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Geology /
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Essential Question How have the geologic sciences developed over time?
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Catastrophism Archbishop James Ussher developed a chronology of the world in the mid-1600s Calculated creation of Earth to have occurred Oct 23, 4004 BC Chronology eventually printed in notes of many Bibles Led to principle of catastrophism
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Catastrophism Earth shaped by major disasters that no longer occurred.
Noah’s Flood for example Allows Earth to be quite young because its landforms can change quickly
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Modern Geology In late 1600s, Nicolas Steno developed:
Principle of original horizontality Law of superposition
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Modern Geology James Hutton wrote “Theory of the Earth” in 1785.
Offered principle of uniformitarianism. Claims the same physical processes that occur today occurred in the past Understanding the present is the key to understanding the past
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Modern Geology Hutton wrote, “We have a chain of facts that clearly demonstrates that the materials of wasted mountains have traveled through rivers.” Claimed only long periods of time were required to shape the Earth as we see it. Modern geologists have calculated American mountains erode at 3cm/year. At that rate, a 10,000 ft mountain would erode in 100 million years.
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It just takes time
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Modern Geology In 1821, Louis Agassiz heard another scientist claim there was evidence for glaciers in places where there were currently no glaciers. He attempted to disprove hypothesis. He ended up accepting hypothesis and forming a hypothesis of a great ice age. Provided evidence for uniformitarianism.
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Glacial valley
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Geology at work today Geologists impact our world in numerous ways
Engineering geologist help design buildings to withstand earthquakes and other forces Petroleum geologists help find oil and natural gas Geology is also an essential part of astronomy, oceanography, paleontology, meteorology and more
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Check-up What is catastrophism? What is uniformitarianism?
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Essential Question How can scientists determine ages of geologic material?
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Relative dating Placing material and events in sequence
Principles and rules of Law of superposition – oldest rocks are on the bottom Principle of original horizontality – sediment is deposited horizontally Principle of cross-cutting relationships – younger feature cuts through an older feature
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Superposition is well illustrated by the strata in the Grand Canyon
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Horizontality Principle of original horizontality claims layers of sediment are deposited horizontally. So, layers that are folded or otherwise distorted were distorted after they were deposited.
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Cross-cutting relationships
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Relative dating Unconformities
An unconformity is a break in the rock record Types of unconformities Angular unconformity – tilted rocks are overlain by flat-lying rocks Disconformity – strata on either side are parallel Nonconformity – igneous or metamorphic rock uplifted, eroded and then covered by sediment
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Formation of an angular unconformity
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Relative dating Principles and rules of Unconformities
Types of unconformities Nonconformity Metamorphic or igneous rocks below Younger sedimentary rocks above
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Several unconformities are present in the Grand Canyon
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Correlation of rock layers
Correlation means to show a relationship Matching rocks of similar age in different regions Often relies upon fossils
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Check-up What is the difference between catastrophism and uniformitarianism? Do people who believe in catastrophism disagree with the law of superposition?
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