Supercontinent
![](https://upload.wikimedia.org/wikipedia/commons/thumb/5/58/Pangea_assembly_250.png/300px-Pangea_assembly_250.png)
![](https://upload.wikimedia.org/wikipedia/commons/thumb/0/01/Afro-Eurasia.png/220px-Afro-Eurasia.png)
Ingeology,asupercontinentis the assembly of most or all ofEarth'scontinental blocksorcratonsto form a single large landmass.[1][2][3]However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leaves room for interpretation and is easier to apply toPrecambriantimes.[4]To separate supercontinents from other groupings, a limit has been proposed in which a continent must include at least about 75% of the continental crust then in existence in order to qualify as a supercontinent.[5]
Moving under the forces ofplate tectonics,supercontinents have assembled and dispersed multiple times in the geologic past. According to modern definitions, a supercontinent does not exist today;[1]the closest is the currentAfro-Eurasianlandmass, which covers approximately 57% of Earth's total land area. The last period in which the continental landmasses were near to one another was 336 to 175 million years ago, forming the supercontinentPangaea.The positions of continents have been accurately determined back to the earlyJurassic,shortly before the breakup of Pangaea.[6]Pangaea's predecessorGondwanais not considered a supercontinent under the first definition since the landmasses ofBaltica,LaurentiaandSiberiawere separate at the time.[7]
A future supercontinent, termedPangaea Proxima,is hypothesized to form within the next 250 million years.[8]
Theories
[edit]ThePhanerozoicsupercontinent Pangaea began to break up215 Maand this distancing continues today. Because Pangaea is the most recent of Earth's supercontinents, it is the best known and understood. Contributing to Pangaea's popularity in the classroom, its reconstruction is almost as simple as fitting together the present continents bordering the Atlantic ocean like puzzle pieces.[4]
For the period before Pangaea, there are two contrasting models for supercontinent evolution throughgeological time.
Series
[edit]The first model theorizes that at least two separate supercontinents existed comprisingVaalbaraandKenorland,with Kenorland comprisingSuperiaandSclavia.These parts ofNeoarcheanage broke off at ~2480 and2312 Ma,and portions of them later collided to formNuna(Northern Europe and North America). Nuna continued to develop during theMesoproterozoic,primarily by lateralaccretionof juvenile arcs, and in~1000 MaNuna collided with other land masses, formingRodinia.[4]Between ~825 and750 MaRodinia broke apart.[9]However, before completely breaking up, some fragments of Rodinia had already come together to formGondwanaby~608 Ma.Pangaea formed through the collision of Gondwana,Laurasia(LaurentiaandBaltica), andSiberia.
Protopangea–Paleopangea
[edit]The second model (Kenorland-Arctica) is based on bothpalaeomagneticand geological evidence and proposes that the continental crust comprised a single supercontinent from~2.72 Gauntil break-up during theEdiacaranperiod after~0.573 Ga.Thereconstruction[10]is derived from the observation that palaeomagnetic poles converge to quasi-static positions for long intervals between ~2.72–2.115 Ga; 1.35–1.13 Ga; and0.75–0.573 Gawith only small peripheral modifications to the reconstruction.[11]During the intervening periods, the poles conform to a unified apparent polar wander path.
Although it contrasts the first model, the first phase (Protopangea) essentially incorporates Vaalbara and Kenorland of the first model. The explanation for the prolonged duration of the Protopangea–Paleopangea supercontinent appears to be thatlid tectonics(comparable to the tectonics operating on Mars and Venus) prevailed duringPrecambriantimes. According to this theory,plate tectonicsas seen on the contemporary Earth became dominant only during the latter part of geological times.[11]This approach was widely criticized by many researchers as it uses incorrect application of paleomagnetic data.[12]
Cycles
[edit]Asupercontinent cycleis the break-up of one supercontinent and the development of another, which takes place on a global scale.[4]Supercontinent cycles are not the same as theWilson cycle,which is the opening and closing of an individualoceanic basin.The Wilson cycle rarely synchronizes with the timing of a supercontinent cycle.[1]However, supercontinent cycles and Wilson cycles were both involved in the creation of Pangaea and Rodinia.[6]
Seculartrends such ascarbonatites,granulites,eclogites,andgreenstone beltdeformation events are all possible indicators of Precambrian supercontinent cyclicity, although the Protopangea–Paleopangea solution implies that Phanerozoic style of supercontinent cycles did not operate during these times. Also, there are instances where these secular trends have a weak, uneven, or absent imprint on the supercontinent cycle; secular methods for supercontinent reconstruction will produce results that have only one explanation, and each explanation for a trend must fit in with the rest.[4]
The following table names reconstructed ancient supercontinents, using Bradley's 2011 looser definition,[7]with an approximate timescale of millions of years ago (Ma).
Supercontinent name | Age (Ma) | Period/Era Range | Comment |
---|---|---|---|
Vaalbara | 3,636–2,803 | Eoarchean-Mesoarchean | Also described as a supercraton or just a continent[13] |
Ur | 2,803–2,408 | Mesoarchean-Siderian | Described as both a continent[2]and a supercontinent[14] |
Kenorland | 2,720–2,114 | Neoarchean-Rhyacian | Alternatively the continents may have formed into two groupingsSuperiaandSclavia[15][4] |
Arctica | 2,114–1,995 | Rhyacian-Orosirian | Not generally regarded as a supercontinent, depending on definition[2] |
Atlantica | 1,991–1,124 | Orosirian-Stenian | Not generally regarded as a supercontinent, depending on definition[2] |
Columbia (Nuna) | 1,820–1,350 | Orosirian-Ectasian | [15] |
Rodinia | 1,130–750 | Stenian-Tonian | [15] |
Pannotia | 633–573 | Ediacaran | [15] |
Gondwana | 550–175 | Ediacaran-Jurassic | From the Carboniferous, formed part of Pangaea,[4]not always regarded as a supercontinent[16] |
Pangaea | 336–175 | Carboniferous-Jurassic |
Volcanism
[edit]![](https://upload.wikimedia.org/wikipedia/en/thumb/7/70/FigureSlabAvalanche.jpg/220px-FigureSlabAvalanche.jpg)
![](https://upload.wikimedia.org/wikipedia/en/thumb/5/5a/FigureSupercontinentBreakup.jpg/220px-FigureSupercontinentBreakup.jpg)
The causes of supercontinent assembly and dispersal are thought to be driven byconvectionprocesses inEarth's mantle.Approximately 660 km into the mantle, a discontinuity occurs, affecting thesurface crustthrough processes involvingplumesandsuperplumes(akalarge low-shear-velocity provinces). When a slab of thesubductedcrust is denser than the surrounding mantle, it sinks to discontinuity. Once the slabs build up, they will sink through to thelower mantlein what is known as a "slab avalanche". This displacement at the discontinuity will cause the lower mantle to compensate and rise elsewhere. The rising mantle can form a plume or superplume.[1]
Besides having compositional effects on theupper mantleby replenishing thelarge-ion lithophile elements,volcanism affects plate movement.[1]The plates will be moved towards a geoidal low perhaps where the slab avalanche occurred and pushed away from the geoidal high that can be caused by the plumes or superplumes. This causes the continents to push together to form supercontinents and was evidently the process that operated to cause the early continental crust to aggregate into Protopangea.[17]
Dispersal of supercontinents is caused by the accumulation of heat underneath the crust due to the rising of very largeconvection cellsor plumes, and a massive heat release resulted in the final break-up of Paleopangea.[18]Accretion occurs over geoidal lows that can be caused by avalanche slabs or the downgoing limbs of convection cells. Evidence of the accretion and dispersion of supercontinents is seen in the geological rock record.
The influence of known volcanic eruptions does not compare to that offlood basalts.The timing of flood basalts has corresponded with a large-scale continental break-up. However, due to a lack of data on the time required to produce flood basalts, the climatic impact is difficult to quantify. The timing of a single lava flow is also undetermined. These are important factors on how flood basalts influencedpaleoclimate.[6]
Plate tectonics
[edit]Globalpalaeogeographyand plate interactions as far back as Pangaea are relatively well understood today. However, the evidence becomes more sparse further back in geologic history. Marine magnetic anomalies,passive marginmatch-ups, geologic interpretation oforogenic belts,paleomagnetism,paleobiogeographyof fossils, and distribution of climatically sensitive strata are all methods to obtain evidence for continent locality and indicators of the environment throughout time.[4]
Phanerozoic (541 Ma to present) and Precambrian (4.6 Gato541 Ma) had primarily passive margins and detritalzircons(and orogenicgranites), whereas the tenure of Pangaea contained few.[4]Matching edges of continents are where passive margins form. The edges of these continents mayrift.At this point,seafloor spreadingbecomes the driving force. Passive margins are therefore born during the break-up of supercontinents and die during supercontinent assembly. Pangaea's supercontinent cycle is a good example of the efficiency of using the presence or lack of these entities to record the development, tenure, and break-up of supercontinents. There is a sharp decrease in passive margins between 500 and350 Maduring the timing of Pangaea's assembly. The tenure of Pangaea is marked by a low number of passive margins during 336 to275 Ma,and its break-up is indicated accurately by an increase in passive margins.[4]
Orogenic belts can form during the assembly of continents and supercontinents. The orogenic belts present on continental blocks are classified into three different categories and have implications for interpreting geologic bodies.[1]Intercratonic orogenic belts are characteristic of ocean basin closure. Clear indicators of intracratonic activity containophiolitesand other oceanic materials that are present in the suture zone. Intracratonic orogenic belts occur as thrust belts and do not contain any oceanic material. However, the absence of ophiolites is not strong evidence for intracratonic belts, because the oceanic material can be squeezed out and eroded away in an intracratonic environment. The third kind of orogenic belt is a confined orogenic belt which is the closure of small basins. The assembly of a supercontinent would have to show intracratonic orogenic belts.[1]However, interpretation of orogenic belts can be difficult.
The collision of Gondwana and Laurasia occurred in the late Palaeozoic. By this collision, theVariscan mountain rangewas created, along the equator.[6]This 6000-km-long mountain range is usually referred to in two parts: theHercynian mountain rangeof the late Carboniferous makes up the eastern part, and the western part is theAppalachian Mountains,uplifted in theearly Permian.(The existence of a flat elevated plateau like theTibetan Plateauis under debate.) The locality of the Variscan range made it influential to both the northern and southern hemispheres. The elevation of the Appalachians would greatly influence global atmospheric circulation.[6]
Climate
[edit]Continents affect the climate of the planet drastically, with supercontinents having a larger, more prevalent influence. Continents modify global wind patterns, control ocean current paths, and have a higheralbedothan the oceans.[1]Winds are redirected by mountains, and albedo differences cause shifts in onshore winds. Higher elevation in continental interiors produces a cooler, drier climate, the phenomenon ofcontinentality.This is seen today inEurasia,and rock record shows evidence of continentality in the middle of Pangaea.[1]
Glacial
[edit]The term glacial-epoch refers to a long episode ofglaciationon Earth over millions of years.[19]Glaciers have major implications on the climate, particularly throughsea level change.Changes in the position and elevation of the continents, the paleolatitude and ocean circulation affect the glacial epochs. There is an association between the rifting and breakup of continents and supercontinents and glacial epochs.[19]According to the model for Precambrian supercontinent series, the breakup of Kenorland and Rodinia was associated with thePaleoproterozoicandNeoproterozoicglacial epochs, respectively.
In contrast, the Protopangea–Paleopangea theory shows that these glaciations correlated with periods of low continental velocity, and it is concluded that a fall in tectonic and corresponding volcanic activity was responsible for these intervals of global frigidity.[11]During the accumulation of supercontinents with times of regional uplift, glacial epochs seem to be rare with little supporting evidence. However, the lack of evidence does not allow for the conclusion that glacial epochs are not associated with the collisional assembly of supercontinents.[19]This could just represent apreservation bias.
During the lateOrdovician(~458.4 Ma), the particular configuration of Gondwana may have allowed for glaciation and high CO2levels to occur at the same time.[20]However, some geologists disagree and think that there was a temperature increase at this time. This increase may have been strongly influenced by the movement of Gondwana across the South Pole, which may have prevented lengthy snow accumulation. Although late Ordovician temperatures at the South Pole may have reached freezing, there were no ice sheets during theearly Silurian(~443.8 Ma)through the lateMississippian(~330.9 Ma).[6]Agreement can be met with the theory that continental snow can occur when the edge of a continent is near the pole. Therefore Gondwana, although located tangent to the South Pole, may have experienced glaciation along its coasts.[20]
Precipitation
[edit]Though precipitation rates duringmonsoonalcirculations are difficult to predict, there is evidence for a large orographic barrier within the interior of Pangaea during the late Paleozoic(~251.9 Ma).The possibility of the southwest–northeast trending Appalachian-Hercynian Mountains makes the region's monsoonal circulations potentially relatable to present-day monsoonal circulations surrounding the Tibetan Plateau, which is known to positively influence the magnitude of monsoonal periods within Eurasia. It is therefore somewhat expected that lower topography in other regions of the supercontinent during theJurassicwould negatively influence precipitation variations. The breakup of supercontinents may have affected local precipitation.[21]When any supercontinent breaks up, there will be an increase in precipitationrunoffover the surface of the continental landmasses, increasingsilicateweatheringand the consumption of CO2.[9]
Temperature
[edit]Even though during the Archaean solar radiation was reduced by 30 percent and theCambrian-Precambrianboundary by 6 percent, the Earth has only experienced threeice agesthroughout the Precambrian.[6]Erroneous conclusions are more likely to be made when models are limited to one climatic configuration (which is usually present-day).[22]
Cold winters in continental interiors are due to rate ratios of radiative cooling (greater) and heat transport from continental rims. To raise winter temperatures within continental interiors, the rate of heat transport must increase to become greater than the rate of radiative cooling. Through climate models, alterations in atmospheric CO2content and ocean heat transport are not comparatively effective.[22]
CO2models suggest that values were low in the late Cenozoic and Carboniferous-Permian glaciations. Although early Paleozoic values are much larger (more than 10 percent higher than that of today). This may be due to high seafloor spreading rates after the breakup of Precambrian supercontinents and the lack of land plants as acarbon sink.[20]
During the late Permian, it is expected that seasonal Pangaean temperatures varied drastically. Subtropic summer temperatures were warmer than that of today by as much as 6–10 degrees, and mid-latitudes in the winter were less than −30 degrees Celsius. These seasonal changes within the supercontinent were influenced by the large size of Pangaea. And, just like today, coastal regions experienced much less variation.[6]
During the Jurassic, summer temperatures did not rise above zero degrees Celsius along the northern rim of Laurasia, which was the northernmost part of Pangaea (the southernmost portion of Pangaea was Gondwana). Ice-rafteddropstonessourced from Russia are indicators of this northern boundary. The Jurassic is thought to have been approximately 10 degrees Celsius warmer along 90 degrees Eastpaleolongitudecompared to the present temperature of today's central Eurasia.[22]
Milankovitch cycles
[edit]Many studies of theMilankovitch cyclesduring supercontinent time periods have focused on the mid-Cretaceous. Present amplitudes of Milankovitch cycles over present-day Eurasia may be mirrored in both the southern and northern hemispheres of the supercontinent Pangaea. Climate modeling shows that summer fluctuations varied 14–16 degrees Celsius on Pangaea, which is similar or slightly higher than summer temperatures of Eurasia during the Pleistocene. The largest-amplitude Milankovitch cycles are expected to have been at mid-to high-latitudes during the Triassic and Jurassic.[22]
Atmospheric gases
[edit]Plate tectonicsand the chemical composition of the atmosphere (specificallygreenhouse gases) are the two most prevailing factors present within the geologic time scale.Continental driftinfluences both cold and warm climatic episodes. Atmospheric circulation and climate are strongly influenced by the location and formation of continents and supercontinents. Therefore, continental drift influences mean global temperature.[6]
Oxygen levels of the Archaean were negligible, and today they are roughly 21 percent. It is thought that the Earth's oxygen content has risen in stages: six or seven steps that are timed very closely to the development of Earth's supercontinents.[23]
- Continents collide
- Super-mountains form
- Erosion of super-mountains
- Large quantities of minerals and nutrients wash out to open ocean
- Explosion of marine algae life (partly sourced from noted nutrients)
- Mass amounts of oxygen produced during photosynthesis
The process of Earth's increase in atmospheric oxygen content is theorized to have started with the continent-continent collision of huge landmasses forming supercontinents, and therefore possibly supercontinent mountain ranges (super-mountains). These super-mountains would have eroded, and the mass amounts of nutrients, includingironandphosphorus,would have washed into oceans, just as is seen happening today. The oceans would then be rich in nutrients essential to photosynthetic organisms, which would then be able to respire mass amounts of oxygen. There is an apparent direct relationship betweenorogenyand the atmospheric oxygen content. There is also evidence for increased sedimentation concurrent with the timing of these mass oxygenation events, meaning that the organic carbon andpyriteat these times were more likely to be buried beneath sediment and therefore unable to react with the free oxygen. This sustained the atmospheric oxygen increases.[23]
At2.65 Gathere was an increase inmolybdenum isotopefractionation. It was temporary but supports the increase in atmospheric oxygen because molybdenum isotopes require free oxygen to fractionate. Between 2.45 and2.32 Ga,the second period of oxygenation occurred, which has been called the 'great oxygenation event.' Evidence supporting this event includesred bedsappearance2.3 Ga(meaning that Fe3+was being produced and became an important component in soils).
The third oxygenation stage approximately1.8 Gais indicated by the disappearance of iron formations.Neodymiumisotopic studies suggest that iron formations are usually from continental sources, meaning that dissolved Fe and Fe2+had to be transported during continental erosion. A rise in atmospheric oxygen prevents Fe transport, so the lack of iron formations may have been the result of an increase in oxygen. The fourth oxygenation event, roughly0.6 Ga,is based on modeled rates ofsulfur isotopesfrom marine carbonate-associatedsulfates.An increase (near doubled concentration) of sulfur isotopes, which is suggested by these models, would require an increase in the oxygen content of the deep oceans.
Between 650 and550 Mathere were three increases in ocean oxygen levels, this period is the fifth oxygenation stage. One of the reasons indicating this period to be an oxygenation event is the increase inredox-sensitivemolybdenumin blackshales.The sixth event occurred between 360 and260 Maand was identified by models suggesting shifts in the balance of34S insulfatesand13C incarbonates,which were strongly influenced by an increase in atmospheric oxygen.[23][24]
Proxies
[edit]![](https://upload.wikimedia.org/wikipedia/en/thumb/7/71/FigureUPbZircons.jpg/220px-FigureUPbZircons.jpg)
Granites and detrital zircons have notably similar and episodic appearances in the rock record. Their fluctuations correlate with Precambrian supercontinent cycles. TheU–Pb zircon datesfrom orogenic granites are among the most reliable aging determinants.
Some issues exist with relying on granite sourced zircons, such as a lack of evenly globally sourced data and the loss of granite zircons by sedimentary coverage orplutonicconsumption. Where granite zircons are less adequate, detrital zircons fromsandstonesappear and make up for the gaps. These detrital zircons are taken from the sands of major modern rivers and theirdrainage basins.[4]Oceanic magnetic anomalies and paleomagnetic data are the primary resources used for reconstructing continent and supercontinent locations back to roughly 150 Ma.[6]
See also
[edit]References
[edit]- ^abcdefghijkRogers, John J. W.; Santosh, M. (2004).Continents and supercontinents.New York: Oxford University Press.ISBN978-0195165890.Retrieved5 January2021.
- ^abcdRogers, J.J.W.; Santosh, M. (2002)."Configuration of Columbia, a Mesoproterozoic Supercontinent"(PDF).Gondwana Research.5(1): 5–22.Bibcode:2002GondR...5....5R.doi:10.1016/S1342-937X(05)70883-2.Archived fromthe original(PDF)on 2015-02-03.
- ^Hoffman, P.F. (1999). "The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth".Journal of African Earth Sciences.28(1): 17–33.Bibcode:1999JAfES..28...17H.doi:10.1016/S0899-5362(99)00018-4.
- ^abcdefghijkBradley, D.C. (2011). "Secular Trends in the Geologic Record and the Supercontinent Cycle".Earth-Science Reviews.108(1–2): 16–33.Bibcode:2011ESRv..108...16B.CiteSeerX10.1.1.715.6618.doi:10.1016/j.earscirev.2011.05.003.S2CID140601854.
- ^Meert, J.G. (2012). "What's in a name? The Columbia (Paleopangaea/Nuna) supercontinent".Gondwana Research.21(4): 987–993.Bibcode:2012GondR..21..987M.doi:10.1016/j.gr.2011.12.002.
- ^abcdefghijFluteau, Frédéric. (2003). "Earth dynamics and climate changes". C. R. Geoscience335(1): 157–174. doi:10.1016/S1631-0713(03)00004-X
- ^abBradley, D. C. (23 December 2014). "Mineral evolution and Earth history".American Mineralogist.100(1): 4–5.Bibcode:2015AmMin.100....4B.doi:10.2138/am-2015-5101.S2CID140191182.
- ^Williams, Caroline; Nield, Ted (October 2007). "Earth's next supercontinent".New Scientist.196(2626): 36–40.doi:10.1016/S0262-4079(07)62661-X.
- ^abDonnadieu, Yannick et al. "A 'Snowball Earth' Climate Triggered by Continental Break-Up Through Changes in Runoff." Nature, 428 (2004): 303–306.
- ^Piper, J.D.A. "A planetary perspective on Earth evolution: Lid Tectonics before Plate Tectonics." Tectonophysics. 589 (2013): 44–56.
- ^abcPiper, J.D.A. "Continental velocity through geological time: the link to magmatism, crustal accretion and episodes of global cooling." Geoscience Frontiers. 4 (2013): 7–36.
- ^Z.X, Li (October 2009). "How not to build a supercontinent: A reply to J.D.A. Piper".Precambrian Research.174(1–2): 208–214.Bibcode:2009PreR..174..208L.doi:10.1016/j.precamres.2009.06.007.
- ^de Kock, M.O.; Evans, D.A.D.; Beukes, N.J. (2009)."Validating the existence of Vaalbara in the Neoarchean"(PDF).Precambrian Research.174(1–2): 145–154.Bibcode:2009PreR..174..145D.doi:10.1016/j.precamres.2009.07.002.
- ^Mahapatro, S.N.; Pant, N.C.; Bhowmik, S.K.; Tripathy, A.K.; Nanda, J.K. (2011)."Archaean granulite facies metamorphism at the Singhbhum Craton–Eastern Ghats Mobile Belt interface: implication for the Ur supercontinent assembly".Geological Journal.47(2–3): 312–333.doi:10.1002/gj.1311.S2CID127300220.
- ^abcdNance, R.D.; Murphy, J.B.; Santosh, M. (2014). "The supercontinent cycle: A retrospective essay".Gondwana Research.25(1): 4–29.Bibcode:2014GondR..25....4N.doi:10.1016/j.gr.2012.12.026.
- ^Evans, D.A.D. (2013)."Reconstructing pre-Pangean supercontinents"(PDF).GSA Bulletin.125(11–12): 1736.Bibcode:2013GSAB..125.1735E.doi:10.1130/B30950.1.
- ^Piper, J.D.A. "Protopangea: palaeomagnetic definition of Earth's oldest (Mid-Archaean-Paleoproterozoic) supercontinent." Journal of Geodynamics. 50 (2010): 154–165.
- ^Piper, J.D.A., "Paleopangea in Meso-Neoproterozoic times: the paleomagnetic evidence and implications to continental integrity, supercontinent from and Eocambrian break-up." Journal of Geodynamics. 50 (2010): 191–223.
- ^abcEyles, Nick. "Glacio-epochs and the Supercontinent Cycle after ~3.0 Ga: Tectonic Boundary Conditions for Glaciation." Paleogeography, Palaeoclimatology, Palaeoecology 258 (2008): 89–129. Print.
- ^abcCrowley, Thomas J., "Climate Change on Tectonic Time Scales". Tectonophysics. 222 (1993): 277–294.
- ^Baum, Steven K., and Thomas J. Crowley. "Milankovitch Fluctuations on Supercontinents." Geophysical Research Letters. 19 (1992): 793–796. Print.
- ^abcdBaum, Steven K., and Thomas J. Crowely. "Milankovitch Fluctuations on Supercontinents." Geophysical Research Letters. 19 (1992): 793–796. Print.
- ^abcdCampbell, Ian H., Charlotte M. Allen. "Formation of Supercontinents Linked to Increases in Atmospheric Oxygen." Nature. 1 (2008): 554–558.
- ^"G'day mate: 1.7-billion-year-old chunk of North America found in Australia".www.msn.com.Archivedfrom the original on 2018-01-25.
Further reading
[edit]- Nield, Ted,Supercontinent: Ten Billion Years in the Life of Our Planet,Harvard University Press, 2009,ISBN978-0674032453