Plumes of steam and smoke

Volcanoes are holes or cracks in the Earth's outer layer from which molten rock or magma - a mixture of liquid lava, solid materials and gases - escapes. Volcanoes are fickle. They can erupt with no warning. Sometimes they just ooze lava. This erratic behaviour makes life difficult for volcanologists, and it probably made life difficult for the dinosaurs 65 million years ago. A volcano suddenly became violent and started spewing plumes of steam and smoke. The sky became dusty and foul, and the fabled beasts choked to death.

Now the argument that a catastrophe wiped out the dinosaurs in the space of a few months or years has three different sides:

■ In the vanilla corner are scientists who believe that a rogue rock from outer space did for the dinosaurs.

■ In the chocolate corner are scientists who believe that a volcano did the job.

■ In the combination vanilla-chocolate corner are scientists who believe in a double whammy - shock waves from the impact of the extra-terrestrial rock immediately spread out beneath the planet's surface, triggering volcanic eruptions on the other side of the planet.

Not all explanations are that simple. 'The trail is littered with dead bodies, but there are few clues to how and why the victims died', laments the American geologist Charles Officer, one of the main supporters of the volcano theory. He argues that there were many active volcanoes 65 million years ago. More than a million cubic kilometres of lava erupted in little more than a few thousand years.

The cornerstone of the extra-terrestrial impact theory is the presence of large amounts of iridium in certain rocks. It is presumed that the iridium came from an asteroid or meteorite. But airborne particles from volcanic eruptions also contain large amounts of iridium, so high levels of iridium in impact craters are not evidence of an asteroid hitting Earth.

Gases from a volcanic eruption can also cause acid rain (from sulphur dioxide), greenhouse warming (from extra carbon dioxide) and depletion of the ozone layer (from chlorine). Sixty-five million years ago these effects would have happened on a much larger scale.

'Although it is difficult to see how an impact in, say, China, could kill and dry trees in Europe, recent major volcanic eruptions have shown that a single event can disrupt the world's climate zone', Officer says. He also cites the example of the eruption of a volcano in Indonesia in 1815, which injected so much sulphur dioxide into the upper atmosphere that it led to a worldwide cooling of the atmosphere in the following year. There were cold spells, frosts and crop failures in New England, and the period is still referred to as 'the year without summer'.

Scientists agree that the consequences of an asteroid impact and a massive volcano would be quite similar. The first effect would have been large amounts of dust (either from the crater or volcanic ash), which would darken the skies. The second effect would have been acid rains. These can be produced by nitric acid from chemical reactions caused by the impact, or by sulphuric acid produced by volcanic eruptions.

The idea of 'a second jolt from volcanoes' after the asteroid hit comes from the American geologist Jon Hagstrum. When the asteroid hit Chicxulub, the Earth acted like a giant mirror and the shock waves were focused at an area opposite the impact point. Taking continental drift into account, he estimates that what is India now was 1,600 kilometres or more away from where the focus point was 65 million years ago. There was indeed extensive volcanic activity in India at that time. The volcanoes produced huge lava flows which formed a series of plateau-like giant steps, known as the Deccan Traps (meaning southern stairs). The lava flow extends well over 10,000 square kilometres. It is estimated that the volume of lava is about a million cubic kilometres. Dinosaur eggs and pieces of bones and teeth have also been found in the Deccan Traps.

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