Chapter 8 Precambrian Earth and Life History The Archean
Introduction Our reference of time, in geology, spans billions of years The Earth is 4.6 billion years old The largest unit of time on Earth is the Precambrian. It lasted more than 4 billion years Fig. 19.1, p. 494 Fig. 19.2, p. 495
Introduction Geologists divide the Precambrian into two eons Eons Archean Proterozoic If all geologic time were represented by a 24-hour clock, the Precambrian would be more than 21 hours long and constitute more than 88% of all geologic time. Fig. 19.1, p. 494
What Happened During the Eoarchean? • The earliest part of the Archean history is the Eoarchean (4.6 bya – 3600 bya). • Volcanism was widespread. Earth was a hot, rapidly rotating, barren waterless planet bombarded constantly by meteorites and comets. Fig. 19.3a, p. 495
What Happened During the Eoarchean? • The earth was still accreting from the planetesimals and differentiating. There are few rocks from this time period. Some crust had formed, possibly by 4.4 bya. • The process of continental accretion was slowly building the continents from small island arcs.
Island arc Sea level Island arc Oceanic lithosphere Asthenosphere
Continental Foundations-Shields, Platforms, and Cratons Each continent today has an ancient, stable craton made up of a Precambrian shield and platform. Areas of exposed Precambrian rocks constitute the shields. Platforms consist of buried Precambrian rocks. A shield and its adjoining platform make up a craton. Fig. 19.5, p. 498
Continental Foundations-Shields, Platforms, and Cratons Direct evidence of continental accretion as a model for the formation of continents comes from the Canadian Shield in North America The Canadian Shield is made up of several smaller cratons that accreted. Fig. 19.8b, p. 500
Archean Earth History • The Archean4.6 bya – 2.5 bya • Archean Rocks Fig. 19.6, p. 498
Another Archean Rock in Greenstone belt • Komatiites • The most interesting rocks in Greenstone belts • They are ultramafic • Have high amount of Iron Means that magma was hot enough to melt-iron rich minerals. • Magmas flowing were a lot hotter that are today. • Komatiites must have cooled from molten rock that was at least 1600°C at the surface of the earth. Today’s highest recorded surface flowis 1350°C
Archean Earth History • Archean Plate Tectonics and the Origin of Cratons • Many geologists think that Archean plates moved faster than plates do now because Earth possessed more radiogenic heat. • Small cratons would have grown more rapidly to become larger continents. • Several small cratons existed, 30-40% of present continental crust existed. • We did not however, have significant amounts of sedimentary rocks which are products of weathering in an oxygen rich atmosphere with abundant water. (.)
Archean Earth History • Archean Plate Tectonics and the Origin of Cratons • Origin of a Greenstone Belt in a Back-Arc Basin Fig. 8.10, p. 1160
Archean Plate Tectonics • The Archean4.6 bya – 2.5 bya • Archean Rocks • Archean rocks are mostly granite-gneiss complexes and subordinate greenstone belts. One model for the origin of greenstone belts holds that they formed in back-arc marginal basins. Fig. 19.8, p. 500
Origin and Evolution of the Atmosphere and Hydrosphere • The Atmosphere • Earth’s earliest atmosphere was derived by the release of gases during outgassing (from volcanoes). • The Archean atmosphere • Lacked water and free oxygen • Rich in carbon dioxide • The Proterozoic atmosphere • 21% free oxygen • Some carbon dioxide • The oceans now covered 71% of the Earth’s surface, close to today’s level. Fig. 19.17a, p. 512
Origin and Evolution of the Atmosphere and Hydrosphere Photochemical dissociation Water vapor in the atmosphere is broken down by UV rays to oxygen and free hydrogen. Results in an ozone barrier but no free oxygen. Photosynthesisaccounts for most of the free oxygen. Cyanobacteria, present since the Archean,did not become common until about 2.3 bya These photosynthesizing organisms added free oxygen to the evolving atmosphere They form structures known as stromatolites. The Atmosphere - The Proterozoic atmosphere had some oxygen. Two processes account for the introduction of free oxygen. Fig. 19.19a, p. 514
Origin and Evolution of the Atmosphere and Hydrosphere • The Hydrosphere • All water on Earth is part of the hydrosphere. Most of it (more than 97%) is in the oceans • Meteorite and comet impacts and outgassing yielded the hydrosphere
Escapes Hydrogen H Water H2O To atmosphere Nitrogen N Water H2O Carbon dioxide CO2 To oceans Volcano Erosional debris Stepped Art Fig. 19-17a, p. 512
Life-Its Origin and Early History • Abiogenesis How life originated from nonliving matter • One point of clarification on abiogenesis—it does not hold that a living organism such as a bacterium, or even a complex organic molecule, sprang fully formed from non-living matter. Rather than one huge step from nonliving to living, the origin of life involved several small steps, each leading to an increase in organization and complexity.
Life-Its Origin and Early History • Fossil as as old as 3.5 bya have been found on Earth. Today we have five kingdoms with millions of species. How did life originate from non-living matter? • The Origin of Life • Energy such as lightning and ultraviolet radiation acting on chemical elements present on Earth may have yielded the first living things. Fig. 19.18, p. 513
Another Hypothesis • That first life was near black smokers • Animation 1 • Animation 2
Life-Its Origin and Early History • Archean Organisms • All known Archean fossils represent prokaryotic bacteria. • Prokaryotes reproduce asexually,(without the recombination of genes) and are single-celled, no internal organs • Not many life forms existed, mostly bacteria and stromatolites • Stromatolites formed by photosynthesizing bacteria may date from 3.5 bya ago.
Prokaryotic organism is also called Cyanobacteria • Cyanobacteria is a single organism also known as Blue-Green Algae. When large colonies of cyanobacteria flourish they create a single green film on the surface of a lake or pond, pond scum
Resources in Precambrian Rocks • Archean Resources • More than 50% of the world’s gold has come from Archean rocks in South Africa. • Massive sulfide deposits containing zinc, copper, and nickel are known from Australia, Zimbabwe (Africa) and Canada. These are similar to the black smokers on the seafloor. • About 25% of the world’s chromium reserves are also in Archean rocks, along with platinum deposits. • Archean pegmatites in the African Shield and the Canadian Shield have yielded valuable gem-quality minerals.