Chapter 8 Precambrian Earth and Life History The Hadean and Archean Archean Rocks The Teton Range is largely Archean gneiss schist and granite Younger rocks are also present but not visible Grand Teton National Park Wyoming Precambrian 4 Billion Years The Precambrian lasted for more than 4 billion years Such a time span is difficult for humans to comprehend Precambrian Time Span 88 of geologic time Precambrian The Precambrian includes time from Earth s origin 4 6 billion years ago to the beginning of the Phanerozoic Eon 545 million years ago No rocks are known for the first 640 million years of geologic time Rocks Difficult to Interpret Precambrian rocks have been altered by metamorphism complexly deformed buried deep beneath younger rocks fossils are rare the few fossils present are of little use in stratigraphy Eon Subdivisions Archean and Proterozoic Eons of the Precambrian The Archean Eon Start coincides with the age of Earth s oldest known rocks 4 billion years old lasted until 2 5 billion years ago the beginning of the Proterozoic Eon Hadean is an informal designation for time preceding the Archean Eon What Happened During the Hadean Earth accreted from planetesimals differentiated into a core and mantle and at least some crust was bombarded by meteorites ubiquitous volcanism atmosphere formed Ocean waters accumulate Hot Barren Waterless Early Earth about 4 6 billion years ago Shortly after accretion Earth was a rapidly rotating hot barren waterless planet bombarded by comets and meteorites with no continents intense cosmic radiation and widespread volcanism Oldest Rocks 3 96 billion year old Acasta Gneiss in Canada and other rocks in Montana Sedimentary rocks in Australia contain detrital zircons ZrSiO4 dated at 4 2 billion years old so continental source rocks at least that old existed during the Hadean Hadean Crust Early Hadean crust was probably thin unstable and made up of ultramafic rock those with comparatively little silica This ultramafic crust was disrupted by upwelling basaltic magma at ridges and consumed at subduction zones Hadean continental crust may have formed by evolution of sialic material Sialic crust contains considerable silicon oxygen and aluminum as in present day continental crust Only sialic rich crust because of its lower density is immune to destruction by subduction Second Crustal Evolution Stage Subduction and partial melting of earlier formed basaltic crust resulted in the origin of andesitic island arcs Partial melting of lower crustal andesites in turn yielded silica rich granitic magmas that were emplaced in the andesitic arcs Second Crustal Evolution Stage Several sialic continental nuclei had formed by the beginning of Archean time by subduction and collisions between island arcs Continental Foundations Continents consist of rocks with composition similar to that of granite Continental crust is thicker and less dense than oceanic crust which is made up of basalt and gabbro Precambrian shields consist of vast areas of exposed ancient rocks and are found on all continents Outward from the shields are broad platforms of buried Precambrian rocks that underlie much of each continent Cratons A shield and platform make up a craton a continent s ancient nucleus and its foundations Along the margins of cratons more continental crust was added as the continents took their present sizes and shapes Both Archean and Proterozoic rocks are present in cratons and show evidence of episodes of deformation accompanied by metamorphism igneous activity and mountain building Cratons have experienced little deformation since the Precambrian Distribution of Precambrian Rocks Areas of exposed Precambrian rocks constitute the shields Platforms consist of buried Precambrian rocks Shields and adjoining platforms make up cratons Canadian Shield The craton in North America is the Canadian shield which occupies most of northeastern Canada a large part of Greenland parts of the Lake Superior region in Minnesota Wisconsin and Michigan and the Adirondack Mountains of New York Canadian Shield Rocks Gneiss a metamorphic rock Georgian Bay Ontario Canada Canadian Shield Rocks Basalt dark volcanic and granite light plutonic on the Chippewa River Ontario Amalgamated Cratons Actually the Canadian shield and adjacent platform is made up of numerous units or smaller cratons that amalgamated along deformation belts during the Early Proterozoic Absolute ages and structural trends help geologists differentiate among these various smaller cratons Drilling and geophysical evidence indicate that Precambrian rocks underlie much of North America in places exposed by deep erosion or uplift Archean Rocks Beyond the Shield Rocky Mountains Colorado Archean metamorphic rocks found in areas of uplift in the Rocky Mtns Archean Rocks Beyond the Shield Archean Brahma Schist in the deeply eroded parts of the Grand Canyon Arizona Archean Rocks The most common Archean Rock associations are granite gneiss complexes The rocks vary from granite to peridotite to various sedimentary rocks all of which have been metamorphosed Greenstone belts are subordinate in quantity but are important in unraveling Archean tectonism Greenstone Belts An ideal greenstone belt has 3 major rock units volcanic rocks are most common in the lower and middle units the upper units are mostly sedimentary The belts typically have synclinal structure Most were intruded by granitic magma and cut by thrust faults Low grade metamorphism makes many of the igneous rocks greenish chlorite Greenstone Belt Volcanics Abundant pillow lavas in greenstone belts indicate that much of the volcanism was under water probably at or near a spreading ridge Pyroclastic materials probably erupted where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone at Marquette Michigan Ultramafic Lava Flows The most interesting rocks in greenstone belts cooled from ultramafic lava flows Ultramafic magma has less than 45 silica and requires near surface magma temperatures of more than 1600 C 250 C hotter than any recent flows During Earth s early history radiogenic heating was higher and the mantle was as much as 300 C hotter This allowed ultramafic magma to reach the surface Ultramafic Lava Flows As Earth s production of radiogenic heat decreased the mantle cooled and ultramafic flows no longer occurred They are rare in rocks younger than Archean and none occur now Sedimentary Rocks of Greenstone Belts Sedimentary rocks are found