The most recent mass extinction is perhaps the best known amongst the general public. It wiped out the dinosaurs, and there is something uncomfortably eerie about victims of that magnitude. It could easily be argued that the dinosaurs were the most successful animals ever to roam the planet. Their dominance was unchallanged for 140 million years. Then all of a sudden they all disappeared. Something must have killed them off. How could such a succussful group of animals fall victim to extinction?
Evidence seems to suggest it happened very rapidly, after an Asteroid impact 65 million years ago. Scientists have claimed the whole world burned. It was like an Old Testament version of hell where up to 80% of all species disappeared. Scientists have been discovering traces of the impact since the 1980s. So what evidence do we have?
In the 1980s Luis Alvarez discovered in the K-T (Cretaceous-Tertiary) boundary at numerous sites around the world a high concentration of an element called Iridium. This K-T boundary is a sharp and thin line and is seen in mines and rock outcrops around the world. Below the line there are lots of dinosaurs fossils. Above the line there are none. Iridium is only found in the Earth's mantle or in Asteroids, so such levels are extremely rare on the Earth's crust and must have come from outer space. Finding such levels at the very moment the dinosuars disappeared suggested that the Earth and an Asteroid had violently collided. There was so much Iridium that Scientist were able to estimate that the Asteroid must have been up to 10 kms in diamater!
The effects of an impact from such a colossal body would have been immense. Scientists have calculated that the shock wave generated would have destroyed all life instantaneously within a radius of hundreds of miles around the impact site.
They estimated that a fireball would have been created equivalent to 10 billion Hiroshma bombs. Sperules (small droplets of basalt) were found in the vincinity of the impact site, evidence that the fireball had vapourised billions of tonnes of rock, which was ejected into the air after impact. The sperules condensed in the upper atmosphere and fell back to Earth.
Also found in the boundary layer are grains of quartz that show evidence of high pressure impact.
High levels of soot were also discovered in the K-T boundary. It looked as if the forests and vegetation had ignited both from the fireball and the falling hot debris and fires raged all over the world.
Scientists also found a high concentration of fern spores. Ferns thrive when all other plants die. They flourish when all other plants are killed off by environmental disasters. So the predominance of fern spores - known as a fern spike - suggested something had
wiped out every plant on the planet.
So vast amounts of dust created by the impact must have blocked out the sun plunging the world into freezing darkness for many months or years. Any dinosaurs that escaped burning either froze or starved to death.
But the mammals were small and could burrow to escape the heat and the subsequent cold. When conditions recovered they would emerge to inherit the Earth.
To be certain the theory was right Scientists needed to find the crater. They estimated it to be about 200 kms in diamater. The sperule layer itself would give a general indication as to where it was, since the sperule layer would be thicker the closer they got to ground zero. So by analysing the sperule layer thickness at various locations around North and South America it was possible to predict that the impact site was somewhere in central America. Eventually it was discovered at Chicxulub (a Maya word), in Mexico's Yucatan peninsula, buried beneth about 1 km of sediment.
But new evidence casts doubt as to whether an impact that size could cause a mass extinction. Impact craters have been found all over the world indicating that the Earth had been hit many times, with little evidence of Mass Extinctions correlated to the times of impact. They seldom coincide with extinction events. And so it seems that impacts might not necessarly be able to cause mass Extinctions.
Also, there are ongoing debates regarding the charcoal levels and some Scientists think the charcoal levels found in the K-T boundary may not be at the high levels originally proposed, suggesting that fires didn't rage all over the planet.
Amphibians survived suggesting that little acid rain fell as had been widely predicted, or what fell was easily diluted and had little effect. Also there are doubts about the survival of Amphibians if the world was plunged into darkness for months.
New evidence suggests Dinosaurs had begun to go extinct 10-3 million years before the K-T boundary. Environmental conditions had changed. Also, as least 500,000 years before the K-T boundary the Deccan traps, in the western part of India, erupted. These mass lava eruptions may have played a leading role in the demise of the Dinosaurs and other species in the Cretaceous mass extinction. The volcanic greenhouse gasses could have initiated global warming and climate change extensive enough to account for the mass extinction. The Deccan traps eruption could also have been the source of the Iridium layer in the K-T boundary as Iridium is found in the mantle of the Earth. The sperules found in the K-T boundary could also have been a consequence of volcanic eruptions.
So it may be that Dinosaurs and other species were already dwindling in numbers, well before any impact. Dinosaurs might have gone extinct anyway if the asteroid missed. The impact probably finished them off. They were the victims of a combination of events; worsening environmental conditions, eruption of the Deccan traps, and an Asteroid impact. They were unfortunate to have all three happen around the same time. Where as we were fortunate, since the age of mammals was ushered in and this made possible our own existance.