What are some mass extinctions that have occurred in Earth history?
Earth as pass through 5 mass extinction, some minor ones end others that are going to this day1–The Cretaceous extintion event which killed 75% of the species2-The Jurassic extinction event which killed 70 to 75% of all species3-The Perminian extinction event which killed 90 to 95% of all species4-The Late Denovian extinction which killed 70% of all species5-The Ordovician extinction which killed 70% of all speciesThose are the large extinction events, there are more minor one.This can explain betterExtinction event
The glaciers extended as far south as the border between which two modern day countries?
Depends on which glaciation you are talking about. There were four "recent" major glacial periods and about 10 minor ones. The problem is that later glaciations tend to destroy the evidence of the former. The 4 largest are the Nebraskan, Kansan, Illinoian, and the Wisconsin. Obviously these are used to describe glaciations in North America. Continental glaciations also occurred in Eurasia and in South America. There have been many glacial periods during the history of the earth. At least twice in earth's history there have been "snowball earth" glaciations, covering the entire planet. One occurred about 2.45 billion years ago. A more recent one occured between 800 and 600 million years ago.
Why did glaciations (a.k.a. Ice Ages) occurred in the late Cenozoic Era?
Im sure it happened in another era but im not sure. What were possible causes for the global cooling and what amplifying mechanisms were there to induce those significant climate oscillations.
Dinosaurs lived for millions of years, as science states, why is it that there were not any ice ages during that time period?
There were glaciations before and after the dinosaurs. Some of the biggest global freeze events occurred long before animals appeared. So, it is reasonable to ask why there weren’t any major glaciations when the dinosaurs lived, 230 and 65 million years ago, the Mesozoic Era.The causes of glaciations are complex. The sun was cooler in the past, making glaciation more likely. But carbon dioxide, which warms the Earth, was generally higher. The overall layout of the continents affects the degree to which glaciers can grow in the winter and shrink in the summer, and that has changed over time. The Earth’s angle relative to the sun, and distance from the sun, vary over time in a complex fashion (Milankovitch cycles) and this also affects the growth of glaciers (Milankovitch Cycles and Glaciation).Elevated CO2 may have been the most important factor preventing glaciations in the Mesozoic. This graph shows CO2 levels in the past compared to ours. A value of 1 means the level was about the same as now. The solid black line is the best estimate and the shaded area shows the possible range of error (Actual CO2 levels could have been anywhere in that range). The levels of CO2 in the Mesozoic era were almost certainly high enough to prevent glaciation.The chart was taken from here Climate and CO2 in the Atmosphere
How many ice ages did humans go through?
Depending on your definition of humans, we emerged as a species between 200,000 and 500,000 years ago.Here is the temperature record for the last 450,000 thousand years:At the conservative estimate of 200,000 years for Homo sapiens, we have survived 2–3 ice ages, one at around 135 k years ago, possibly one at 65 k years ago, and the last one which ended about 12 k years ago. We also went through one previous warm period which was warmer than the present day warm period.If you push humanity back to 500 k years, we went through several more ice ages and 4 previous warm periods.One thing I find interesting is that the temperature anomaly graph places zero at the modern average temperature. The range is between 2 and -8 degrees C. Which is a range of 10 degrees. The mean is 5 degrees, so the zero line should be at -3 on the graph. Th current arrangement makes the warming periods seem more unusual.It is also interesting that civilization, at least as defined by the invention of writing began around 5,000 years ago.This is well after the last glaciation maximum. The Lascaux cave paintings may be up to 20 k years old, which was around the height of the last glaciation. It is unlikely that we biologically evolved much in the 15 thousand years so why did civilization not arise until several thousand years after the melting began?Did a warming and relatively stable climate stimulate the development of civilization?
What caused previous ice ages to occur?
As Keith P noted, the Milankovitch cycles are one possible theory that explain the glacial/interglacial cycle. However, there are many problems with this particular theory--mainly that the small variations require huge (and unlikely) feedbacks and the periodicity of the Milankovitch cycle does not always match that of previous glacials/interglacials (which is a big problem). Another possible theory is actual changes in the magnetic activity of the sun: http://www.unisci.com/stories/20022/0606... This is a relatively new theory, and obviously, requires more research. --------- Edit: Bob wrote "Solar radiation has been decreasing while temperatures increase." Come one Bob, that is only if you use one dataset out of the two main dataets (there are more). ACRIM has shown a slight increase in solar activity. -------- Edit2: "Some folks think having the Antarctic plate at the south pole is important." There is general agreement that landmass at the south pole makes it easier for ice ages to occur (different than a glacial), but it does not cause ice ages.
History help please??
A couple ways that maybe you would not expect. One would be that glaciation causes evolutionary islands. Little eco-pockets get created when glaciers that are inconvenient to cross cut off one population from another. Fundamentally, cold weather would make poeple good at making fire and other tools that are useful in winter or changing seasons. Lower sea levels from having a lot of ice tied up in glaciers would allow movement you otherwise would not see - that one is more obvious.
When was the last ice age?
Most people think of the Ice Age as something that happened so long ago that not a sign of it remains. But did you know that geologists say we are just now reaching the end of the Ice Age? And people who live in Greenland are actually still in the Ice Age as far as they’re concerned.About 25,000 years ago, any people who may have been living in North America saw ice and snow year round. There was a great wall of ice that stretched from coast to coast, and the ice extended northward without an end. This was the latest Ice Age, and all of Canada, much of the United States, and most of northwestern Europe were covered by a sheet of ice thousands of feet thick.This didn’t mean that it was always icy cold. The temperature was only about 10 degrees lower than it is now in Northern United States. What caused the Ice Age was that the summers were very cool. So there wasn’t enough heat during the summer months to melt away the winter’s ice and snow. It just continued to pile up until it covered all the northern area.But the Ice Age really consisted of four periods. During each period the ice formed and advanced, then melted back toward the North Pole. It is believed this happened four times. The cold periods are called “glaciations,” and the warm periods are called “interglacial” periods.It is believed that in North America the first period of ice came about 2,000,000 years ago, the second 1,250,000 years ago, the third about 500,000 years ago, and the last about 100,000 years ago.The last Ice Age didn’t melt at the same rate everywhere. For example, ice that reached what is now Wisconsin began to melt about 40,000 years ago. But ice that had covered New England melted about 28,000 years ago. And there was ice covering what is now Minnesota until about 15,000 years ago!In Europe, Germany got from under the ice 17,000 years ago and Sweden remained covered with ice until about 13,000 years ago!World