Evolution of the Earth 地球的發(fā)展演化

1、Evolution of the EarthSeventh EditionProthero DottChapter 8Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Cryptozoic History: Introduction to the Origin of Continental CrustFigure 4.7Figure 4.8Complete Geologic Time ScaleHadean to RecentPhanerozoic “visible

2、 life”Geologic Time Scale for 1st 3.8 Billion Years of Earth ExistenceProterozoic - “hidden life”Archean life first appears (?) and remains viableHadean meteorite bombardment, life started and restarted?Chap. 8 - Origin of Continental Crust Main Topics Earth cooled sufficiently to permit formation o

3、f early continental (granitic) material Isotopic age dates within continents “cluster” suggesting several periods of “orogeny” Early continents seem to represent “partial melts” of andesitic volcanics or early sediments. Most of the present-day volume of continental material had formed by 2.5 billio

4、n yrs. ago.Chap. 8 - Origin of Continental CrustMain Topics (cont.) Archean (3800 2500 Bya) rocks characterized by “greenstone” belts and texturally immature sediments (graywackes), largely form oceanic arcs. Suggesting plate tectonics may have started? Proterozoic (2500 540 Bya) rocks are texturall

5、y and compositionally mature, include chemical sediments (carbonates and evaporites). Stromatolites are present showing life had evolved while evaporites suggest that sea water had also evolved to its present compositionFig. 8.1Atrists conception of what surface of earth looked like during its first

6、 500 million years.Surface was largely molten, with a few of the original microcontinents beginning to form.Intense meteorite bombardment heated surface to melting. Moon was twice as close, exerting a very strong gravitational pull.Early atmosphere had no O2, but probably consisted of N2, CH4, NH3,

7、CO2 and H2O. Note no oceans.Evidence of Crustal Development from Igneous and Metamorphic Rocks Importance of Granite Rock-types surviving from early Cryptozic are mainly granitic in composition and they are arrangemed in highly deformed orogenic belts. This has led to hypothesis of continential accr

8、etion of early granitic masses into protocontinents and then continents.Evidence of Crustal Development from Igneous and Metamorphic Rocks However field evidence suggests that granitic continental crust was not original and must have increased in volume through time. Original crust was thin and main

9、ly basalt. Weathering, erosion and igneous activity converted some of the original crust to granite to form embryonic continents. Embryonic continents persisted on surface of earth and accreted slowly to form larger continents.Fig. 8.10Archean granite (light) intruding metavolcanic (metamorphosed vo

10、lcanic ash, etc.) sediments. Nestor Falls. Ontario. Granite is about 2.5 By (Algoman orogeny).Fig. 8.2High-grade metamorphic rock (gneiss) typical of ancient “shield” regions. Sondre Stromfjord, SW Greenland.Age of rocks in this picture are 3.8 By.Cryptozoic (“hidden life”) EonFig. 8.6Cross-section

11、from N. Shore of L. Superior to northern Michigan. Numbers refer to relative age (1 = oldest).Development of a Cryptozoic Chronology Age dating of ancient rocks showed patterns of old rocks bounded by younger rocks in patterns that suggested accretion of younger material onto a core of older, mostly

12、 granitic, rock. Thus the modern continents have a history of growth by addition of smaller granitic masses, which persisted through time because of their greater buoyancy. Fig. 8.3Map showing locations of all Cryptozoic and early Paleozoic rocks in the world. Numbers refer to age in By.Fig. 8.11The

13、se geologic provinces form the core of the North American craton.The older rocks probably accreted about 1.8 - 1.9 Bya. The Grenville Province was sutured about 1.0 Bya.(craton = stable nucleus of a continent)Isotopic age dates show great discordance when mapped over the entire N. American craton.Gr

14、eenstone Belts “Greenstone Belts” are basically metamorphosed basalts and graywacke (discussed below) sandstones deposited as pillow lavas and turbidity flows on the floors of ancient seas. When protocontinents collided and accreted, the ocean floors filled with these basalts and graywackes collapse

15、d, forming greenstone belts that also accreted to the growing protocontinent. Thus some of the early seafloor survived destruction (by subduction) and became part of the stable craton.Fig. 8.12Evolution of greenstone belts. A. Basins between protocontinents fill with basalts, B. when protocontinents

16、 collide, they “collapse” the oceans filled with basalts and graywackes, forming greenstone belts.Fig. 8.13Hypothetical scenario for assembly of N. American craton during Proterozoic. Based on dates and tectonic patterns in previous figure.Interpretation of Crustal Development from Sediments Terrige

17、nous vs. nonterrigenous sediments Composition of sedimentary rock reflects source Clastic sediments primarily silicates, derived from erosion of older rocks in land areas Chemical sediments evaporites (salt NaCl, gypsum CaSO4) and carbonates. Precipitates or bio-precipitates in warm, shallow seasFig

18、. 8.14Stages in the development of textural maturity in a sand through abrasion and sorting of grains. Size tends to decrease with time and transport distance. Clay minerals form, from from chemically unstable minerals such as feldspars and amphiboles and quartz is concentrated in residue. Final sta

19、ge is a pure quartz sandstone, but often only after several tectonic (erosion, burial, uplift) cycles.Fig. 8.15Steps in the evolution of a mature sand from initial weathering of a granite.Texturally mature sand is mono-minerallic (quartz), well-rounded and of a uniform grain size. This indicates a l

20、ong time spent in transport or washing around on a beach. It may also be 2nd or even 3rd cycle. Graywacke suggests rapid transport and burial (why?) while arkosic sands suggest longer transport or more intense weathering in place, since most unstable minerals (amphiboles) are missing.graywacke arkos

21、e quartziteFig. 8.16aPhotomicrograph of a graywacke sandstone showing lack of textural maturity (angular grains, many unstable minerals and poor sorting (a wide range of grain sizes. This rock is 1st cycle, deposited rapidly, perhaps as a turbidite and spent little or no time in a high-energy enviro

22、nment such as a beach.This type of rock would be expected to be common on the early (Archean) earth.Fig. 8.8aGraded bedding (grain size decreases upward in the gray beds) in Archean graywacke from Ely, Mn.Fig. 8.8bArchean graywacke showing multiple graded beds and interstratified limestones.East of

23、Great Slave Lake, Northwest Territories, Canada.Fig. 8.20Fig. 8.16bPhotomicrograph of a pure quartz sandstone characterized by good sorting (mono-minerallic, one dominant grain size) well-rounded grains and absence of clay and unstable minerals.This type of rock would be expected to be found on a st

24、able craton where it could spend a lot of time (millions (?) of years ) washing around as loose grains on a beach.This rock could be 2nd or 3rd cycle from pre-existing sediments as they were buried, consolidated and then uplifted and eroded.One example of a classification chart for sedimentary rocks

25、Sediment composition triangle The diagram shows the range of sedimentary rock types represented as mixtures of three components: calcium (plus magnesium) carbonates, clay minerals (represented by the hypothetical hydrated aluminum and iron oxides as the end member), and silica (silicon dioxide). Sed

26、iments and sedimentary rocks have the same ranges of composition. Iron-rich laterites and aluminum-rich bauxites are the products of intense weathering. Sandstones are primarily composed of indurated sandy sediments, in many cases dominantly quartz. Argillaceous rocks are formed by lithification of

27、clay-rich muds. Sediments or sedimentary rocks rarely, if ever, have compositions represented by the white area of the triangle. Cherts are the sedimentary rock equivalent of biologically deposited siliceous deposits. During the transformation into rock, the amorphous silica, originally deposited by

28、 diatoms and radiolarians, is transformed into very hard microcrystalline quartz-rich rock. A simple model showing how different tectonic regimes lead to different types of sandstone deposition. QFL triangular diagrams are usual method of depicting sandstone composition and hence provenance (source)

29、 and history.QFL = Quartz, Feldspar, Lithic fragmentsSEDIMENTARY DEPOSITIONAL ENVIRONMENTS“Long” vs “short” system models for sedimentary deposition environments. Note both systems eventually result in submarine fans but long reach has more and varied environments.Fig. 8.9Cross-bedded 1.75 By sandst

30、ones from the Big Bear Formation, Coppermine River, NW Territories, Canada. Cross-beds are produced when coarse sand is deposited by water (fluvial) or wind (aeolian). These are probably aeolian._Fig. 8.17Ripple marks in early Proterozoic (Huronian) quartzite. 30 miles east of Sault Ste. Marie, Onta

31、rio. Ripple marks contain information on direction of sediment transport as well as being “tops” indicators.Block diagram showing origin of cross-stratification by migration of ripples. Cross-bedding reveals top and bottom as well as current direction.Fig. 8.19Comparison of relative sorting of sand

32、grain sizes by different sedimentary processes. Sorting can help determine the origin of a sandstone.Origin of Life - Stromatolites A special type of rock exists throughout the geologic record, called stromatolites, which record the very first visible evidence of life, as early as 3.465 billion year

33、s ago. These rocks are actually comples colonies of different types of bacteria, each type occuping a special niche in the colony. The most important are the photosynthetic cyanobacteria (formerly blue green algae) common pond scum. These amazing life forms are highly adaptable and form the base of

34、the first food chain. Oh yes, they also are responsible for all the oxygen in the air. O2 is a waste product of their photosynthesis. Plants later likely simply incorporated a version of cyanobaterial to carry out their photosynthesis. Nature rarely reinvents a wheel. Fig. 8.22Outcrop of a stromatol

35、ite “reef” from 1.6-billion year old Proterozoic carbonate in the Wopmay orogen. These reefs were formed by colonies of photosynthetic “blue-green” algae, cyanobacteria and represent some of the first life forms on earth.Fig. 8.22Modern algae from Shark Bay Australia. They survive in the hypersaline

36、 lagoons because predators cannot tolerate the high salt content.Shark Bay A Glimpse into the ArcheanFig. 8.28Model showing schematically how cyanobacteria changed the world. Note the iron minerals (BIFs) in A and the oxygen segregation in the oceans (B).Fig. 8.7Banded Iron Formation (“BIF”) near Ja

37、sper Nob, Ishpeming MI. Chert (red) iron (gray).Fig. 8.30Oolites in Banded Iron Formation (BIF), N. Michigan. Oolites are now chert (SiO2) but were most likely originally deposited as carbonate (CaCO3). Jolters Key in the Bahamas may be a modern analog for the original depositional environment.Moder

38、n habitat of ooids Jolters Cay in Bahamas (Island in center of picture). Modern ooids form in the warm, shallow waters in the lee of the islandFig. 8.29SEM photographic of chert showing the sponge spicules that make up the bulk of the rock. Magnification 160 x.Fig. 8.23Fig. 8.24Continental growth by accretion of small plates (“strange terrains”). Note the “suture” zone between the two colliding granitic masses. The following slides of E. Africa show a modern “aulacogen” in the process of developing.Fig. 8.26Another product of a failed rift,