notes-5 - Glaciation and surficial deposits E1) Past...

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Glaciation and surficial deposits E1) Past glaciations For most of the last 3.5 billion years of the earth ’s history (and possibly for a quite a bit longer) the temperature has stayed at a level that is moderate and suitable for life to flourish. The present global mean annual temperature (MAT) is 15° C, which is probably a few degrees cooler than the average MAT over geological time. We have evidence that past temperatures have been as much as 10° C warmer than this for extended periods (tens to hundreds of millions of years). There is also abundant evidence that it has been colder for shorter periods (millions of years) and that the earth has been glaciated to varying degrees many times in the past. The most recent glacial period, known as the Pleistocene glaciation, has lasted for about the past 2 m.y., and while we are not in a deep glaciation right now, we are still within this glacial period and it is quite likely that more intense glacial conditions will return within the next few tens of thousands of years. (The Pleistocene glaciation is covered in greater depth below.) As we ’ll discuss later, in the context of Plate Tectonics, there was a significant glaciation during the Permian and Carboniferous. At that time most of the earth’s land masses were all part of one continent (known as Pangea) situated near to the South Pole, and large parts of Africa, South America, India, Australia and Antarctica were glaciated. This appears to be one of the most enduring glaciations in the geological record, lasting as long as about 40 m.y. There was also a less extensive and less well understood glaciation in the latter part of the Devonian. Again, most of the evidence for glaciation at this time is in rocks of the land masses that were south of the equator at the time. It appears that another glacial event took place near to the end of the Ordovician. In addition to these Phanerozoic glaciations, geological evidence from ancient rocks show that some of the most intense glaciations took place in pre-Cambrian times at 635 m.y. and at 750 m.y., and also at 2200 m.y. ago. These glaciations, accompanied by global MATs as cold as -50° C, appear to have been so intense that the entire oceans froze over a so- called ―snowball earth‖— for millions of years. It is proposed that the conditions that allowed such intense glaciation to develop included: a concentration of landmass near to the equator, continental breakup that would have led to enhanced weathering of rocks and hence consumption of atmospheric CO 2 , and a powerful positive feedback effect as the build-up of ice led to increased albedo (reflectivity) of the earth, which led to more cooling and more ice and so on.
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Vancouver Island University Geology 111 Discovering Planet Earth Steven Earle 2010 2 There is abundant geological evidence for snowball earth episodes. In numerous locations rocks have been found that show that glaciation existed in equatorial regions at sea level
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notes-5 - Glaciation and surficial deposits E1) Past...

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