Effects of Continents and Supercontinents on Organic Evolution

The earth’s organic life has changed continually for more than 3.5 billion years. This evolution may have resulted partly from environmental stress generated by tectonic activity within the earth and partly from processes independent of the earth’s interior. This chapter investigates these different effects in an attempt to determine the role that continents played in the evolution of organisms. Continents and tectonics associated with them may have influenced organic evolution in both active and passive ways. Active effects include several processes that partly controlled the earth’s surface environment. Climate change was caused partly by movements of continents and construction of orogenic belts. Continental rifting increased the area of shallow seas as new continental margins subsided. Changes in volume of ocean ridges and epeiric movements of continents caused marine transgressions and regressions. Temperatures of water in shallow seas increased or decreased as continents moved across latitudes. The major passive effects of continents and supercontinents result from their influence on diversity of organisms. When continents were broadly dispersed and occupied most latitudes, as on the present earth, this isolation resulted in shallow-water and subaerial families that contained numerous genera, genera with large numbers of species, and species divided among many different varieties. This diversity was clearly smaller at times when continents were aggregated into a few landmasses and particularly low when supercontinents permitted exchange of organisms throughout most of the world’s land and shallow seas. During times of major environmental stress, these differences would have restricted extinction of organisms to local species and genera during times of high diversity but might have permitted disappearance of whole orders and classes when diversity was low. Organic evolution was almost certainly affected by species diversity, but it may have occurred without any active control by tectonic processes. Although evolution probably occurs only when changing environments place stresses on organisms that enhance the competition among them, it is also possible that competition between organisms can cause evolution even without significant environmental change. Furthermore, some environmental change probably resulted from processes that are not related to the tectonics of the solid earth.

Effects of Continents and Supercontinents on Climate

Continents affect the earth’s climate because they modify global wind patterns, control the paths of ocean currents, and absorb less heat than seawater. Throughout earth history the constant movement of continents and the episodic assembly of supercontinents has influenced both global climate and the climates of individual continents. In this chapter we discuss both present climate and the history of climate as far back in the geologic record as we can draw inferences. We concentrate on longterm changes that are affected by continental movements and omit discussion of processes with periodicities less than about 20,000 years. We refer readers to Clark et al. (1999) and Cronin (1999) if they are interested in such short-term processes as El Nino, periodic variations in solar irradiance, and Heinrich events. The chapter is divided into three sections. The first section describes the processes that control climate on the earth and includes a discussion of possible causes of glaciation that occurred over much of the earth at more than one time in the past. The second section investigates the types of evidence that geologists use to infer past climates. They include specific rock types that can form only under restricted climatic conditions, varieties of individual fossils, diversity of fossil populations, and information that the 18O/16O isotopic system can provide about temperatures of formation of ancient sediments. The third section recounts the history of the earth’s climate and relates changes to the growth and movement of continents. This history takes us from the Archean, when climates are virtually unknown, through various stages in the evolution of organic life, and ultimately to the causes of the present glaciation in both the north and the south polar regions. The earth’s climate is controlled both by processes that would operate even if continents did not exist and also by the positions and topographies of continents. We begin with the general controls, then discuss the specific effects of continents, and close with a brief discussion of processes that cause glaciation. The general climate of the earth is determined by the variation in the amount of sunshine received at different latitudes, by the earth’s rotation, and by the amount of arriving solar energy that is retained in the atmosphere.

Gondwana and Pangea

Pangea, the most recent supercontinent, attained its condition of maximum packing at ~250 Ma. At this time, it consisted of a northern part, Laurasia, and a southern part, Gondwana. Gondwana contained the southern continents—South America, Africa, India, Madagascar, Australia, and Antarctica. It had become a coherent supercontinent at ~500 Ma and accreted to Pangea largely as a single block. Laurasia consisted of the northern continents—North America, Greenland, Europe, and northern Asia. It accreted during the Late Paleozoic and became a supercontinent when fusion of these continental blocks with Gondwana occurred near the end of the Paleozoic. The configuration of Pangea, including Gondwana, can be determined accurately by tracing the patterns of magnetic stripes in the oceans that opened within it (chapters 1 and 9). The history of accretion of Laurasia is also well known, but the development of Gondwana is highly controversial. Gondwana was clearly a single supercontinent by ~500 Ma, but whether it formed by fusion of a few large blocks or the assembly of numerous small blocks is uncertain. Figure 8.1 shows Gondwana divided into East and West parts, but the boundary between them is highly controversial (see below). We start this chapter by investigating the history of Gondwana, using appendix SI to describe detailed histories of orogenic belts of Pan-African age (600–500-Ma). Then we continue with the development of Pangea, including the Paleozoic orogenic belts that led to its development. The next section summarizes the paleomagnetically determined movement of blocks from the accretion of Gondwana until the assembly of Pangea, and the last section discusses the differences between Gondwana and Laurasia in Pangea. The patterns of dispersal and development of modern oceans are left to chapter 9, and the histories of continents following dispersal to chapter 10. By the later part of the 1800s, geologists working in the southern hemisphere realized that the Paleozoic fossils that occurred there were very different from those in the northern hemisphere. They found similar fossils in South America, Africa, Madagascar, India, and Australia, and in 1913 they added Antarctica when identical specimens were found by the Scott expedition.

Supercontinents Older than Gondwana

The configurations of Gondwana and Pangea are well known because the histories of oceans that opened to disperse Pangea can be reconstructed from their patterns of magnetic stripes (chapters 1 and 9). The configurations of older supercontinents cannot be easily determined because the oceanic lithosphere formed when they dispersed is so old that it has been completely subducted and destroyed. Thus the histories, and even existence, of these older continents must be inferred from indirect evidence. The four most widely used techniques for reconstructing old supercontinents are: paleomagnetic data; correlation of orogenic belts that developed during accretion of the supercontinent: correlation of extensional features that developed when the supercontinent fragmented; and recognition that sediment in one present continent was derived from a source now in another continent. Paleomagnetic information can be used in two ways. One is to compare APW curves for different continental blocks to determine whether there were periods of time when two or more blocks seem to have been joined (appendix C). If similar movements are found for several continental blocks that are now separated, then we can infer that they formed a single block, perhaps a supercontinent, during the period when they had identical APW paths. Another method of using paleomagnetic data is simply to compare the apparent latitudes of numerous continental blocks. Even though longitudes cannot be specified, latitudes can be used to infer proximity of different blocks, thus supporting other information that suggests the configuration of a supercontinent. Correlation of orogenic belts starts with identification of belts of different ages in present continents. Belts of the same age are now scattered all over the earth’s land surface because of fragmentation of supercontinents and movement of modern continents to their present positions. The configurations of older supercontinents can be inferred by placing modern continents into positions in which these orogenic belts line up to form a pattern that would be expected to develop during accretion of a supercontinent. We demonstrate this technique below in our discussion of the configurations of Rodinia and Columbia.

Growth of Cratons and their Post-Stabilization Histories

As we have seen in chapter 3, continental crust evolved from regions of the mantle that contained higher concentrations of LIL elements than regions that underlie typical ocean basins. The most complete record of this evolutionary process is in cratons, which passed through periods of rapid crust production to times of comparative stability over intervals of several hundred million years. After the cratons became stable enough to accumulate sequences of undeformed platform sediments, they moved about the earth without being subjected to further compressive tectonic activity. Because many of the cratons are also partly covered by sediments that are unmetamorphosed or only slightly metamorphosed, they appear to have undergone very little erosion since the sediments were deposited. Thus, a craton may be considered as a large block of continental crust that has been permanently removed from the crustal recycling process. This chapter starts with a discussion of the history of cratons as interpreted from studies of the upper part of the crust. We describe the Superior craton of the Canadian shield and the Western Dharwar craton of southern India within the chapter and use appendix E for brief summaries of other typical cratons. These cratons and numerous others elsewhere developed at different times during earth history, and we look for similarities and differences that may have been caused by progressive cooling of the earth (chapter 2). This section concludes with a summary of the general evolution of cratons and the meaning of the terms “Archean” and “Proterozoic.” The following section is an investigation of processes that occurred following stabilization, all of which take place in the presence of fluids that permeate the crust. We include a summary of these fluids and their effects on anorogenic magmatism and separation of the lower and upper crust. The final section discusses the relationship between cratons and their underlying subcontinental lithospheric mantle (SCLM). Continual metasomatism and metamorphism of the SCLM after cratons develop above it apparently has not destroyed the relationship between the ages of the cratons and the concentrations of major elements in the SCLM. This provides us with an opportunity to determine whether cratons evolved from the mantle beneath them or by depletion of much larger volumes of mantle. The discussions in this chapter are based partly on information summarized in appendices B (heat flow) and D (isotopes).

Plate Tectonics Now and in the Past

The concepts known as plate tectonics that began to develop in the 1960s built on a foundation of information that included: • The earth’s mantle is rigid enough to transmit seismic P and S waves, but it is mobile to long-term stresses. • The earth’s temperature gradient is so high that convective overturn must occur in the mantle. • The top of the mobile part of the mantle is a zone of relatively low velocity at depths of about 100 to 200 km. This zone separates an underlying asthenosphere from a rigid lithosphere, which includes rigid upper mantle and crust. • Seismic activity, commonly accompanied by volcanism, occurs along narrow, relatively linear, zones in oceans and along some continental margins. • The zones of instability surround large areas of comparative stability. • Ocean lithosphere is continually generated along mid-ocean ridges and destroyed where it descends under the margins of continents and island arcs. This causes oceans to become larger, but shrinkage of oceans can occur where lithosphere is destroyed around ocean margins faster than it is formed within the basin. • Some of the belts of instability are faults with lateral offsets of hundreds of kilometers. • Some continental margins are unstable (Pacific type), but others are attached to oceanic lithosphere without any apparent tectonic contact (Atlantic type). • Different areas containing continents and attached oceanic lithosphere move around the earth independently of each other. Most of this chapter consists of a summary of plate tectonics in the present earth, including processes along plate margins and the types of rocks formed there (readers who want more detailed information are referred to Rogers, 1993a; Kearey, 1996; and Condie, 1999). We also briefly discuss plumes and then finish with a word of caution about interpreting the history of the ancient and hotter earth with the principles of modern plate tectonics. Starting from the body of continually expanding information summarized above, numerous earth scientists in the 1960s and 1970s began to establish a conceptual framework that would organize scientific thinking about the earth’s tectonic processes. This required a new terminology, and it arrived rapidly (Oreskes, 2002). Geologists decided to call the stable areas “plates” and the unstable zones around them “plate margins.” Thus, the concept became known as “plate tectonics.” Plates are essentially broad regions of lithosphere, although the failure to detect low-velocity zones under many continents leaves unresolved questions.

Continental Drift—The Road to Plate Tectonics

Alfred Wegener never set out to be a geologist. With an education in meteorology and astronomy, his career seemed clear when he was appointed Lecturer in those subjects at the University of Marburg, Germany. It wasn’t until 1912, when Wegener was 32, that he published a paper titled “Die Entstehung der Kontinente” (The origin of the continents) in a recently founded journal called Geologische Rundschau. This meteorologist had just fired the opening shot in a revolution that would change the way that geologists thought about the earth. In a series of publications and talks both before and after World War I, Wegener pressed the idea that continents moved around the earth independently of each other and that the present continents resulted from the splitting of a large landmass (we now call it a “supercontinent”) that previously contained all of the world’s continents. After splitting, they moved to their current positions, closing oceans in front of them and opening new oceans behind them. Wegener and his supporters referred to this process as “continental drift.” The proposal that continents moved around the earth led to a series of investigations and ideas that occupied much of the 20th century. They are now grouped as a set of concepts known as “plate tectonics.” We begin this chapter with an investigation of the history of this development, starting with ideas that preceded Wegener’s proposal. This is followed by a section that describes the reactions of different geologists to the idea of continental drift, including some comments that demonstrate the rancorous nature of the debate. The next section discusses developments between Wegener’s proposal and 1960, when Harry Hess suggested that the history of modern ocean basins is consistent with the concept of drifting continents. We finish the chapter with a brief description of seafloor spreading and leave a survey of plate tectonics to chapter 2. Although Wegener is credited with first proposing continental drift, some tenuous suggestions had already been made. We summarize some of this early history from LeGrand (1988).

Assembly and Dispersal of Supercontinents

Supercontinents are assemblies that contain all, or nearly all, of the earth’s continental blocks. The concept arose with the recognition of Gondwana in the late 1800s (chapters 1 and 8), and it has been greatly expanded since then. In this chapter we build on the ideas developed in chapters 2 through 5 to discuss the origin and dispersal of supercontinents. The first section considers various mechanisms for the accretion of supercontinents, and the second section considers the reasons for their assembly. The last two sections consider evidence that former supercontinents have broken up and the reasons for their dispersal. We emphasize that the processes of accretion and dispersal overlap, with rifting of some parts of a supercontinent occurring at the same times as suturing in other areas. This overlapping produces a time when the supercontinent has its largest coherent area, which we refer to as the time of “maximum packing.” All supercontinents contain the same types of terranes that occur in individual continents (chapter 4). All models of assembly recognize that some terranes accreted as small individual blocks and some as continental-sized masses that contained several individual blocks that had been previously sutured together. Differences between models involve the area of the supercontinent that consisted of previously sutured large blocks and the area formed by accretion of small individual blocks. Resolution of this problem requires an understanding of the nature of orogenic belts developed during assembly, and we discuss this issue first. All orogenic belts have many similarities. They all underwent lateral compression that led to rock deformation and crustal thickening. Thickening pushed some rocks down into realms of higher temperature and pressure, causing metamorphism, and magmatic intrusion locally raised temperatures even higher in some areas. Almost all orogens contain magmatic rocks from various sources, including rocks partially melted within the orogen and magmas from subducted lithosphere and mantle below the deformed belt. Despite these similarities, different orogens contain features that enable us to distinguish different environments of formation.

Assembly of Continents and Establishment of Lower Crust and Upper Mantle

Continents are very large areas of stable continental crust. After their initial accretion, they rift and move about the earth but undergo compressional deformation almost entirely on their margins. We start our discussion of continents by identifying the varieties of terranes that come together to create them. Because accretion of terranes requires closure of oceans between them, we continue our discussion by describing two different processes of closure. Then we recognize that assembly merely develops a group of terranes, and they must be “fused” or “welded” together before they can be a coherent continent. This process takes place partly during assembly, but most of it appears to be the result of post-collisional processes that continue for tens to hundreds of millions of years. This fusion develops lower continental crust and subcontinental lithospheric mantle (SCLM), the part of the upper mantle directly underlying continental crust, that have similar, although slightly variable, properties across the entire continent. The lower crust and SCLM are separated by the seismic discontinuity known as the Moho (chapter 1), and we finish this chapter by describing the lower crust and SCLM and variations in the depth of the Moho through time. Many of the blocks involved in continental accretion are “exotic” terranes that formed somewhere away from the continent and became “allochthonous” when they accreted to the continent. They include large continental blocks that collide with each other, small continental fragments that accrete to the margins of existing continents, intraoceanic island arcs, and small amounts of oceanic lithosphere. Terranes formed on the margin of a growing continent are regarded as “autochthonous” terranes. We recognize two of them: continental-margin magmatic arcs and sediments accumulated on passive margins. Collision of large continental blocks causes intense orogeny. We illustrate this process with the collision of the Russian platform and the Siberian plate to form the Urals in the Late Paleozoic (fig. 5.1; Fershtater et al., 1997; Puchkov, 1997; Friberg and Petrov, 1998; Brown and Spadea, 1999). The East European (Russian) platform was formed by the fusion of the Baltic and Ukrainian cratons at ~2 Ga.

History of Continents after Rifting from Pangea

As continents moved from Pangea to their present positions, they experienced more than 100 million years of geologic history. Compressive and extensional stresses generated by collision with continental and oceanic plates formed mountain belts, zones of rifting and strike-slip faulting, and magmatism in all of these environments. In this chapter we can only provide capsule summaries of this history for each of the various continents, but many of their salient features have been discussed as examples of tectonic processes in earlier chapters. The final section analyzes the breakup of Pangea as part of the latest cycle of accretion and dispersal of supercontinents. Because it involves continuation of this cycle into the future, it is necessarily very speculative. Figure 10.1 shows approximate patterns of movement of each continent from its position in Pangea to the present. The dominant feature of this pattern is northward movement of all continents except Antarctica, which has remained over the South Pole for more than 250 million years. Shortly after geologists recognized the concept of continental drift, this movement was referred to by the German word “Polflucht” (flight from the pole) because all of the continents were seen to be fleeing from the South Pole. The only continent that did not simply move northward was Eurasia, which essentially rotated clockwise and changed its orientation from north–south to east–west. Comparison of fig. 10.1 with fig. 8.12a (locations of continents shortly before the assembly of Gondwana) shows that the net effect of the last 580 million years of earth history has been a transfer of most continental crust from the southern hemisphere to the northern hemisphere. Accretion and compression against the southern margin of Eurasia constructed a series of mountain belts from the Pyrenees in the west to the numerous ranges of Southeast Asia in the east. This collision generated extensional and transtensional forces that opened rifts and pull-apart basins. Tectonic loading created foreland basins with sediment thicknesses of several kilometers. Opposite the area where the collision of India caused the most intense compression, the extensional basins are interspersed with mountain ranges that were lifted up intracontinentally. We divide the discussion of Eurasia into a section where compression dominates to the south (present orientation) of the former margin of Pangea and a section that describes processes within the landmass to the north.