The Nature of Geomorphology

Alistair F. Pitty

• Publisher: Routledge
• Year: 2020
• ISBN:
  9781000046427
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Géologie et sciences de la Terre

In this re-evaluation of the basic postulates of geomorphology, first published in 1982, Alistair Pitty examines the subject within its scientific context, arguing that coherence in geomorphology can be demonstrated despite the many apparent divergences, which should themselves be regarded as poles within a spectrum of opinion. Not least, the particularly geological and geographical aspects of geomorphology are carefully identified and explained within this coherence.

Scientific context of geomorphology

1

First, then, as to the configuration of the earth’s surface. We have here a bone of contention between the geographers and the geologists. The latter hold that the causes which have determined the form of the lithosphere are dealt with by their science, and that there is neither room nor necessity for the physical geographer. (Mackinder, 1887)

Geomorphology itself has suffered, and will continue to suffer, from attempts to include it in the geographic realm. In the history of its development, in its methods, and in its affiliations it is a part of geology. (Johnson, 1929) The place in geography of geomorphology is still a debated question, and though it enjoys a privileged place abroad, its claim to inclusion within the main terrain of study is commonly denied by geographers of the ‘inner faith’ in this country. (Wooldridge, 1932)

Introduction

Wooldridge (1958) regarded it ‘as quite fundamental that geomorphology is primarily concerned with the interpretation of forms, not the study of processes. The latter can be left to physical geology’. On the other hand, ‘It would not be possible to predict the path that a drop of water falling on the slope would take. I cannot imagine a geologist wanting to do so’ (Kitts, 1976). Despite reiteration of such disciplinary guidelines, however, some contributions from geography concentrate on non-geographical aspects of geomorphology and others, professionally designated as geologists, are leading practitioners of what Kitts finds unimaginable. Although the 1960s saw much reshaping of geomorphology, the question of where it belongs in research, applied study and practice, or in advanced teaching, was unresolved (Dury, 1972). Thus today the scientific study of landforms and landforming processes comprises a range of differing understandings of its purpose, context and scope. Regional emphases are easily recognized and appreciated, but disciplinary shades merge or clash whilst institutional styles or personality tinges may glare or pass unnoticed.

Geomorphologists are academic migrants, a vociferous, squabbling yet cohesive flock, tracking continually across the political boundaries of the established nation-states of science. Roosting comfortably in some geological branches, they are the ugly ducklings in other nests, and some uneasy clutches of geographers suspect that the raising of a boisterous cuckoo has been foisted on them (Brown, 1975). Thus, Gould (1973) reflects that ‘the excising of physical geography is not nearly so traumatic as many would have us believe’, whereas Chorley (1971) recommends that physical geography must be sensitive to the changing aims and objectives of human geography and the allied humanities. It is, therefore, important to identify points at which geomorphologists’ interests merge or contrast with the main contemporary purposes of either geology or geography, not least because both have changed substantially during the 1970s.

Implications of recent changes of emphasis in geology

PLATE TECTONICS

Since 1970, when the term ‘plate tectonics’ began to appear in print, geologists have restructured much of their knowledge of geophysical processes within this new, developing conceptual framework. In contrast to the early scepticism which first greeted A. Wegener’s ideas on continental drift, geologists now generally accept that there is ‘a continuous network of mobile belts about the earth which divide the surface into several large rigid plates’ (Wilson, 1965). For geomorphology, the concept of lithospheric plates is probably most important in outlining areas of relative vertical stability and zones of intense deformation (Rice, 1977; 76). These rigid plates, which move on top of a mechanically soft layer, the asthenosphere, have three types of boundary that depend on directions of plate motion. First, trailing edges are identified where plates move away from each other, with material being added from great depth, mostly along mid-oceanic ridges. Secondly, where plates collide or converge, the leading edge boundary is identified, with an ocean trench marking the zone where the overridden plate begins to descend the subduction zone which leads down to the asthenosphere (Fig. 1). Where the converging plates carry no continents, the main morphological expression of plate boundaries is ocean trenches and island arcs, as in the Pacific. If the leading edge of such an oceanic plate is underthrust below a continental plate, high mountain ranges like the Rockies and Andes are created and maintained. The Himalayas owe their greater altitude to the collision of two continental plates. A third type of plate boundary may be marked by deep fractures along which plates slip past each other laterally, with little gain or loss of surface material. In such cases, like California, surface strike-slip faulting has occurred during historically recorded earthquakes and major faults have moved during the Quaternary. In contrast, in some continental interiors, stable ‘cratons’ of old, resistant rocks may have remained largely unmoved since Precambrian times.

Figure 1 Geographical contrasts and similarities in major earth-surface features explained by subduction hypotheses

A Some features of island arcs at the convergence of two lithospheric plates. LP/HT indicates low-pressure/high-temperature metamorphism.

B Tectonic history of Japan; related to collision and descent of the formerly mid-oceanic Kula-Pacific Ridge in Late Cretaceous times.

(i)120 million years BP. Ridge approaches Asiatic continent.

(ii)90 million years BP. Thermal effect of underthrust plate causes extensive vulcanism.

(iii)70 million years BP. Thermal effect further reduces thickness of part of the continental plate, which is eventually broken by tensional force, the fragment drifting away to form the Japanese islands.

C Possible generation of the Coastal Batholith of Peru in the Lower Tertiary along deep-penetrating faults intersecting east-migrating subduction zone.

D Tectonic and geothermal history of the Sierra Nevada and the initiation of the Basin-and-Range province.

(i)Before 80 million–100 million years BP. Possible generation of the Sierra Nevada batholith from underthrusting slab in Mesozoic times.

(ii)80 million–100 million years BP to 20 million–30 million years BP. Cold lithospheric slab beneath Sierra crust with steeply inclined portion of subduction zone and associated magma generation having shifted east, attributable to increased westerly drift of the North American plate.

Sources: Adapted from A Mitchell and Reading, 1971; B Uyeda and Miyashiro, 1974; C Pitcher, 1978; D Henyey and Lee, 1976.

Clearly, the plate-tectonic model illuminates the distinctive character of many of the earth’s major surface configurations. Some of these are reflected in larger-scale geomorphological responses, such as continental asymmetry, with high orogenic belts on the collision side of continents explaining the general absence of large rivers on the ‘leading-edge’ sides of North and South America (Inman and Nordstrom, 1971). Conversely, trailing-edge continental coasts receive more sediment than collision coasts because of the larger drainage areas (Potter, 1978), and a further geomorphological consequence is the correlation of percentage of coastline length occupied by barrier islands with plate-margin type (Glaeser, 1978). Where strike-slip faulting is active, deposits and landsurfaces broken and offset by these displacements can be surveyed and mapped, with numerous examples from New Zealand already enshrined in the works of Cotton (1960). Not least, the combined effect of continental drift and the wandering of the earth’s poles may have initiated the late Cenozoic glaciations when the South Pole and the drifting antarctic continent began to overlap.

DEEP-SEA EXPLORATION

Since 1968, when the vessel Glomar Challenger began deep-sea drilling, a worldwide network of ocean-floor cores has been established (Wyllie, 1976). This Deep Sea Drilling Project, advised by the Joint Oceano-graphic Institution for Deep Earth Sampling (JOIDES) has supplemented and extended the intensive explorations of the oil industry which first moved offshore in the 1940s. Thus, the evidence for plate tectonics and the opportunities for the oil industry have led the emphasis of geology away from the dry land and its forms, into offshore, continental shelf and deep-sea areas. However, evidence indirectly relevant to geomorphology has been obtained, simply because accumulation rates and stratigraphy of offshore sedimentation are the natural complement to the erosional record on land. For instance, the scale of the Navy Fan, offshore from the Tijuana River and adjacent streams draining into San Diego Bay, suggests that the mean rate of late-Quaternary denudation of northernmost Baja California is 60 cm/1000 yr (Normark and Piper, 1972). Further south, in enclosed basins on the Pacific continental margin of Central and South America, a median sedimentation rate of 8.8 cm/1000 yr has been recorded.

Episodes of tectonic activity, vulcanism and climatic change can be inferred from cores. For example, the 800 × 300 km Meiji sediment tongue occurs in the north-west corner of the Pacific floor. Because no major Siberian river could be involved, this 1800 m thick sedimentary body may date the initiation of the steep coastal drainage system and the associated uplift phase of an early Upper Tertiary orogenic pulse (Scholl et al., 1977). Recurrence and intensity of volcanic activity is identifiable in some cores, particularly for basin-wide episodes of stagnation in the Mediterranean Sea. More than forty tephra layers occur in deeper horizons of the Tyrrhenian Sea and, by matching the mineralogy and petrochemistry of deep-sea ashes with their source regions, a time scale for many of the major Mediterranean eruptions has been established (Keller et al., 1978). The influence of climatic fluctuations can be estimated from oceanic and continental responses because both pelagic and terrigenous components are recognizable in deep-sea cores. The distribution and accumulation rates of clay and terrigenous silts and sands indicate continental responses to climatic fluctuations. Glacial–interglacial change in the temperature and salinity of the ocean is reflected by planktonic foraminifera and, in lower latitudes, by amounts of pelagic carbonate. In higher latitudes, glacial intervals can be identified by increases in the amount of ice-rafted detritus in cores and by the occurrence of erratics beyond the present-day limit of icebergs Core analyses from JOIDES leg 28 in the Ross Sea suggest a slow rate of sedimentation in the Miocene up to about 5 million years BP, followed by rapid sedimentation of ice-rafted debris at the Mio-Pliocene boundary. The sequence suggests a glacial maximum followed by melting, collapse and retreat of the ice sheet and implies extensive erosion in the Transantarctic Mountain source area (Drewry, 1975).

ENGINEERING AND ENVIRONMENTAL GEOLOGY

Significantly, the founder of modern geology, William Smith, was a canal engineer, and the escalation of urbanization and the extension of associated road networks in recent decades is reflected in renewed attention to engineering geology. This practice involves the general principles of geology as they may influence constructional project designs, particularly the physical properties of rock stability and bearing strength (Woodland, 1968). The variation in lithology and the structural relations of rocks beneath the ground-surface must be fully appreciated in the appropriate siting and development of major civil-engineering projects. For example, the layout of the engineering works of the Cruachan Pumped-Storage Scheme followed closely geological mapping which delineated uniform conditions within a subsidence pluton of granodiorite. Geology is also a major influence on motorway construction costs, as these are dependent on the character and scale of earthworks. Geological influences are partly expressed in the configuration of the land-surface over which routes might be planned. These influences are accentuated where landsurface conditions vary significantly along the route, modifying the suitability of materials for embankment construction, the declivity for safe side slopes for embankments and cuttings and the volume of rock to be excavated (Newbery and Subramaniam, 1977). The renewed growth of engineering geology increases knowledge about rock mechanics, a fundamental geomorphological property. Equally important is the attention now paid to ‘soft’ rocks, since the youngest drift deposits are commonly a major consideration in siting civil engineering projects, with hidden, drift-filled valleys a particular problem in reservoir construction. Not least, many engineering failures are found to be related to instability of hillslopes. For example, a whole section of a partially completed bypass road near Sevenoaks, Kent, was abandoned. Construction reactivated movement in extensive solifluction deposits and in landslips in the Hythe Beds and the underlying Atherfield and Weald clays. It was revealed that several solifluction lobes were superimposed, each with a marked slip-plane of low shear strength at its base (Woodland, 1968).

Environmental geology is based on hydrogeology and branches of economic geology as well as incorporating engineering geology. It is linked with other earth-science specialisms, including marine geology, sedimentology, seismology and geomorphology, thus embracing ‘those aspects of geology which are concerned with the use, by society, of the Earth and its resources and, in addition, the implications of geological processes on man’ (Knill, 1970). In addition to the urban planning and site-foundation responsibilities of engineering geology, environmental geology considers natural hazards, seen at their most sensational in earthquakes and volcanic eruptions. For example, it is possible to plot on a relief map the area in which an eruption, should it occur, would produce flows long enough to encounter a town (Guest and Murray, 1979). Man-made hazards are also considered, with the location of sites where waste products can be safely dumped becoming an increasingly exacting task.

Since 1970 environmental geology has been incorporated into the curriculum of an increasing number of colleges and universities, attracting significantly increased student enrolment (Hoffman, 1979). In addition, a model for a humanistically orientated undergraduate geology course has been outlined (Romey, 1972).

Divergence between geomorphology and certain emphases in geology

For the geomorphologist, the receding sea-cliff is a striking erosional landform; to the geologist, it is a fresh exposure of rocks (Brown and Waters, 1974). Similarly, the geomorphological weathering processes that have fashioned and continue to modify landforms are, for the geologist, essentially ‘subaerial diagenesis’, the processes by which new rocks are being formed, with comparable products identifiable in the stratigraphic record. Differences in perception of the same phenomena are common. Perhaps the clearest instance of divergence is seen in the intense interest of geologists in vulcanicity. Admittedly, geomorphologically significant volumes of lava and clastic material can be derived from active volcanoes and the geographical spread of ash can be on intracontinental scales. However, geologically crucial glimpses are offered into the largely inscrutable characteristics of magma generation in the earth’s molten interior and guides to the unaltered composition of the magmatic melt phase. Thus, ash sheets, uncommon and largely expressionless as landforms, are exceptionally interesting to geologists. Where ash composition is zoned, it reflects, in inverted order, the original compositional zonation within their magma-chamber source. Study of Quaternary volcanic domes and intrusive rocks also provides vital insights into the processes of magma generation, but these are either small or geographically uncommon landsurface features.

There are instances where geological enquiry ceases to be directly concerned with the study of shape of the present landsurface. These arise because geology, being an historical science, is concerned with occurrences at particular times and places in the past. However, due to the time-span duri...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Acknowledgements
• Preface
• 1 Scientific context of geomorphology
• 2 The nature of geomorphology
• 3 Basic postulates
• Glossary
• Bibliography
• Index