Miocene: The World Assumes a Modern Configuration


20 million years ago, Antarctica was covered by ice and the northern continents were cooling rapidly. The world has taken on a "modern" look, but notice that Florida and parts of Asia were flooded by the sea.

The Miocene was a time of warmer global climates than those in the preceeding Oligocene, or the following Pliocene. It is particularly notable in that two major ecosystems first appeared at this time: kelp forests and grasslands. The expansion of grasslands is correlated to a drying of continental interiors as the global climate first warmed and then cooled.

Global circulation patterns changed as Antarctica became isolated and the circum-polar ocean circulation became established. This reduced significantly the mixing or warmer tropical water and cold polar water, and permitted the buildup of the Antarctic polar cap. Likewise, the African-Arabian plate joined to Asia, closing the seaway which had previously separated Africa from Asia, and a number of migrations of animals brought these two faunas into contact.

The collision of India with Asia is just one of a series of continental collisions that has all but closed the ocean great Tethys Ocean. From east to west these continent-continent collisions are: Spain with France forming the Pyrenees mountains, Italy with France and Switzerland forming the Alps, Greece and Turkey with the Balkan States forming the Hellenide and Dinaride mountains, Arabia with Iran forming the Zagros mountains, India with Asia, and finally the youngest collision, Australia with Indonesia.

This phase of continental collision has raised high mountains by horizontally compressing the continental lithosphere. Though the continents occupy the same volume, their area has decreased slightly. Consequently, on a global scale, the area of the ocean basins has increased slightly during the Cenozoic, at the expense of the continents. Because the ocean basins are larger, they can hold more water. As a result, sea level has fallen during the last 66 million years. In general, sea level is lower during times of continental collision (early Devonian, Late Carboniferous, Permian, and Triassic).

mioceneDuring times of low sea level the continents are emergent, land faunas flourish, migration routes between continents open up, the climate becomes more seasonal, and probably most importantly, the global climate tends to cool off. This is largely because land tends to reflect the Sun's energy back to space, while the oceans absorb the Sun's energy. Also, land masses permit the growth of permanent ice sheets, which because they are white reflect even more energy back to space. The formation of ice on the continents, of course, lowers sea level even further, which results in more land, which cools the Earth, forming more ice, and so on, and so on. The lesson here is: once the Earth begins to cool (or warm-up) positive feed-back mechanisms push the Earth's climate system to greater and greater cooling (or heating). During the last half of the Cenozoic the Earth began to cool off. Ice sheets formed first on Antarctica and then spread to the northern hemisphere. For the last 5 million years the Earth has been in a major Ice Age. There have been only a few times in Earth's history when it has been as cold as it has been during the last 5 million years.

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