DEFINITION OF DECCAN FLOOD BASALTS, INDIA

The end of the CretaceousPeriod at 66.4 million years ago was marked by a global

mass extinction event made most famous by the death of the

dinosaurs. One of the major events on the planet that contributed

to the mass extinction event was the eruption of

massive quantities of mafic lava through fissures in western

India, forming the Deccan traps or flood basalt province. Latest

Cretaceous through Eocene basalts cover more than 315

square miles (510,000 km2) in western and central India,

reaching more than 1.2 miles (2,000 m) in thickness near

Mombai (Bombay). Estimates of the original size of the flood

basalt province range up to 600,000 square miles (1.5 million

km2), making this one of the largest volcanic provinces on

Earth. The volume of preserved basalts is roughly 12,275

cubic miles (512,000 km3), compared with 1 cubic kilometer

of material erupted during the 1980 Mount St. Helens eruption.

The basalt plateau is made of at least 48 separate flows

with some intervening pyroclastic layers, and locally tens to

hundreds of feet of interbedded sediments. Tholeiitic basalts

are by far the dominant lava type, along with minor alkaline

olivine basalt, picritic basalt, mugearite, and volcanic glass

(obsidian). Ultramafic to acidic plutons and a ring dike complex

are reportedly associated with the traps, and minor trachyte,

andesite, and granophyre are also reported. The

variations in the composition of the lavas can be explained by

fractional crystallization of a tholeiitic parent magma with

sinking and separation of early-formed heavy minerals such

as pyroxene.

See also CRETACEOUS; FLOOD BASALT; MASS EXTINCTIONS.

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