And what fraction of the time are they in effect?
Is the Earth by default most a snowball, briefly melting to let life flourish beyond the tropics? I mean I never heard anything about the fossil record that seemed to indicate that, but it just seems there’s so many fn ice ages…
For most of its history, the Earth has been mostly ice-free. There have been major ice ages at long intervals in the past.
The current “Ice Age,” actually a series of major glaciations of the Northern Hemisphere alternating with warming periods, began about 2.6 million years ago. We are currently in one of these interglacial stages in the midst of this Ice Age.
However, the climate has generally been cooling since the Oligocene. Ice began to build up in Antarctica in the Miocene about 20 million years ago.
There was another major Ice Age about 350 to 260 million years ago, during the Carboniferous and early Permian.
The most severe Ice Age we know about happened about 850 to 630 million years ago, when virtually the entire Earth seems to have been covered in ice (“Snowball Earth”).
The current round of ice ages goes back about one million years to the beginning of the Pleistocene (which is largely defined by the onset of ice ages). In the past million years there have been IIRC four periods of glacial advance and glacial retreat. Officially we’re currently in an “interglacial period” where the glaciers have retreated but might return again in the next 10,000 - 100,000 years. (Not counting man-made climate change, which means anyone’s guess.) In the distant past there were other rounds of ice ages, back in the Permian for example.
Overall though as far back as reliable data on the climate goes, ice ages have been the exception rather than the rule. I can’t remember where I heard it (one of the newer Star Trek shows?) but there was a line to the effect of “Humans! The only species in the galaxy that thinks it’s NORMAL for a planet to have ice caps!”
A bit earlier than that. The start of the Pleistocene is generally given as about 1.8 million years ago. As I mentioned above, glaciation seems to have started a bit earlier than that, at 2.6 million years ago.
What are the geological markers of a past ice age since ice (itself) vanishes. What evidence would tell a geologist that the entire earth was “icebound” at some point in half a billion years ago. The effects of glaciers grinding up the earth?
Here’s some of the geological evidence for Snowball Earth.
Glacial deposits are generally easily recognized. Glaciers produce characteristic scraping and scouring of the bedrock they pass over from the rocks they carry within the ice. They deposit moraines and other features on their sides and leading ends. Ice bergs carry rocks out to sea and deposit them in oceanic sediments when they melt. There are also various kinds of isotopic and chemical cues characteristic of glacial conditions.
Wow. So one Ice Age led to multicellular life, and the other to human intelligence. Fucking awesome.
Do other planets undergo ice ages? Does Mars melt periodically, or Europa?
I don’t believe we have enough geological evidence to know.
Actually, more than four. They have been on approximately 100,000 year cycles for the last ~million years. Before that, they seemed to be on even faster cycles of ~40,000 years. There are various orbital variations (changes in orbital eccentricity, degree of tilt of the earth’s axis, and the precession of the direction of the axial tilt) with various cycles that are believed to explain the periodicity to some degree, although the reason for the switch from the ~40,000 year cycle to the ~100,000 year cycle is not well-understood.