GEOL 1020 Lecture Notes - Lecture 26: Water Vapor, Photosynthesis, Cosmic Dust
7 Dec 2017
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Document Summary
Great oxidation event that occurred ~2. 3 ga. Pyrite (fes2) is relatively common in archean rocks, but is rare after 2. 3 ga. Banded iron formations (bifs) are common in archean rocks, but rarely formed after about 1. 85 ga. Bands of reddish brown chert alternating with iron-rich minerals (silver- black in color) accumulated in oxygen-poor ocean waters. Must have formed in low oxygen, iron-rich, deep waters. Iron and silica are hypothesized to have come from hydrothermal vents on seafloor at rifting zones. Red beds show the opposite pattern to bifs. They are rarely found in rocks older than 2. 3 ga Acasta gneiss ~ 4 ga (among the oldest known rocks). This is a metamorphic rock, and comes from the igneous rock granite and granodiorite (mostly made up of quartz and feldspar) after metamorphism. Even older crystals of zircon have been dated at 4. 4 ga (hadean in age) from gneisses in jack hills, western australia.
