From: Andy Soos, ENN
Published August 17, 2011 09:01 AM

Ancient Lava and Whence It Came

The Sylhet Traps lava flows of the Shillong Plateau in northeastern India lie some 340 miles to the east of the Rajmahal Traps at the bend of the Ganges River as it flows south to the Bay of Bengal. Almost 1,000 miles to the south is the 3,000 mile-long Ninetyeast Ridge rising a mile above the surrounding Indian Ocean floor, still beneath the seawater. To the east from the southern edge of this Ridge, some 1,600 miles away is the edge of western Australia. And finally, 2,500 miles to the southwest is the underwater Kerguelen Plateau, just off of Antarctica. Despite these vast distances, research by University of Rochester Geochemistry Professor Asish Basu shows great similarities in the chemical and isotopic signatures of lava rock samples from all these regions.

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Basu, whose findings are published in the journal Earth and Planetary Science Letters, says the samples all came from the same lava plume that seems to have broken apart the Gondwana supercontinent—which formed some 500 million years ago by the amalgamation of the continental landmasses of Antarctica, South America, Africa, Madagascar, Australia, the Arabian Peninsula, and the Indian subcontinent. Basu says his findings confirm the plume's role in breaking up this Gondwana supercontinent.

In paleogeography, was the southernmost of two supercontinents (the other being Laurasia) that formed part of the Pangaea supercontinent from approximately 510 to 200 million years ago (Mya). Gondwana is believed to have sutured to the Pangaea formation between ca. 570 and 510 Mya, joining East Gondwana to West Gondwana. It separated from Laurasia 200-180 Mya (the late Mesozoic era) during the breakup of Pangaea, drifting further south after the split.

Gondwana began to break up in the early-Jurassic (about 184 Mya) accompanied by massive eruptions of basalt lava, as East Gondwana, comprising Antarctica, Madagascar, India and Australia, began to separate from Africa. South America began to drift slowly westward from Africa as the South Atlantic Ocean opened, beginning about 130 Mya during the Early Cretaceous, and resulting in open marine conditions by 110 Mya. East Gondwana then began to separate about 120 Mya when India began to move northward.

The fragmenting of Gondwana started when three plumes—Kerguelen, Marion, and Reunion—heated the lithosphere (the earth's crust and upper mantle). The Kerguelen plume began as heated deep mantle rocks, which rose 1,800 miles from the core-mantle boundary and spread out near the Earth's surface. As the tectonic plates moved north, the head of the plume remained attached to the core by its stem, while a north-south plume-tail developed, tracking the northward movement of the Indian plate. Basu's work shows the plate also took with it part of the broken plume-head, which ended up in disjointed fragments at the foothills of the Himalayas, Rajmahal Hills and in the Shillong Plateau, as well as beneath the Bengal Basin, all in the northeastern part of India.

The rest of the plume-head can be found in Australia, Antarctica, and the Kerguelen Plateau. Basu's findings are based on the detailed chemical and isotopic analysis of lava rock samples from all the locations, which show similar geochemical concentrations of 25 trace elements, as well as isotopes of the elements strontium, neodymium, and lead.

"It's important to understand large regions of igneous rock formations—called large igneous provinces, because they often break apart continents and are sometimes associated with environmental catastrophes, like mass extinctions." said Basu.

Basu says the Ninetyeast Ridge, which lies in a north-south direction and points to the Sylhet and Rajmahal Traps, shows the direction of the plate movement that broke up the supercontinent into today's configuration.

For further information: http://www.rochester.edu/news/show.php?id=3898

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