|theImage.com     Notes on Basic Geology|
|Notes created & information organization based on the book:
"The Dynamic Earth - an introduction to physical geology"
Brian Skinner & Stephen C. Porter   (further book information here)
also look at www.wiley.com for additional resource information
|Glaciers & Glaciation - Page 4|
Beyond the limit of glaciers, but features that reside in a permafrost environment are called periglacial. Permafrost is a ground condition that exits where the temperature never fully thaws the regolith.
Most permafrost is thought to have been created in one of the last glacial ages. This type of ground as been the resting place for well preserved woolly mammoths and other animals that are long ago extinct.
The permafrost areas must have been north of any ice sheets as the weight of the ice would have been sufficient to cause melting at depth, and the ice sheets would have acted a temperature thermostats isolating the ground from extreme cold and holding it no more than about freezing temperature.
Areas of soil and rock exposed to the air north of the ice sheets would have been subjected to far colder temperatures and would have frozen to greater depth. Permafrost has been measured to over 1500 m in Siberia and just over 600 m in Alaska.
|Ice Ages? Stupidest thing I ever heard!
First postulated in the early 1800's, they were regarded at first as a silly concept. But with time they became accepted.
Today, ice ages are the accepted norm, and have been happening on a regular basis. They show us rapid global climactic changes, and show us that they have happened on a semi-regular basis.
They even provide clues as to how parts of the crust and upper mantle operate.
Is it time yet to buy stock in energy companies that provide power for heating in colder climes?
Ice ages are long term events on a people time scale although fast events on the geologic time scale.
About 30,000 years ago a mass of glaciers began their march south, and new glaciers formed in most high mountain ranges. At their peak they cove about 29% of all available land area. Today ice and glaciers cover about 10%.
This glacial period changed the route of the Missouri and Ohio rivers by pushing them south. (They probably weren't called those names at the time!)
During the same period sea level fell about 100 m, that's over 300 feet. The coast lines would have been substantially different all around the world. The land bridge between Siberia and Alaska would have been totally above water level. England and France would also have been connected at the time.
The largest ice mass was located in central North America (in what is now Canada). Since he loss of the massive ice sheet the area in northern Canada that was covered is still rising due to the loss in weight. (Remember the principle of isostasy?)
My measuring the rate of change (rate of rise in this case) of crustal rocks Geologist can begin to understand how the lithosphere and asthenosphere react to heavy loads.
Radio carbon dating has shown that the average age of a glacial period is about 100,000 years. There have been at least 20 such cycles during the measurement period. Originally based on evidence in the Alps it was thought that there were only 4 major periods. (That is why we continually question theories and hypotheses.)
Glacial --> Interglacial period. Glacial period when the ice is moving south and ice masses are growing, Interglacial period when the ice masses move north ad the ice sheet are decreasing in size.
Today we are standing at about the end of an Interglacial period. The oldest recorded glaciation dates may be as old as 2.3 billion (with a "B") years ago.
|So what causes the cycle?
The ultimate answer is that no one knows yet for sure, but there are theories ...
1.) the movement of the continents as they are moved along on the lithosphere.
2.) The large scale uplift of crust as continents collide.
3.) The creation of mountain ranges
4.) The opening or closing of ocean basins between continents.
So what's the relationship?
Glaciers tend to form on land masses at high latitudes (south). And at high altitudes. Where winds can supply high moisture content . It appears that the continental drift theory has shown us that the movement has had major effects in changing ocean currents and also atmospheric circulation.
Geologic evidence (various rock formations related to glaciers) points to a time of very high ice coverage when most of the continents were south of the equator. The drift theory shows that during the last several million years the continent were drifting in more or less a northern direction and for much of the time existed just above or below the equator.
It appears that the big ice ages need to happen when the continents are near a pole (South or North). As the continents continued their northerly journey, large mountain ranges were being formed in the western US and in Central Asia. So we began to enter another long ice age.
This explains the long term changes but does not explain the 100,000 year cycle.