinctions, belching large amounts of gases and particles into the atmosphere, and re-paving the ocean floor. The result? Loss of species, increased greenhouse gases in the atmosphere, and changes in ocean circulation. Despite their global impact, the origin and triggering mechanism of these eruptions remain poorly understood. New data collected during a recent Integrated Ocean Drilling Program (IODP) scientific research expedition in the Pacific Ocean may provide clues to unlocking this unsolved mystery in Earth's geologic record.

In fall 2009, an international team of scientists participating in IODP Expedition 324 'Shatsky Rise Formation,' drilled five sites in the ocean floor to study the origin of the 145 million-year-old Shatsky Rise volcanic mountain chain. Located approximately 1500 kilometres (930 miles) east of Japan, Shatsky Rise measures roughly the size of California. This underwater mountain chain represents one of the largest supervolcanoes in the world: the top of Shatsky Rise lies three and a half kilometres (about two miles) below the sea surface, while its base plunges to nearly six kilometres (four miles) below the surface. Shatsky Rise is composed of layers of hardened lava, with individual lava flows that are up to 23 metres (75 feet) thick.

'Seafloor supervolcanoes are characterised by the eruption of enormous volumes of lava. Studying their formation is critical to understanding the processes of volcanism and the movement of material from the Earth's interior to the surface,' remarked Dr William Sager of Texas A and M University, who led the expedition together with co-chief scientist Dr Takashi Sano of Japan's National Museum of Nature and Science in Tokyo.

About a dozen supervolcanoes exist on Earth - some are found on land, while others lie at the bottom of the ocean. Those found on the seafloor are often referred to as 'large oceanic plateaus.' Current scientific thinking suggests that these supervolcanoes were caused by eruptions occurring over a period of a few million years or less - a rapid pace in geologic time. Each of these supervolcanoes produced several million cubic kilometres of lava - about three hundred times the volume of all the Great Lakes combined - dwarfing the volume of lava produced by the biggest present-day volcanoes such as Hawaii.

Since the 1960s, geologists have debated the formation and origin of these large oceanic plateaus. The mystery lies in the origin of the magma, which is molten rock that forms within the Earth. A magma source rising from deep within the interior of the Earth has a different chemical composition than magma that forms just below the Earth's crust. Some large oceanic plateaus exhibit signs of a deep-mantle origin. Others exhibit chemical signatures indicative of magma originating from a much shallower depth.

The IODP Shatsky Rise expedition focused on deciphering the relationship between supervolcano formation and the boundaries of tectonic plates, which may prove crucial to understanding what triggers supervolcano formation. A widely-accepted explanation for oceanic plateaus is that they form when a huge blob of magma source (a 'plume head') rises from deep in 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,' but this mechanism is poorly understood. Shatsky Rise could play a key role in this debate, because it formed at a triple junction, but also displays certain 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,' Sager pointed out. 'What makes Shatsky Rise unique is the fact that it is 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. As Sager explained, 'We can use these magnetic stripes to decipher the timing of the eruption and the spatial relationship of Shatsky Rise to the surrounding tectonic plates and triple junctions.'

According to preliminary results, 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 further show that much of the lava erupted rapidly and that Shatsky Rise formed at or near the equator. As analyses continue in the months and years ahead, data collected during this expedition may help scientists to resolve the 50 year-old debate about the origin and nature of large oceanic plateaus.

IODP Expedition 324 'Shatsky Rise Formation' took place onboard the scientific ocean drilling vessel JOIDES Resolution from September 4 to November 4, 2009. The JOIDES Resolution is one of the primary research vessels of IODP, an international marine research program dedicated to advancing scientific understanding of the Earth through drilling, coring, and monitoring the subseafloor. The vessel is operated by the U.S. Implementing Organisation of IODP, consisting of the Consortium for Ocean Leadership, Texas A and M University, and Lamont-Doherty Earth Observatory of Columbia University.