Washington ? The most extensive extinction event in history, the die-off of about 90 percent of all species 250 million years ago, may have been caused by a massive flow of molten rock that covered an area half the size of Australia to a depth of more than a mile.

A study appearing today in the journal Science suggests the flood of molten rock that created a plateau known as the Siberian Traps in Russia was almost twice as big as previously believed and could have continued for thousands of years, changing the climate of the entire planet.

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A group of United Kingdom and Russian scientists say in Science that such an eruption of flood basalt would have filled the planet’s atmosphere with a choking concentration of sulfur dioxide, carbon dioxide and other gasses, making it difficult for any species to survive.

Samples from the lava flow have been age-dated at about 250 million years. Other studies have shown that during this period the Earth experienced its most extensive extinction crisis a die-off that killed at least 90 percent of ocean species and more than 70 percent of land creatures.

Called the Permian-Triassic extinction, it is a key event in the planet’s history. It was followed by the rise of the dinosaurs, the animal species that dominated the Earth until they too went extinct about 65 million years ago.

Some earlier studies suggested the Permian-Triassic extinction was caused by an asteroid wiping out much of life with a sudden, single blow.

But the evidence from the new study points toward a prolonged extinction event, stretching over hundreds of thousands of years.