h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hi folks! I would like to give additional background information to this story. It was torture trying to constrain it to a thousand words!

The event I went to see, the Zanclean Flood, must be one of the most impressive displays of the force of Nature ever to have happened on this planet.

The Mediterranean Sea is a fairly recent phenomenon. It only came into existence as a result of the plate on which the continent of Africa lies, drifting northwards and colliding with the plate on which Europe and Asia lie. Before that, there was an ocean in between Africa and the northern continents called the Tethys Sea. Northward movement of Africa relative to Eurasia caused a narrowing of this sea until it was more of a seaway, a strait between the continents.

The shoving of Africa against Eurasia caused the earth's crust to buckle ... in North Africa the Atlas mountains formed, and in Europe the Alps and Pyrenees, and in between the crust buckled *downward*, to form the Mediterranean Basin.

That is how the Mediterranean Sea formed - and it's still long before our story. What happened next, is that the basin closed off on the west as well as the east - due to the uplift of land, *or* the lowering of the global sea level - or both. But the Mediterranean received no more inflow of water from the sea ... and the rivers feeding into it from Africa and Europe, were not enough to maintain it. It dried up ... the whole sea evaporated. It can happen, if enough time goes by ... and we're talking of hundreds of thousands of years here.

The drying up of the waters of the Mediterranean exposed the basin. The bottom of the sea today is over five kilometres deep. So, when the water dried up, this was the deepest depression on Earth. Without the weight of the water, the bottom might have lifted a bit, but would perhaps still have been over four kilometres below sea level.

At such a depth, air pressure would be almost double that at sea level, and even worse ... temperatures would be inhumanly high. That is a result of air warming up as it descends. Air blowing in from the high continental parts of Europe, Africa and Asia, would descend into the basin and produce unbearable heat.

So, I'm imagining the deep part of the basin as having been an intolerable desert. High evaporation rates would leave it fairly dry, even if some rains fall from moisture coming in from elsewhere. But complex animal life would probably have been absent. There might be some remnants of the sea that do not completely evaporate, like the 'salt lake' I described. On the dry land, there would likely be massive deposits of salt, gypsum, and other minerals left behind when huge amounts of seawater evaporated.

I do think simple plants could survive in the deep basin ... hence the salt bushes of this story. (Over here in South Africa I've seen plants with incredible heat tolerance... they grow on rocks that in summer becomes so hot you can't touch it with your bare hands and feet.)

But 'higher up' the climate would be more tolerable. There are many higher parts in the basin. What today are the large islands such as Crete, Cyprus, Sicily, Corsica, Sardinia, and the Balearic Islands, would in this period have been huge mountain massifs rising from the basin. Their tops and their slopes would have been habitable for a diversity of animals and plants. Some of the hardier mammals and plants would have ventured from there down into the hotter, deeper parts.

At some point, though, the sea flooded back into the basin.

In fact this did happen - and it happened several times! There were several cycles of the basin closing up, drying out, opening and getting flooded, closing up and drying out again.

The Zanclean Flood is simply the last and final flood that we know about, and it took place about 5.3 million years ago.

It was difficult for me to imagine how the flood would have looked. *How* did the water manage to come through what is today the straits of Gibraltar, and finally reach the basin and initiate the flood? I'm thinking, the first thing that was necessary was a valley between mountains ... on the south, the Atlas Mountains, on the north, the hills (the Betic Cordillera, today) of which the Rock of Gibraltar forms part. So, there was already a kind of pass between the two land masses there. Next, either a lowering of the land level in general, or a raising of the sea level, could actually allow the sea to flow into this 'valley'. The sea would flow like a river, seeking out the lowest portions, and cutting out a 'bed' for itself. The articles I've read say that this 'river' from the sea into the Mediterranean basin, might have been about 200 km long!

In all probability though, this was not a single event (I mean just in the Zanclean Flood case, not all those other earlier times when the basin flooded ... so each 'flood' also needed multiple smaller floods to get started). There might have been several cycles in which the ocean flowed into the strait region, *without* making it all the way to the basin. So, gradually, a riverbed was gouged out. Then at some final point, the channel was carved all the way to the basin. I am fairly sure that it would be a single flood event then. The basin represents a zone of very low gravitational potential (a 'hole' into which things 'want to' fall). When this is *reached*, the full force of the entire Ocean would be pouring down into this 'low-potential hole'. The riverbed would rapidly be scoured out deep enough to ensure that even at low tide, the sea level would still be higher than the opening of the river into the basin, meaning that the 'river' can keep on flowing continuously. (Tides at Gibraltar today differ by at most 3 ft).

The entire Mediterranean could then have been filled up in a single non-stop flood ... taking a few months, to a few years. But the volume of water pouring down into the basin would have completely dwarfed any waterfall or river that exists today. *Seeing* it would have been awe-inspiring.

With water levels raising by up to 10 m a day, some animals could have escaped the flood, but many might have become trapped on islands the tops of which would eventually have been below the new sea level.

During the times when the basin was dry, animals could reach what are islands today, following overland routes. Furthermore, animals could travel from Africa to Europe overland, and vice versa. Hence, animals like elephants, deer, and hippos, could reach the islands of Crete, Sicily, Sardinia and others. When the basin was filled by the sea, they were isolated from each other and from the mainland, and evolved into numerous different species. On most of the islands, the animals became smaller - due to less food being available on the islands. This is called 'insular dwarfism'. So there were numerous species of pygmy elephants and hippos found on the Mediterranean Islands. Antelopes were also able to spread from Africa to Europe and Asia, partly thanks to the drying up of the basin.