Lecture 14 // Geologic Time and Earth History Flashcards
Geologic Time Scale
- Eons: hundreds of millions to billions
- Eras: tens to hundreds of millions
- Periods: millions to tens of millions
- Epochs: Tens of thousands to millions
Ga = giga-annum (billion yrs) Ma = mega-annum (million yrs)
The age of the Earth is assumed to be the same age as other rock bodies in our solar system.
The Phanerozoic Eon
- The Phanerozoic Eon time scale divisions are based on significant changes in life on Earth.
- Both the Paleozoic and Mesozoic end with mass extinction events, and a biotic turnover follows.
Relative age
dating
The geologic time scale was first assembled utilizing relative age dating techniques.
• Determining a sequence of geologic events in an area.
• Comparison with many other areas of a similar time span (based on fossil content).
• Nowhere on Earth is there a record of all geologic time. All areas have experienced times of uplift and erosion, and times of non-deposition.
Absolute ages
Absolute ages were eventually added (and tweaked) once radioactive decay was discovered (1898) and radioisotopic age dating methods were developed (early 1900s).
Precambrian: Hadean Oldest crustal (Earth) rocks
Age of Hadean time have been determined by applying radiometric dating to moon, meteorites and moon rocks. Acasta Gneiss (4.03 Ga) The oldest crustal rock on Earth is found in the Canadian Shield of the Northwest Territories.
Stromatolites (Archean)
Our rock record truly begins. The oldest fossils of ancient life are 3.8 Ga trace fossils called stromatolites.
Rodinia (Proterozoic)
Earth’s first supercontinent (Rodinia) sutures together 1.1 Ga, and then begins to rift apart due to plate divergence around 800 mya.
The Ediacaran Biota (Proterozoic)
Earth’s first multicellular life evolves in the Ediacaran Period (635-542 million years) of the Late Proterozoic. These larger, simple soft-bodied marine life forms are called the Ediacaran Biota.
Phanerozoic / Cambrian
During the Cambrian life with shells and hard external bodies appeared for the first time representing a huge beginning of the Cambrian period.
The Cambrian Explosion &
Cambrian Substrate Revolution (Phanerozoic)
Hard parts evolve and thus the fossil record ‘explodes’ = ‘Cambrian Explosion’. Complex infaunal feeding (seafloor burrowing) begins = ‘Cambrian Substrate Revolution’.
The Burgess Shale (Phanerozoic: 510 Ma)
The middle Cambrian (~510 Ma) Burgess Shale of Field, BC is a World Heritage Site boasting exceptional preservation of an early Paleozoic marine ecosystem.
Even soft-bodied organisms are preserved in the Burgess Shale, including marine worms.
Marrella
The tiny 1cm long crustacean Marrella is the most abundant Burgess Shale animal.
Anomalocaris
The biggest predator in the Cambrian seas at about 1 m long.
Paleozoic (Phanerozoic)
The Paleozoic Era (542 – 251 Ma) is the ‘Age of Invertebrates’. - The Paleozoic ends in the Permian with the full assembly of Pangea.
Mesozoic (Phanerozoic)
- The explosion of diversity in the reptiles created the dinosaurs during the Triassic period.
- The first amphibious appeared during the Jurassic period.
- Finally during the end of the Cretaceous a mass extinction occurred that finished with 5 main reptiles group, including the dinosaurs.
