Evolution of the Atmosphere
- the early atmosphere was probably rich in carbon dioxide, nitrogen, methane, ammonia, hydrogen, and water vapor; this reducing atmosphere was devoid of free oxygen (O2)
- the carbon dioxide rich oceans were very acidic (carbon dioxide + water = carbonic acid)
- the photodissociation of water vapor may have produced some free oxygen
- photosynthesis produced large quantities of oxygen and removed much carbon dioxide from the atmosphere once life evolved by 3.5 b.y.
- by about 3 b.y. banded iron formations (BIFs) show that free oxygen was available periodically, perhaps seasonally, to oxidize and precipitate the abundant iron cations that were dissolved in ocean water (remember these iron cations were the product of carbonic acid weathering silicate rocks)
- by about 2 - 1.7 b.y. redbeds (hematite-cemented sandstones and shales) started to form, indicating a continuous supply of free oxygen
- by about 2 - 1.7 b.y. carbonate rocks/limestones and other carbonate rocks became common indicating that oxygen had increased and carbon dioxide reduced through photosynthesis so that ocean acidity had decreased enough to allow calcium carbonate to precipitate
Archean Geology: 4 - 2.5 b.y. formation of the first stable crust
- Algoman/Kenoran orogeny between 2.6 and 2.5 b.y. in the Great Lakes region and Canada was a period of major crustal formation (granitization), crustal thickening, and aggregation of mini continents from smaller pieces of Archean crust
Proterozoic Geology: 2.5 b.y. - 543 m.y. formation of continents
- The Hudsonian orogeny in the mid-Proterozoic around 1.7 b.y. sutured together the various Archean mini continents into the core of the stable continent Laurentia
- the Grenville orogeny in eastern Laurentia about 1 b.y. apparently resulted from assembly of the supercontinent Rodinia which contained ancestral cores of today's continents
- Laurentia apparently rifted out of Rodinia about 600 to 700 m.y.; western Laurentia rifted from Australia; southern and southeastern Laurentia rifted from Antarctica; eastern Laurentia rifted from ancestral South America; the breakout apparently assembled the southern supercontinent Gondwana (Antarctica, Africa, South America, India, and Australia)
- passive margin subsidence yielded thick Upper Proterozoic/Lower Paleozoic sedimentary sequences in present day western Canada and the eastern US