Physical Geology lecture
Rates, geologic rhythm, and a host of -isms
What is the nature of history? What mix of revolutions and evolutions, of sudden change vs. slow change? How sensitive is the course of history to small events and changes? What variations in behavior are possible? For these and many other very basic questions geology provides fundamental insight because it provides the longest and possible accessible record of events that we have access to. A somewhat newer focus is on the history of climate change, as we seek to know changes to come.
The time frame being considered very much shapes our thinking about history. Consider fault motion as an example. On a short time frame and earthquake is a sudden episode of movement on a fault. Movement from this perspective would be a discontinuous history of distinct earthquake-movement events. However, on a larger time scale and considering the length of the fault that history appears more continuous, and indeed is connected to the slow continuous movements of the plates at rates of centimeters per year.
Diagram show the difference between a long term and short term view of fault movement. From a human and environmental perspective, which view of fault movement is more instructive? From the perspective of thinking about and trying to understand plate motions and geologic history which perspective might be better?
The character of history and a host of -isms (these will be explained in greater depth in class):
What is the biggest event of its kind possible? The case history of the wave that paved Lanai:
Accidents happen -the Mesozoic-Cenozoic boundary clay:
One of the species of big dinosaurs that went extinct at the end of the Mesozoic.
Milankovich cycles: the regular and predictable vs. chaos?
Photo from the National Ice Core Lab. Each tube contains ice that was cored from some glacier around the world. Some of these cores go over a mile deep and retrieve ice that is hundreds of thousands of years old. Image source: http://www.usgs.gov/core_science_systems/access/summer_2012/article-7.html .
We have cored deeply into major glaciers, including on Antarctica, to obtain the record within the layered ice that has accumulated with time. This graph shows how an isotope signature that is sensitive to atmospheric Temperatures changed in the Vovstok ice core over time. Clearly it is a complex signal, but one with some periodicity. The best explanation for this periodicity is orbital fluctuations (the Milankovitch cycles) that influence climate.
Major events in earth history
Here are some examples - there are more.
Precambrian ironstone from Lookout Mountain in the Black Hills. These were originally almost pure iron oxide sediments, and these Banded Iron Formations (BIFs) formed mostly during a certain part of the Precambrian Era and are related to the development of life on earth, which in turn is connected to the oxygenation of our atmosphere.
Niobrara Chalks exposed on the South Dakota side of Lewis and Clark Reservoir on the Missouri River. These chalks extend down into Kansas, and even down into Texas (where they are known as the Austin Chalk), and were deposited in a shallow sea (the Western Interior Seaway) that used to occupy the middle of the U.S. as a long north-south arm of the Gulf. Similar age chalks make up the White Cliffs of Dover, and can also be found in Denmark and France. Cretaceous, the geologic period in which these formed, literally translates as the "Age of Chalk", and is so named because of all these chalks. Why did so much chalk form during the Cretaceous??
We have a course in Historical Geology where one can explore all of this and much more.
Earth in its 4.5 billion years really has an amazing history.
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