Introducing Geology

Park Graham

Description

Our world is made of rock. Although much of the Earths surface is covered by vegetation, concrete or water, if one digs down far enough solid rock will always be found. Those who live in a landscape where rock outcrops are obvious will have wondered about the kind of rocks they are looking at and how they came to be where they are now. Graham Parks introductory book has swiftly established itself as a key resource for those looking for a straightforward explanation of what geology tells us about the world. Many objects of great beauty and which excite our curiosity, such as crystals or fossils, are to be found by examining rocks. In particular fossils, whilst interesting in themselves, tell us from their context in geological time of biological evolution and these clues give an insight into the origins of life on earth. Copiously illustrated this book is intended for those whose interest in geology has been awakened, perhaps by media coverage of earthquakes or of dinosaurs, and want to know more. It has proved an ideal primer for those considering the study of earth sciences more formally. Technical terms are kept to a minimum and are explained in a glossary.

Sujets

• Sciences et techniques

Informations

• Publié par: Dunedin Academic Press
• Date de parution: 08 novembre 2018
• Nombre de lectures: 1
• EAN13: 9781780465968
• Langue: English
• Poids de l'ouvrage: 17 Mo

Extrait

IutrodUciug Geology

A GUide to the World of Rocks

THIRD EDITION

Graham Park

Contents

Acknowledgements List of tables and illustrations Sourced illustrations Preface: A world of rock

1Crystals, minerals and gemstones 2Volcanoes and melted rock

3Shaping the land

4Mud, sand and other deposits

5Moving continents and making mountains

6Earthquakes and faults

7Squeezing and stretching – rock deformation

8Geological time and the age of the Earth

9Fossils – a history of life

10Turning the pages – Earth history

11Geology and industry

Glossary Further Reading

Acknowledgements

I am indebted to Mrs Anne Shelley of the Orcadian S tone Company, Golspie, Sutherland, for allowing me to photograph some of the excellent specimens of fossils and minerals in her geological museum. My thanks ar e also due to Anne Morton of Dunedin Academic Press and an anonymous reviewer of the draft of the first edition for their numerous helpful suggestions, and to Prof essor Charles Holland of Trinity College, Dublin for his careful review ofchapters 9and10. The second edition has benefited from a number of h elpful suggestions for improvement from various reviewers of the first edi tion. I am grateful in particular to John Winchester, who pointed out a number of mistak es and drew my attention to the revised geological timescale. Any remaining ina dequacies in this book are entirely the author’s responsibility. Finally I wish to thank my wife Sylvia for her cons tant support and encouragement, and as a non-geologist, for ‘test driving’ the firs t draft.

The following sources of data were particularly use ful.

Duff, P. McL. D.,Holmes’ Principles of Physical Geology, 4th edition, Chapman & Hall, London, 1993.

Gradstein, F. M., Ogg, J. G. & Smith, A. G.,A Geologic Time Scale, Cambridge University Press, 2005.

Keary, P. (ed.),The Encyclopedia of the Solid Earth Sciences, Blackwell, Oxford, 1993.

Lambert, D.,The Cambridge Field Guide to Prehistoric Life, Cambridge University Press, 1985.

Stanley, S. M.,Exploring Earth and Life through Time, Freeman, New York, 1993.

Note to the third edition

I have taken the opportunity to thoroughly revise t he text and improve many of the line drawings, with greater use of colour, taking i nto account the many helpful suggestions made by reviewers. Some of the photogra phs have been exchanged for better versions, and a number of others added.Graham Park, July 2018.

Table 2.1 Table 4.1 Table 6.1 Table 8.1 Table 9.1

Figure 1.1 Figure 1.2 Figure 1.3 Figure 1.4 Figure 1.5 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 3.5

Figure 3.6 Figure 3.7

Figure 3.8 Figure 4.1 Figure 4.2 Figure 4.3

Figure 4.4 Figure 5.1 Figure 5.2 Figure 5.3 Figure 5.4 Figure 5.5 Figure 5.6 Figure 5.7 Figure 5.8 Figure 5.9 Figure 5.10 Figure 5.11 Figure 6.1 Figure 6.2

List of tables and illustrations

Main types of igneous rock. Some common types of sedimentary rock. Measuring earthquake effects: Mercalli and Richter scales. The stratigraphic column. The family tree of man.

Crystal structure. A, quartz crystal; B, calcite crystal; C, geode; D, muscovite mica. A, gypsum; B, fluorspar (fluorite). A, hematite; B, pyrite; C, malachite; D, azurite; E, galena. A, agate; B, garnet Vulcanicity: A, erupting volcano; B, ropy lava; C, pyroclastic flow. A, volcanic breccia; B, geyser. A, igneous dyke; B, basalt pillow lava. Magma chambers and channels: types of ign eous intrusion. A, granite; B, basalt; C, dolerite dyke; D, photomicrograph of gabbro Main regions of the Earth’s interior. Origin of oceanic basaltic lavas. Erosion: granite tor. Erosion: A, effects of erosion on soft be ach sand; B, debris apron. River systems: A, potholes; B, river mean ders. Effects of rejuvenation on a river profil e. Coastal effects: A, raised beach; B, ripp le marks on beach sand; C, mud cracks. Desert erosion: A, canyon; B, mesa and bu ttes. Glaciation: A, valley glacier; B, the end of the Briksdal Glacier; C, crevasse. Glaciation: A, glacial erratic; B, glacia ted corrie; C, the Matterhorn. Sedimentary rock layering: A, bedding; B, formations, Grand Canyon. Clastic sedimentary rock: pebbles in coar se sandstone. Sedimentary structures: A, cross-bedding; B, mud cracks; C, load casts in greywacke. A, boulder tillite; B, red and green bedd ed cherts. Gondwana. Climatic zones of Pangaea. 200 million-year-old north pole positions . Main topographic features of continents a nd oceans. The ‘conveyor-belt’ model. Pattern of recent earthquake and volcanic activity. The plates and their boundaries. The Red Sea–Gulf of Aden–African Rift sys tem. Subduction and collision. Mechanisms for plate motion. A, Iceland hot-spot; B, model of a plume . Earthquake damage. Earthquake waves: recording, wave paths a nd location.

Figure 6.3 Figure 6.4 Figure 6.5 Figure 6.6 Figure 6.7 Figure 6.8 Figure 7.1 Figure 7.2 Figure 7.3 Figure 7.4 Figure 7.5 Figure 8.1 Figure 9.1 Figure 9.2 Figure 9.3 Figure 9.4

Figure 9.5

Figure 10.1 Figure 10.2 Figure 10.3 Figure 10.4 Figure 10.5 Figure 10.6 Figure 11.1 Figure 11.2 Figure 11.3

Types of fault: normal, reverse and wrenc h. Thrust faults. Normal fault. Extension by faulting. Strike-slip faulting. Columnar jointing, Staffa. Fold shapes. A, folding associated with faulting; B, f low folds. Rock fabric. Fabric: A, foliation; B, cleavage; C, lin eation; D, shear zone. Transition from fault to shear zone: sect ion through the crust. Hutton’s unconformity. Evolution of the graptolites. Evolution of the cephalopods. A, goniatite; B, trilobite. A, coral; B, brachiopodTerebratulina; C, echinoidClypeaster; D, ammonite. A, Jurassic fish; B,Tyrannosaurus rex; C,Archaeopteryx; D, fly and spider in amber. Early Proterozoic supercontinent. Rodinia: a mid-Proterozoic supercontinen t. Pannotia: a late Proterozoic supercontin ent. Mid-Palaeozoic reconstruction of the con tinents. The assembly of Pangaea. The break-up of Pangaea. Structural traps for oil and gas. Oil production platform. Artesian aquifer.

Sourced illustrations

The following illustrations are rebroduced By bermission: ritish Geological Survey. ©NERC. All rights reserved. IPR/73-34C, 122-06CT:Figures 1.2D,2.1A,C,2.5A,D,3.1,3.3,3.5A, 3.7A,3.8A,,8.1,9.4A,,C, and9.5D; ©Science Photo LiBrary:Figure 9.5,C; Shutterstock:1.2C,1.3,1.4D,E,1.5A,2.2,3.2,3.6,3.8C,6.1,9.5A,11.2.

The following illustrations have Been adabted from buBlished sources:

Figure 2.6: HamBlin, W.K. 1989.The Earth’s dynamic systems, 5th edition. Macmillan, New York. Figure 2.7: ott, M.P. 1982.The interior of the Earth, 2nd edition, Edward Arnold, London. Figure 5.1,5.2: HamBlin, W.K. 1989.The Earth’s dynamic systems, 5th edition. Macmillan, New York. Figure 5.3: McElhinny, N.W. 1973.Palaeomagnetism and plate tectonics, CamBridge University Press. Figure 5.4: Wyllie, P.J. 1976.The way the Earth works. Wiley, New York. Figure 5.6: Chadwick, P. 1962. Mountain-Building hybotheses. In: S.K. Runkorn (ed) Continental drift. Academic Press, New York, London (seismicity);galleryhib.com (volcanoes) via Wikimedia commons. Figure 5.7: Vine, F.J. and Hess, H.H. 1970. In:The Sea, v4. Wiley, New York. Figure 5.8:weeBly.com/somaliblate via Wikimedia Commons. Figure 5.11A: Saemundsson, K. 1974. Evolution of the axial rifting zone in northern Iceland. Bulletin of the Geological Society of America,85, 495–504. Figure 6.4C: Elliott, D. and Johnstone, M.R.W. 1980. Structural evolution in the northern bart of the Moine thrust zone.Transactions of the Royal Society of Edinburgh: Earth Sciences, 71, 69–96. Figure 6.6: Lister, G.S. & Davis, G.A. 1989. The origin of metamorbhic core comblexes. Journal of Structural Geology, 11, 65–94. Figure 7.5: Ramsay, J.G. 1980. Shear geometry: a review.Journal of Structural Geology, 2, 8399. Figure 10.1: uchan, K.L., Mertanen, S., Park, R.G., Pesonen, L.J., Elming, S.A., ABrahamsen, A. & ylund, G. 2000. Combaring the drift of Laurentia and altica in the Proterozoic: the imbortance of key balaeomagnetic boles.Tectonophysics,319, 167–98. Figures 10.2,10.3,10.4: Dalziel, I.W.D. 1997. Neobroterozoic-Palaeozoic geograbhy and tectonics: review, hybothesis, environmental sbeculation.Geological Society of America Bulletin,109, 1, 16–42. Figure 10.5: Matte, P. 1986. Tectonics and blate tectonics model for the Variscan Belt of Eurobe.Tectonophysics,126, 329–74. Figures 10.5,10.6: Torsvik, T.H. and Cox, L.R.M. 2017.Earth history and palaeogeography. CamBridge University Press.

Preface: A world of rock

Our world is made of rock. This fact might be diffi cult to comprehend at first, since much of the Earth’s surface is covered by vegetatio n, concrete or water, but if one digs down far enough, solid rock will always be fou nd. Those fortunate to live in a landscape where rock outcrops are obvious features will maybe have wondered what kinds of rock they are looking at and how they have come to be where they are. The answers to these Puestions are in the province of t he geologist, and this book is an attempt to explain what geology can tell us about t he world we live in. Many objects of great beauty, or which excite our curiosity – su ch as crystals or fossils – are to be found by examining rocks, and those searching for a nd finding such objects gain much more by knowing how they originated. Fossils, for example, are interesting in themselves of course, but set in the context of geo logical time, provide us with the evidence for biological evolution and the clues tha t give us an insight into the origins of life itself. The science of geology covers many different subjec ts, and embraces the methods of physicists, chemists and biologists in o rder to understand the rocks that make up the Earth and the processes that formed and modified them. Broadly speaking, geological phenomena can be divided into materials and processes. Materials include rocks, and those objects that are contained within rocks, such as minerals and fossils. One set of processes is respo nsible for shaping the landscape; these wear down rocks to form gravel, sand and mud that eventually end up as sedimentary deposits in rivers, lakes and the sea. Rocks formed in this way are te rm e dsedimentary rocksch. Another type of process involves molten rock, whi forms deep within the Earth and makes its way towar ds the surface to form volcanoes. On its way up, molten rock (magma) is injected into the surrounding solid rock, forming a variety of structures. Rocks formed by the solidification of magma are term edigneous rocks. A third group, known astectonic processes, results from large-scale movements of the Earth’s crust (‘plate tectonics’) which in turn lead on a smaller scale to the formation of faults and to the sPueezing and stretching of solid rocks into new shapes. Tectonic processes taking pl ace at great depth are accompanied by changes in the rocks themselves brou ght about by the greater heat and higher pressure; rocks, of whatever origin, cha nged in this way in the solid state are termedmetamorphic rocks. The almost incomprehensible vastness of geological time is one of the most fascinating aspects of the study of the Earth. We s hall look at some of the methods that are used to measure the age of rocks and to da te geological events. Using these methods, Earth history can be seen as a series of e vents extending from the formation of the Earth some 4600 million years ago until the present day.

Note:all terms highlighted inboldare defined in the Glossary at the end of the book

Crystals, minerals and gemstones

Crystals are among the most beautiful objects of th e natural world. How are they formed, and where can we find them? To answer these questions, we have to un derstand something of the processes by which crysta ls are formed, and of the relationships between crysta ls, minerals and rocks. All rocks are made up of minerals, so that we may r egard minerals as the ‘building blocks’ of rock. A mineral is a substance with a fixed chemical compos ition and atomic structure. For example,quartz, one of the most common of the rock-forming minerals, has t he chemical composition silicon dioxide (silica – SiO ) 2 and has a simple atomic structure based on a single silicon atom surrounded by four oxygen atoms, each of which is shared in turn by neighbouring silicon ato ms (Figure 1.1A). This chemical composition and atomic structure uniquely characterises the mineral quartz . Whether or not quartz assumes a crystalline form de pends on how it is formed, and on its individual history. Some rocks are entirely, or nearly entirel y, made up of quartz minerals. Such a rock is calle d a quartzite, and the quartz particles making up this rock are tightly packed together and do not show an obvious crystalline form. For a crystalline form to develop, the crystal ideally has to have space to grow without being subject to the pressure of neighbouri ng rock material. Thus quartz crystals may develop in a cavity within otherwise solid rock, where fluids co ntaining silica are available, or within a rock mel t of a suitable chemical composition. Laboratory experimen ts can easily be carried out to demonstrate how cry stals can form quite rapidly from solutions of the approp riate chemical composition, and we are all familiar with how ice crystals can form as snow from atmospheric water. The principle is the same with rock-forming minerals except that the time scale is typically ve ry much longer. The shapes of the crystals of a particular mineral can be used to identify the mineral and are geometr ically related to the atomic structure. Thus crystals of c ertain minerals have an easily recognised symmetry and form, such as cubic, or tetrahedral (four-sided) or they may have a hexagonal cross-section, as inquartz (Figure 1.2Am is the angle between any two of the various). A diagnostic feature of a particular crystal for crystal faces, rather than the shape of the faces t hemselves, which can vary considerably.

Figure 1.1Crystal structure. The diagram shows the basic building block of a silicate mineral, which is the 4-sided silica pyramid, or tetrahedron (A). This is made up of a central small silicon atom (black) surrounded at each of the 4 corners of thetetrahedronby a larger oxygen atom (green). The tetrahedron can be joined to others in the form of a chain (B) or a sheet (C) by sharing oxygen atoms (black circles) with adjoining tetrahedra. The regular geometry of these arrangements is reflected in the crystal shape. In quartz, all 4 oxygen atoms are shared, so that the molecular formula is SiO . In the silicate minerals, some of the oxygen atoms are shared by other elements such as iron or 2 aluminium; these join adjacent chains or adjacent sheets, for example.

To find natural examples of crystals, we have to lo ok for cavities or voids within the rock where crys tal growth could occur, and we also have to determine w hether the rock composition is appropriate. Thus, f or example, crystalline quartz is commonly found in op en cracks within silica-rich rocks, and crystallinecalcite (calcium carbonate – CaCO ) (Figure 1.2B) may often be found in caves within limestone rock s. The less 3 common minerals are not so easily found. Many occur inmineral veins, which are structures of variable, often sheet-like, form composed entirely of a parti cular mineral or group of minerals. Although such v eins need not be composed of minerals in their crystalli ne form, they are the most likely places to find cr ystals. Unfortunately, many mineral veins that contain valu able ore minerals, such as copper or lead ores, hav e been heavily exploited and are no longer accessible . Another source of crystals isgeodes, which occur in volcanic rock; these commonly have an almost spheri cal shape and represent cavities within which cryst als have grown inwards from the outer shell of the geod e (Figure 1.2C). A favourite source of minerals is the spoil heaps that accompany old mine workings, altho ugh in many cases these have been thoroughly worked over by amateur geologists and mineral collectors!

Some common minerals

The most common rock-forming mineral isfeldspars,, of which there are a number of different varietie depending on their chemical composition. The feldsp ars aresilicates, which combine silica with aluminium and various combinations of sodium, potassium and c alcium. Feldspar is the chief component of most volcanic rocks; it is usually white or pink in colo ur and is the most obvious mineral to be seen in th e polished granite slabs adorning many of our banks – a good p lace, incidentally, to look for examples of mineral s. Another common mineral, also to be found in many gr anites, ismica, of which there are two main varieties, biotite andmuscovite. Biotite is brown or black in colour, whereas musc ovite is almost colourless (Figure 1.2D). Micas occur as thin sheets or flakes and their f lat surfaces gleam in the light so that they are ea sily distinguishable from the accompanying feldspar and quartz. Muscovite is a hydrated potassium aluminium silicate, with the complex chemical formula KAl (OH) (AlSi O ). Biotite is similar but contains iron and 2 2 3 10 magnesium in place of some of the aluminium. The pr esence of iron is responsible for the darker colour of this mineral. The sheet-like or tabular form of mic a crystals is due to their atomic structure (e.g.seeFigure 1.1Cm), in which relatively strong silicate sheets are h eld together by weaker bonds consisting of potassiu and aluminium atoms.