Thursday, August 11, 2005
Are volcanoes responsible for the extinction of dinosaurs?
At a scientific conference on Wednesday, a French researcher presented a study of Indian lava fields that suggested ancient volcanic activity was intense enough to have caused the climate changes believed to lie behind the extinction of the giant lizards.
“We can see that due to the surface degassing of the lava flow we have a potential impact on climate,” said Anne-Lise Chenet of the Institut de Physique du Globe de Paris.
Some scientists theorize that dinosaurs died out about 65 million years ago — along with more than 70 per cent of all species on Earth — as a result of climate change caused by the impact of a massive asteroid crashing into our planet.
That impact would have sent trillions of tonnes of dust into the air and sparked huge fires. That combination of dust and soot — some estimates suggest 25 per cent of earth's vegetation burned — would have lowered temperatures and acidified rainfall around the globe, destroying dinosaur habitat.
An undersea crater off Mexico's Yucatan coast is thought to be the impact site.
But other scientists believe earth's climate changed as a result of gases belched into the air from an upswing in volcanic activity.
Eruptions release both carbon dioxide, a major greenhouse gas, and sulphur dioxide, a source of acid rain. They also load the atmosphere with fine dust and soot.
Pro-volcano researchers have long pointed to the Deccan Trap in India, a vast area of volcanic rock thought to have originally contained up to three million cubic kilometres of ancient lava. That many eruptions could have released enough gas to be climate-altering — if they happened in rapid enough succession.
According to Dr. Chenet, that's exactly what happened.
“We found that the volcanism duration is certainly shorter than it was estimated before,” she said.
“A succession of different eruptions could lead to climate disequilibrium. If we have no time between two eruptions, we have no time to re-find climatic system balance.”
Dr. Chenet and her team dated the ancient lava flows by taking advantage of how the earth's magnetic field has shifted over time.
Lava contains magnetic minerals that align themselves with the earth's magnetic field before the lava cools. By comparing that alignment with what is known about how the magnetic field has shifted, scientists can date the flow.
Using that method, Dr. Chenet estimates that a layer of lava 600 metres thick may have piled up in as little as 30,000 years, and that the entire volcanic episode lasted about a million years.
“Volcanism might be a key player in mass extinctions,” Dr. Chenet said.
As well, right in the middle of the lava, Dr. Chenet also found a layer of iridium thought to have come from the Yucatan asteroid, which would prove the volcanoes predated it.
“Our view is that impact added to the stress already generated by an ongoing massive eruption, enhancing significantly the extent of the extinction — which would have taken place even if the impact had not occurred.”
Dr. Chenet's next task will be to calculate the amount of gas and sulphur compounds that would have been emitted during the period of the eruptions. Scientists will then attempt to estimate the changes those gases might have caused.
Dr. Chenet points out that over the last 300 million years, all mass extinctions have coincided with major volcanic activity.
The extinction that killed off the dinosaurs, known as the Cretaceous-Tertiary mass extinction, is the largest the earth has ever known. All land animals, everywhere on earth, that were larger than 25 kilograms were killed off.