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November 14, 2013
Determining Earth's History Rocks and Ice Cores
One goal of geology is to develop a timeline of Earth's past, including both geological and biological events. Geologists study rock layers and fossils to learn about Earth's history. They study ice cores to understand major climate changes that affected life on Earth.
Scientists use the Principle of Uniformitarianism to help them figure out Earth's history. This law states that we can interpret past events by understanding presentday processes. In other words, we assume a volcano eruption, earthquake, tsunami, etc. in the past was like a volcano eruption, earthquake, or tsunami now.
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ABSOLUTE DATING
When studying rocks, scientists use different techniques to determine their absolute or relative ages. Absolute age tells the actual, numerical age of a rock or fossil, or how long ago an event occurred. Scientists do this through radioactive dating. by comparing the amount of the radioactive form of an element such as Uranium 238 (nonliving things) or Carbon 14 (living things) to the amount of its half life. Uranium 238 can date rocks 10 million years old to the beginning of time. Carbon 14 can date the remains of organisms to about 50,000 years ago.
Relative age describes the age of an object or event in relation to another object or event. It can tell you which events happened earlier or later, without giving a definite date. Relative age is often determined by looking at different rock layers and at index fossils.
Relative Dating
Relative age: youngest middle oldestAbsolute age: 2 1/2 yrs 5 yrs 7 yrs
Index fossils are fossils that are known to have lived in a certain
geologic age, so can be used to date the rock layer in which they're found.
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Law of Superposition
Drop the numbers on the layers to order each from oldest to youngest.
Law of Superposition Younger layers of rock are deposited on top of older layers.
Relative Dating
The Law of Superposition a rock layer is younger than layers below it and older than layers above it, unless tectonic forces have overturned or disturbed the layers.
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Based on the Law of Original Horizontality, if sediment is deposited in flat, horizontal layers, then anything that cuts through layers, such as a fault or magma, must be younger than what it crosses. This is called crosscutting. The magma that cuts in is called an intrusion.
Unconformities
Intrusions, crosscutting, or any other gap in the rock record (missing layers) is called an unconformity.
Unconformities can also be overturned rock beds (where the rocks on the bottom are now the youngest) or an angular bed (where the horizontal rocks are younger than the tilted rocks).
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Since there is no single place on Earth where all of geologic time is represented in the rock layers, scientists have to use pieces from many different areas to put together a complete picture. This process is called correlation. Correlation uses similar rock types, geologic features and index fossils. (More about fossils later...)
Scientists use ice cores to understand how the climate has changed on Earth. Ice cores form in layers similar to rocks lower layers are older than higher layers. Because gases from the atmosphere are trapped in the ice as it forms, scientists can determine the concentration of the gases in the atmosphere and elements that were in Earth's water at that time.
Ice Cores
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One ice core from Antarctica shows the history of Earth's climate for the past 740,000 years and is 3 kilometers long! During that time there were 8 ice ages, when glaciers covered much of the Earth.
Scientists know this because of low levels of deuterium (a form of hydrogen) that signals colder temperatures. High deuterium and carbon dioxide levels are signs of warmer temperatures.
Ice Cores
