Academic literature on the topic 'Slopes (Physical geography) – South Africa'

In comparison to the rest of Europe, Africa, and Asia, most rivers arising and flowing within the Mediterranean watershed typically drain small catchments with mountainous headwaters. The hydrology of Mediterranean catchments is strongly influenced by the seasonal distribution of precipitation, catchment geology, vegetation type and extent, and the geomorphology of the slope and channel systems. It is important to appreciate, as the preceding chapters have shown, that the area draining to the Mediterranean Sea is large and enormously variable in terms of the key controls on catchment hydrology outlined above, and it is therefore not possible to define, in hydrological terms, a strict single Mediterranean river type. However, river regimes across the basin do have a marked seasonality that is largely controlled by the climate system (Chapter 3) and, in most basins, the dominant flows occur in winter—but autumn and spring runoff is also important in many areas. These patterns reflect the general water balance of the basin as a whole, but there are key geographical patterns in catchment hydrology and sediment yield and a marked contrast is evident between the more humid north and the semi-arid south and east (Struglia et al. 2004; Chapter 21). Also, because of the long history of vegetation and hillslope modification by human activity and the more recent and widespread implementation of water resource management projects, there are almost no natural river regimes in the Mediterranean region, especially in the middle and lower reaches of river catchments (Cudennec et al. 2007). Runoff generation on hillslopes in the Mediterranean is very closely related to rainfall intensities and land surface properties as discussed in Chapter 6. While this is probably true of most catchments, runoff generation in the Mediterranean is very sensitive to vegetation cover because of the seasonal dynamics of rainfall and the role played by extreme events. The cumulative effect of these characteristics is a specific set of management problems and restoration issues and, although these are rather different in the various socio-political regimes of the region, it can be argued that they are in many ways unique to Mediterranean catchments.

The effects of climate change are intrinsic features of Earth’s landscapes, and South America is no exception. Abundant evidence bears witness to the changes that have shaped the continent over time—from the glacial tillites inherited from late Paleozoic Gondwana to recent terrigenous sediments and life forms trapped in alluvial, lacustrine, and nearby marine deposits. Preeminent among this evidence are the landforms and sediments derived from the late Cenozoic glaciations of the Andes, which have been the focus of so much recent and ongoing research. Because South America has long been a mainly tropical and subtropical continent, most of it escaped the direct effects of these glaciations. Nevertheless, portions of the continent extend sufficiently far poleward and rise high enough to attract snowfall and promote glaciers today. Glaciers were more emphatically present during Pliocene and Pleistocene cold stages, and it is their legacies that provide information about the changing environments of those times, and more especially of the past 30,000 years. There is evidence for glaciation in the tropical and extratropical Andes as early as Pliocene time (Clapperton, 1993). In southern South America, along the eastern side of the Patagonian Andes, Mercer (1976) dated a series of basalts interbedded with glacial tills that suggest multiple glacial advances after ~3.6 Ma (million years before present). In the La Paz Valley, Bolivia, volcanic ashes dated by K/Ar (potassium/argon) methods are interbedded with glacial tills indicative of at least two phases of glaciation in the late Pliocene, at 3.27 and 2.20 Ma (Clapperton, 1979, 1993). This evidence for early glaciation in disparate parts of the Andes indicates that portions of the cordillera were high enough and climatic variations were great enough in the Pliocene for glaciers to form long before the cold episodes of the Pleistocene. Glacial deposits in Ecuador, Perú, and Bolivia provide evidence for climate variability in tropical South America in the recent geological past. In the late Pleistocene, glacier equilibrium-line altitudes were as much as 1,200 m lower than they are today on the eastern slopes of the Andes, indicative of a significant depression in mean annual temperature in the tropics at maximum glaciation (e.g., Klein et al., 1999).

Arid and semi-arid ecosystems in South America are best illustrated by two desert regions, the Peruvian and Atacama Deserts of the Pacific coast and the Monte Desert of central Argentina. The caatinga of northeast Brazil is often described as semi-arid, but mostly receives 500–750 mm of annual rainfall and is better regarded as dry savanna. Small areas of Venezuela and Colombia near the Caribbean coast, and nearby offshore islands, support desert-like vegetation with arborescent cacti, Prosopis, and Capparis, but generally receive up to 500 mm annual rainfall. Substrate conditions, as much or more than climate, determine the desert-like structure and composition of these communities, and thus they are not discussed further here. Extensive areas of Patagonian steppe also have semi-arid conditions, as discussed in chapter 14. The Peruvian and Atacama Deserts form a continuous belt along the west coast of South America, extending 3,500 km from near the northern border of Perú (5°S) to north-central Chile near La Serena (29°55’S), where the Mediterranean- type climate regime becomes dominant. The eastward extent of the Peruvian and Atacama Deserts is strongly truncated where either the coastal ranges or Andean Cordillera rise steeply from the Pacific coast and, as a biogeographic unit, the desert zone may extend from 20 to 100 km or more inland. A calculation of the area covered by these deserts depends in part on how this eastern margin is defined. Thus the Peruvian Desert covers between 80,000 and 144,000 km2, while the Atacama Desert of Chile extends over about 128,000 km2 if the barren lower slopes of the Andes are included. Actual vegetated landscapes are far smaller and for the lomas of Perú change dramatically between years depending on rainfall. Only about 12,000 km2 of the Atacama contain perennial plant communities, largely in the southern portion known as the Norte Chico but also including a narrow coastal belt of lomas extending northward almost to Antofagasta and the Prosopis woodlands of the Pampa del Tamarugal. The vegetated areas of the coastal lomas of Perú and Chile together probably do not exceed 4,000 km2 as a maximum following El Niño rains.

Soil is a physical, chemical, and biological medium at the upper surface of Earth’s land areas capable of accepting plant roots and thereby enabling plants to extend their photosynthetic tissues upward and intercept radiant energy from the sun. Each day, chemical and biological activities in soil change in response to temperature and moisture dynamics. Each soil has a range of physical, chemical, and biological properties determined by inherited mineral composition and biogeochemical processes existing in a quasi-steady state of flux. Most primary minerals in soil formed in geologic environments of high temperature and pressure. When exposed to lower temperatures and pressures, meteoric water, and organic compounds near Earth’s surface, primary minerals slowly decompose in response to weathering processes. As primary minerals weather, some elements necessary for plant growth are released as inorganic ions, some reassemble to form secondary silicate and oxide clay minerals, and some elements are lost via dissolution and leaching. After prolonged or intense weathering, few minerals containing elements necessary for plant growth remain. Weathering most often occurs in or slightly below the soil but may not be entirely related to the present soil. Material from which a present soil is formed may have been weathered in a soil environment, and eroded and deposited many times before coming to rest in its present location. Such materials are often almost devoid of nutrient- bearing minerals, and the soils formed provide scant amounts of the elements essential for plant growth. In contrast, minerals exposed to a soil environment for the first time on rapidly eroding slopes, fluvial deposits, or as volcanic ejecta succumb more rapidly to weathering and release essential elements in forms needed by plants. Plants ingest inorganic ions and water from the soil through their roots and combine them with carbon secured as carbon dioxide from the air, and with hydrogen and oxygen from water to form organic tissues. Organic residues are added to the soil as various plant parts, insects, and animals die. Microorganisms in and above the soil then decompose the organic residues, carbon returns to the air as carbon dioxide, and essential elements contained in the organic compounds are released as inorganic ions within the soil.

Tectonism is the science of Earth movements and the rocks and structures involved therein. These movements build the structural framework that supports the stage on which surface processes, plants, animals and, most recently, people pursue their various roles under an atmospheric canopy. An appreciation of this tectonic framework is thus a desirable starting point for understanding the physical geography of South America, from its roots in the distant past through the many and varied changes that have shaped the landscapes visible today. Tectonic science recognizes that Earth’s lithosphere comprises rocks of varying density that mobilize as relatively rigid plates, some continental in origin, some oceanic, and some, like the South American plate, amalgams of both continental and oceanic rocks. These plates shift in response to deep-seated forces, such as convection in the upper mantle, and crustal forces involving push and pull mechanics between plates. Crustal motions, augmented by magmatism, erosion, and deposition, in turn generate complex three-dimensional patterns. Although plate architecture has changed over geologic time, Earth’s lithosphere is presently organized into seven major plates, including the South American plate, and numerous smaller plates and slivers. The crustal mobility implicit in plate tectonics often focuses more attention on plate margins than on plate interiors. In this respect, it is usual to distinguish between passive margins, where plates are rifting and diverging, and active margins, where plates are either converging or shearing laterally alongside one another. At passive or divergent margins, such as the present eastern margin of the South American plate, severe crustal deformation is rare but crustal flexuring (epeirogeny), faulting, and volcanism occur as plates shift away from spreading centers, such as the Mid-Atlantic Ridge, where new crust is forming. Despite this lack of severe postrift deformation, however, passive margins commonly involve the separation of highly deformed rocks and structures that were involved in the earlier assembly of continental plates, as shown by similar structural legacies in the facing continental margins of eastern South America and western Africa. At active convergent margins, mountain building (orogeny) commonly results from subduction of oceanic plates, collision of continental plates, or accretion of displaced terranes.

South America forms the greater part of the Neotropical faunal realm, which extends northward through Central America to tropical southern Mexico. Although making up only 12% of the world’s land area, South America is the richest continent for virtually all organismal groups, including vertebrates. For example, of the known 23,250 species of fish (Eschmeyer, 1998), 41% or 9,530 species are freshwater, and of these, more than 2,800 species (29%) are in South America (Moyle and Cech, 2000). A comparable level of diversity exists for amphibians and birds. Of Earth’s 5,900 species of amphibians, at least 1,749 or 30% occur in South America (Duellman, 1999a, 1999b; Köhler et al., 2005; www.amphibiaweb.org). More than 3,200 (or nearly 32%) of Earth’s 9,900 species of birds occur in South America (Sibley and Monroe, 1990). For reptiles and mammals, diversity is only slightly lower; at least 1,560 (19%) of 8,240 reptile species (Uetz and Etzold, 1996; www.reptiledatabase. org), and 1,037 (19%) of 5,416 mammal species (Nowak, 1999; Wilson and Reeder, 2005) are found in South America. Four major geological events or features are important to understanding South America’s contemporary zoogeography. The first was the breakup of Pangea, and then of Gondwana. South America and Africa remained close for an extended period of the Mesozoic, and thus share important similarities in their faunas, including groups not fully evolved at the time of separation. South America also maintained connections to other Gondwanan continents, directly with Antarctica, indirectly with Australia, until the early Cenozoic. The second major feature was South America’s long period of isolation in the Cenozoic, particularly from North America pending establishment of the late Pliocene land bridge after 3 Ma (million years before present). The latter resulted in “The Great American Interchange” (Webb, 1976; Marshall et al., 1982), which had profound consequences for the fauna. The third major feature of South America has been the Andes, which, in addition to modifying climate, have been a center of speciation, a dispersal route, and a barrier. The cordillera has had an overriding effect on distributions and histories of both past and current biotas on the continent.

This chapter reviews the state of North American geographical research on Africa in the 1990s. During the 1980s research on Africa dwelt on the many crises, some real and some imagined, usually sensationalized by the media, such as the collapse of the state in Sierra Leone, Liberia, Somalia, and Rwanda and the economic shocks of structural adjustment programs. The 1990s witnessed momentous positive changes. For example, apartheid ended in South Africa and emerging democratic systems replaced dictatorial regimes in Malawi and Zambia. Persuaded that Africa had made progress on many fronts largely due to self-generated advances, some scholars began to highlight the positive new developments (Gaile and Ferguson 1996). Due to space limitations, selecting works to include in this review has been difficult. In many instances we stayed within five cited works (first authorship) for anyone scholar to ensure focus on the most important works and to achieve a sense of balance in the works cited. Thus, research reviewed in this chapter should be treated as a sample of the variety and quality of North American geographical work on Africa. One major challenge was where to draw the boundary between “geography,” “not quite geography,” and “by North American authors” versus others. In these days of globalized research paradigms, geography has benefited tremendously from interchanging ideas with other social and natural science disciplines. Thus, separating North American geographic research in the 1990s from other groundbreaking works that profoundly influence the discipline of geography is difficult. For example, while the empirical subject matter included agriculture, health, gender, and development issues, the related theoretical paradigm often included representation, discourse, resistance, and indigenous development within broader frameworks influenced by the ideas of social science scholars such as Foucault (1970, 1977, 1980), Said (1978), Sen (1981, 1990), and Scott (1977, 1987). This chapter engages these debates. Building upon T. J. Bassett’s (1989) review of research in the 1980s, the chapter develops a typology for the growing research on African issues and related theoretical orientations (Table 36.1). The reviewed works fall into the three main subdisciplines of geography—human geography (by far the most dominant), physical geography now commonly referred to as earth systems science or global change studies, and geographic information systems (GIS).

Southeast Asia is a corner of the continent of Asia which ends in an assemblage of peninsulas, archipelagos, and partially enclosed seas. Towards the northwest, the physical contact of this region with the rest of Asia is via a mountainous region that includes the eastern Tibetan Plateau, the eastern Himalaya Mountains, the hills and plateaux of Assam (India) and of Yunnan (China). From this high region a number of large, elongated river basins run north–south or northwest–southeast. These are the basins of rivers such as the Irrawaddy, Salween, Chao Phraya, Mekong, and Sông Hóng (Red). An east–west traverse across the mainland part of Southeast Asia, therefore, is a repetition of alluvium-filled valleys of large rivers separated by mountain chains or plateaux. To the south and to the east are coastal plains, rocky peninsulas, and a number of deltas. Beyond lies the outer margin of Southeast Asia, the arcuate islands of Indonesia, and the Philippines with steep volcanic slopes, intermontane basins, and flat coastal plains of varying size. This assemblage of landforms has resulted from a combination of plate tectonics, Pleistocene history, Holocene geomorphic processes, and anthropogenic modifications of the landscape. Most of the world has been shaped by such a combination, but unlike the rest of the world, in Southeast Asia all four are important. The conventional wisdom of a primarily climate-driven tropical geomorphology is untenable here. The first two factors, plate tectonics and the Pleistocene history, have been discussed in Chapters 1 and 2 respectively. In the Holocene, Southeast Asia has been affected by the following phenomena: • The sea rose to its present level several thousand years ago. • The present natural vegetation, a major part of which includes a set of rainforest formations, achieved its distribution. • A hot and humid climate became the norm, except in the high altitudes and the extreme northern parts. • The dual monsoon systems blowing from the northeast in the northern hemispheric winter and from the southwest in the summer (and in general producing a large volume of precipitation) became strongly developed.

North-western Europe has on various counts a very heterogeneous character. Crystalline and metamorphic bedrocks of various ages and Tertiary and Quaternary deposits define its geology and geomorphological features. The area belongs to several climatic zones and parts of it went through quite different processes during their Quaternary development. All these aspects were of essential importance for forests—their origin, development, species composition, structural features, and the character of their environments. During the postglacial period favourable climatic conditions enabled trees to migrate from the refuges in the south and south-east of Europe to the north and north-west. With the exception of extreme conditions all the dry land of north-western Europe was covered with forests whose species composition varied, depending on local conditions of the physical environment. Natural woods and forests, both closed and open and continuously changing in time, contributed greatly to natural landscape diversity. Since the Neolithic and especially in the Middle Ages, human influence becomes the crucial factor of forest development, the impact being superimposed on natural conditions and evolutionary processes. Man not only drastically reduced the forested area in Europe, but the use of forests over several millennia also strongly changed the conditions for the functioning of forests as natural ecosystems. As a result, the man-made forests of today often have little in common with natural forest communities, which once covered the European continent. Nevertheless, even these man-made forests have important functions: they greatly influence the local climate and the hydrological regime of the landscape; they protect steep slopes against erosion and are an important source of biodiversity; and they contribute strongly to the variety of landscape structure as well as to the protection of the environment. This chapter provides a general survey of the phytogeographical, palaeoecological, and environmental aspects of forests in north-western Europe. For a proper insight the following components are taken into consideration: • the abiotic component (the physical environment: topography, climate); • the phytogeographical component (horizontal distribution and altitudinal zonation); • the historical component (postglacial development, early impact of humans on forests); • the ecological component (distribution and ecological properties of trees, main forest types); • the forest use component (organized forestry and its development and the present situation of forests and forestry.

ABSTRACT Important syn-rift hydrocarbon discoveries in the Tertiary East African Rift and in the South Atlantic subsalt basins have in recent years promoted renewed interest in the variability of source and reservoir rock facies in continental rifts. This talk considers several important new observations and developments in our understanding of the sedimentary evolution of lacustrine rift basins. Offshore subsalt basins in the South Atlantic demonstrate the importance of lacustrine carbonates, and especially microbialites, as reservoir facies in extensional systems. The role of rift-related ma