The Building of a Continent. Delving into Deep Time

The Building of a Continent Delving into Deep Time

Methods for Studying the Past Identifying orogenies Mountain building events Mountains erode Can t study topography Study the evidence they leave behind Igneous activity metamorphism Date the rock record radiometrically Foreland basins Mountains depress the crust; collects sediment eroded from the mountains; study sed seqiemces

Even after mountains have eroded, they leave their roots behind W. W. Norton

Recognizing the Growth of Continents Not all continental crust formed at the same time Must determine the age of different regions of the crust Determine when rock was formed and when other rocks were metamorphized during a subsequent orogeny. Indications of tectonic environment in which the crust formed

Recognizing Past Environments Environments in specific locations change through geologic time Determines the type of sediment deposited Determines the type of organisms that live there

Recognizing Past Changes in Sea Level Transgressions/Regressions Can be determined by the vertical succession of sedimentary strata Correlating lithostratigraphic columns from around the world global sea level can be determined

Recognizing Past Positions of Continents Apparent polar wandering Latitude and longitude of continent in the past More recent data From magnetic stripes in the sea floor Oldest: Jurassic Matching fossils from different regions To see if continents once adjacent

Using Apparent Polar Wandering W. W. Norton

Using Marine Magnetic Anomalies W. W. Norton. Modified from Pitman and Talwani, 1972

Recognizing Past Climates From fossil record If tropical organisms lived near the poles during a given time period, then the climate in that region at that time must have been warmer Using ratios of radioisotopes 18 O/ 16 O Measure of past temperature

Recognizing Organic Evolution Accepting evolution and the principle of faunal succession Progressive changes in fossil assemblages in a sequence of strata represents changes in assemblages or organisms inhabiting Earth through time

The Hadean Hell on Earth

The Hadean Eon Hell on Earth Oldest mineral grains from Australia 4.1-4.2 bya Oldest complete rocks Gneissic rocks in outcrops in Canada, Wyoming, Greenland and China 4.3 bya But meteorites date to 4.6 bya No direct record of Earth s first 600 my

Hadean Solar system/earth formation Molten surface magma ocean Ultramafic minerals flooded the surface Heat from Meteorite impact Radioactive decay Iron catastrophe Iron sank to the middle Layered Earth

The Archean Birth of the Crust, Oceans and Life

The Archean Eon Birth of the Crust, Oceans and Life 4 bya the crust cooled and the surface solidified Lithospheric plates Subduction Partial melting of ultramafic rocks produced mafic to intermediate magma Rose and erupted at volcanic arcs Hot spots Huge hot spot volcanoes

The Archean Eon The first rock record: 4.03-2.5 bya Protocontinents This magma too buoyant to to be subducted Collisions Stuck together Formed first continents

Fig. 13.05 a, b W. W. Norton

Archean By 2.7 bya first long-lived blocks of crust are formed Start of formation of Cratons By end of Archean 80% of continental crust formed

Atmospheric Gases Volcanic activity continuously added gases to the atmosphere outgassing Comets passing near the Earth contributed gases

Archean Principle Rock Types Gneiss Relicts of Archean metamorphism from collisions Greenstone Metamorphosed relicts of ocean crust trapped between colliding blocks Or of basalts that filled early continental rifts Granite From magmas generated from partial melting Volcanic arcs and hot spots

Archean Principle Rock Types Graywacke Mixture of sand and clay eroded from volcanoes and deposited in the nearby deep ocean basins Chert Formed from the precipitation of silica in the deep sea Nearshore sediments rare Continents too small sed depositional environments didn t exist Or have been eroded away

Archean Oceans Earth cooled below the boiling point of water Water vapor in the atmosphere condensed and rained onto the planet Around 3.8 bya global oceans accumulated and rivers flowed over the unvegetated land Rounded grains from transportation in water Salts weathered out of rocks made the ocean salty

Archean high rate of continental formation W. W. Norton

The Beginning of Life 3.8 bya Prokaryotic cells First cells No nucleus Cyanobacteria (blue green algae) Rocks contain traces of organic carbon Evidence Chert beds in western Australia contain fossils that resemble cyanobacteria 3.2 bya stromatolites

Stromatolites Geological Survey of Newfoundland & Labrador W. W. Norton

The Proterozoic First Life Transition to the Modern World

The Proterozoic First Life Transition to the Modern World Named before discovery of Archean fossil bacteria Spans approximaely 2 by 2.5 bya to beginning of Cambrian 545 mya Half of Earth s known history Large lithospheric plates, large continents, oxygen rich atmosphere

Rate of continental formation slowed By middle of the Proterozoic 90% of continental crust formed Tectonic processes slowed Oceanic plates grew larger Proterozoic

Proterozoic Collisions between large Archean cratons and the accretion of new volcanic island arcs and hot spot volcanoes resulted in large cratons Interiors far from orgenic activity Some still exist and remain intact today Formed around 1.8 bya

Distribution of PreCambrian Crust W. W. Norton

Shield Proterozoic Canadian Shield Broad, low-lying region of exposed Precambrian rocks Cratonic Platform (Continental Platform) In U.S. most Precambrian rocks covered over by Phanerozoic (younger than 540my) strata Only see PreC rocks where exposed by erosion of the sedimentary cover

Fig. 13.09 Distribution of PreC crust. The cratons formed in Proterozoic time (2.5by 540my), either from magmas rising from the mantle or by the metamorphism and deformation of existing Archean crust.

NA Craton built from fragments joined during PreC collision: large blocks joined accretion: volcanic island arcs, hot-spot volcanoes and small continents W. W. Norton. Modified from Hoffman, 1988

Super continents at the end of the PreC Rodinia formed around 1 bya; lasted until around 700 mya Breakup resulted in rift basins and eventually a passive margin basin along western NA

Changing Rock Types Sedimentary Shallow marine sediments common Quartz sandstone Limestone Due to abundance of calcite-secreting microorganisms (bacteria and algae)

Change in Oxygen present O levels at ~21% reached during the Phanreozoic eon W. W. Norton

Evidence for Oxygen rich atmosphere Banded iron formations Alternating layers of hematite or magnetite (iron oxides) and chert. In the presence of abundant O, iron becomes less soluble in sea-water With higher concentration of O in the atmosphere iron precipitated out of the sea to form the BIF

Evolution of Earth and Life original art by Gary Hincks

Phanerozoic: Visible Life 545 million years of Earth s time

Paleozoic Era From Rodinia to Pangaea Distribution of continents during the Cambrian; about 510 mya Passive margin basin on the east coast of Laruentia (now eastern NA) Sea level rose Continents flooded with shallow seas Well lit; abundant life Epicontinental Seas Moon closer; huge tides

North America during the Paleozoic Dry land Shallow seas Passive margins

Early Ordovician Sea level fell briefly Formation of erosional surface on top of existing strata Leading to unconformity

Middle Ordovician Second great flooding of continents Unconformity New sedimentary strata blanketed These high seas lasted approximately 50 my Eastern margin of Laurentia rams into volcanic island arc Taconic Orogeny First stage in development of the Applachians

Fig. 13.16 a, b W. W. Norton

Life Evolution Cambrian Explosion Diversification of life Occurred during the breakup of a supercontinent New ecological niches Isolation of populations

Fig. 13.18 W. W. Norton

Fig. 13.19 W. W. Norton. Modified from Dott and Prothero

Fig. 13.20 W. W. Norton

Fig. 13.21 W. W. Norton

Fig. 13.22 W. W. Norton

Fig. 13.23

Fig. 13.24 W. W. Norton

Fig. 13.25 W. W. Norton

Fig. 13.26 W. W. Norton

Fig. 13.29 a W. W. Norton

Fig. 13.29 b W. W. Norton. Modified from Sloss, 1962

Fig. 13.29 c W. W. Norton. Adapted from Haq et al., 1987

Fig. 13.30 W. W. Norton

Fig. 13.31 a, b W. W. Norton. Adapted from Atwater, 1989

Fig. 13.31 c

Fig. 13.32 a, b W. W. Norton

Major land and sea regions in North and South America during the Carboniferous Period. (Adapted from Ross, C. A. 1967. J. Paleontol. 41(6): 1341-1354.)

Fig. 13.33 W. W. Norton

Fig. 13.34 W. W. Norton