5.1 Principles of Earth’s History Unit 5: Earth’s History February 2, 2012 Sanders

History of Earth’s History Planet Earth probably formed about 4.6 billion years ago. Geologists locate, observe, and interpret the clues recorded in Earth's rocks. August 4, 2019 Unit 5: Earth’s History

Geologists’ Role A primary role of geologists is to take evidence from rocks and reconstruct a sequence of geologic events. There are several fundamental principles that guide geologists in interpreting Earth's history. Hutton's unconformity at Siccar Point on 17 June 2008, looking NE. Both geological units consist of parallel sediment layers (Red Sandstone and Greywacke) deposited almost horizontally in a water body. The significant change in orientation across the unconformity signals that a major reorganization of what was up and down must have taken place during the time between the deposition of these two units. August 4, 2019 Unit 5: Earth’s History

Uniformitarianism The processes that shape Earth s surface today are the same processes that occurred in the geologic past is called uniformitarianism. "The present is the key to the past." Earthquakes occur in the same way, the rock cycle and the water cycle have been the same since the beginning of the planet. August 4, 2019 Unit 5: Earth’s History

Volcano near the Eyjafjalla glacier in Iceland Lava spurts out of the site of a volcanic eruption at the Eyjafjallajökull volcano near the Eyjafjalla glacier in Iceland on March 27, 2010 August 4, 2019 Unit 5: Earth’s History

Law of Superposition Rocks at the bottom of an undisturbed area are usually the oldest. There are some exceptions to the law of superposition. August 4, 2019 Unit 5: Earth’s History

Law of Superposition August 4, 2019 Unit 5: Earth’s History

Law of Original Horizontality This principle states that layers of sediment are originally deposited horizontally under the action of gravity. August 4, 2019 Unit 5: Earth’s History

Law of Original Horizontality August 4, 2019 Unit 5: Earth’s History

Law of Cross Cutting Relationships A rock is always older than the processes that changed it. August 4, 2019 Unit 5: Earth’s History

Law of Cross Cutting Relationships August 4, 2019 Unit 5: Earth’s History

Law of Cross Cutting Relationships August 4, 2019 Unit 5: Earth’s History