Leg 183

Kerguelen Plateau and Broken Ridge reveal solid Earth dynamics and environmental consequences of massive volcanism

11 February 1999
Today, scientists from the Ocean Drilling Program (ODP) completed an expedition to one of the most remote places on Earth, the Kerguelen Plateau. This plateau is located in the southern Indian Ocean, is one-third the size of the United States, and is described as a large igneous province (LIP). LIPs are areas where magma wells up from deep beneath Earth's surface and forms molten rock. The major objectives of the expedition were to determine through drilling: when volcanism was active; how much of the plateau formed above sea level and how long portions of the feature remained above sea level; and if continental fragments form part of the plateau. The scientists met these objectives, and future shore-based research will be devoted to the recovered samples.

Using evidence from fossils as well as terrestrial plant remains, the scientific team constrained the time period during which the Kerguelen LIP formed. They found that the southern Kerguelen Plateau, only hundreds of kilometers from Antarctica, formed approximately 110 million years ago. To the north, the central Kerguelen Plateau and the once-contiguous Broken Ridge formed between 85 and 95 million years ago. In contrast, the northern Kerguelen Plateau is much younger, having formed less than 35 million years ago. These results indicate that several intense episodes of volcanism formed this large plateau LIP over a long time period, rather than from a single massive volcanic event.

"We found abundant evidence that much of the Kerguelen LIP formed above sea level," states co chief Dr. Mike Coffin of The University of Texas Institute of Geophysics. "Wood fragments, a seed, spores and pollen recovered in 90 million year old sediment of the central Kerguelen Plateau, just southeast of Heard Island, unambiguously indicates that this region was above sea level."

"A spectacular result was finding uniquely continental rocks in a conglomerate that was probably deposited in a river on Elan Bank, a western salient of the central and southern Kerguelen Plateau," explains co-chief Professor Fred Frey of the Massachusetts Institute of Technology. "Understanding how pieces of an ancient continent were incorporated into the oceanic environment of Elan Bank will have significant impact on our understanding of the approximately 130 million year old breakup among Australia, India and Antarctica."

A surprising finding at three drill sites was evidence for large-scale explosive volcanism, apparently a common phenomenon as volcanic construction of the plateau came to an end. Explosive volcanism can perturb the earth-atmosphere system significantly by injecting material into the stratosphere where, depending on the magnitude and altitude of the injection, it may persist for several years. This can cause considerable short-term environmental effects, such as those resulting from the recent eruption of Mount Pinatubo in the Philippines.

In contrast to explosive volcanism, the major type of volcanism that created most of the Kerguelen LIP, similar to that forming Hawaii and Iceland today, is generally viewed as environmentally benign. However, the scale of Kerguelen LIP volcanism lies beyond the realm of human experience. The 1783-84 eruption of Laki on Iceland, which produced a lava field only one percent of the size of a typical flood basalt flow in a LIP such as Kerguelen, provides some insight into the environmental effects of prolonged effusive-type volcanism. Laki's eruption over six months resulted in the deaths of 75 percent of Iceland's livestock and 25 percent of its population, and altered Europe's climate for several subsequent years. Given the large former land area of the Kerguelen LIP, both the prolonged effusive volcanism that formed most of the LIP's upper crust and the culminating explosive volcanism likely caused substantial and catastrophic short- and long term environmental effects during some of these time intervals.

Background on Kerguelen LIP

Vast volcanic eruptions between 110 and 85 million years ago created most of the Kerguelen large igneous province (LIP), a landmass one-third of Australia's size in the then-young Indian Ocean. Of a scale far greater than any volcanism in recorded history, these eruptions originated from a heat source deep within the Earth. Through time, as the tectonic plates carried the landmass away from the heat source, the crust of Kerguelen LIP cooled, contracted, and gradually subsided below sea level to depths of 1 to 2.5 km. Kerguelen Plateau and Broken Ridge are two components of the LIP that formed together, but were subsequently separated by seafloor spreading some 40 million years ago.

ODP

The international team of 45 scientists conducted their research aboard the world's largest scientific drilling vessel JOIDES Resolution. The drillship departed Fremantle, Australia on 13 Dec. 1998 and is scheduled to arrive in Fremantle on 11 Feb. 1999. Australia is home to the Australian Geological Survey Organisation (AGSO). AGSO, French, and United States scientists provided geophysical data used to select drill sites for this expedition.

The Ocean Drilling Program, an international partnership of scientific institutions and governments, explores the history and evolution of Earth's history. The Ocean Drilling Program is funded principally by the U.S. National Science Foundation, with substantial contributions from its international partners. These include the Federal Republic of Germany, Japan, the United Kingdom, the Australia/ Canada/ Chinese Taipei/ Korea Consortium for Ocean Drilling, the European Science Foundation Consortium for Ocean Drilling (Belgium, Denmark, Finland, Iceland, Italy, The Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and Turkey), France and the People's Republic of China. The program is managed by Joint Oceanographic Institutions, a consortium of 11 U.S. institutions, with Texas A&M University responsible for science operations. Lamont Doherty Earth Observatory is the operator for downhole logging.

Additional information can be found on the ODP web site.