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Principle of Uniformitarianism:
James Hutton challenged the previous view and came up with the idea that provides the foundation for geology today. Father of Geology. He had the idea of the principle of uniformitarianism which simply states that physical processes we observe today also operated in the past at roughly the same rates and were responsible for the formation of the geological features we see in outcrops. It means the present is key to the past.
Relative Age v. Numeric Age
Relative Age: Specify the age of one feature with respet to another as its relative age.
Numeric Age: Age of a feature given in years.
Principles for definining Relative Age:
1. Principle of Uniformitarianism;
2. Priniple of Original Horizontality;
3. Principle of Superposition;
4. Principle of Original Continuity;
5. Principle of Cross-Cutting Relationships;
6. Principle of Baked Contacts:
7. Principle of Inclusions;
- Uniformitarianism: Processes we observe today happened in the past at comparable rates.
- Original Horizontality: Sediments on Earth settle out of a fluid in a gravitational field. Typically surfaces on which sediments accumulate are fairly horizontal and therefore when sediments are originally deposited they were deposited horizontally.
- Superposition: In a sequence of sedimentary rock layers, each layer must be younger than the one below; a layer of sediment can't accumulate unless there's something for it to accumulate on.
- Principle of Original Continuity: Sediments generally accumulate in continuous sheets.
- Principle of Cross-Cutting Relationships: If one geological feature cuts across another, the feature that has been CUT IS OLDER. If igneous dikes cut across a sequene of sedimentary beds, the beds must be older than the dike.
- Principle of Baked Contacts: An igneous intrusion "bakes" (metamorphoses) surrounding rocks. The rock that has been baked must be older than the intrusion.
- Principle of Inclusions: If a layer of sediment deposited on an igneous layer includes pebbles of the igneous rock, the fragments must be older than the intrusion. The fragments (xenoliths) in an igneous body and the pebbles in the sedimentary layer are inclusions, or pieces of one material into another. Rock containing the inclusion must be younger than the inclusion.
Numerical dating: many relative ages can now be assigned actual dates.