The Earth's dynamic surface: an overview

Abstract

Debate about the relative roles of catastrophic v. continuous processes of landform evolution is as old as the discipline of Earth Science itself. Over the last 10 years or so, research in the Earth Sciences has focussed strongly on the Earth’s surface and particularly in terms of quantifying rates of processes. This research parallels developments in geomorphology and sedimentology in the quantification of surface processes since the 1950s and 1960s. These surface processes are the manifestation of the large-scale interaction of climate and tectonics operating over a wide range of spatial and temporal scales. Thus, recent research had required integration of the historically distinct subjects of geomorphology, sedimentology, climatology and tectonics. Partly as a cause and partly as a consequence of this integration, there have been many recent developments in quantitative modelling and both laboratory and field-based analytical tools. Together, these have provided new insights into absolute and relative rates of denudation, and the factors that control the many dynamic processes involved. One of the outstanding issues concerns the balance between tectonics, climate and denudation, and in particular the limiting effects of one on the others and the nature of dynamic feedback mechanisms. The fact that processes can be considered catastrophic or continuous, depending on the timescale of observation or interest, can hinder the predictability of models, depending on how they are formulated. Certain conditions may lead to a steady-state situation in which denudation balances tectonic uplift, leading to a more or less constant topography. Steady-state topography means that detailed study of present day landforms can provide important insights into the nature of surface processes back in time. Such assumptions underpin debates in geomorphology relating to the process-form linkage and the understanding of characteristic forms in the landscape. Alternatively, the recognition of non-steady-state situations and a clearer understanding of why these situations occur provide the key for resolving the climate– tectonics–landscape evolution feedback loop. The transition between the two states will reflect the process response time, and therefore the transitory state may provide a clearer picture of the time lag of topographic response to changes in the rates of climate change and tectonic forcing. However, the response time is not necessarily constant and may have changed considerably at key points in the past, such as the evolution of plants on land in the Palaeozoic and the acceleration of human activity within the landscape in the Holocene. In terms of denudation (physical erosion and chemical and mechanical weathering), there are clearly catastrophic processes, such as landsliding, which operate discretely and on short timescales and more continuous processes, such as chemical weathering, which can be considerably more protracted. The distinction between discrete catastrophe and continuous modification depends also on the time and spatial scales of interest. These considerations also impact directly on the questions of if and how steady-state topography can be achieved, how the processes controlling this state can be quantified and resolved, what causes departures from a steady-state condition and how topography reflects the coupling between denudation, climate and tectonics. Some of the key current research areas in the world are tectonically active regions, such as New Zealand (southern Alps), Taiwan and Olympic Mountains (USA). However, the link between tectonics and denudation is complicated in these convergent zones (e.g. Willett et al. 2001), as there is a significant horizontal component to the deformation and, additionally, climatic variations often produce marked asymmetry in denudation, which itself then feedbacks into the isostatic component of vertical motion. In practice, this research field necessarily involves a broad range of disciplines including field geologists, geomorphologists, structural geologists, geochemists, climatologists and geophysical modellers. These researchers address the observational constraints on