The Origin of Mountains

Cliff Ollier, Colin Pain

• Publisher: Routledge
• Year: 2004
• eBook ISBN:
  9781134638789
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

The Origins of Mountains approaches mountains from facts about mountain landscapes rather than theory. The book illustrates that almost everywhere, mountains arose by vertical uplift of a former plain, and by a mixture of cracking and warping by earth movements, and erosion by rivers and glaciers, the present mountainous landscapes were created. It also gives evidence that this uplift only occured in the last few million years, a time scale which does not fit the plate tectonics theory.
Another fascinating part of the evidence, shows that mountain uplift correlates very well with climatic change. Mountain building could have been responsible for the onset of the ice age. It certainly resulted in the creation of new environments. Fossil plants and animals are used in places to work out the time of mountain uplift, which in turn helps to explain biogeographical distributions.

1
Introduction

Mountains are a common feature of the Earth, easily recognised by ordinary people and scientists alike. Folded rocks are also observed in many places, and a dominant idea arose very early that the same forces that folded the rocks also formed the mountains. Over the past two centuries many theories of mountain formation have been proposed, mostly based on a mechanism that could also fold rocks. An early idea was the shrinking Earth, where fold mountains were created like folds in the skin of a shrivelled apple. Since about 1965 plate tectonic ideas have dominated geology. In this hypothesis mountains are formed where plates of the Earth’s crust collide, and the movement of one plate below another (subduction) causes both folding and mountain building.

These grand theories go straight to the ultimate mechanism and driving force, and miss out on some of the vital landscape-forming processes, especially the former existence of plateaus where the mountains stand today.

Geomorphologists (scientists who study landscapes) have long studied plains, many of which are erosion surfaces cut across varied rock structures such as folds and faults. If an extensive plain is raised by regional uplift it becomes a plateau, such as the Tibetan Plateau, the Highveld of southern Africa, or the Tablelands and High Plains of Australia. Erosion of plateaus can create rugged topography—in fact it makes mountains. So to understand mountains we must first know more about their geomorphic history. In most mountain studies this is not done: instead the rock structures inside the mountains are described in detail, with the tacit assumption that whatever made the structures also made the mountains.

This assumption is not warranted. The plains of Western Australia and Africa are about as flat as any erosional land surface can get (Figure 1.1). Very complex structures including folds, faults and highly sheared metamorphic zones underlie the plains (Figure 1.2). Nobody suggests that these structures formed the planation surface. Yet when similar structures are found beneath mountains many geologists assume that the forces that made the structures also formed the mountains. Even when the rock structure includes great thrusts, as in the Carpathians, the reasoning should be the same: the major structures were planated long before the present mountains came into existence (Figure 1.3). Thus structures in the Appalachian Mountains are commonly thought of as related to the Appalachian orogeny that folded the Palaeozoic rocks and also formed the Appalachian Mountains. In reality the Palaeozoic structures were planated, and it was later uplift of the planation surface followed by further erosion of valleys that created the Appalachians of today. Similarly in Scandinavia we are told that the Caledonian orogeny (which deformed the rocks) made the Caledonian Mountains. The reality is that the Caledonian structures were eroded to a plain, the planation surface was warped up much later to form a plateau, and later erosion made the mountains of Norway. Planation surfaces are formed by erosion to a base level, usually sea level. Uplifted planation surfaces (plateaus) indicate vertical uplift of a former low-lying plain. Widespread planation itself indicates a period of tectonic stability when erosion was not offset by uplift. Uplift is a vertical movement. Uplift does not affect the whole world, but a broad area. This is epeirogeny, and it is ironic that while the meaning of orogeny has changed from ‘mountain building’ to ‘rock deformation’ it now seems that epeirogeny is what makes mountains.

Figure 1.1 The Acholi Plain in northern Uganda (photo C.D.Ollier).

Figure 1.2 A typical cross-section in northern Uganda (J.V.Hepworth, pers. comm.).

Figure 1.3 Cross-section of the Western Carpathians. The thrusting affects virtually all the crust, and in comparison all the structures are planated. The mountains of today are caused by much younger forces than those that caused the great structures. (after Plasienka et al. 1997).

The study of the geomorphology of mountains all over the world leads to a remarkable conclusion. Most of the world’s mountains are formed by erosion of plateaus, which are themselves uplifted erosional plains. Folding in these areas must be older than the formation of the planation surface, and of course older than the mountains. This means we must separate two processes, or sets of processes—one set causes folding and other structures, the other set causes plateau uplift. In fact we can distinguish four sets of processes:

1. processes that cause folds and other structures;
2. processes that make planation surfaces;
3. processes that cause uplift of a plain to form a plateau (gentle bends, monocline, fault block (horst), tilt block);
4. erosional processes that dissect a plateau into mountains (basically fluvial and glacial).

These four sections will be dealt with in the next chapters.

Furthermore, many of the world’s mountain belts are on sites of geologically young uplift, which also leads to many interesting conclusions. Later in the book we shall present evidence for the age of mountains throughout the world, and examine the implications.

It turns out that uplift of land to make mountains results in tectonic, climatic and geomorphic changes. In other words there are feedback mechanisms between mountain building and other processes.

Volcanoes are different from other mountains in their mode of origin, and are treated separately in Chapter 9.

This book presents a detailed account of mountain building including both tectonic and geomorphic evidence so far as can be done in a single book. We also have to digress at times to discuss some basic ideas of geology and geomorphology. Finally we discuss some of the implications of this view of earth history.

Terminology

Uplift, orogeny and mountain building

This section is all about the confusing nomenclature associated with mountains. If you are not interested in details you can skip this part, but if you have a little learning, and especially if you think orogeny makes mountains, it will be best to read it before you read the rest of the book.

Nearly all modern books on mountain-building and orogeny are confused about the origin of mountains and the origin of structures inside them. Hsü’s Mountain Building Processes (1982) is all about structures, and it is simply assumed by most contributors that ‘orogeny’ creates both internal structures and the present-day topographic mountains. In that book only Gansser, in his chapter on the ‘morphogenetic phase’ of mountain building, distinguishes the late, vertical mountain building from earlier compression. Schaer and Rogers’ book The Anatomy of Mountain Ranges (1987) is likewise about internal structures, tacitly assumed to be related to present day mountains. Orogeny is still equated with mountain building by many geologists.

Orogeny

Orogeny is a word literally meaning the genesis of mountains, and when proposed it meant just that. Unfortunately in later years the idea of folding and mountain building being the same thing became entrenched, and with a further swing the term ‘orogeny’ came to mean the folding of rocks. Orogeny is now used to refer to the folding of rocks in fold belts. It does not mean mountain building, despite its etymology. We shall have to use the longer phrase mountain building to be clear.

If authority is needed for this practice we may note the following:

King (1969) wrote in his influential paper: ‘In this account, and on the legend of the “Tectonic map of North America”, “orogeny” is therefore used for the processes by which the rock structures within the mountain chains or fold belts are created.’

In Orogeny Through Time Burg and Ford (1997) claim that: ‘To field geologists the term orogeny represents a penetrative deformation of the Earth’s crust.’ We are not convinced that all field geologists really appreciate this not-so-subtle change of meaning.

Jackson (1997), in what is virtually the bible for English-speaking geologists, wrote:

orogeny literally, the process of formation of mountains. The term came into use in the middle of the 19th Century, when the process was thought to include both the deformation of rocks within the mountains, and the creation of the mountainous topography. Only much later was it realised that the two processes were mostly not closely related, either in origin or in time. Today, most geologists regard the formation of mountainous topography as postorogenic. By present geological usage, orogeny is the process by which structures within fold-belt mountainous areas were formed, including thrusting, folding, and faulting in the outer and higher layers, and plastic folding, metamorphism, and plutonism in the inner and deeper layers. Only in the very youngest, late Cenozoic mountains is there any evident casual relation between rock structure and surface landscape. Little such evidence is available for the early Cenozoic, still less for the Mesozoic and Paleozoic, and virtually none for the Precambrian—yet all the deformation structures are much alike, whatever their age, and are appropriately considered as products of orogeny.

However the modern usage has not filtered down to lower levels, and elementary books and dictionaries still commonly follow the old usage, and have figures like that in Figure 1.4. For example, the Hutchin Pocket Dictionary of Geography (1993) has the following entry:

orogeny or orogenesis—the formation of mountains. It is brought about by the movements of the rigid plates making up the Earth’s crust (described by plate tectonics). Where two plates collide at a destructive margin rocks become folded and lifted to form chains of fold mountains (such as the young fold mountains of the Himalayas).

The prestigious National Geographic Society in Exploring Your World (1993) states:

Fold mountains are formed when two of the large plates that carry the earth’s crust slowly collide and compress, or when one plate gradually folds and wrinkles as a result of this action.

They illustrate the idea with a diagram like Figure 1.4, leaving no doubt that they consider the folded rocks to be the direct origin of the fold mountain. Some more serious sources also maintain the old story, such as the definition in The Cambridge Encyclopedia of Earth Sciences (1981):

Orogeny An episode of tectonic activity (folding, faulting, thrusting) and mountain building usually related to a destructive plate margin.

Some writers seem to adopt their own personal definitions of mountain building and orogeny. Miller and Gans (1997), for instance, wrote: ‘Mountain building and orogeny (i.e. thickening of continental crust) are not necessarily the result of subduction.’ We do not equate either mountain building or orogeny with crustal thickening, and suspect that few other workers do so.

Allmendinger and Jordan (1997) wrote: ‘The term “Andean Orogeny” refers collectively to all tectonism that occurred between the Jurassic and Recent.’ This would probably not be most people’s idea of orogeny, but it certainly would not be synonymous with ‘mountain building’ for, as we shall see, the Andean Cordillera were created in only a twentieth of that time.

Epeirogeny

In contrast with orogeny, early geologists used the term ‘epeirogeny’ to mean the uplift of broad areas, as opposed to the narrow fold belts of mountain chains. Gilbert (1890, p. 40) coined the word ‘epeirogeny’ and was also one of the first to use the term ‘orogeny’, so it is useful to get his views: ‘The process of mountain formation is orogeny, the process of continent formation is epeirogeny, and the two collectively are diastrophism.’ Probably nobody follows this usage any more. ‘Epeirogeny’ is still a valid term for the uplift of broad areas, but it does not mean continent formation. We believe that mountains result from erosion of areas that have been uplifted epeirogenically.

Figure 1.4. Two naive illustrations of fold mountains

Top. Elementary diagram of fold mountains, in which the actual mountains have the same shape as the folds. This does not happen in the...

Table of contents

• Cover Page
• Half Title page
• Title Page
• Copyright Page
• Dedication
• Contents
• List of figures
• List of tables
• Preface
• Acknowledgements
• 1 Introduction
• 2 Simple plateaus and erosional mountains
• 3 Fault block mountains
• 4 European mountains
• 5 Western North America
• 6 The Andes
• 7 Asian Mountains
• 8 Mountains with gravity structures
• 9 Volcanoes and granite mountains
• 10 Mountains on passive margins
• 11 Plains and planation surfaces, drainage and climate
• 12 Problems of mountain tectonics
• 13 Science and the origin of mountains
• References
• Index