Answer: changes in the Earth's orbit cause ice ages.
The Earth's orbit is not perfectly circular, nor is it perfectly constant over time. The ice ages over the past two million years have resulted largely from subtle differences in the amount of sunlight reaching the poles. These changes happen because the Earth is not the only planet in the Solar System, and the effects of the other planets' gravity do things to our orbit and orientation. By changing the amount of sunlight reaching the poles, ice at the North Pole becomes more or less stable with time - and when ice forms, it reinforces itself by reflecting additional light back into space.
Now, ice melting is an indication of it getting warmer. However, there are other climate cycles due to things beyond our control that can cause these cycles. For example, there was a cold period a couple hundred years ago called the "little ice age". Skeptics of global warming, or, more accurately, human-caused global warming, suggest that the melting ice we are seeing now is a natural consequence of the end of whatever caused the little ice age - the best explanation for which we have at the moment that the sun was a tiny bit fainter then than it is now (this is based on sunspot records).
By the way, the Great Lakes are not the water left over from the ice melting. They are holes in the ground scooped out by the glaciers, and where you have a hole, water collects and you get a lake.
Yep, ice just melts! When the melting temperature of ice is reached, ice melts.
b) What would have caused that ice to melt?
Temperatures at/ or above the melting temperature cause ice to melt.
c) Were there more volcano eruptions then than now?
I don't know. But I think what you're trying to get at is the question, "Did greenhouse gases emitted by volcanoes or other sources thousands of years ago equal the anthropogenic (human-caused) greenhouse gases emitted today?" And the answer to that question is no. There are more greenhouse gases in the atmosphere today than there were before the industrial revolution. We know this because gas measurements have been made from samples collected from air pockets in ice. Ages are assigned to layers in the ice both from looking at the layers in the ice (much like looking at tree rings) and by using age models.
The melting temperature has to be reached in order to melt ice. Prior to the industrial revolution, ice was forming and also melting in various places around the globe as it does today. However, with more greenhouse gases in our atmosphere today, global temperatures are rising higher, causing more ice to melt from our glaciers today.
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