Introduction
Mountains are created by two complementary processes – uplift and erosion. While the role of erosion in moulding the landscape has been generally understood by scientists since the work of James Hutton in the late eighteenth century, the mechanism causing uplift has been a puzzle until comparatively recently, and it is necessary to explain how and why this elevation occurs if we are to understand the origin of mountains.
Most mountains occur within relatively well-defined, narrow belts or mountain chains separated by wide expanses of much lower-lying ground. Their distribution is not random, but is caused by the now well-understood geological processes of plate tectonics. Some mountains mark the site of a former plate collision – where one continental plate has ridden up over another, resulting in a zone of highly deformed and elevated rocks. Others are essentially volcanic in origin – the volcanoes occurring either singly or in linear zones.
Hollow mountains
Early investigators were puzzled by the fact that the great mountain ranges such as the Himalayas appeared to be less heavy than their volume would suggest; that is, they did not exert the amount of gravitational attraction that a body of their size should have done – they behaved as if they were hollow! This was then explained by the discovery that more of a mountain belt occurs below the surface than above; this phenomenon is explained by the principle of ‘isostasy’, which describes a state of general gravitational equilibrium in which the extra weight of a mountain should be balanced by a deficiency of the denser material beneath, thus implying that denser material at a lower level must have been displaced to accommodate the additional mass of the less-dense material. The usual analogy made is with an iceberg floating in water, where most of the ice is submerged. Without this insight, it is impossible to explain why mountains can exist.
Thus, to the geologist, a mountain is more than just the topographical shape of the mountain itself, which is largely due to the way in which it has been carved out by erosion, but is part of a structure that is much larger and extends both vertically downwards and, apart from single volcanoes, also horizontally along the belt or mountain chain of which it forms a part. Moreover, the geological composition and structure of the rocks composing the mountain are an essential clue to its origin.
The terms ‘mountain’ and ‘orogenic belt’
It is important, at the outset, to explain the difference in meaning between the terms ‘mountain belt’ and ‘orogenic belt’. The former is used in a geographical sense and describes a topographic feature. The terms ‘orogenic belt’ or ‘orogen’ are used by geologists to describe a suite of geological features in which the presence of mountains, either now or in the geological past, is an integral and essential part.
The age of a mountain
While geologists now have a good understanding of how mountains are formed, this is by no means the case for the average non-geologist. A popular misconception is that the age of a mountain is represented by the age of the rocks it contains, and it is important to emphasise that the age is given by the date at which the mountain was elevated and eroded, and that this bears no simple relationship to the age of its included rocks. For example, the grand mountains of Assynt in Northwest Scotland are often referred to as ‘the oldest mountains in Scotland’ by virtue of the fact that they are composed of relatively old rocks (mostly late Precambrian). However, the age of these mountains is in fact determined by the period during which they were elevated and consequently sculpted by erosion – which was in this case considerably later, and culminated in substantial modification during the last Ice Age.
Modern ideas on the origin of mountains evolved gradually over several centuries. Chapter 2 traces the historical development of these ideas and the contributions of the geologists who have played an important part in it, summarising the various mechanisms that have been proposed historically for the origin of mountains until the1960s, when our ideas were completely transformed by the plate-tectonic theory.
The historical development of the plate-tectonic theory, with its implications for mountain formation, is summarised in chapter 3, followed by an outline of the structure and composition of a well-studied example of an existing mountain belt, the Himalayas, providing a template which can be applied to the various examples that follow in the succeeding chapters.
The Earth’s mountain systems may be divided into three categories: 1) the currently active Alpine–Himalayan and circum-Pacific belts; 2) the ocean ridge network; and 3) the older mountain belts that are no longer active.
The geologically recent Alpine–Himalayan mountain belt, described in Chapters 4–8, follows a set of collisional boundaries between (largely) continental plates. The circum-Pacific system, described in Chapters 9–12, follows the complex set of subduction zones that border the Western Pacific Ocean, and the American Cordilleran system in the Eastern Pacific.
Chapter 13 is devoted to the great submerged mountain chains of the deep oceans: these include currently active spreading ridges as well as those following active faults, inactive fracture zones and volcanic island chains.
Figure 1.1 Main sub-divisions of geological time. Note that none of the subdivisions of the Mesozoic and Palaeozoic Eras, nor of the Archaean Eon are shown, as they are not referred to in the text. Generally, only the first-order subdivisions of the Proterozoic are used – Early, Mid, and Late; these terms are equivalent to the more correct Palaeo-Proterozoic, Meso-Proterozoic and Neo-Proterozoic, respectively.
The final chapter explores some of the older (pre-Mesozoic) mountain chains, including the Palaeozoic Caledonian and Hercynian orogenic belts.
Units of time and motion
The age of geological events is always given in Ma (million years) before present. Tectonic events are rarely known with much precision, and in any case usually span time periods of many millions of years. Most frequently they are quoted in terms of part of the geological timescale (Fig. 1.1) in terms such as ‘mid-Cretaceous’ or ‘early Miocene’, in which case the Table will give the exact time range referred to.
Rates of movement are usually in the range 1–20 millimetres per year and are quoted in the form: mm/a (mm per annum). These are invariably mean measurements or estimates based over several years, or in the case of plate movements, millennia.