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Orogenesis: The Making of Mountains

A new book describes the processes involved in mountain-building and sheds light on the significance of orogenesis to the petroleum industry.
This article appeared in Vol. 9, No. 5 - 2013

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Through the centuries, people have been drawn to mountains for a variety of reasons: sports, adventure, war, exploration, mining, pilgrimage, and so on. When George Mallory was asked why he wanted to climb Mount Everest, the English mountaineer gave his often-quoted answer: “Because it is there.” The existence of mountains has a special significance for geologists: the very science of geology owes its existence to mountains. In the late 18th century when geology was born, Horace Benedict de Saussure, the Swiss founder of Alpine geology (and indeed one of the earliest scientists to use the term ‘geology’), remarked: “It is above all the study of mountains that will accelerate progress in the Theory of the Earth. Plains are uniform; it is not possible to investigate in them a section of the earth and its different layers.” Ironically, there is no particular branch of geology that exclusively studies mountains, possibly because all of geology is concerned with mountains: igneous and metamorphic rocks, faults and folds, fossils and sediments, landscape development, and even many types of sedimentary basins which are formed and fed by mountains.

Nevertheless, the rapid specialisation of numerous disciplines in geology over the past century has created a void in our geoscience education and research. It is high time we developed an integrative, multi-disciplinary field that focuses on the study of mountains in a holistic way, not only for purely scientific reasons but also because mountains exert great influences on highland and lowland populations, river courses, regional climate, agriculture, tourism industry, and so forth.

Orogenesis: The Making of Mountains fills an important niche in our geoscience education. Perhaps the first of its kind as a textbook, it is written by two geologists who each have decades of teaching and research experience. Michael Johnson, a structural geologist, and Simon Harley, a metamorphic geologist, are both from the University of Edinburgh, the very place James Hutton laid the foundation of geology more than two centuries ago.

Review of Mountain Geology

The term orogenesis (‘mountain-building’) usually refers to the formation of mountains by the convergence of tectonic plates. This takes place by ocean-continent collision (e.g., the Andes), continent-continent collision (the Alps and the Himalayas), or island arc-continent collision (e.g., New Guinea). All these tectonic processes create sedimentary basins of various types. (‘Orogen’ is a mountain system, and ‘orogeny’ refers to a particular mountain-building event such as the Alpine or the Andean. These terms were introduced by the German geologist, Leopold Kober.)

This book focuses on mountain belts like the Alps, the Himalayas, the Andes and the Cordilleras. (Continental rift-shoulder mountains are not discussed in this book.) These are all modern mountains with high relief (over 600m, the definition of a mountain) but to the geologist, even the tectonically old and low-relief features like the Appalachians and the Caledonides are also orogens because they allow us to investigate deep-seated tectonic processes.

Through twelve chapters, the book offers an up-to-date review of various aspects of mountain geology. The first two chapters are an introduction to the plate-boundary types and driving mechanisms of plate tectonics. This is followed by basic information about geodynamics, geochronology, and thrust tectonics (Chapters 3 and 4). The meat of the book is two chapters which together account for 40% of the entire volume: Chapter 5 is on the tectonic evolution of orogens, analysed both in theory and with examples from around the world; and Chapter 7 deals with metamorphism and metamorphic rocks in mountain belts. The erosion and denudation of mountains and the formation of foredeep basins are treated in Chapters 8 and 9. One chapter titled ‘Mountains and climate’ discusses the role of the Himalayas in shaping the monsoons in southern Asia. The last chapter of the book, ‘Secular change in orogeny’, touches on a hot debate: are there particular periods in the Earth’s history during which mountain-building processes predominate or is orogenesis a uniform process through geologic time? This last question obviously has important implications for the global distribution of foredeep basins. A set of 47 colour photographs and a list of some 500 references for further reading come at the end of the book.

Mountain outcrops provide a unique opportunity to observe formations and structures which are responsible for petroleum systems in the subsurface. Moreover, mountain-building and basin formation processes are geodynamically coupled. A modern and comprehensive understanding of orogenesis, as presented in this new book, deserves to be part of the petroleum geologist’s reading.

Orogenesis: The Making of Mountains
Published by Cambridge University Press in 2012.

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