Note: The following texts are for the master students who are preparing for the English state
exam. The texts will be followed by reading comprehension and vocabulary exercises. Also will
be added speaking and grammar tasks in the exam.
School of Geology and Petroleum Engineering
Text 1
An Invitation to Geology
Geology is the study of the earth. Physical geology, in particular, is concerned with the materials
and physical features of the earth, changes in those features, and the processes that bring them
about. Intellectual curiosity about the way the earth works is one reason for the study of geology.
It is not an isolated discipline but draws on the principles of many other sciences. The earth is
challenging subject, for it is old, complex in composition and structure, and large in scale.
Physical geology focuses particularly on the physical features of the earth and how they have
formed. Observations suggest that, for the most part, those features are the result of many
individually small, gradual change continuing over long periods of time, punctured by
occasional, unusual, cataclysmic it’s formation, the events. Shortly after its formation, the earth
underwent melting and compositional differentiation into core, mantle, and crust.
There are also practical aspects to the study of geology. Certain geologic processes and events
can be hazardous, and a better understanding of such phenomena may help us to minimize the
risks.
The scientific method in a means of discovering basic scientific principles. The systematic study
of the earth that constitutes the science of geology has existed as an organized discipline for only
about250 years. It was first formally developed in Europe. Two principal opposing schools of
thought emerged in the eighteenth and nineteenth centuries to explain geologic observation. One,
popularized by James Hutton and later named by Charles Lyell , was the concept of
uniformitarianism. Uniformitarianism comprises the ideas that the surface of the earth has been
continuously and gradually changed and modified over the immense span of geologic time and
that, by studying the geologic processes now active in shaping the earth. It is not assumed that
the rates of all processes have been the same throughout time, but rather that the nature of
processes in similar that the same physical principles operating in the present. The second,
contrasting theory was catastrophism. The catastrophists, led by French scientist Georges Cuvier
, believed that a series of immense, worldwide upheavals were the agents of change that,
between catastrophes, the earth was static. Violent volcanic eruptions followed by torrential rains
and floods were invoked to explain mountains and valleys and to bury animal populations that
later became fossilized. In between those episodic global devastations, the earth’s surface did not
change, according to catastrophist theory. Catastrophists also believed that entire plant and
animal populations were created a new after each such event, to be wholly destroyed by the next.
For all practical purposes, the earth is a closed system, meaning that the amount of matter in and
on the earth is fixed. No new elements are being added. There is, therefore, an ultimate limit to
how much of any metal we can exploit. There is also only so much land to live on.
The early atmosphere and oceans formed at the same time. heat from within the earth and from
the sun together drive many of the internal and surface processes that have shaped and modified
the earth throughout its history and that continue to do so. The earliest life forms date back
several billion years. Organisms with hard parts became widespread about 600 million years ago.
Humans only appeared 3 to 4 million years ago, but their large and growing numbers and
technological advances have had significant impacts on natural systems, some of which may not
readily be erased by slower- paced geological processes.
Text 2
Minerals and Rocks
It is difficult to talk at length about geology without talking about rocks or the minerals of which
they are composed. All natural and most synthetic substances on earth are made from the 90
naturally occurring chemical elements. An element is the simplest kind of chemical; it cannot be
broken down further by ordinary chemical or physical processes.
Chemical elements consist of atoms, which are, composed of protons, neutrons, and electrons.
Isotopes are atoms of one element (having; therefore, the same number of protons) with different
numbers of neutrons; chemically, isotopes of one element and thus acquired a positive or
negative charge. Atoms of the same or different elements may bond together. The most common
kinds of bonding in minerals are ionic (resulting from the attraction between oppositely charged
ions) and covalent (involving sharing or electrons between atoms). When atoms of two or more
different elements bond together, they form a compound.
The nucleus, at the center of the atom, contains the protons and neutrons; the electrons move
around the nucleus. The number of protons in the nucleus is unique to each element and
determines what chemical element that atom is. Every atom of hydrogen contains one proton in
its nucleus; every carbon atom, six protons; every oxygen atom; eight protons; every uranium
atom, ninety-two protons. The characteristic number of protons in the atomic number of the
element.
In an electrically neutral atom, the number of protons equals the number of electrons. The
negative charge of one electron just equals the positive charge of the proton.
Most atoms, however, can gain or lose electrons. When this happens, the atom acquires a
positive or negative electrical charge and is termed an icon. If it loses electrons, it becomes a
positively charged cation, as the number of protons exceeds the number of electrons. If it gains
electrons, the resulting ion has a negative electrical charge and is termed an union.
A mineral is a naturally accruing, inorganic, solid element or compound. With a definite
composition (or range in composition) and a regular internal crystal structure. When appropriate
instruments for determining composition and crystal structure are unavailable, minerals can be
identified from a set of physical properties, including color, crystal form, cleavage or fracture,
hardness, luster, specific gravity, and others. Minerals are broadly divided into silicates and non
silicates. The silicates are subdivided into structural types(for example, chain silicates, sheet
silicates, framework, silicates) on the basis of how the silica tetrahedral are linked in each
mineral. Silicates may alternatively be grouped by compositional characteristics. The non
silicates are subdivided into several groups, each of which has some compositional characteristic
in common. Examples, include the carbonates (each containing the CO3 group), the sulfates
(SO4 ), and the sulfides.
Rocks are cohesive mineral aggregates. Certain of their physical properties are a consequence of
the ways in which their constituent mineral grains are assembled. All rocks are part of the rock
cycle, through which hold rocks are continually being transformed into new ones. A
consequence of this that no rocks have been preserved throughout earth’s history, and many
early stages in the development of one rock may have been erased by subsequent events.
Text 3
Volcanoes
A volcano is a vent through which magma, fragments of rock and ash, and gases erupt, or the
structure built around the vent by such eruption. Most volcanic activity is concentrated near plate
boundaries. Active volcanoes send out smoke and steam and occasionally erupt. An erupting
volcano gushes out ash, molten lava and smoke. Volcanoes form either at the edges of tectonic
plates, or at hot-spots in the earth’s crust. Volcanoes differ widely in eruptive style, and thus in
the kinds of dangers they present. Seafloor rift zones and hot spots are characterized by the more
fluid, basaltic lavas. Subduction -zone volcanoes typically produce much more viscous, silicarich, gas – charged andesitic magma, so, in addition to lava, they may emit large quantities of
pyroclastics and other deadly products like nuees ardentes. Lava is perhaps the leats serious
hazard associated with volcanoes. It moves slowly, it can sometimes be predicted. The results of
explosive eruptions are less predictable and the eruptions themselves more sudden. One
secondary affect of volcanic eruptions, especially explosive ones, which occurs as a result of dust
and gases being thrown into the atmosphere and blocking incoming sunlight. Lava is not
generally life threatening. Most lava flows advance at speeds of at most a few kilometers an
hour. So one can evade the advancing lava readily even on foot. The lava will destroy or bury
any property over which it flows. Lava temperatures are typically over 500° C and may be over
1,400° C.
Pyroclastics are often more dangerous than lava flows. They may erupt more suddenly.
Explosively, and spread faster and farther. Cinders and ash are examples of free- falling
pyroclastics. Another special kind of pyroclastic outburst is a deadly, denser-than-air flow of
mixed hot gases and fine ash known as a nuee ardente, from the French for ‘glowing cloud.” A
nuee ardente is very hot-temperatures can be over 1,000°C in the interior – and it can rush down
the slopes of the volcano at more than 100 kilometers per hour, charring everything in its path
and flattening trees and weak buildings.
The volcanic structure divided into following parts based on their eruptive patterns and
characteristic form, shield volcano, stratovolcano, dormant volcano, extinct volcano and active
volcano. When a volcano emits lava as well as pyroclastics, a concave- shaped composite
volcano or stratovolcano is built of alternating lava flows and beds of pyroclastics. Dormant
volcano is a volcano that is not now erupting but that has erupted within historic time and is
considered likely to do so in the future. A volcano that is erupting or is expected to erupt is an
active volcano. Shield volcano is a volcano in the shape of a flattened dome, broad and low, built
by flows of very fluid, basaltic lava. Extinct volcano is a volcano that is not presently erupting
and that is not considered likely to do so in the future.
Precursors are changes observed in or near a volcano that herald an impending eruption. There
are several types of advance warnings of volcanic activity. A common one is seismic activity.
Early signs of potential volcanic activity include bulging and warming of the ground surface and
increased seismic activity. Volcanologists cannot yet predict the exact timing or type of eruption
very precisely, except insofar as they can anticipate eruptive style on the basis of historic
records, the nature of the products of previous eruptions, and tectonic setting.
A single volcanic eruption can have a global impact on climate, although the effect may be only
brief. Intense explosive eruptions put large quantities of volcanic dust high into the atmosphere,
from which it may take years to settle. In the interim, it par- tially blocks out incoming sunlight,
thus causing measurable cooling. After Krakatoa’s 1883 eruption, worldwide temperatures
dropped nearly half a degree centigrade, and the cooling effects persisted for almost ten years.
The larger 1815 eruption of Tambora caused still more dramatic cooling. 1816 became known in
the Northern Hemisphere as the “year without a summer.” Such past experience forms the basis
for fears of a “nuclear winter” in the event of a nuclear war, for modern nuclear weapons are
powerful enough to cast volumes of fine dust into the air, and more dust and ash would be
generated by ensuing fires. The climatic impacts of volcanoes are not confined to the effects of
volcanic dust. The 1982 eruption of the Mexican volcano EI Chichon did not produce a
particularly large quantity of dust, but it did shoot volumes of unusually sulfur-rich gases into the
atmosphere. These gases produced clouds of sulfuric acid droplets that spread around the earth.
Not only do the acid droplets block some sun-light, like dust, but in time they also settle back to
the earth as acid rain.
Text 4
How Old is the Earth?
The ability to determine the numerical ages of rocks has changed the way you think about the
world. But how can we determine the age of the Earth itself? Time earliest and preserved in the
Earth come from the great assemblage of most morphic and igneous rocks formed during
Precambrian time. The oldest radiometric dates, about 4.4 billion years, have been obtained from
individual miner grains in sedimentary rocks from Australia. These grains – of the mineral zircon
– show evidence of having experienced wet partial melting during the formation a magma, which
then crystallized into an igneous rock, which in turn was weathered, eroded and redeposited,
eventually to be incorporated into a sedimentary rock. Dates almost as old – 4.0 billion years –
have been obtained from granite igneous rocks from Canada. The existence of such ancient rocks
proves that continental crust was present 4.0 billion years ago, while the 4.4 billion year old
mineral grains prove that the cycle of weathering, erosion, deposition, and cementation was
operating then. Because we see wet melting and ancient sediment that was transported by water,
we know that there must have been water on the surface of the Earth at the time the sediments
were deposited.
These ancient rocks are all from the Archean eon. Recall that the Hadean eon predates the
Archean. No rocks that might provide radiometric dates are preserved from early Hadean time –
none that we have yet found, at any rate. How long did the Hadean eon last, and, therefore, how
much older might our planet be? Strong evidence from astronomy suggests that Earth formed at
the same time as the Moon, the other planets, and meteorites. Through radiometric dating, it has
been possible to determine the ages of meteorites and of Moon rocks brought back by astronauts.
The ages of the most primitive of these objects cluster closely around 4.56 billion years. By
inference, the time of formation of Earth, and indeed of all the other planets and meteorites in the
solar system, is believed to be 4.56 billion years ago.
Text 5
Metamorphic Rocks
Metamorphism is change in rocks (short of complete melting) brought about by changes in
temperature, pressure, or chemical conditions. With progressive metamorphism, existing
minerals are commonly recrystallized, the crystals often growing larger in the process, and
minerals stable at low temperatures and pressures may break down, to be replaced by other
minerals stable at higher-grade conditions. Metamorphic rocks are subdivided into foliated and
nonfoliated rocks. Directed stress may also lead to formation of foliated rocks, in which
elongated or platy minerals assume a preferred orientation. Foliated rocks are named on the basis
of their particular texture. Nonfoliated rocks are most often named on the basis of mineralogic
composition.
An amphibolites is a metamorphic rock rich in amphiboles. Amphibolities are not
necessarily unfoliated rocks, for amphiboles commonly form in elongated or needlelike crystals
that may take on a preferred orientation in the presence of directed stress. Most metamorphism is
either contact metamorphism, which occurs in country rock close to the contacts of invading
plutons and is characterized by relatively low-pressure mineral assemblages, or regional
metamorphism, which is caused principally by platetectonic activity and emplacement of large
batholiths, and is characterized by elevated pressure as well as elevated temperature.
Metamorphic rocks are assigned to a particular facies, corresponding to a specific range of
pressures and temperatures, on the basis of the mineral assemblages they contain. Index minerals
are useful in assessing the general metamorphic grade of a rock and in determining regional
trends in metamorphic grade.
A variety of geologic settings and events lead to metamorphism. Most metamorphic
processes can be sub- divided into regional, contact, and fault-zone metamorphism.
Regional metamorphism is, as its name implies, metamorphism on a grand scale, a
regional event. Regional metamorphism is commonly associated with mountain building events,
when large areas are uplifted, downwarped or stressed severely and deformed, as during plate
collisions. Rocks pushed to greater depths are subjected to increased pressures and temperatures,
and are metamorphosed. Collision adds directed stress, producing abundant lineated and foliated
rocks. Regional metamorphism involving changes in both pressure and temperature is also called
dynamothermal metamorphism. The emplacement of large batholiths, which may or may not be
associated with mountain building, raises crystal temperature over broad areas as heat is released
from cooling plutons, so batholith formation may likewise result in metamorphism on a regional
scale.
Contact metamorphism is named for the setting in which it occurs, near the contact of a
pluton. The pluton, emplaced from greater depths, is hotter than the country rock, and if it is
significantly hotter, the adjacent country rock is metamorphosed. The pluton then is surrounded
by a zone of metamorphic rock, also known as a contact aureole, or halo. (The term aureole
comes from the Latin for “golden,” as a crown or halo might be.) Higher – temperature
metamorphic minerals are found close to the contact, lower – temperature minerals farther away.
Metasomatism may not occur in the contact aureole. Contact metamorphism, by definition, is a
more localized phenomenon than regional metamorphism, since it is confined to the immediate
environs of the responsible pluton. Contact – metamorphic effects also tend to be most marked
around plutons emplaced at shallow depths in the crust.
A metamorphic facies is a set of physical conditions that give rise to characteristic mineral
assemblages. That is rock of a particular metamorphic facies contain one or more minerals
indicative of a particular, restricted range of pressures and temperatures. Contact – metamorphic
facies are the facies of low pressure and a range of temperatures. They consist of sanidinite
facies pyroxene – hornfels facies, hornblende – hornfels facies and zeolite facies. The zeolites
are group of hydrous silicates, stable only at low pressure and temperature. Regional –
metamorphic facies are characterized by elevated pressure and temperature. The greenschist
facies is so named because greenschist – facies rock commonly contain one or both of the
greenish silicates chlorite and epidote. The blueschist facies is characterized by high pressure but
low temperature. It’s name derives from the several bluish silicates kyanite, an aluminum
silicate, and that form under these conditions.
Text 6
Earthquakes
Rocks subjected to stress may behave elastically or plastically Earthquakes result from
sudden rupture of brittle or elastic rocks, or sudden movement along fault zones in response to
stress. Most earthquakes are confined to the cold, rigid lithosphere. The pent – up energy of an
earthquake is related through seismic waves, which include ground rupture and shaking, fire,
liquefaction, landslides, and tsunamis.
While earthquakes cannot be stopped, their negative effects can be limited by: seeking ways
to cause locked faults to slip gradually and harmlessly, perhaps by using fluid injection to reduce
frictional resistance to shear; designing structures in active fault zones to be more resistant to
earthquake damage; identifying and, wherever possible, avoiding development in areas at
particular risk from earthquake – related hazards; increasing public awareness of and
preparedness for earthquakes in threatened areas; and learning enough about earthquake
precursor phenomena to make accurate and timely predictions of earthquakes.
Strain is deformation a change in shape, or volume, or both resulting from stress. It may be
either temporary or permanent, depending on the amount and type of stress and the ability of the
material to resist it. If the deformation is elastic, the amount of deformation is proportional to the
stress applied, and the material returns to its original size and shape when the stress is removed.
A gently stretched rubber band shows elastic behavior. Rocks, too, may behave elastically,
although much greater stress is needed to produce detectable strain. Once the elastic limit of a
material is reached, it may go through a phase of plastic deformation with increasing stress.
During this stage, relatively small added stresses yield large corresponding strains, and the
changes of shape are permanent. The material does not return to its original dimensions after
removal of the stress. A glassblower, an artist shaping clay, a carpenter fitting caulk into cracks
around a window, and a blacksmith shaping a bar of hot iron into a horseshoe are all making use
of plastic behavior of materials. Faults and fractures come in all sizes, from microscopically
small to hundreds of kilometers long. Likewise, earthquakes come in all size, from tremors so
small that even sensitive instruments can barely detect them, to massive shocks that can level
cities.
The point on a fault at which the first movement or break occurs during an earthquake is
called the earthquake’s focus, or hypocenter. In the case of a large earthquake, a section of fault
many kilometers long may slip, but there is always a point at which the first movement occurs,
and this is the focus. The point on the earth’s sur-face directly above the focus is the epicenter.
When an earthquake occurs, the stored-up energy is released in the form of seismic waves that
travel away from the focus. There are several types of seismic waves. Body waves (P waves and
S waves) travel through the interior of the earth. Surface waves, as their name suggests, travel
along the surface. The use of body waves to explore the earth’s internal structure is explored P
waves are compressional waves. As P waves travel through matter, the matter is alternately
compressed and expanded. P waves travel through the earth, than, much as sound waves travel
through air.
S waves are shear waves, involving a side-to-side sliding motion of material. Ground shaking
may cause a further problem in areas where the ground is very wet in filled land, near the coast,
or in places with a high water table. This problem is liquefaction. When wet soil is shaken by an
earthquake, the soil particles may be jarred apart, allowing water to seep in between them. This
greatly reduces the friction between soil particles that gives the soil strength, and it causes the
ground to become somewhat like quicksand. When this happens, buildings can just topple over
or partially sink into the liquefied soil the soil has no strength to support them. The effects of
liquefaction were dramatically after a major earthquake in Niigata. Apartment building tipped
over to settle at an angle of 30 degrees to the ground while the structure remained intact. In some
areas prone to liquefaction, improved underground drainage systems may be installed to try to
keep the soil drier, but little else can be done about this hazard beyond avoiding areas at risk. Not
all areas with wet soils are subject to liquefaction. The nature of the soil or fill plays a large role
in the extent of the danger.
Coastal areas, especially around the Pacific Ocean basin where so many large earthquakes
occur, may also be vulnerable to tsunamis. These are seismic sea waves, sometimes improperly
called “ tidal waves” although they have nothing to do with tides. When an undersea or near
shore earthquake occurs, sudden movement of the sea floor may set up waves travelling away
from that spot, like ripples on a pond caused by a dropped pebble. Tsunamis can also be
triggered by major submarine landslides or by violent explosion of volcanoes in ocean basis. In
the open sea, the tsunami is only an unusually broad swell or ripple on the water surface. Like all
waves, tsunamis only develop into breakers as they approach shore and the undulating waters
touch bottom. The breakers associated with tsunamis, however, can easily be over 15 meters
high and may reach up to 65 meters in the case of larger earthquakes. Several such breakers may
crash over the coast in succession: between waves, the water may be pulled swiftly seaward,
emptying a harbor or bay, and perhaps pulling unwary onlookers along. Tsunamis can travel very
quickly speeds of 1,000 kilometers per hour are not uncommon and a tsunami set off on one side
of the Pacific may still cause noticeable effects on the other side of the ocean.
Text 7
STRESS AND STRAIN
In discussing rock deformation, geologists often use the word stress, which refers to the force
acting on a surface. The definition of pressure is exactly the same. However, the term stress often
refers to “differential stress, strain” that is, a situation in which the force acting on the surface of
a body is greater from one direction than from another. Pressure is commonly used to mean
“uniform stress, “in which the force on a body is equal in all directions. For example, the
pressure on a small body floating within a liquid is uniform-the same from all directions.
Uniform stress is also called confining pressure. A rock in the lithosphere is confined by the
rocks all around it and is uniformly stressed by those surrounding rocks. The related terms
“lithostatic pressure” and “hydrostatic pressure” also describe uniform stress on a rock, but they
convey additional information about how the pressure is transmitted to the rock: by overlying
rocks (lithostatic, from lithos, the Greek root that means “rock”), or by water (hydrostatic, from
hydro, the Greek root that means “water”).
In response to stress, a rock will change its shape or its volume, sometimes both. This change is
called strain. Uniform stress causes rocks to change their volume. For example, if a rock is
subjected to uniform stress by being buried deep in the Earth, its volume will decrease. If the
spaces (pores) between the grains become smaller, or if the minerals in the rock are transformed
into more compact crystal structures, the volume change may be relatively large. Differential
stress causes rocks to change their shape, and sometimes their volume as well.
There area several kinds of differential stress (Figure 8.1). Tension acts in a direction
perpendicular to and away a surface; this kind of stress pulls or stretches rocks. Compression acts
in a direction perpendicular to and toward a surface; compressional stress squeezes rocks,
shortening or squashing them and decreasing their volume. Shear stress acts parallel to a surface.
It causes rocks to change shape by bending, or breaking. In response to shear stress, different
parts of the rock may slide past each other like cards in a deck.
Text 8
THE ROCK-FORMING MINERALS
Geologists have identified approximately 3,500 minerals, but fewer than 30 of them are common
in the crust of the Earth. Why aren’t there more minerals in the Earth’s crust? The reason
becomes clear when we consider the relative abundances of the chemical elements in the Earth’
crust. Only 12 elements – oxygen, silicon, aluminum, iron, calcium, magnesium, sodium,
potassium, titanium, hydrogen, manganese, and phosphorus – occur in the amounts greater than
0.1 percent (by weight). Together, these 12 elements make up more than 99 percent of the mass
of a limited number of minerals in which one or more of these 12 abundant elements is an
essential ingredient. Minerals containing scarcer elements occur only in small amounts and only
under special circumstances.
Two elements – oxygen and silicon – make up more than 70 percent of the crust in atomic by
weight. Oxygen itself constitutes more than 60 percent of the crust in atomic proportion – that is,
the actual number of atoms of oxygen in the crust – and more than 90 percent by volume.
Oxygen is a large, lightweight atom; not only are there lots of oxygen atoms in the crust, they
also take up a lot of space. Oxygen forms a simple anion, O2- ; compounds that contain this anion
are called oxides. Oxygen and silicon together form an exceedingly strong anionic complex
called a silicon anion, (SiO4)4-; minerals that contain this anion are called silicates. Silicates are
the most abundant of all minerals; oxides are the second most abundant. Other mineral groups
based on different anions are less common.
Text 9
EROSION BY WATER UNDER THE GROUND
Water can also cause erosion underneath the ground. As soon as rainwater infiltrates the ground
to become groundwater, it begins to react with the minerals in the regolith and the bedrock,
causing chemical weathering. Among the minerals of the Earth’s crust, the carbonates are most
readily dissolved. Carbonate rocks such as limestone are almost insoluble in pure water, but are
easily dissolved by carbonic acid, a common constituent of rainwater. The attack occurs mainly
along joints and other opening in the rock. When limestone weathers, it may be dissolved and
carried away in slowly moving groundwater. In some carbonate terrains, the rate of dissolution is
even faster than the average rate of erosion of surface materials by streams and mass wasting.
When carbonate rock is dissolved by circulating groundwater, a cave may form. Caves are
dissolution cavities that are closed to the surface, or have only a small opening. Cave formation
begins with dissolution along interconnected fractures and bedding planes, where two different
sedimentary rock units meet. A passage eventually develops along the most favorable flow route.
The rate of cave formation is related to the rate of dissolution. As the passage grows and the flow
of groundwater becomes more rapid and turbulent, the rate of dissolution also tends to increase.
The development of a continuous passage by slowly moving groundwater may take up to 10,000
years, and enlargement of the passage to create a fully developed cave system may take as long
as a million years. Terrains that are underlain by extensive cave systems are called karst terrains.
Sinkholes are dissolution cavities, like caves, but open to the sky. Some sinkholes are
formed when the roof of a cave collapses. Others are formed at the surface, when rainwater is
freshly charged with carbon dioxide and hence is most effective as a solvent. Some sinkholes
form slowly; others form catastrophically. An example of the latter occurred in Winter Park,
Florida, in 1972. In a period of just 10 hours, a sinkhole developed and consumed part of a
house, six commercial buildings, several automobiles, and a municipal swimming pool. The total
cost of the damage was over $2 million. Events as dramatic as the Winter Park sinkhole are rare,
but sinkhole collapse is a common occurrence in areas underlain by carbonate rocks.
Text 10
EROSION BY WIND
Wind is an important agent of erosion, especially in arid and semiarid regions. Processes related
to wind are called eolian processes after Aeolus, the Greek god of wind. Because the density of
air is far less than that of water, air cannot move as large a particle as water flowing at the same
velocity. In most regions with moderate to strong winds, the largest particles that can be lifted by
the air are grains of sand. Only the finest dust particles remain aloft long enough to be moved by
suspension.
Flowing air erodes the land surface in two ways. The first, abrasion, results from the impact
of wind-driven grains of sand (Figure 6.4). Airborne particles act like tools, chipping small
fragments off rocks that stick up from the surface. When rocks are abraded in this way they
acquire distinctive, curved shapes and a surface polish. A bedrock surface or stone that has been
abraded and shaped by wind-blown sediment is called a ventifact (“wind artifact”). The second
wind erosional process, deflation (from the Latin word meaning “to blow away”), occurs when
the wind picks up and removes loose particles of sand and dust (Figure 6.5). Deflation on a large
scale takes place only where there is a little or no vegetation and loose particles are fine enough
to be picked up by the wind. It is especially severe in deserts, but can occur elsewhere during
times of drought when no moisture is present to hold soil particles together.
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