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The geological Time SCale:
The geological Time SCale:
The geologic time scale divides up the history of the earth based on life-forms that have existed during specific times since the creation of the planet. These divisions are called geochronologic units (geo: rock, chronology: time). The Geologic Time Scale is divided by the following divisions; Eons: Longest subdivision; based on the abundance of certain fossils, Eras: Next to longest subdivision; marked by major changes in the fossil record, Periods: Based on types of life existing at the time, Epochs: Shortest subdivision; marked by differences in life forms and can vary from continent to continent (Geologic Time - South Carolina Department of Natural Resources).
Principles behind Geolic Time:
Principles behind Geolic Time:
The principle of superposition: The layer on the bottom was deposited first and so is the oldest
The principle of horizontality: All rock layers were originally deposited horizontally.
The principle of original lateral continuity: Originally deposited layers of rock extend laterally in all directions until either thinning out or being cut off by a different rock layer.
- The principle of biologic succession: Each age in the earth‟s history is unique such that fossil remains will be unique. This permits vertical and horizontal correlation of the rock layers based on fossil species.
Geological time has been seperated into four eons; Hadean, Archean, Proterozoic, Phaneorzoic. The Phaneorzoic is divided into three eras: the Paleozoic "early life", the Mesozoic "middle life", and the Cenozoic "new life".
The principle of cross-cutting relationships: A rock feature that cuts across another feature must be younger than the rock that it cuts.
Inclusion principle: Small fragments of one type of rock but embedded in a second type of rock must have formed first, and were included when the second rock was forming.
“Geologic Time - South Carolina Department of Natural Resources.” Geologic Time and Earth's Natural History, https://www.dnr.sc.gov/geology/pdfs/education/Geologic%20Time.pdf
Relative Dating Methods
Relative Dating Methods
We can determine the relative aged of different rocks by observing/interpreting relationships amoung them, such as superstition, cross cutting and inclusions. Gaps in the geological record are represented by various types of unconformities (Panchuk, Karla, and Steven Earle).
Panchuk, Karla, and Steven Earle. “Chapter 7 Metamorphism and Metamorphic Rocks.” Physical Geology 2nd Edition, BCcampus, 23 Sept. 2019, https://opentextbc.ca/physicalgeology2ed/part/chapter-7-metamorphism-and-metamorphic-rocks/.
Dating Rocks Using Fossils:
Dating Rocks Using Fossils:
Fossils are usefull for dating rocks date back to about 600 Ma. If we know the age range of a fossil, we can date the rock... Index fossils represnt shorter geological times, and if a rock has several different fossils with known age ranges, we can normally narrow the rock down to tge time it was formed.
Isotopic Dating Methods:
Isotopic Dating Methods:
Radioactive isotopes decap at predictable/known rates and can be used to date igneous and metamorphic rocks. Some of the more useful isotope systems are potassium-argon, rubidium-strontium, uranian-led and carbon-nitrogen. Radiocarbon dating can be applied to sediments/sedimentray rocks but ONLY if they are younger than 60 ka.
Other Dating methods:
Other Dating methods:
There are many methods for dating geological materials, two that are widely used are dendrochronology and magnetic chronology. Denochronology,based on the study of tree rings, is widely applied to dating glacial events. Magnetic chronolgy is based on the known record of the earth's magnetic field reversals.
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