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Transcript
Week 2 Students get to understand the concepts of geomorphology, hydrology, land form systems, the earth building and d~nudation processes.. The scope and l1ature of the environment and its components are referred to. CHAPTER 1 Environmental Science Definition, Scope and Importance INTRODUCTION The science of Environment studies is a multi-disciplinary science because it comprises various branches of studies like chemistry, physics, medical science, life science, agriculture, public health , sanitary engineering etc. It is the science of physical phenomena in the environment. It studies of the sources, reactions, transport, effect and fate of physical a biological species in the air, water and soil and the effect of from human activity upon these. Environment Explained Literary environment means the surrounding external conditions influencing development or growth of people, animal or plants; living or working conditions etc. This involves three questions: 1. What is Surrounded The answer to this question is living objects in general and man in particular. 2. By what Surrounded The physical attributes are the answer to this question, which become environment. In fact, the concern of all education is the environment of man. However, man cannot exist or be understood in isolation from the other forms oflife and from plant life. Hence, environment refers to the sum total of condition, which surround point in space and time. The scope of the term Environment has been changing and widening by the passage of time. In the primitive age, the environment consisted of only physical aspects of the planted earth' land, air and water as biological communities. As the time passed on man extended his environment through his social, economic and political functions. 3. Where Surrounded The answer to this question. It is in nature that physical component of the plant earth, viz land, air, water etc., support and affect life in the biosphere. According to a Goudie 2 ENVIRONMENTAL SCIENCE environment is the representative of physical components of the earth where in man is an important factor affecting the environment. (i) Definitions of Environment: Some important definitions of environment are as under: 1. Boring: 'A person's environment consists of the sum total of the stimulation which he receives from his conception until his death.' It can be concluded from the above definition that Environment comprises various types of forces such as physical, intellectual, economic, political, cultural, social, moral and emotional. Environment is the sum total of all the external forces, influences and conditions , which affect the life, nature, behaviour and the growth, development and maturation of living organisms. 2. Douglas and Holland: 'The term environment is used to describe, in the aggregate, all the external forces, influences and conditions, which affect the life, nature, behaviour and the growth, development and maturity of living organisms.' (ii) Scope of Environment: The environment consists of four segments as under: 1. Atmosphere: The atmosphere implies the protective blanket of gases, surrounding the .earth: (a) It sustains life on the earth. (b) It saves it from the hostile environment of outer space. (c) It absorbs most of the cosmic rays from outer space and a major portion of the electromagnetic radiation from the sun. (d) It transmits only here ultraviolet, visible, near infrared radiation (300 to 2500 nm) and radio waves. (0.14 to 40 m) while filtering out tissue-damaging ultra violate waves below about 300 nm. The atmosphere is composed of nitrogen and oxygen. Besides, argon, carbon dioxide, and trace gases. 2. Hydrosphere: The Hydrosphere comprises all types of water resources oceans, seas, lakes, rivers, streams, reserviour, polar icecaps, glaciers, and ground water. (i) Nature 97% of the earth's water supply is in the oceans, (ii) About 2% of the water resources is locked in the polar icecaps and glaciers. (iii)Only about 1% is available as fresh surface water-rivers, lakes streams, and ground water fit to be used for human consumption and other uses. 3. Lithosphere: Lithosphere is the outer mantle of the solid earth. It consists of minerals occurring in the earth's crusts and the soil e.g. minerals, organic matter, air and water. 4. Biosphere: Biosphere indicates the realm of living organisms and their interactions with' environment, viz atmosphere, hydrosphere and lithosphere. 1 COMPONENTS AND SUBCOMPONENTS OF ENVIRONMENT 1.1 CLASSIFICATION OF ENVIRONMENT The tenn Environment can be broadly defined as one's surroundings. To be more specific we can say that it is the physical and biological habitat that surrounds us, which can be felt by our physical faculties (seen, heard, touched, smelled and tasted.) The two major classifications of environment are : (A) Physical Environment: External physical factors like Air, Water, and Land etc. This is also called the Abiotic Environment. (B) Living Environment: All living organisms around us viz. plants, animals, and microorganisms. This is also called the Biotic Environment. Earth 's environment can be further subdivided into the following four segments: (I) Lithosphere (2) Hydrosphere (3) Atmosphere (4) Biosphere. LITHOSPHERE The earth's crust consisting of the soil and rocks is the lithosphere. The soil is made up of inorganic and organic matter and water. The main mineral constituents are compounds or mixtures derived from the elements of Si, Ca, K, AI , Fe, Mn, Ti, 0 etc. (Oxides, Silicates, and Carbonates). The organic constituents are mainly polysaccharides, organo compounds of N, P and S. The organic constituents even though fonn only around 4% - 6% of the lithosphere, they are responsible for the fertility of the soil and hence its productivity. HYDROSPHERE This comprises all water resources both surface and ground water. The world's water is fOUlld in oceans and seas, lakes and reservoirs, rivers and streams, glaciers and snowcaps in 3 4 PRINCIPLES OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY the Polar Regions in addition to ground water below the land areas. The distribution of water among these resources is as under Table 1.1 Table 1.1 Oceans and Seas 96-97 % Glaciers and polar icecaps 2-3 % Fresh water < 1% The water locked up in the Oceans and Seas are too salty and cannot be used directly for human consumption, domestic, agriculture or Industrial purposes. Only less than I % of water resources are available for human exploitation. Water is considered to be a common compound with uncommon properties. These uncommon properties (e.g. anomalous expansion of water) are mainly responsible for supporting terrestrial and aquatic life on earth. BIOSPHERE The biosphere is a capsule encircling the earth's surface wherein all the living things exist. This portion extends from IOOO()' m below sea level to 6000 m above sea level. Life forms do not exist outside this zone. The biosphere covers parts of other segments of the environment viz. Lithosphere, Hydrosphere and Atmosphere. Life sustaining resources like food, water and oxygen present in the biosphere are being withdrawn and waste products in increasing quantities are being dumped. The biosphere has been absorbing this and assimilating them. However the rate of waste dumping has gone beyond the assimilating capability of the biosphere and signals of this stress is becoming evident. ATMOSPHERE It is the gaseous envelope surrounding the earth and extends upto 500 kms above the earth's surface. The composition of the atmosphere is given in Table 1.2 Table 1.2 Constituent Volume Nitrogen 78.1 Oxygen 20.9 Water vapour 0.1-5 Argon 0.9 Carbon dioxide 0.03 Trace constituents* 0/0 Balance *The trace constituents include Helium, Neon, Krypton, xenon, S02' N0 2, Ammonia, Ozone, and Carbon monoxide etc. The atmosphere, which is a gaseous cover, protects the earth from cosmic radiations and provides life sustaining Oxygen, the macronutrient Nitrogen and Carbon dioxide needed for photosynthesis. The atmosphere screens the dangerous UV radiations trom the sun and allows only radiations in the range of 300 nm - 2500 nm (near UV to near IR) and radio waves. The atmosphere plays a major role in maintaining the heat balance of the earth by absorbing the ~~~M~cJ~ ~ ~ ~t.o w-.o"'fNl\'0t'J ~~t~+ 1 WHAT IS GEOMORPHOLOGY? Geomorphology is rhe srudy of landforms and rhe processes rhar crean; rhem. This chaprer covers: • historical, process, applied. and orher geomorphologies • • • • rhe form of rhe land land -forming processes and geomorphic sysrems rhe hisrory of landForms merhodological isms INTRODUCING GEOMORPHOLOGY The word geomorphology derives from rhree Greek words: yEW (the Emh), ~lopCPl1 (Fopn), and A.oyor:, (discourse). Geomorphology is therefore 'a discour.~c on Earth Forms'. Iris rhe srudy of Earrh's physical land sUl'face features, irs landforms - rivers, hills, plains, heaches, sand dunes, and myriad orhers. Some work ers include submarine landforms wirhin rhe scope of geomorphology. And some would add rhe landforms of orher rerre.m ial-rypc planers and sarellire.< in rhe Solar Sysrem - Mars, rhe Moon. Venus, and so on. Landforms are conspicuous fearures of rhe Earrh and occur every where. They range in size from molehills ro mounrains ro major recronic plates, and rheir 'Iifespans' range from days ro millennia ro aeons (Figure 1.1). Geomorphology wasfirsr used as a rerm ro describe rhe morphology of[he Earrh's surface in rhe 18705 and 1880s (e.g. de Margcrie 181l(i, 315). Ir was originally defined as 'rhe gcncric srudy of ropographic forms' (McGee 1888, 547), and was used in popular parlance by 1896. Despire rhe modern acquisirion of irs name, geomorphology is a venerable discipline (Box 1.1). Ir invesrigares landforms and rhe processes [har fashion rhem. A large corpus of geomorphologi.m expends much sweat in researching relarionships herwecn landforms and rhe processes acr ing on rhem now. These are rhe process or functional geolllorphoIogists. Many geomorphic processes affecr, '.J:>. Z -i ;;0 , oo c Exogenic processes Microclimate and meteorolollical e\>ents ~ Animal and plant activities I- Local climate Short-term cli matic chanlle loCal vegetation change J t i 1 Microscale landforms km Lifespan: ., 0 years Area: < 0.25 fxogen;c examples: Fluvial Glacial Pools and riffles Small cirques Aeolian Ripples Coastal Beach cusps Mesosc.ale landforms , Area: 0 .25-100 km Lifespan: 1,000 years , - Meanders Valley glaciers Dunes Small iault scarps i t Individual earthquakes and vokanie eruptions i- Regional climate f--+- 1- Regional vegetation change Lifespa n: 10 - , t 1 I I Major drainage basins ~ Ice sheets and inlandsis Large sand ~eas Rt>gional geology I+- Endogenic processes and factors Figure 1. J Landforms at different scales and their inreractions with exogenic and endogenic processes. i ! f--+- long-term uplift and sobsidence r.ontinenlal motion 1- Continental geology o ~ C/l » z I :; I I Major mountain ranges 1 1 Regional uplift and ~ subsi d,.nce Z o ., o Continental coastlines Blod-faulted terrain :; I f+-- I- Isostatic uplift ~ LocaliLed volcanism and seismic activity Local geologic.al structure I+- Geological structure Bedrock properties I I caps G) ;;0 I I Capes and ba}'S Small volcanoes I , km Lifespan: > 1a million years Area: > 1 ,000,000 million years Large river floodplains ice Binloglcal evolution Cl Megascale landform~ Macroscale landforms km long-term climatic change (e.g i(ehoust'> and hothouse states) i t 1 1 Area: 100-1 ,000,000 Z Climatic zones Medium-term climatic change (e.g. glacial-interglacial cycles) Sand seas Deltas IEndogenic examples: f---+ () i ~ Z oC/l () » "mC/l WHAT IS GEOMORPHOLOGY? Box 1.1 THE ORIGIN OF GEOMORPHOLOGY Anciem Greek ;U1d Roman philosophers wondered how mountains and other syrface features in the natural landscape had formed. Arisrode. Herodotus. Sem:ca. Srraho, Xenophallcs, and many ot.hers dis coursed on topics such as the origin of river valleys and deltas, and the presence of seashells in moumains. Xenophanes of Colophon (c. 580-480 1lC) speculated thar. as seashells are fOllnd on the tops of moun tains, the surface of the Earth must have risen and fallen. H~rod()ru.' k 41\4-420 lie) thought that the lower part of Egypt was a former marine hay. reput edly saying 'Egypt is the gift of the river', referring to the year-by-year accumulation of river-borne silt in the Nile delta region. Aris'rorle Ol:l4-322 lie) conjec tured that land and sea change places, with areas that arc (lOW dry land once heing sea and areas that arc now sea once heing dry land. Straho (64/63 flC-AD 2Y) observed that the land rises and falls, and sug gested that the size of a river delta depends on the nature of its catcbmcnt, the largest deltas being found where the catchment areas are: large and rill: surface roLks within it are weak. Lucius Annaeus Seneca (4 Be A/). (5) appears ro have appreciated that rivers possess the power to erode their valleys. Ahout a millennium later, the illustrious Arab scholar ibn-Sina, also known as Avicenna (9l:l0-10J7), who translated Aristorle, propounded the view that some moullluins are pro duced hy differential erosion, running water and wind and are affected by, human activities. Applied geomor phologists explore this rich area of <:nquiry, which is largely an extension of process geomorphology. Many landforms have a long history, and {heir present form docs not always relate ro the currelH processes acring upon them. The nature and rate of geomorphic processes change with time, and som~ landforms were produced under different environmental conditions, surviving today as relict fcatUl'es. In high latitudes, many land forms are relicts from the Quaternary glaciarions; bur, in hollowing out soner rocks. During the Renaissance. many scholars debated Earth history. Leonardo da Vinci (1452-1519) believed that changes in the levels ofland and sea explained the presence of fossil rnarine shells in mountains. He also opin.::d that valleys were cur by StreanlS and that screams carried material from one place and deposited it elsewhere. In the eighteenth century, Giovanni Targioni-Tozzetti (1712-84) recog nized evidence of stream erosion. He argued that the valleys of the Arno, Val di Chaina, and Omhrosa in Italy were excavated hy rivers and floods resulting from the bursting of barrier lakes, and suggested that [he irregular courses of srrcanlS relate to the difFerences in the rocks in which they cut. a process now called difFercnrial erosioll. Jeau-Etienne Guerrard (1715-86) argucd that streams destroy mountains and the sedi ment produced in the process builds floodplains before being carried to the sea. He also pointed to the effi cacy of marine erosion, noting the rapid destruction of cbalk cliffs in northern France by the sea, and the faa [hat the mountains of rhe Allvergne were cxr.inct volcanoes. Horace-Benedict de Saussurc (1740-99) contended that valleys were produced hy the streams that Row within them, and that glaciers may erode: rocks. From these early ideas on the origin oflandforms arose modern geomorphology. (See Chorley u aI. 1964 alld KClmedy 2005 for details on [he development of the SUhjcCL) parts of the world, some landforms survive from millions and hundreds of millions of years ago. Geomorphology, thcn, has an important hismrical dimension, which is the domain of the historical geomorphologists. In short, modern geomorphologists study three chief aspects of landforms - form, process, and history. The first rwo are sometimes termed funcrional geomorphology, the last historical geomorphology (Chorley 1978). Process slUdies have enj()yed hegemony for some three or four decades. Historical studies were sidelined by process 5 6 INTRODUCING LANDFORMS AND LANDSCAPES srut/ie, blH are making a strong comeback. AldltlUgh proce.,s and hisrorical stlldie., dominare much modern geomorphological enquiry, particularly in English speaking nations, other types of srudy exist. For exam ple, structural geomorphologists, who were once a very influenrial group, argued that underlying geological ,trucrun.:s are the kcy to understanding many landforms. Climatic geomorphologist5, who are found mainly in France and Germany, believe thar climate exerts a profound inAuence on landforms, each climatic region cre<lting a distinguishing suite of land forms (p. 13). Historical geomorphology · Traditionally, historical geomorphologis5s srrove to work our landscape history by mapping morphological and sedimentary features. Their golden rule was the dictllm that 'the present is the key to the past'. This was a warral\[ to assume rhar rhe effecrs of geomorphic pro cesses , een in action coday may be legitimately used to infer the causcs of assuroed landscape changes in rhe past. Before rdiable daring techniques were available, such studies were difficulr and largely educated guess work. However, the brilliant .successes of early historical gcomorphologisrs should not be overlooked. William Morris Davis The 'geographical cycle', expounded by Wuliam Morris Davis, was rhe first modern rheory of land scape evolution kg. Davi, 188~, 18~~, 190~) . Il assumed rhat uplift takes place quickly. Ceomorphic processes, wirhout further complications from tccronic movemenrs. rhen gradually wear down rhe raw topog raphy. Furthermore, slopes within landscapes decline through rime - maximum slope angles slowly lessen (though few licld srudies have .~ubslanriared [his claim). So topography i.~ reduced, lirrle by lildc. {O an exten sive Aar region close to baselevel _. ' a peneplain wirh occasional hills. called monadnocks afrer Mounr Monadnock in New Hampshire, USA, which are local erosional remnanrs, standing conspicuously above (he general level. The reducrion process creare, a rirne sequence of landforms rhar progresscs rhrough the srages of youth. maturity, and old age. However. these rerms. borrowed from biology, are mi sleading and much ccnsured (e.g. Oilier 1% 7; Oilier and Pain 19%,204-5). The 'geographical cycle' was designed to accoul1( for rhe developmenr of humid temperate landforms produced by prolonged wearing down of uplifted rocks offering uniform resisrance to erosion. Ie was extended to other landforms. incillding arid landscapes, glacial landscapes, pcriglacial landscapes, to landforms produced by shore processes, and to karsr landscapes. William Morris Davis's 'geographical cycle' - in which landscapes are seen to evolve through stages of youth, maturity, and old age - must be regarded as a classic work, even if ir ha.~ been superseded (Figure 1.2). Ir, appeal seems (() have lain in irs theoretical tenor and in its simpliciry (Chorley 19(5). It had al) all-pervasive influence on geomorphological rhought and spawned rhe once highly inAuential field of denudarion chronology. The work of denudation chronologisrs, who dealr mainly wirh morphological evidcnce, was subsequently criticized for seeing Aat surfaces everywhere. Walther Penck A variarion on Davis's scheme was offered by Walther Penck. According [() the Davisian model, upJifi: and pla nation take place alrernately. Bur, in many landscapes, uplift and denudarion occur ar (he .lame rime. The con rinuoll.'; and gradual interacrion of rectonic processes and denudation leads ro a different model of landscape evo lution , in which the evolurion of individual slopes is rhoughr to determine the evoluciofl of the enrire land scape (Pcnck 1924. 1953). Three main slope forms evolve with differenr combinations of uplifr and denudarion rares. First, convex slope profiles, resulring from wax ing developmenr (aufiteigende Emwicklung), form when (he uplift rate exceeds the denudarion rate. Second, srraighl slopes. resulring from srationary (or steady-slare) developmenr (gleichformige Entwicklung), form when uplifr and denudarion rares march one another. And, rhird, concave slopes, resulring from waning develop ment (absteigende Entwicklung) , form when rhe uplift rate is less rhan the denudation rate. Later work has shown thar valley-side shape depends not on rht: simple interpl ay of erosion rates and uplifr rart's, bur on slope marerials and rhe nature of slope-eroding processes. W 0. '1" ~.u.. &to.."t"") \j \ e.f,s, wu;\. ")"\ -' .h -. L - 1- e.1,.,J I ~ Chapter 1 Introduction • Prologue The purpose of this chapter is to: • • • • Define hydrology. Give a brief history of the evolution of this important earth science. State the fundamental equation of hydrology. Demonstrate how hydrologic principles can be applied to supplement decision support systems for water and environmental management. 1.1 HYDROLOGY DEFINED Hydrology is an earth science. It encompasses the occurrence, distribution. move ment, and properties of the waters of the earth. A knowledge of hydrology is funda mental to decisiomnakingprocesses where water is a component of the system of concern. Water and environmental issues are inextricably linked, and it is important to clearly understand how water is affected by and bow water affects ecosystem manipulations. 1.2 A BRIEF HISTORY Ancient·philosophers focused their attention on the nat\l (tl of processes in\'olved in the production of surface water flows and other phenomena relaled to Ihe origin and occurrence of water in various stage of the perpetua l cycle of water being mnveyed from the sea to the atmosphere to lhe laud and back ag<ljn 10 rhe sea. Untortunate[y. early speculation was often faulty.! -'+ For example , Homer beJieved in the exi~lencc of large subterranean reservoirs that supplied rivers, seas, springs, and deep wells. It is interesting to -note. however, that Homer understood the dependence Of flow in rhe • Superior numbel'1l indicate references at the end of the chapter. 4 CHAPTER 1 INTRODUCTION Greek aqueducts on both conveyance cross section and velocity. This knowledge was lost to the Romans, and the proper relation between area, velocity, and rate of flow remained unknown until Leonardo da Vinci rediscovered it during tbe Italian Renais sance. During the first century B.C. Marcus Vitruvius, in Volume 8 of his treatise De Architectura Libri Decem (the engineer's chief handbook during the Middle Ages), set forth a theory generaJly considered to be the predecessor of modern notions of the hydrologic cycle . He hypothesized that rain and snow Mling in mountainous areas infiltrated the earth's surface and later appeared in the lowlands as streams and springs. In spite ofthe inaccurate theories proposed in ancient times, it is only fair to state that practical application of various hydrologic principles was often carried out with considerable success. For example, about 4000 B.C. a dam was constructed across the Nile to permit reclamation of previously barren lands for agricultural production. Several thousand years I~ter a canal to convey fresh water from Cairo to Suez was built. Mesopotamian towns were protected against floods by high earthen walls. The Greek and Roman aqueducts and early Chinese irrigation and flood control works were also significant project~. Near the end of the fifteenth century the trend toward a more scientific approach to hydrology based on the observation of hydrologic phenomena became evid.ent. Leonardo da Vinci and Bernard Palissy independently reached an accurate under standing of the water cycle. They apparently based their theories more on observation than on purely philosophical reasoning . Nevertheless, until the seventeenth century it seems evident that little if an)' effort was directed toward obtaining quantitative measurements of hydrologic variables. The advent of what might be called the "modern" science of hydrology is usually considered to begin with the studies of such pioneers as Perrault, Mariottc, and HaJley in the seventeenth century. 1.4 Perrault obtained measurementR of rainfull in the Seine River drainage basin over a period of 3 years. Using these and measurements of runoff, and knowing the drainage area size, he showed that rainfall was adequate in quantity to account for river flows. He also made measurements of evaporation and capillarity. Marioue gauged the velocity of flow of the River Seine. Recorded veloc ities were translated into terms of discharge by introducing measurements of the river cross section. The English astronomer Halley measured the rate of evaporation of the Mediterranean Sea and concluded that the amount of water evaporated was sufficient to account for the outflow of rivers tributary to the sea. Measurements such as these, although crude, permitted reliable conclusions to be drawn reg(lrding the hydrologic phenomena being studied. The eighteenlh century brl) ught lorth numerous advances in bydraul.ic theory and .iLl~trumenta(lo n. The B moulli piezometer, the Pitot tube, Bernoulli's tbeorem, and the eh zy fO rnlUJa ar ~ some examples. 8 During the nineteenth century. experimental hydrology flourished. Significant advance. were made in gr undwater hydrology and in the measurement of surface water. Such significant contributions as Hagen-Poiseuille's capillary flow equation, Darcy's law of flow in porous media, and the Dupuit-Thiem well formula were evolved. 9 .. " The beginning of systematic stream gauging can also be traced to this period. Although the basis for modern hydrology was well established in the nine , t 1.4 THE HYDROLOGIC BUDGET 5 teenth century, much of the effort was empirical in nature. The fundamentals of physical hydrology had not yet been well established or widely recognized. Tn the early years of the twentieth century the inadequacies of many earlier empirical formula tions became well known . As a result, interested governmental agencies began to develop their own programs of hydrologic research. From about 1930 to 1950, rational analyses began to replace empiricism.) Sherman's unit hydrograph, Horton'S infiltration theory, and Theis's nonequilibrium approach to well hydraulics are out standing examples of the great progress made. :2 - 14 Since 1950 a theoretical approach to hydrologic problems has largely replaced less sophisticated methods of the past. Advances in scientific knowledge permit a better U1;derstanding of the physical basis or hydrologic relations, and the advent and continued development of high-speed digital computers have made possible, in both a practical and an economic sense, extensive mathematical manipulations that would ' have been overwhelming in the past. For a more comprehensive historical treatment, the reader is referred to the works of Meinzer, Jones. Biswas, and their co-workers. I , 2.4.5 , 15 1.3 THE HYDROLOGIC CYCLE The hydrologic cycle is aconliouous process by which water is transported from the oceans to the atmosphere to the land and back to the sea. Many subcycles elCist. The evaporation of inland water and its subsequent precipitation over land before return ing to the ocean is one example. The driving force for the global water transport system is provided by the sun, which furnishes the energy required for evaporation. Note that the water quality. also changes during passage through the cycle; for example, sea water is converted to fresh water through evaporation. The complete water cycle is global in nature. World water problems require studies on regional, national, international, continental, and global scales. 16 Practical significance of the fact thai the total supply of fresh water available to the earth is limited and very small compared with the salt waler content of the oceans has received little attention. Thus waters flowing in one country cannot be available at the same time for use in other regions of the world. Raymond L. Nace of the U.S . Geological Survey has aptly stated that "water resources are a global problem with local roots."16 Modern hydrologists are obligated to cope with problems requiring definition in varying scales of order of magnitude difference. In addition, developing techniques to coou'oi weather must receive careful attention, since climatological changes in one area can protoundly affect the hydrology and therefore the water resources of other regions. 1.4 THE HYDROLOGIC BUDGET Because the total quantity of water available to the earth is finite and indestructible, the global hydrologic system may be looked upou as closed. Open hydrologic subsys~ terns are abundant, however, and these are usually the type analyzed. For any system, . a water budget can be developed to account for the hydrologic components. - ---------- - - .--- - - .- ... .. --~ 1. V\~~' e1A- --\"1:J f\ < \-'v.M -r ~ eM So EFORE the morning of November 14, 1963, cartographers thought they had fin ished their work in Iceland . They certainly had been at it long enough since the island's first appearance on a world map by Eratosthenes some two centuries before Christ. It was a well-mapped area early tn the Middle Ages and could be set aside as one of the certainties of the North At lantic. Then with a roar, a pillar of steam and fire, and a hail of ash. nature demanded that the cartographers go back to work. Surtsey, the Dark One-god of fire and destruction-rose from the sea 20 miles off the Icelandic coast to become new land-1 mile long, 600 feet high , 670 acres of rock. aSh. and lava. Cartographers had another job. as would have been their lot had they been around for, say, the last 100 million years. Things just won't stand still. And not only little things, like new islands, or big ones, like mountains rising and bei!lg worn low to swampy plains, but . also monstrous things: continents that wander about like nomacL~ and ocean basins that expand. contract, and split up the middle like worn-out coats. It is a fascinating story, this forma tion and alteration of the home of humans, ~hich at first glance seems so eternal and unchanging. Geologic time is long, but the rorces that give shape to the land are timeless and constant . Processes of crea tion and destruction are continually at work to fashion the seemingly eterna.l structure upon which humankind, lives and works. Two types of forces interact to produce those infinite local variations in the surface of the earth called landforms: (1) forces that push. mov.e, and raise the earth's surface; and (2) forces that scour, wash. and wear down the .surface. Mountains rise and then arc worn away. The eroded material oil. sand. pebbles, rocks-is trans porled to new locations and helps to create new landforms . How long these processes have worked, how they work , and their effects are the subjecL of this chapter. Much of the research needed to create the story of landforms results from the work of geomorphologists . Geomorphology , a branch of the fields of geology and physica l geography, is the study of the origin , character lsti~s, and development of landforms. Geomorphology emphasizes the study of the various processes that influ ence the erosion, transportation. and deposition of mate rials . A modern thrust is in the area of the interrela tionships between plant and animal life and landforms . In a single chapter we can but begin to explore the many and varied contributions of geomorphologists . After dis cussing the context within which landform change takes place, we consider the forces that are building up the earth's surface and then review the forces wearing it down . Since most earth surface-changing processes occur over 46 The Earth-SCience Tradition ./ ~.e..o~'f~ j \A~ .\-\,. ~ ~ ~ .,.. j ~ Y'cw-9- t:el\ ~ ~ '" long periods of time, amounting to millions of years, per spective can be gained by developing a sense of what is meant by the time span within which the drama of earth change takes place. GeologiC Time The earth was formed about 4.7 billion years ago. Wben we think of a person who lives to be 100 years old as having had a long life, it becomes clear that the earth is incred ibly old indced . Because our usual concept of time is dwarfed when we speak of billions of years, it is useful to compare the age of the earth with someth ing that is more familiar. Imagine that the height of the World Trade Center in New York City represents the age of the earth . The twin towers are 110 stories, or 135) fect (412 m), tall. In relative terms, even the thickness of an average piece of paper laid on top of the roof would be too great to rep resent an average person's lifetime. or the totaJ building height, 4.7 stories represent the 200 million years that have elapsed since the present ocean basins began to form. The first hominids, or humanlike creatures, made their ap pearance on earth about 15 million years ago, or the equivalent of the hcight of one-third of a story. Earth his tory is so long and involves so many major geologic events that scientists have divided it into a series of recognizable. distinctive stages. These are depicted in Figure 3.1. At this moment, the landforms on which we live arc evcr so slightly being created and destroyed. The pro cesses involved have been in operation for so long that any given location most likely was the site of ocean and land at a number of different times in its past. Many of the landscape features on earth today can be traced back sev eral hundred million years . The processes responsible for building up and for tearing down those features are oc curring simultan~usly but usually at different rates . In the last 40 years ~cientists have developed a useful framework within which one can best study our constantly changing physical environment. Their work is based on the early 20th century geological studies of Alfred We gener, onc of the pioneers of the rapidly evolving scientific theory of plate tectonics. He believed that the present con tinents were once united in one supercontinent and that ovcr many millions of years the continents broke away from each other and slowly drifted to their current positions. New evidence and new ways of rethinking old knowledge have led to wide acceptance in recent years by earth sci entists of the idea of moving continents. Movement of the Continents The landforms mapped by cartographers are only the sur face features of a thin cover of rock, the earth's crust. Above the core and the lower mantle of the earth, there is a partially molten plastio layer called the asthenosphere " 4.7 billion Y/~rs ago Figure 3.1 A diagra"!matlc history of the earth. The sketch depicts some of the known characteristics of the named geologic periods. (Figure 3.2). The asthenosphere supports a thin but strong solid shell of rocks called the lithosphere. of which the outer, lighter portion is the earth's crust. The crust con sists of one set of rocks found below t he oceans and an other set that makes up the continents. The lithosphere is broken into about ten large, rigid plates, each of which, according to the theory of plate tec tonics, slides or drifts very slowly over the heavy semi molten asthenosphere. A single plate may contain both oceanic and continental crust. Figure 3.3 shows that the North American plate, for example, contains the north west Atlantic Ocean and most, but not all , of North America. The peninsula of Mexico (Baja California) and part of California are on the Pacific plate. SCientists are not certain why the lithospheric plates move . One reasonable theory suggests that heat and heated material from the earth's interior rise by convection into particular crustal zones of weakness . These zones are sources for the divergence of the plates. The cooled ma terials then sink downward in subduction zones (discussed below). In this way, the plates arc thought to be set in motion. Strong evidence indicates that about 200 million years ago the entire continental crust was connected in one super continent, to which Wegener gave the name Pangaea ("all earth"). Pangaea was broken into plates as the seafloor began to spread. the major force coming from the widening of what is now the Atlantic Ocean (Figure 3.4). Materials from the asthenosphere have been rising along the Atlantic Ocean fracture and, as a result, the sea floor has continued to spread. The Atlantic Ocean is now 4300 miles (6920 km) wide at the equator. If it widened by a bit less than! inch (2.5 cm) per year, as scientists have estimated, one could calculate that the separation of the continents did in fact begin about 200 million years Physical Geography: Landforms 47 Figure 3.2 (II) The outer zones of the earth (not to scale). The IIthosphflre includes the crust and the upp&rmost mantle. The asthenosphere lies entirely within the upper mantle. (b) The very thin crust of the earth overlies a layered planetllry interior. Zonation of the earth occurred early in its history. Radioactive heating melted the original homogeneous planet. A dense iron core settled to the center; a surface and the remnant lower mantle. overlain by II trans ition zone and the asthenosphere, formed between them. Escaping gases eventually created the atmosphere and the oceans . (a) 48 The Earth-Science Tradition Figure 3.3 The large lit'hospheric plates move as separate entities and collide. Assl)mlng the African plate to be stationary. relative plate movements are shown by arrows. Seafloor spreading. the triggering mechanism. takes place along the axes 01 the ridges . The drifting of the continents. Two hundred million yea'rs ago the continents were connected as one large landmass. After they split apart, the continents moved to their present positions . Notice how India broke away from Antarctica and collided with the Eurasian landmass. The Himalaya·s weJe formed at the :zone of contact . Physical Geography: Landforms 49 ago . Notice on Figure 3.~ how the ridge line that makes up tbe axis of the ocean runs parallel to the eastern coast of North and South America and the western coast of Europe and Africa. Scientists were led to the theory of the continental drift of 1.ithospheric plates by the amazing fit of the continents. According Lo this theory, collisions occurred as the lithospheric plafes moved . The pressure exerted at the in tersc;ctions of plates resulted in earthquakes, which over periods of many years combined to change the shape and the features of the landforms. Figure 3.6 shows the lo cation of near-surface earthquakes for a recent time period. Comparison with Figure 3.3 illustrates that the areas of greatest earthquake activity are at plate boundaries. The famous Sun Andreas fault of California is part of a long fracture separating two lithospheric plates. the North American and the Pacific. Earthquakes occur along faults (sharp breaks in rocks along which there is slip page) when the tension and the compression at the junc tion become so great that only an earth movement can release the pressure. The San Andreas case is called a transform fault, which occurs when one plate slips past another in a horizontal motion. Because the Atlantic Ocean is stil,1 widening at the rate of about I inch (2.5 em) per year"earthquakes must occur from time to time to relieve the stress afong the tension zone in the mid-Atlantic and along other fracture Jines, such as the San Andreas fault. Despite the availability of scientific knowledge about earthquake zones. the general disregard for this danger is a difficult cultural phenomenon with which to deal. Every year there are h'undreds and sometimes thousands of ca sualtjes resulting f~om inadequate preparation for earth quakes. In some well-popuLated areas, the chances that damaging earthquakes will occur are very great. The dis tribution of earthquakes shown in Figure 3.6 implies the potential dangers to densely settled areas of Japan, the Philippines. parts of Southeast Asia. and the western rim of the Americas. In the aftermath of the devastating earthquake in t he A rmenian region of the Soviet Union on December 7, 1988, there has been a hastening of activity to develop worldwide networks of highly sensitive seismic stations. There are at least seven major national and international net works now under construction. The purpose of the net works is to provide warnings of future earthquakes and to learn more about the forces at work in the earth's interior that keep the cbntinental plates in motion. • \1ovemcnt of the lithospheric plates results in the formation of deep-sea trenches and continental-scale mountain ranges as well as in the occurrence of earth quakes . The continental crust is made up of lighter rocks than is the oceanic crusL Thus, where plates with dif ferent types of crust at their edges push against each other. there is a tendenty for the denser oceanic crust to be forced dow!) into the asthenosphere, causing long and deep 50 The Earth-Sci!lnce Tradition Figure 3.5 The configuration of the Atlantic Ocean floor is evidence of the dynamic forces shaping continents and ocean basins. trenches to form below the ocean. This type of collision is termed subduction (Figure 3.7a). The edge of the over riding continental plate is uplifted to form a mountain chain that runs close to and parallel with the offshore trench. The subduction zones of the world are shown in Figure 3.7b. Most of the Pacific Ocean is underlain by a plate that. like the others. is constantly pushing and being pushed. The continental crust on adjacent plates is being forced to rise and fracture. making an active earthquake World Ocean Floor by Bruce C. Heezen and· Marie Tharp. 1977 and copyright by Marie Tharp 1977. Reproduced by permission of Marie Tharp, 1 Washington Ave. South Nyack. NY 10960. and volcano zone of the rim of the Pacific Ocean (some times called the "ring of fire"). In recent years, major earthquakes and volcanic activity have occurred in Co lombia, Mexico, Central America, the Pacific Northwest of the United States. Alaska •. the Armenian region of the USSR, China, and the Philippines. Figure 3.8 shows the location of volcanoes that are known to have erupted at some time in the past. The tremendous explosion that rocked Mount SI. Helens in the state of Washington in 1980 is an example of continuing volcanic activity along the Pacific rim. The many scientists who believed that a damaging earthquake would occur along the San Andreas fault were not surprised by the October 17, 1989, earth quake whose epicenter on the fault was 40 miles south of San Francisco (see box "Danger near the San Andreas fault"). Plate intersections are not the only locations that are susceptible to readjustments in the lithosphere. In the process of continental drift, the earth's crust has been cracked or broken in virtually thousands of places. Some of the breaks are weakened to the point that they allow molten material from the asthenosphere to find its way (0 Physical Geography: Landforms 51 Da'?9 r Near the San Andreas .Fault The earthquakes of June 12,1989, in Los Angeles and October 17, 1989, near Santa Cru ~ south of San Fran cisco reminded many that the vast ma jority of people in California live close to the San Andreas fault and its related faults. Since October 1987, in the Los Angeles basin there have been te~ earthquakes registering between 4.3 and 5 .9 , on the Richter scale (see box Oil ttie Richter scale), and in the San Francisco area a major earthquake oc curre'd measuring 7.1 . Experts who have been monitoring seismic pat terns in that area predict that more major earthquakes are Ifkely to occur along the fault be fore the year 2010. Great earthquakes have occurred to the east ot LoS 'Angeles about every 145 years, plus or minus several decades. The last major quake took place there in 1857. well before the city became the huge metropolis it Is today. The San Francisco earthquake of 1906 caused fires to break out. which de stroyed the city . In the 1989 earth quake. which caused billions of dollars of damage. a freeway collapsed during rush hour, killing many people . ~. " "' I i " ....."" ·t· t /''' ~""L, Figure 3.6 World earthquake epicenters, 1961-1967. 52 The Earth-Science Tradition • ,~'.' ", . '" . ~ "" • " . '.. " '," , ; 'i... the surface. The molten material may explode out of a volcano or ooze out of cracks. Later in this chapter, when we discuss the earth building forces, we wiU return to the discussion ~f volcanic activity. First, however, it is nec essary to describe the materials that make up the earth's surface. silicon, aluminum, iron, ani:! calcium, together with less abundant elements. A particular chemical combination that has a hardness, density, and definite crystal structure of its own is called a mineral. Some well-known minerals are quartz, feldspar, and silica. Depending on the nature of the minerals that form them, rocks may be hard or soft, dense or open, one color or another, chemically stable or not. While some rocks resist decomposition, others are very easily broken down. Among the more common varieties of rock are granites, basalts, limestones, sandstones, and sla tes. Although one can classify rocks according to their physical properties, the more common approach is to clas sify them by the way they evolved. The three main groups of rocks are igneous, sedimentary, and metamorphic. Earth Materials The rocks of the earth's crust vary according to mineral composition . Rocks are made up of particles tha t contain various combinations of such common elements as oxygen, Igneous Rocks Igneous rocks are formed by the cooling and hardening of earth material. Weaknesses in the crust give molten ma terial from the asthenosphere an opportunity to find its way into or onto the crust. When the molten material cools, it hardens and becomes rock. The name for underground' (a) Paolflo ({ • (b) Figure 3,7 The process of subduction. (a) When lithospheriC plates collide , the heavier oceanic crust is usually forced beneath the lighter continental material. Deep-sea trenches, mountain ranges, volcanoes , and earthquakes occur along the plate collision lines . (b) The subduction zones of the world. Physical Geography: Landforrns 53 Figure 3.8 The distribution of volcanoes. Note the association of volcanic activity with plate boundaries as shown on Figure 3.7 . molten material is magma; above ground it is lava. Intru sive igneous rocks were formed below ground level by the hardening of m~gma, while extrusive igneous rocks were crea ted above nearby ground level by the hardening of lava (FigiJre 3.9). The chemicals making up lava and magma are fairly uniform, but depending on the speed of cooling, different minerals form. Because it is not exposed to the coolness of the air, magma hardens slowly, allo.wing silicon and oxygen to unite and form quartz, a hard, dense mineral. With other components, grains of quartz combine to form the rock called granite. The Java that oozes out onto the earth's surface and makes up a large part of the ocean basins contains a con siderable amount of sodium or calcium aluminosilica tes. These form the mineral called feldspar and together with several other minerals make up basalt, the most common rock on earth. If instead of oozing the lava erupts from a volcano crater, it may cool very rapidly. Some of the rocks formed in this manner contain air spaces and are light and angttlar, such as pumice. Some may be dense, even glassy, as is obsidian. The glassiness occurs when lava meets standing water and suddenly cools . Sedimentary Rocks Sedimentary rocks are composed of particles of gravel, sand; silt, and clay that were eroded from already existing 54 The Earth-Science Tradition rocks. Surface waters carry the sediment to oceans, marshes, lakes, or tidal basins. Compression of these ma terials by the weight of additional deposits on top of them, and a cementing process brought on by the chemical action of water and certain minerals, causes sedimentary rock to form. Sedimentary rocks evolve under water in horizontal beds called strata. Usually one type of sediment collects in a given area. If the particles are large-for instance, the size of gravel-a gravelly rock called conglomerate forms. Sand particles are the ingredient for sandstone, while silt and clay form shale or siltstone. Sedimentary rocks also derive from organic mate rial, such as coral, shells, and marine skeletons. These ma terials settle into beds in shallow seas and congeal, forming limestone. If the organic material is mainly vegetation, it can develop into a sedimentary rock called coal. Petro leum is also a biological product, formed during the mil lions of years of burial by chemical reactions that transform some of the organic material into liquid and gaseous compounds. The oil and gas are light, therefore they ooze through the pores of the surrounding rock to places where dense rocks block their upward movement. Sedimentary rocks vary considerably in color (from coal black to chalk white), hardness, density, and resis tance to chemical decomposition. Large parts of the con tinents contain sedimentary rocks. Nearly the entire Figure 3.9 Extrusive and Intrusive forms of volcanism. Lava and ejecta (ash and cinders) are extrusions of rock material onto the earth's surface in the form of cones or hori.zontal flows. Ba.tholiths and laccoliths are irregular masses of crystalline rock that have cooled slowly below the earth's surface (intrusions), and in this diagram they have become surface features because of the erosion of overlying material. eastern half of the United States is overlain with these rocks, for example . Such formations 'indicate that in the geologic past, seas covered even larger proportions of the earth than they do today. Metamorphic Rocks Metamorphic rocks are formed from igneous and sedi mentary rocks by earth forces that generate heat, pres sure, or chemical reaction. The word metamorphic means "changed shape." The internal earth forces that cause the movement and collision of lithospheric plates may be so great that by heat and pressure, the mineral structure of a rock changes, forming new rocks. For example, under great pressure, shale, a sedimentary rock, becomes slate, a rock with different properties. Limestone under certain conditions may become marble, and granite may become gneiss (pronounced "nice"). Materials metamorphosed at great depth and exposed only after overlying surfaces have been slowly eroded a way are among the oldest rocks known on earth. Like igneous and sedimentary rocks, however, their forma tion is a continuing process. Rocks are the constituent ingredients of most land forms. Their strength or weakness, their permeability, and their chemical content control the way they respond to the forces that shape and reshape them. Two principal pro cesses are at work altering rocks: the tectonic forces that tend to build landforms up and the gradational processes that wear landforms down. All rocks are part of the rock cycle through which old rocks are continually being trans formed into new ones by these processes. No rocks have been preserved unaltered throughout the earth's history. Tectonic Forces ' The earth's crust is altered by the constant forces re sulting from plate movement. Tectonic (generated from within the earth) processes shaping and reshaping the earth's crust are of two types: diastrophic and volcanic. Diastrophism is the great pressure acting on the plates that deforms the surface by folding, twisting, warping, breaking, or compressing rock. Volcanism is the force that transports heated material to or toward the surface of the earth. When particular places on the continents are under pressure, the changes that take place can be as simple as the bowing or cracking of rock or as dramatic as lava ex ploding from the crater and sides of a Mount St. Helens. Diastrophism In the process of continental drift, pressures build in var ious parts of the earth's crust, and slowly, over thousands of years, the crust is transformed. By studying rock for mations, geologists are able to tr,ace the history of the de velopment of a region . Over geologic time, most continental areas have been subjected to both tectonic and grada tional activity-to building up and tearing down. They usually have a complex history of broad warping. folding, faulting, and leveling. Some fiat plains in existence today may hide a history of great mountain development in the past. Broad Warping Great forces resulting perhaps from the movement of con tinents may bow an entire continent. Also, the changing weight of a large region, perhaps due to melting conti nental glaciers, may result in the warping of the surface. For example, the down-warping of the eastern United States is evident in the many irregularly shaped stream estuaries. As the coastal area was warped downward, the sea has advanced, forming estuaries and underwa ter can yons. Folding When the pressure caused by moving continents is grea t, layers of rock are forced to buckle. The result may be a PhySical Geography: Landforms 55 . warping or bending effect, and a ridge or a series of par allel ridges or folds may develop. If the stress is pro nounced , great wavelike folds form (Figure 3.10). The folds can be thrust upward many thousands of feet and laterally for many miles. The folded ridges of the eastern United States are at present low parallel mountains (lOOO- 3000 feet-300 to 900 m-above sea level), but the rock evidence suggests that the tops of the present mountains were once the valleys between 30,000-foot (9100-m) crests (Figure 3.11) . Faulting A fault is a break or fracture in rock. The stress causing a Fault results in displacement of the earth's crust along the fracture zone. Figure 3. 12 depicts examples of fault types. There may be uplift on one side of the rault or downthrust on the other. I n some cases, a steep slope known as a fault escarpment, which may be several hundred miles long, is formed. The stress can push one side up over the other side, or a separation away from the fault may cause sinking of the land and create a rift valley (Figure 3.13) . Many faults are merely cracks (called joints) with little noticeable movement along them, but in other cases, mountains, such as the Sierra Nevada of California, have risen as the result of faulting. In some inst.ances, the movement has been horizontal along the surface rather than up or down. The San Andreas transform fault men tioned earlier and pictured in Figure 3.14 is such a case. Anticline . Syncline Close folds ~r------- Figure 3,11 The ridge and valley region of Pennsylvania, now eroded to hill lands, is the relic of 30,000-foot (9100-m) folds that were reduced to form synClinal (downarched) hills and anticlinal (uparched) valleys. The rock in the original troughs, having been compressed, was less susceptible to erosion. Overtumed folds - _ J ' - _ _...------.. (a) Figure 3.10 DegreEls of folding vary from slight undulations of strata with little departure from the horizontal to highfy compressed or overturned beds . (a) Diagram of styliZed forms of folding. (b) An overturned fold in the Appalachian Mountains. (b) 56 The Earth-Science Tradition Fault-block mountain Fault escarpment Strike-Slip fault or transform fault Normal fault Fault steps Horst Overthrust fault Graben (a) Figure 3.12 Faults , in their great variation, are common features of mountain belts where deformation is great. (a) The different forms of faulting are categorized by the direction of movement along the plane of fracture . (b) The major 'aults in California with the epicenters of twO strong 1986 and one strong 1989 quakes noted . - - 0_ 10 100 110 0;:;:-'=:-'50 Pacific (b) Physical Geography: Landforms 57 (b) Earthquakes (a) Figure 3.13 (a) Great fractures in the earth's crust resulted in the creation, through subsidence, of an extensive rift val/ey system in East Africa. The parallel faults, some reaching more than 2000 feet (610 m) below sea level, are bordered by steep walls of the adjacent plateau. which rises to 5000 feet (1500 m) above sea level and from which the structure dropped, (b) In Tanzania, the same tectonic forces have created a rift valley, one edge of w~iCh is shown here , 58 The Earth-Science Tradition Whenever movement occurs along a fault or at some other point of weakness, an earthquake results. The greater the movement, the greater the magnitude of the earthquake. Tension builds in rock as tectonic forces are applied, and when finally a critical point is reached, the earthquake occurs and tension is reduced, The earthquake that oc curred in Alaska on Good Friday in 1964 was one of the strongest kn<>wn. Although the stress point of tha t earth quake was below ground 75 miles from Anchorage, vi brations caJled seismic waves caused earth movement in the weak clays under the city. Sections of Anchorage lit erally slid downhill, and part of tlhe business district dropped 10 feet (3 m) . Table 3.1 indicates the kinds of effects associated with earthquakes of different magni tudes, and Figure 3.15 SMWS various types of earthquake induced damage. If an earthquake occurs below an ocean, the move ment can cause a tsunami, a large destructive sea wave. Though not noticeable on the open sea, a tsunami may become 30 or more feet high as it approaches land some times thousands of mUes from the earthquake site. The islands of Hawaii now have a ,tsunami warning system that was developed following the devastation at Hilo in 1946. The Richter Scale In 1935, C. F. Richter devised a scale of earthquake magnitude. An earth quake is really a form of en.ergy ex pressed as wave motion passing through the surface layer of the earth. Radiating in all directions from the earthquake focus, seismic waves gradually dissipate their energy at in creasing distances from the epicenter (the point on the earth's surface di rectly above the focus). On the Richter scale, the amount of energy released during an earthquake is estimated by measurement of the ground motion that OCcurs . Seismographs record earthquake waves, and by compar ison of wave heights, the relative strength of quakes can be determined. Although Richter scale numbers run from 0 to 9, there is no absolute upper limit to earthquake severity. Presum ably, nature could outdo the magni tude of the most intense earthquakes so far recorded, which reached 8.5 8 .6. Table 3.1 Richter Scale of Earthquake Magnitude The Tsunami C"-,actertatlc Effecta Because magnitude, as opposed to intensity, can be measured accu rately, the Richter scale has been widely adopted. Nevertheless, it is still only an approximation of the amount of energy released in an earthquake. In addition, the height of the seismic waves can be affected by the rock ma terials under the seismographic sta tion, and some seismologists beHeve that the Richter scale underestimates the magnitude of major tremors. HawaII of I!arthquaka. Magnitude" Occurring Near the Eiltrth'. Surface" 0 not felt not felt 2 not felt 3 felt by some 4 windows rattle 5 windows break 6 poorly constructed buildings destroyed; others damaged 7 widespread damage; steel bends 8 nearly total damage 9 total Clestruction 'Since the Richter scale IS legarithmic. eaen mcremen t .of a whele number signifies a 10-f.old Increase in magnitude . Thus a magnilude 4 earthquake preduc es a registered effect upen Ihe seismegraph 10 times greater than a magnitude 3 earthquake . "The damage levels .of earthquakes are p resented in terms .of the censequences that are fell or seen ,n pepulaled areas : the recorded seismic wave he ighls remain the same whether or not there are structures en the surface to be damaged . The acluallmpact .of earthquakes upen humans varres net .only with the severity .of the quake and such sec.ondary eHects as tsunamis or landslides but als.o with the density .of pepulation in the area aHected. A tsunami follows any submarine earthquake that causes fissures or cracks in the earth's surface. Water rushes in to fill the depression caused by the falling away of the ocean bot tom . The water then moves outward, building in momentum and rhythm, as swells of tremendous power. The waves that hit Hawaii following the April 1, 1946, earthquake off Dutch Harbor. Alaska, were moving at approximately 400 miles (640 km) an hour with a crest-to-crest spacing of some 80 miles (130 km). The long swells of a tsunami a're largely unnoticed in the open ocean . Only when the wave trough scrapes sea bottom in shallow coastal areas does the water pile up into precipitous peaks. The seismic sea waves at Hila on the exposed northeast part of the island of Hawaii were estimated at be tween 45 and 100 (14 and 30 m) feet in height. The water smashed into the city, deposited 14 feet (4.25 m) of silt in its harbor, left fish stranded in palm trees, caused many millions of dollars in damage, and resulted in 173 deaths; many were people who had gone to the shore to see the giant waves arrive . Physical Geography: Landforms 61 The Mexican Earthquake An earthquake registering 8.1 on the Richter scale struck at the edge of the North American plate along Mexico's Pacific Coast about 220 miles (350 km) from Mexico City on September 16, 1985. Pushed by the huge Nazca plate to the south, the Cocos plate slammed into and under the North American plate (Figure 3.3). About one thousand times as much energy as was released ' at Hiroshima was expended over western Mexico . Destruction was heaviest in the teeming capital of Mexico City. Wave after wave of vio lent earth shaking continued for about three minutes. The next day a violent aftershock registering 7,6 on the Richter scale added more fear and grief to the people of Mexico. A large part of Mexico City sits on an old lake basin made up of silt, clay, sand, and gravel-a very unstable foundation for a city with tall buildings . The rock between the epicenter and Mexico City absorbed most of the quake's force, but, nonetheless, the buildings in the downtown swayed. Fortunately, relatively few collapsed. Most were built to high standards spe cifically designed to resist earth quakes, and they held. Unfortunately, the 7000 buildings that did collapse Earthquakes occur daily in a number of places throughout the world . Most are slight and are noticeable only on seismographs, the instruments that record seismic waves, but from time to time there are large-scale earth quakes, such as those in Guatemala and China, in J 976 or the 1986 EI Salvador quake tbat killed more than 1000 people and left 200,000 homeless. Most earrhquakes take place on the rim of the Pacific (Figure 3.7), where stress from the outward-moving lithospheric plates is greatest. The Aleutian Islands of Alaska, Japan, Central America, and Indonesia experience many moderately severe earth quakes each year. Volcanism The ~econd tectonic force is volcanism. The most likely places through which molten materials can move toward the slJrface are at the intersections of plates, but other fault-weakened zones are also subject to volcanic activity (compare Figures 3.3 and 3.8). If sufficient internal pressure forces the magma upward, weaknesses in the crust, or faults, enable molten materials to reach the surface. The material ejected onto the earth's surface may arrive as a great explosion forming a steey-sided cone termed a strato or composite volcano (Figure 3.16), or the eruption may be without explosions, forming a gently sloping shield volcano. Figure 3.16 Sudden decompression 01 gases contained within lavas results in explosions of rock material to form ashes and cinders. Composite volcanoes. such as the one diagrammed . are composed of alternate layers of solidified lava and of ash and Cinders . 62 The Earth-Science Tradition killed 9000 people, caus.ed 30,000 in Juries, and left 95,000 people home less. A similar quake at the edge of the Cocos plate caused additional wide spread death and destruction in Oc tober, 1986, in San Salvador, the capital of EI Salvador. Earthquakes do their greatest damage not directly but indirectly, by putting people living in dense settlements In Jeopardy of being destroyed by buildings that faU, by landslides and mudflows that descend on them, and by tsunamis that ravage coastal areas. The major volcanic belt of the world coincides with the major earthquake and fault zones. This is the zone of convergence between two plates. A second zone of vol canic activity is a t diverging pia te boundaries, such as in the center of the Atlantic Ocean. Molten material can either flow smoothly out of a crater or be shot into the air with great force. Some relatively quiet volcanoes have long gentle slopes indicative of smooth flow, while explosive volcanoes have steep sides. Steam and gases are con stantly escaping from the nearly 300 volcanoes active in the world today. When pressure builds, a era ter can become a boiling cauldron with steam, gas, lava, and ash all billowing out (Figure 3.]7). In the case of Mount St. Helens in 1980, a large bulge had formed on the north slope of the moun tain. An earthquake preceded an explosion in the buLging area shooting debris into the air, completely devastating an area of about 150 square miles, causing about 4 inches Vent partly plugged with lava fragments of ash to rain down on most of Washington and pans of Idaho and Montana, and reducing the elevation of the mountain by ov,er 1000 feet. In many cases the forces beneath the crust are not grea't enough to aLlow the magma to reach the surface. In these instances the magma hardens into a variety of un derground formations of igneous rock that do little to affect surface landform fea tures. However, grada tional forces may erode overlying rock, so that the igneous rock, which usually is hard and resists erosion, becomes a surface fea ture. The Palisades. a rocky ridge facing New York City from the west, is such a landform. On other occasions, a weakness below the earth's surface may allow the growth of a mass of magma that is denied ex.it to the surface be cause of firm overlying rock. Through the pressure it exerts, however, the magmatic intrusion may still buckle, bubble, or break the surface rocks, and domes of consid erable size may develop. such as the Black Hills of South Dakota (Figure 13 .5). Evidence from the past shows that sometimes lava has flowed through fissures or fractures without volcanoes forming. These oozing lava flows have covered large areas to great depth. The Deccan Plateau of India and the Co lumbia Plateau of the U.S. Pacific Northwest are exam ples of this type of process (Figure 3.(8). G~adauonalProcesses Gradational processes are responsible for the reduction of land surface. If a land surface where a mountain once stood is now a low, fiat plain, gradational processes have been at work. The material that has been worn, scraped, or blown away is deposited in new places . and as a result, new landforms are created. In terms of geologic time, the Himalayas are a recent phenomenon; gradational pro cesses, although active there just as they are active on all land surfaces, have not yet had time to reduce the huge mountains. There are three kinds of gradational processes: weathering. gravity (rans/er, and ·erosion. Weathering processes, both mechanical and chemical, prepare bits of ~ock for tbeir role in the creation of soils and for their movement to new sites by means of gravity or erosion. The force of gravity acts to transfer any loosened, higher lying material, and the agents of running water. moving ice. wind. waves, and currents erode and carry the loose ma terials to other areas, where landforms are created or changed. . Mechanical Weathering Mechanical weathering is the physical disintegration of earth materials at or near the surface. A number of pro cesses cause mechanical weathering, the three most im portant being frost action, the development o(sa It crystals, and root action. 64 The Earth-Science Tradition Figure 3.18 Fluid basaltic lavas created the Columbia Plateau, covering an area of 50,000 square miles (130,000 km'). Some individual flows were more than 300 feet (100 m) thick and spread up to 40 miles (60 km) from their original fissures . If the water that soaks into a rock (between parti cles or along joints) freezes, ice crystals grow and exert pressure on the rock. If the process is repeated-freezing, thawing, freezing, thawing, and so on-there is a ten dency for the rock to begin to disintegrate. Salt crystals act similarly in dry climates, where groundwater is drawn to the surface by capillary action (water rising because of surface tension) . This action is similar to the process in plants whereby liquid plant nutrients move upward through the stem and leaf system. Evaporation Leaves behind salt crystals, which help disintegrate rocks. Roots of trees and other plants may also find their way into rock joints and, as they grow, break and disintegrate rock. These are all mechanical processes because they are physical in nature and do not alter the chemical composition of the material upon which they act. Chemical Weathering A number of chemical weathering processes cause rock to decompose rather than to disintegrate, that is, to separate into component parts by chemical reaction rather than to