Scientifica

Humanity always ponders over it's extinction. (How will the human race end ?) To put things a little in context : humans are one of the millions of species on this planet. According to estimates, the average lifetime of a species is only about 4 million years. In general, at any given time, about 5% of all species are going extinct.

However, once in a while something happens which wipes out more than 80 % of the species living at that point in time. I was really amazed at this information. 80 % of the living species ! ! whoops...Till date there have been five such 'mass extinctions' and many other minor ones.
So what were these five mass extictions and when did they happen ? (Let me just say that this dying out does not happen in an instant. Though they may be spread over tens of thousands of years, they really are an instant in geological time.)

To get a better idea about these extictions - the time period, the affected species, the geological setting and the probable causes go here.

Of the above the Cretaceous -Tertiary (KT) is the most famous, allegedly wiping out the dianosours. Though the most devastating was the Permian extinction where around 95 % of the species were eliminated. It has also been suggested inconclusively that there is some periodicity to these extinctions.

The next question is what causes these extictions ? Well, so far scientists have been able to broadly identify two sources :
1. Catastrophic Agents- such as meteorite impacts and comet showers
2. Earth Agents- such as volcanism, glaciation, variations in sea level, global climatic changes, and changes in ocean levels of oxygen or salinity.

Let us just look at what could have happened to the mighty dianosaurs in their heyday. It is suggested that the earth was rammed by a rocky asteroid, some 6-15 kilometers in diameter, which burned through the earth's atmosphere and struck it with a force of 100 million (100,000,000) megatons. To put this in perspective, the Hiroshima A-bomb was a mere 0.015 megatons of TNT. As it struck the ground, most of its mass and an equal amount of the Earth turned into liquid and vapor. This mass, on the rebound swiftly flew out of the atmosphere and condensed into grains of dust.

On its way back, flying in ballistic trajectories and low orbits these were spread around the whole planet. When they came down, every one of those quadrillions of particles became a shooting star, and the sky lit up at an effective temperature of over 1000 K (about 1500°F), from horizon to horizon. The brightness stabbed downward for a quarter-hour, and when it ended, whatever could burn was in flames. The resulting smoke, and the fine dust from the impactor, together plunged the world into complete darkness for a period of months. The numerical simulation models suggest that by the end of that time, most of the world's land surface was near freezing.

The resulting crater (named Chicxulub) is somewhere between 145 -180 kilometers in diameter and is located on Mexico's Yucatan Peninsula.

And the order of nature has not changed. There can still be catastrophic agents, and mass extinctions will happen...It is a humbling thought for a foolish ape-species to think that we can control our enviornments. Oh the hubris of man !