Download Week 2 Essential Reading

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
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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
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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
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z
I
:;
I
I
Major mountain ranges
1 1
Regional uplift and
~ subsi d,.nce
Z
o
.,
o
Continental coastlines
Blod-faulted terrain
:;
I
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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:
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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.
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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-
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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