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 !

One thing that keeps on intriguing me is the 'smallness' of the human race in the larger scale of things. And when people (including me) try make our petty jelousies, agendas, issues magnified it seems so futile. Or perhaps because we are so small, we try to make 'something' permanent in this ephermal lifetime.

Why do i say we are 'small' ? Well for one, take the earth. Best estimates put it at 4.5 billion years old. This one little planet of ours, part of a single-star planetary system, which in turm is just one of the billion stars in our galaxy. And our galaxy is one of the billions of galaxies in the universe which is estimated to be around 15 billion years old.

And our earth itself. Lets take our puny little planet and see how it's surface (which we take for granted) has morphed since it's inception ? Currently i am reading an intersting book about the history of the earth and combining it with some bit of internet searching. The latest understanding is that our earth's crust follows a 'super-continent' cycle. Supercontinets, by definition are huge land-masses. The supercontinent cycle implies that supercontinent break up into smaller fragments and then again coalesce. This happens due to 'plate tectonics'.

But what could the mechanism for such movement be? A simplistic explanation is shown in the four drawings on right (see hyperlink). It is generally accepted that the crustal movement is caused by convection currents, originating from differences in temperature between spots in the liquid mantle. The Earth cools its interior by leaking heat to the outside. Where the crust is thin, such as underneath oceans, heat is lost more easily, resulting in a cooler area. But since continents are four times thicker, they also insulate better. A hotspot could appear underneath a large continental slab. As the solid mantle heats up, the coninent is bulged up and cracks. It allows a convection zone to form, pushing the broken halves apart and ending up as a spreading mid-ocean ridge. At some other place on the globe, continents ae pushed together again, and a new hot spot is formed, while the old hot spot shrinks. Continents break up again and reverse their travel.*


-First came the supercontinent Columbia (called such as the strongest evidence of it's existence is found in present-day Columbia) around 1.5 years ago. To put this in context, the earth is 4.5 billion years old. At that time, not all the continents as we know today were above the sea level but were parts of shallow seas.
-This first initial supercontinent broke up into fragments and 500 million years or so later new supercontinent, called Rodinia ('homeland' in Russian) was formed.
-This again broke up and a the most recent supercontinent Panagea was formed. This continent again broke up and formed two land-masses : Laurasia (Europe & North America) and Gondwanaland (Africa, India, Australia) from where our current continents and countries have formed.

This is really a humbling thought. To imagine that your country, the land you are standing on never even existed a mere few million years ago (a wink in cosmological time). It is a humbling thought that time conquers all. Though the speed of plate movement is about equal to a growing fingernail but given enough time, it creates the loftiest mountains (himalayas), e deepest trench (mariana) and the vast oceans (atlantic). And that these will also wither away in a few million years perhaps forming the next supercontinent. And so the story continues...

PS : This blog will continue with an overview of 'The Great Extinctions' the 'Major Earth Impacts'