Lecture 3 – life and time on earth (1).pdf

Lecture 3 – life and time on earth Uniformitariansim – geologic process operating at present are the same as those that operated in the past How to decipher earth/life history: ● Relative time ○ E.g. interpret that trilobites predated dinosaurs who predated humans ○ From understanding the rock forma...

Lecture 3 – life and time on earth Uniformitariansim – geologic process operating at present are the same as those that operated in the past How to decipher earth/life history: ● Relative time ○ E.g. interpret that trilobites predated dinosaurs who predated humans ○ From understanding the rock formations ○ Sedimentary rocks are deposited in horizontal layers ■ Principle of original horizontality ○ Infer that young strata overlay older ■ Principle of stratigraphic superposition ○ Disrupted pattern is older than the cause of disruption ■ Principle of cross-cutting relationships ■ E.g. faults, magma injections Oldest to youngest: C > B > A > D >E ● Absolute ages ○ Based on radioactive decay of unstable isotopes ○ Since these materials are unstable and decay at very precise rates, we can use them as geological chronometers – geochronology A short history of earth: ● Earth was created 4.7 Ga (giga annum) of years ago ● Earth scientists use the following divisions ○ Eons ○ Eras ○ periods ● Divisions are agreed on by a certain feature (e.g. the appearance of a fossil) anZd described for a specific location Paleozoic (540-250 Ma) Cambrian explosion: ● At ~540 Ma, rapid appearance of complex life in fossil record ● Massive diversification of species (“big bang of ecology”) ● E.g. Gurgess shale, AB Hadean eon: (started 4,567 million years ago) ● Solar system forms from nebula ○ Contracts and spins → disc ○ Sun at center ○ Inner rocky planets (includes earth) ○ Outer gas giants ● Date of formation from radiometric dating of meteorites ● Lots of impact, including a huge one which formed the moon ● Magma ocean, water vapour, lots of impacts Middle Paleozoic: ● First land plants in Silurian ● Amphibians in Devonian ● First seed plants ● First forests ● First soil Mesozoic era: (250-65 Ma) ● Time of dinosaurs ● Emergence of dinosaurs in Triassio, also swimming and flying reptiles ● Mammals also develop in Triassio ● Bipedal movement ● Atlantic opens, first birds in Jurassic ● Gymnsperms, cycad trees K-T extinction event: ● At 65 Ma, almost vertebrates (in land, sea, air) become extinct; many invertebrates, land plants too ○ ~60% of species on planet dies out ● Probably an asteroid ~10km in diameter collides with Earth ● “Nuclear winter” atmospheric effect Evidence for Bolide Impact: ● Spike for iridium (extra-terrestrial element) at K-T boundary ● Chicxulub impact crater at Yucatan ● Massive tsunamis along Atlantic coasts at ~65 Ma Cenozoic (65 Ma to present) – time of the mammals The future: ● We can predict next supercontinent “Pangea Ultima” in 250 Myrs ● Earth will cease to exist in about 5 billion yrs ○ Sun will run out of fuel and briefly expand into a red giant ○ Earth will evaporate