المدير العام المدير العام
عدد المساهمات : 1713 السٌّمعَة : 1 تاريخ التسجيل : 26/08/2011
| | quaternary periodالعصر الجليدى | |
The Quaternary Period
The Quaternary is the most recent geological period of time in Earth’s history, spanning the last two million years and extending up to the present day. The Quaternary period is subdivided into the Pleistocene (“Ice Age”) and the Holocene (present warm interval) epochs, with the Pleistocene spanning most of the Quaternary and the Holocene covering the past 10 000 years. The Quaternary period is characterized by a series of large-scale environmental changes that have profoundly affected and shaped both landscapes and life on Earth. One of the most distinctive features of the Quaternary has been the periodic build-up of major continental ice sheets and mountain ice caps in many parts of the world during long lasting glacial stages, divided by warm episodes (interglacials) of shorter duration, when temperatures were similar to or higher than today. During long periods of these climatic cycles, perhaps 8/10th of the time, temperatures were cool or cold. The number of Quaternary interglacial-glacial cycles is probably in the order of 30-50.
Oxygen isotope record for the past 2,6 million years. Peaks
represent warm Earth, troughs a cold Earth
There have been shifts in the frequency of climate oscillations and amplitude of temperatures and glaciations through the Quaternary. At the onset of the Quaternary, many arctic areas were comparatively warm, with trees and bushes growing far north of the present treeline. Prior to about 800 000 years ago each interglacial-glacial cycle lasted for about 40 000 years, but after that the periodicity shifted to a prevailing rhythm of about 100 000 years. Prior to this shift in frequency there was a repeated build-up of relatively small-to-moderate sized ice sheets at high northern latitudes. After c. 800 000 years ago there occurred a major intensification of glaciations, with repeated growth of continental-scale ice sheets reaching mid-latitudes and with ice volumes much larger than during the earlier Quaternary glaciations. There have occurred 8-10 major glaciations during the past 800 000 years. Two of the largest Northern Hemisphere glaciations are the last one (called the Weichselian/Wisconsin glaciation, at its maximum about 20 000 years ago) and the one occurring prior to the last interglacial (called the Saalian/Illinoian glaciation, occurring prior to c. 130 000 years ago). During the peak of both glaciations, ice sheets covered extensive areas north of 40-50oN in both Eurasia and N America. The Saalian glaciation was particularly extensive in the high Eurasian north, covering vast areas of N Russia, coastal Arctic Ocean and Siberia | |
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الجمعة أكتوبر 07, 2011 6:19 pm من طرف المدير العام