Periglacial Geomorphology

Colin Ballantyne, DSc 2000

Periglacial geomorphology is the study of non-glacial surface and near-surface processes that operate in the cold regions of the Earth to produce distinctive landscapes, landforms, sediments and soil structures. The hallmark of of periglacial environments is the dominant role of ground freezing and thawing in affecting surface and near-surface geomorphological activity. Many areas affected by periglacial processes are underlain by permafrost, defined as ground where the temperature remains below 0°C throughout the year; others are affected by deep seasonal ground freezing throughout much of the year, interrupted by summer thawing.

At present, about 25% of the Earth's land surface falls within the periglacial realm, mainly in the Arctic and Subarctic, unglacierized parts of Antarctica, the higher parts of alpine-scale mountains and the Tibetan plateau, but even the highest parts of Scottish mountains experience a distinctive 'maritime periglacial' climatic regime. Many midlatitude areas also experienced prolonged periglacial conditions during numerous glacial stages over the last 2.6 million years. At such times, for example, permafrost underlay all of southern England and much of Europe beyond the limits of successive Pleistocene ice sheets; the present landscapes of such mid-latitude areas are essentially the result of periglacial processes operating over long timescales .

'Periglacial Geomorphology' is the first fully comprehensive text on the topic to be published since 1980. The book falls into six parts.

The first (Chapters 1 and 2) outlines the history of the research field and explores the environmental characteristics of the periglacial regions of the Earth in terms of their climatic regime, soils and vegetation cover.

Part 2 (Chapters 3–5) are devoted to the physics of ground freezing and thawing, the characteristics of permafrost and the nature and origin of underground ice, which often constitutes 10–50% of near-surface permafrost.

Part 3 (Chapters 6–9) deals with the surface expression of ice in the ground and recurrent freeze-thaw activity, introducing the origins of permafrost landforms such as frost polygons, ice wedges and ice-cored hills, the effects of thermal degradation of ice-rich permafrost (thermokarst), and the effects of freezing and thawing of seasonally-frozen ground in producing a wide range of phenomena such as patterned ground and cryoturbation structures in soils.

Part 4 (Chapters 10–12) addresses the physical and chemical processes that act to break down rock in cold environments, and the characteristic landforms that result, such as blockfields and tors. It provides a rigorous analysis of the wide range of processes that cause downslope movement of soil in periglacial environments: solifluction (slow downslope creep of the soil due to recurrent freezing and thawing of the ground), permafrost creep, and more rapid slope movements due to rupturing of the soil over permafrost. The final part of this section addresses rockfall, avalanche and debris-flow activity and the origin and dynamics of resultant landforms, such as talus accumulations, boulder tongues and rock glaciers.

Part 5 of the book (chapters 13–15) considers the effects of fluvial, aeolian and coastal processes in cold environments. The runoff regime of periglacial rivers is dominated by snowmelt floods, and the effects of rivers flowing across permafrost terrain produce distinctive landforms that are absent in other environments. Wind action has resulted in the deposition of cold-climate sand sheets and dune fields, and during Pleistocene cold periods about 10% of the Earth land surface was mantled in thick deposits of cold-climate loess (windblown silt) that constitute fertile, well-drained soils that are of crucial importance to global food security. Extensive areas of the Arctic Ocean littoral consist of ice-rich permafrost lowlands that are increasingly vulnerable to erosion as the Arctic warms.

The final section (Chapters 15 and 16) considers first how relict periglacial phenomena in mid-latitude temperate environments can inform our interpretation of changing climatic conditions during the most recent Glacial Stage. The final chapter is devoted to consideration of how present and future climate change will impact periglacial environments, particularly in the circum-arctic permafrost zone and in alpine environments. In both areas, permafrost is warming and thawing, triggering a cascade of landscape changes that includes subsidence and flooding of terrain, increased incidence of slope failure and destabilisation of mountain rockfalls. Summer sea-ice cover in the arctic is shrinking, the duration of the snow-free season on land is increasing, the runoff regime of periglacial rivers is changing, and there is evidence that ice-rich permafrost coasts are receding at increasing rates. The greatest concern is that thawing permafrost, particularly in the subarctic zone, contains huge amounts of partly decomposed frozen plant matter. When the permafrost thaws, such organic material undergoes microbial decomposition, releasing carbon dioxide and methane that contribute to further atmospheric warming.

The major challenge now is to understand how periglacial processes, landforms and landscapes are responding and will continue to respond to climate change, and the effects of these responses on future climate. Periglacial environments are in the front line of the battle to limit global climate change, and the study of periglacial geomorphology has never been so relevant to society as is now the case.

ISBN: 978-1-405-10006-9