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44-464: The Oella Formation is a Late Proterozoic or early Cambrian schist in Howard and Baltimore Counties, Maryland. It is described as "Medium-grained biotite - plagioclase - muscovite - quartz schist, locally garnetiferous , interlayered on a centimeter to decimeter scale with fine-grained biotite-plagioclase-quartz gneiss , commonly bearing muscovite but less commonly garnet." The type locality

88-414: A e f f T 1 κ t + d r {\displaystyle d(t)={\frac {2}{\sqrt {\pi }}}a_{\rm {eff}}T_{1}{\sqrt {\kappa t}}+d_{\rm {r}}} where κ is the thermal diffusivity of the mantle lithosphere ( c. 8 × 10   m / s ), a eff is the effective thermal expansion coefficient for rock ( c. 5.7 × 10  °C ), T 1

132-589: A few billion years in age. It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic . Studies by Condie (2000) and Rino et al. (2004) harvp error: no target: CITEREFRinoKomiyaWindleyet_al2004 ( help ) suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it

176-481: A few plausible models that explain tectonics of the early Earth prior to the formation of Columbia, but the current most plausible hypothesis is that prior to Columbia, there were only a few independent cratons scattered around the Earth (not necessarily a supercontinent, like Rodinia or Columbia). The Proterozoic can be roughly divided into seven biostratigraphic zones which correspond to informal time periods. The first

220-526: A period of increasing crustal recycling, suggesting subduction . Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6  Ga . The occurrence of eclogite (a type of metamorphic rock created by high pressure, > 1 GPa), is explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor

264-407: A result of continental collision (compressing the continents increases ocean area and lowers sea level). Increasing sea level will flood the continents, while decreasing sea level will expose continental shelves . Because the continental shelf has a very low slope, a small increase in sea level will result in a large change in the percent of continents flooded. If the world ocean on average is young,

308-500: Is along the Patapsco River at Oella , southwest Baltimore County . Proterozoic The Proterozoic ( IPA : / ˌ p r oʊ t ər ə ˈ z oʊ ɪ k , ˌ p r ɒ t -, - ər oʊ -, - t r ə -, - t r oʊ -/ PROH -tər-ə- ZOH -ik, PROT-, -⁠ər-oh-, -⁠trə-, -⁠troh- ) is the third of the four geologic eons of Earth's history , spanning the time interval from 2500 to 538.8   Mya ,

352-436: Is based on the observation that if only small peripheral modifications are made to the primary reconstruction, the data show that the palaeomagnetic poles converged to quasi-static positions for long intervals between about 2.7–2.2 Ga; 1.5–1.25 Ga; and 0.75–0.6 Ga. During the intervening periods, the poles appear to have conformed to a unified apparent polar wander path. The paleomagnetic data are adequately explained by

396-413: Is concurrent with the shorter-term Wilson Cycle named after plate tectonics pioneer John Tuzo Wilson , which describes the periodic opening and closing of oceanic basins from a single plate rift. The oldest seafloor material found today dates to 170 Ma, whereas the oldest continental crust material found today dates to 4 Ga, showing the relative brevity of the regional Wilson cycles compared to

440-469: Is currently in a short greenhouse phase of an icehouse climate. Periods of icehouse climate include much of the Neoproterozoic , late Paleozoic , late Cenozoic , while periods of greenhouse climate include early Paleozoic , Mesozoic –early Cenozoic . The principal mechanism for evolution is natural selection among diverse populations. Diversity, as measured by the number of families, follows

484-524: Is known that sea level is generally low when the continents are together and high when they are apart. For example, sea level was low at the time of formation of Pangaea ( Permian ) and Pannotia (latest Neoproterozoic ), and rose rapidly to maxima during Ordovician and Cretaceous times, when the continents were dispersed. Major influences on sea level during the break up of supercontinents include: oceanic crust age, lost back-arc basins , marine sediment depths, emplacement of large igneous provinces, and

SECTION 10

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528-547: Is the temperature of ascending magma compared to the temperature at the upper boundary ( c. 1220 °C for the Atlantic and Indian Oceans, c. 1120 °C for the eastern Pacific) and d r is the depth of the ridge below the ocean surface. After plugging in rough numbers for the sea floor, the equation becomes: for the eastern Pacific Ocean: d ( t ) = 350 t + 2500 {\displaystyle d(t)=350{\sqrt {t}}+2500} and for

572-575: The Paleozoic era. There are two different views on the history of earlier supercontinents. The first theory proposes a series of supercontinents: starting with Vaalbara (3.6 to 2.8 Ga); Ur (c. 3 Ga); Kenorland (2.7 to 2.1 Ga); Columbia (1.8 to 1.5 Ga); Rodinia (1.25 Ga to 750 Ma); and Pannotia ( c. 600 Ma), whose dispersal produced the continents that ultimately collided to form Pangaea. The kinds of minerals found inside ancient diamonds suggest that

616-661: The 300 million years-long Huronian glaciation (during the Siderian and Rhyacian periods of the Paleoproterozoic) and the hypothesized Snowball Earth (during the Cryogenian period in the late Neoproterozoic); and the Ediacaran period (635–538.8  Ma ), which is characterized by the evolution of abundant soft-bodied multicellular organisms such as sponges , algae , cnidarians , bilaterians and

660-399: The Atlantic and Indian Oceans: d ( t ) = 390 t + 2500 {\displaystyle d(t)=390{\sqrt {t}}+2500} where d is in meters and t is in millions of years, so that recently-formed crust at the mid-ocean ridges lies at about 2,500 m depth, whereas 50-million-year-old seafloor lies at a depth of about 5,000 m. As the mean level of

704-706: The Ediacaran, proving that multicellular life had already become widespread tens of millions of years before the Cambrian Explosion in what is known as the Avalon Explosion . Nonetheless, the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian , which is currently placed at 538.8 Ma. Supercontinent cycle The most recent supercontinent , Pangaea , formed about 300 million years ago (0.3 Ga ), during

748-737: The Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations. One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere. Though oxygen is believed to have been released by photosynthesis as far back as the Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur and iron had been exhausted. Until roughly 2.3 billion years ago, oxygen

792-592: The Palaeoproterozoic or Mesoproterozoic, according to molecular data. Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian Period when the first fossils of animals, including trilobites and archeocyathids , as well as the animal-like Caveasphaera , appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, particularly in ones from

836-545: The breakup of Pannotia. A north–south arrangement of continents and oceans leads to much more diversity and isolation than east–west arrangements. North-to-south arrangements give climatically different zones along the communication routes to the north and south, which are separated by water or land from other continental or oceanic zones of similar climate. Formation of similar tracts of continents and ocean basins oriented east–west would lead to much less isolation, diversification, and slower evolution, since each continent or ocean

880-486: The breakup of the supercontinent Columbia and prior to the assemblage of the supercontinent Gondwana (~500 Ma). The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the Pan-African orogeny . Columbia was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are

924-533: The cycle of supercontinental formation and breakup began roughly 3 Ga. Before 3.2 Ga, only diamonds with peridotitic compositions (commonly found in the Earth's mantle ) formed, whereas after 3.0 Ga eclogitic diamonds (rocks from the Earth's crust ) became prevalent. This change is thought to have come about as subduction and continental collision introduced eclogite into subcontinental diamond-forming fluids. The hypothesized supercontinent cycle

SECTION 20

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968-522: The deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as orogenic belts or ophiolite complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian, the Earth went through several supercontinent breakup and rebuilding cycles ( Wilson cycle ). In

1012-523: The effect of passive margin extension. Of these, oceanic crust age, and marine sediment depths seem to play some of the largest roles in creating a sea level model. The addition of the other controlling parameters help stabilize models when data is sparse. The age of the oceanic lithosphere provides a first order control on the depth of the ocean basins, and therefore on global sea level. Oceanic lithosphere forms at mid-ocean ridges and moves outwards, conductively cooling and shrinking , which decreases

1056-426: The eon continued the massive continental accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity ( orogeny ). There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during

1100-453: The existence of a single Protopangea–Paleopangea supercontinent with prolonged quasi-integrity. The prolonged duration of this supercontinent could be explained by the operation of lid tectonics (comparable to the tectonics operating on Mars and Venus) during Precambrian times, as opposed to the plate tectonics seen on the contemporary Earth. However, this approach is widely criticized as an incorrect application of paleomagnetic data. It

1144-678: The first half of the Ediacaran from 0.63–0.55 Ga, and the Belomorian, spanning from 0.55–0.542 Ga. The emergence of advanced single-celled eukaryotes began after the Oxygen Catastrophe . This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria . It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved. By

1188-419: The formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon. As a result of remelting of basaltic oceanic crust due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes. The long-term tectonic stability of those cratons is why we find continental crust ranging up to

1232-665: The late Palaeoproterozoic, eukaryotic organisms had become moderately biodiverse. The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria – in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago. The earliest fossils possessing features typical of fungi date to the Paleoproterozoic Era, some 2.4 billion years ago; these multicellular benthic organisms had filamentous structures capable of anastomosis . The Viridiplantae evolved sometime in

1276-625: The late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called Laurentia . An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the Grenville orogeny located in Eastern North America. Rodinia formed after

1320-606: The longest eon of the Earth's geologic time scale . It is preceded by the Archean and followed by the Phanerozoic , and is the most recent part of the Precambrian "supereon". The Proterozoic is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic , Mesoproterozoic and Neoproterozoic . It covers the time from the appearance of free oxygen in Earth's atmosphere to just before

1364-498: The proliferation of complex life on the Earth during the Cambrian Explosion . The name Proterozoic combines two words of Greek origin: protero- meaning "former, earlier", and -zoic , meaning "of life". Well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; the evolution of eukaryotes via symbiogenesis ; several global glaciations , which produced

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1408-428: The sea floor decreases, the volume of the ocean basins increases, and if other factors that can control sea level remain constant, sea level falls. The converse is also true: younger oceanic lithosphere leads to shallower oceans and higher sea levels if other factors remain constant. The surface area of the oceans can change when continents rift (stretching the continents decreases ocean area and raises sea level) or as

1452-399: The seafloor will be relatively shallow, and sea level will be high: more of the continents are flooded. If the world ocean is on average old, seafloor will be relatively deep, and sea level will be low: more of the continents will be exposed. There is thus a relatively simple relationship between the supercontinent cycle and the mean age of the seafloor. There will also be a climatic effect of

1496-554: The sessile Ediacaran biota (some of which had evolved sexual reproduction ) and provides the first obvious fossil evidence of life on Earth . The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas ; furthermore, many of those rocks are less metamorphosed than Archean rocks, and many are unaltered. Studies of these rocks have shown that

1540-556: The supercontinent cycle that will amplify this further: There is a progression of tectonic regimes that accompanies the supercontinent cycle: During break-up of the supercontinent, rifting environments dominate. This is followed by passive margin environments, while seafloor spreading continues and the oceans grow. This in turn is followed by the development of collisional environments that become increasingly important with time. First collisions are between continents and island arcs, but lead ultimately to continent-continent collisions. This

1584-471: The supercontinent cycle very well. As genetic drift occurs more frequently in small populations, diversity is an observed consequence of geographic isolation. Less isolation, and thus less diversification, occurs when the continents are all together, producing one continent, one continuous coast, and one ocean. In late Neoproterozoic to early Paleozoic, when the tremendous proliferation of diverse metazoa occurred, isolation of marine environments resulted from

1628-462: The thickness and increases the density of the oceanic lithosphere, and lowers the seafloor away from mid-ocean ridges. For oceanic lithosphere that is less than about 75 Ma, a simple cooling half-space model of conductive cooling works, in which the depth of the ocean basins d in areas in which there is no nearby subduction is a function of the age of the oceanic lithosphere t . In general, d ( t ) = 2 π

1672-567: The time was virtually all obligate anaerobic . A second, later surge in oxygen concentrations is called the Neoproterozoic Oxygenation Event , occurred during the Middle and Late Neoproterozoic and drove the rapid evolution of multicellular life towards the end of the era. The Proterozoic Eon was a very tectonically active period in the Earth's history. The late Archean Eon to Early Proterozoic Eon corresponds to

1716-424: The whole-planetary pulses seen in the arrangement of the continents. The second view, based on both palaeomagnetic and geological evidence, is that supercontinent cycles did not occur before about 0.6 Ga (during the Ediacaran period). Instead, the continental crust comprised a single supercontinent from about 2.7 Ga until it broke up for the first time, somewhere around 0.6 Ga. This reconstruction

1760-444: Was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period. Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. Paleomagnetic and geochronological dating mechanisms have allowed

1804-511: Was probably due to two factors: Exhaustion of the chemical sinks, and an increase in carbon sequestration , which sequestered organic compounds that would have otherwise been oxidized by the atmosphere. The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the Great Oxygenation Event , or alternately the Oxygen Catastrophe – to reflect the mass extinction of almost all life on Earth, which at

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1848-487: Was probably only 1% to 2% of its current level. The banded iron formations , which provide most of the world's iron ore , are one mark of that mineral sink process. Their accumulation ceased after 1.9 billion years ago, after the iron in the oceans had all been oxidized . Red beds , which are colored by hematite , indicate an increase in atmospheric oxygen 2 billion years ago. Such massive iron oxide formations are not found in older rocks. The oxygen buildup

1892-761: Was the Labradorian, lasting from 2.0–1.65  Ga . It was followed by the Anabarian, which lasted from 1.65–1.2 Ga and was itself followed by the Turukhanian from 1.2–1.03 Ga. The Turukhanian was succeeded by the Uchuromayan, lasting from 1.03–0.85 Ga, which was in turn succeeded by the Yuzhnouralian, lasting from 0.85–0.63 Ga. The final two zones were the Amadeusian, spanning

1936-526: Was the situation during the Paleozoic supercontinent cycle; it is being observed for the Mesozoic – Cenozoic supercontinent cycle, still in progress. There are two types of global earth climates: icehouse and greenhouse. Icehouse is characterized by frequent continental glaciations and severe desert environments. Greenhouse is characterized by warm climates. Both reflect the supercontinent cycle. The Earth

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