Hi Dave. Let me see if I have this straight. You said “There are scientists who can pinpoint an asteroid impact for the beginning of each one. Generally the ice ages last about 200 million years.” What your saying is that an asteroid caused each ice age. Is that correct.

I only know of one such ice age that an asteroid most certainly played a part in the ice age about 65 million years ago. The Deccan basalts may have played a role in the extinction of the dinosaurs. Most of the basalt was erupted between 65 and 60 million years ago. Gases released by the eruption may have changed the global climate and lead to the demise of the dinosaurs 65 million years ago. Take Care…Don in creepy town

Although scientists cannot answer this question with certainty, they know that a number of factors interact to produce conditions favoring the formation of ice sheets. Some of these factors include
changing continental positions
uplift of continental blocks
reduction of CO2 in the atmosphere
changes in the Earth's orbit

Long ice age intervals did not just suddenly occur. Instead, they seem to have been the culmination of even longer periods of worldwide climatic cooling. This cooling took place for tens of millions of years before the beginning of glaciation.
Once ice sheets start to grow, they probably contribute to their own further development. This positive feedback occurs because ice sheets reflect more sunlight back into space than does ground not covered by ice. The reflected sunlight would otherwise warm the Earth's surface. Consequently, the presence of ice sheets may lead to more cooling and continued development of ice sheets.

Changing continental positions
Plate tectonics is an important process influencing when ice ages occur, and the position of the continents is probably one of the most important factors controlling long periods of multiple glaciations. The presence of large landmasses at high latitude appears to be a prerequisite for the development of extensive ice sheets, because the large accumulations of ice associated with ice sheets cannot form over the ocean.

During the current ice age, which began slightly less than 3 million years ago, several large landmasses have been at high latitude. These include Antarctica, much of North America and much of Eurasia. This continental configuration led to extensive glaciation of both North America and Eurasia.

During the ice age that occurred in the Pennsylvanian and Permian, the southern portion of the super continent Pangea was at the South Pole. The result was extensive glaciation of what are now Africa, South America, India, Antarctica, Australia, and the Arabian Peninsula.

The position of the continents during the Late Proterozoic glaciation (around 700 million years ago) is not well known. Evidence of glaciers exists from North America, Australia, and Africa.

Uplift of continental blocks
Plate tectonics probably contribute to the development of long periods with many glaciations in a second, more subtle way. Plate movements sometimes cause uplift of large continental blocks. Major uplift can cause profound changes in the global oceanic and atmospheric circulation patterns. Changing circulation patterns cause climate change. Some scientists hypothesize that climatic changes cause by uplift are critical to the development of ice ages.

Over the past 15 million years, the continents have risen about 600 meters (2000 feet) on average. The uplift of the Himalayas and the Tibetan Plateau probably contributed to the initiation of the current cool period.

Similar tectonic uplift appears to have been involved in the three other long, ice age intervals.

Reduction of CO2 in the atmosphere
A general reduction in amount of CO2 in the atmosphere may contribute to the development of ice ages. Carbon dioxide is an important greenhouse gas. Decreases in the amount of CO2 in the atmosphere may lead to global cooling.

Many processes can cause a long-term decrease in the amount of CO2 in the atmosphere. These processes include many complex interactions among organisms, ocean currents, erosion, and volcanism. Important relationships exist between ice ages and the composition of the atmosphere; however, many scientists are unsure whether the changes in atmosphere cause cool periods or whether cool periods cause atmospheric changes. Also, many scientists are not sure the magnitude of past CO2 changes was large enough to initiate ice ages.

Changes in the Earth's orbit
The Earth's orbit varies through time. Important parameters that vary include the eccentricity of the orbit around the sun, the tilt of the Earth's axis, and the direction the North Pole points. Variation in these three factors changes the amount and distribution of incoming solar radiation. Variations in the distribution of solar radiation affect and initiate glaciations.

However, the variation of the orbital parameters seems to be on too short a time scale to explain the timing of the long, cool intervals with many glaciations. Variations in orbital factors are probably more important in controlling the advance and retreat of large glaciers during the four long, cool periods than they are for controlling the larger-scale patterns.

• Deccan Traps