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IN THE REMOTE southern Indian Ocean, straddling the Antarctic Polar Front, lies a mostly submarine plateau one-third the size of the contiguous United States. The Kerguelen Plateau is one example of a unique type of Earth feature, a large igneous province (LIP). One of the least understood features in the ocean basins, LIPs are believed to be the surface manifestations of massive pulses of volcanism that originated deep within Earth's mantle in association with narrow upwelling systems, known as mantle plumes or hotspots. The episodic nature of LIP eruptions documented in the geologic record, especially between 150 and 50 million years ago, is evidence that a more dynamic, unpredictable mode of mantle circulation, very different from that driving present-day plate motions, existed during Earth's past. LIPs preserve a record of mantle dynamics and may have also affected the Earth's environment in the past by potentially altering ocean circulation, climate conditions and sea level.

ODP LEG 183 (December 1998 to February 1999). Despite the challenging winds and seas of the Kerguelen "Triangle," where two Vendée Globe round-the-world racing yachts capsized in 1997, the Ocean Drilling Program (ODP) mounted an expedition to study the Kerguelen Plateau and its now separate, conjugate feature, Broken Ridge.

MIKE COFFIN of The University of Texas Institute for Geophysics (UTIG) and Fred Frey of the Massachusetts Institute of Technology led the scientific team of 28 scientists representing eight countries to study the plateau's eruption history by analyzing samples of sediment and lava collected from deep beneath the seafloor. Since its formation the plateau has subsided to great water depths (1 to 2.5 km). Prior to Leg 183, Mike Coffin and other UTIG researchers participated in three successful geophysical surveys on the Kerguelen Plateau undertaken by Australia (1997) and France (1998). The new site survey data from these cruises set the stage for ODP Leg 183 to the remote and turbulent waters of the southern Indian Ocean.

THE EARLIEST known volcanism associated with the Kerguelen hot spot began roughly 130 million years ago when Africa, Antarctica, Australia, India, and Madagascar were just beginning to break apart, thus creating the Indian Ocean. Changing tectonic plate motions over the ensuing eons left a continuous record of the hot spot's eruptive history, with peak volcanic events creating much of the Kerguelen Plateau and Broken Ridge. Leg 183 recovered volcanic rocks and interbedded sediments, which revealed two significant pulses of volcanism during Cretaceous time at about 110 and 85-95 million years. UTIG's PLATES software was used to generate this reconstruction of regional tectonic plates at 98 million years during Cretaceous time. The Kerguelen LIP is shown in red. Subsequent plate motions over the Kerguelen plume resulted in the formation of the Ninetyeast Ridge (~82-38 million years) and the northern Kerguelen Plateau (~35 million years). The Kerguelen hot spot continues to erupt today at Heard and McDonald islands, albeit at rates much lower than those of more than 80 million years ago.

RESULTS. Surprising evidence recovered on Leg 183 revealed that the Kerguelen plateau had existed as a large landmass above sea level at three different times during an 80 million year period, before finally become submerged about 20 million years ago. The evidence consists of dark brown sediment overlying subaerially erupted lava flows, charcoal and wood fragments in sediments overlying igneous rocks, and conglomerates, which formed from sediments deposited in river beds, interlayered with lava flows. The final stage of volcanism forming the Kerguelen LIP produced magmas rich with gases such as carbon dioxide, oxides of sulfur, and water vapor, which may have caused global environmental change.

EDUCATION AND TRAINING. The Ocean Drilling Program provides opportunities for graduate students from participating countries to sail as part of the science party on ODP legs.

HISTORICAL BACKGROUND. During the late 1700s, many explorers were searching for a legendary large southern continent they called Terra Australis, which they believed to exist and which they were anxious to claim and colonize for the country for which they sailed.

Yves Joseph de Kerguélen-Trémarac discovered La France Australe (today's Kerguelen chain in the Indian Ocean just north of Antarctica) in 1772, and in the same year claimed this "southern motherland" for France. Upon returning home, his eloquent rhapsodizing to the reigning French monarch about Kerguelen's superb agricultural and mineral potential resulted in the king dispatching three ships and 700 men to colonize Kerguelen in 1773. However, 699 pairs of eyes couldn't lie. They saw only the tiny volcanic islands but no massive continent as Kerguelen had claimed. Indeed, they had no way of knowing then that the volcanic islands they saw were the tips of a small continent which lay submerged about two kilometers below the ocrean surface. Upon Kerguélen's second homecoming to France, he was court-martialed, sentenced to 20 years in prison, and dismissed from the Navy for exaggerating his findings.

Another explorer, Captain James Cook, later set sail, also in search of Terra Australis. Cook discovered a large landmass, which he named Australia, but not the legendary southern continent. Terra Australis was eventually discovered in the 19th century. It was located farther south and its present-day name is Antarctica./

Footnote: Captain James Cook also landed at La France Australe on Christmas Eve 1776. After one of his men found a bottle containing a parchment inscribed in Latin telling of Kerguélen's visits, he named the locale the "Isles of Desolation."

For information about Leg 183 to the Kerguelen Plateau, as well as other LIPs, please contact Mike Coffin: mikec@ig.utexas.edu