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 Suppose that a 24 hour clock represented all 4 6 billion years of geologic time then the Precambrian would be slightly more than 21 hours long constituting about 88 of all geologic time Precambrian Time Span 88 of geologic time Precambrian The term Precambrian is informal but widely used referring to both time and rocks The Precambrian includes time from Earth s origin 4 6 billion years ago to the beginning of the Phanerozoic Eon 545 million years ago It encompasses all rocks older than Cambrian age rocks No rocks are known for the first 640 million years of geologic time The oldest known rocks on Earth are 3 96 billion years old Rocks Difficult to Interpret The earliest record of geologic time preserved in rocks is difficult to interpret because many 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 Subdivisions of the Precambrian have been difficult to establish Two eons for the Precambrian are the Archean and Proterozoic Eons of the Precambrian The onset of the Archean Eon coincides with the age of Earth s oldest known rocks approximately 4 billion years old and lasted until 2 5 billion years ago the beginning of the Proterozoic Eon Hadean is an informal designation for the time preceding the Archean Eon Precambrian eons have no stratotypes unlike the Cambrian Period for example which is based on the Cambrian System a time stratigraphic unit with a stratotype in Wales Precambrian eons are strictly terms denoting time US Geologic Survey Terms Archean and Proterozoic are used in our following discussions of Precambrian history but the U S Geological Survey USGS uses different terms Their Precambrian W begins within the Early Archean and ends at the end of the Archean Precambrian X corresponds to the Early Proterozoic 2500 to 1600 million years ago Precambrian Y from 1600 to 800 million years ago overlaps with the Middle and part of the Late Proterozoic Precambrian Z is from 800 million years to the end of the Precambrian within the Late Proterozoic What Happened During the Hadean Although no rocks of Hadean age are present on Earth except for meteorite we do know some events that took place then Earth accreted from planetesimals and differentiated into a core and mantle and at least some crust Earth was bombarded by meteorites Volcanic activity was ubiquitous Atmosphere formed quite different from today s Oceans began to 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 Judging from the oldest known rocks on Earth the 3 96 billion year old Acasta Gneiss in Canada and other rocks in Montana some continental crust had evolved by 4 billion years ago Sedimentary rocks in Australia contain detrital zircons ZrSiO4 dated at 4 2 billion years old so source rocks at least that old existed These rocks indicted that some kind of Hadean crust was certainly present but its distribution is unknown Hadean Crust Early Hadean crust was probably thin unstable and made up of ultramafic rock rock 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 This second stage in crustal evolution began as Earth s production of radiogenic heat decreased 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 Dynamic Processes During the Hadean various dynamic systems similar to ones we see today became operative but not all at the same time nor in their present forms Once Earth differentiated into core mantle and crust million of years after it formed internal heat caused interactions among plates as they diverged converged and slid past each other in transform motion Continents began to grow by accretion along convergent plate boundaries 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 It s topography is subdued with numerous lakes and exposed Archean and Proterozoic rocks thinly covered in places by Pleistocene glacial deposits Canadian Shield Rocks Gneiss a metamorphic rock Georgian