Ice Age
Ice Age
An ice age is a period of long-term reduction in the temperature of Earth’s climate, resulting in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers (“glaciation”). Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age (because the Greenland and Antarctic ice sheets still exist). More colloquially, when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and Eurasian continents: in this sense, the last ice age ended about 10,000 years ago. This article will use the term ice age in the former, glaciological, sense; and use the term glacial periods for colder periods during ice ages and interglacial for the warmer periods.
Many glacial periods have occurred during the last few million years, initially at 40,000-year frequency but more recently at 100,000-year frequencies. These are the best studied. There have been four major ice ages in the further past.
Evidence for ice ages
There are three main types of evidence for ice ages: geological, chemical, and paleontological. Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics. Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. It took some time for the current theory to be worked out.
The chemical evidence mainly consists of variations in the ratios of isotopes in sedimentary rocks, ocean sediment cores, and for the most recent glacial periods, ice cores. This evidence is also difficult to interpret since other factors can change isotope ratios. For example a major mass extinction increases the proportion of lighter isotopes in sediments and ice because biological processes preferentially use lighter isotopes and a reduction in biological processes makes larger quantities of lighter isotopes available for deposition.
The paleontological evidence consists of changes in the geographical distribution of fossils - during a glacial period cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or are squeezed into lower latitudes. This evidence is also difficult to interpret because it requires: (1) sequences of sediments which cover a long time-span and wide range of latitudes and are easily correlated, (2) ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed, and (3) the finding of the relevant fossils, which requires a lot of luck.
Despite the difficulties, analyses of ice cores and ocean sediment cores clearly show the record of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available.
Major ice ages
There have been at least four major ice ages in the Earth’s past. The earliest hypothesized ice age is believed to have occurred around 2.7 to 2.3 billion years ago during the early Proterozoic Eon.
The earliest well-documented ice age, and probably the most severe of the last 1 billion years, occurred from 800 to 600 million years ago (the Cryogenian period) and may have produced a Snowball Earth in which permanent sea ice extended to or very near the equator. It has been suggested that the end of this ice age was responsible for the subsequent Cambrian Explosion, though this theory is recent and controversial.
A minor ice age occurred from 460 to 430 million years ago, during the Late Ordovician Period. There were extensive polar ice caps at intervals from 350 to 260 million years ago, during the Carboniferous and early Permian Periods, associated with the Karoo Ice Age. The Industrial Revolution and consumption of fossil fuels have accelerated conditions that precursor an Ice Age and erratic climate conditions. These cycles are measured in thousands to tens of thousands of years.
Information Courtesy of Wikipedia, Visual References Courtesy of NOAA
