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UNIT -2MATERIALS OF THE EARTH’S CRUST
TOPIC
CLASSIFICATION OF SEDIMENTARY ROCKS ON THE
BASIS OF GENESIS AND TEXTURE
SHAFIQA BUKHARI
ASSOCIATE PROFESOR
AS COLLEGE
Sedimentary rocks are those rocks which have been formed by the mechanical
or chemical action of the agents of denudation like running water, blowing
wind, moving ice, percolating ground water etc on pre-existing rocks (igneous,
metamorphic or even sedimentary rocks). The word sedimentary has been
derived from Latin word ‘SEDIMENTUM’ meaning setting down. Thus these
rocks have been formed by the deposition, compaction and cementation of
sediments.
The single most distinctive feature of sedimentary rocks is STRATIFICATION i-e
the sediments are laid down in a series of individual beds or layers separated
from one another from bedding planes. For this reason they are also called
STRATIFIED ROCKS. They cover about 75% of the land surface and extend only
5% downwards of the earth’s crust.
The branch of Geology dealing with description, correlation and classification
of strata in sedimentary rocks is called STRATIGRAPHY.
Classification of sedimentary rocks is a complicated task because of their
diverse nature and variations in their mechanical and chemical compositions.
For this reason a single classification of sedimentary rocks is not sufficient.
Therefore, they need to be classified differently depending upon different
criteria.
CLASSIFICATION OF SEDIMENTARY ROCKS ON THE BASIS OF TEXTURE;
Sedimentary rocks are made up of particles of various sizes. Depending upon
the size, particles of sediments are classified into boulders, cobbles, pebbles,
gravel, very coarse sand, coarse sand, medium sand, fine sand, silt and clay.
Following table shows grade and grain size of sediments;
GRADE
Boulders
Cobbles
Pebbles
Gravel
Very coarse sand
Coarse sand
Medium sand
Fine sand
Silt
Dust, mud and clay
GRAIN SIZE
>200 mm
50-200 mm
10-50 mm
2-10 mm
1-2 mm
0.5-1 mm
0.25-0.5 mm
0.1-0.25 mm
0.01-0.1 mm
<0.01 mm
Sediments which contain a large number of grades in more or less equal
amount are said to be un-assorted or ill-assorted. On the other hand sediments
containing a large proportion of one grade are said to be well-assorted or
graded. Thus on the basis of texture i-e grain size, sedimentary rocks have
been divided into the following categories;
a.
b.
c.
d.
RUDACEOUS ROCKS
ARENACEOUS ROCKS
SILT ROCKS
ARGILLACEOUS ROCKS
a. Rudaceous Rocks; Sedimentary rocks having very coarse grained texture in
which the individual grains forming the rocks are larger than 2 mm in
diameter are called Rudaceous Sedimentary rocks. Their fragments may be
boulders, cobbles or pebbles. If the grains are rounded, the rock is called
‘Conglomerate’ and if they are angular, the rock is termed as ‘Breccia’.
b. Arenaceous Rocks; Sedimentary rocks in which the individual grains are of
the size of common sand i-e 1-2 mm are called “Arenaceous Rocks”. When
the individual grains are rounded, the rock is called “Sandstone” and if the
individual grains are angular, the rock is called “Grift”.
c. Silt Rocks; These are the sedimentary rocks in which the individual grains
are finer than common sand but coarser than clay i-e the size is between
0.01 mm – 1 mm the example is silt stone and loess.
d. Argillaceous Rocks; Sedimentary rocks which are made up of particles of
the finest grade i-e less than 0.01 mm are called Argillaceous Rocks e.g;
Shale, Mudstone etc.
CLASSIFICATION OF SEDIMENTARY ROCKS ON THE BASIS OF GENESIS
On the basis of nature of sediments and their mode and mechanism of
accumulation and consolidation, sedimentary rocks may be classified into the
following broad types:
a. Clastic Sedimentary Rocks
b. Non-Clastic Sedimentary Rocks
a. Clastic Sedimentary Rocks; This group includes all those rocks which are
composed of fragments of pre-existing minerals and rocks. These are
formed under the mechanical action of the agents of denudation. These
are also called Mechanical or Detrital Sedimentary Rocks. These have
formed due to deposition under suitable conditions of the pebbles, sand,
silt etc which were carried in suspension with the surface run of or
blown by wind or transported by glaciers. These are also called
Transported Sedimentary Rocks. Common examples of these rocks are
sandstone, conglomerate, breccias, grift, mudstone etc.
Some clastic sedimentary rocks are known as Residual Sedimentary Rocks
which are formed due to accumulation and consolidation of those materials
which were left as residue during the operation of weathering and
transportation. Comman examples of these rocks are Laterite, Bouxite. The
grain size of the component particles of clastic sedimentary rocks varies within
wide limits from boulder and cobbles to silt and clay. On the basis of grain size,
these rocks are sub-divided into Rudaceous Rocks, Arenaceous Rocks, Silt
Rocks and Argillaceous Rocks.
b. Non-Clastic Sedimentary Rocks; Sedimentary rocks which are formed by
evaporation, precipitation or biological agents are known as “NonClastic Sedimentary Rocks. These can be further sub-divided into:
1- Chemically formed sedimentary rocks
2- Organic sedimentary rocks
1. Chemically formed sedimentary rocks; These are formed due to
precipitation and subsequent settling down and accumulation of the
soluble constituents travelling along with the surface run-off. The
precipitation of material may occur directly as the result of inorganic
process or directly as the result of the life processes of water dwelling
organisms. Sediments formed in the second way is said to have biochemical origin. On the basis of chemical composition of chemically
formed sedimentary rocks these may be categorised into following
types:
1- Carbonates → Travertine, Dolomite, Limestone etc.
2- Sulphates → Gypsum, Anhydrite etc
3- Chlorides → Rock salt.
4- Silicates → Sinter, Flint, Chert etc.
5- Iron stones or Ferruginous → Limonite, Haematite, Siderite etc.
6- Phosphorites → Phosphorite , Apatite, Chlorapatite etc.
7- Borates → Borax and Tincal.
8- Nitrates → Caliche.
2. Organically formed sedimentary rocks; These rocks are formed directly
or indirectly due to the activities of animals and plants. These represent
the products of accumulation of organic matter and are preserved under
suitable conditions e.g Coal, Fossilferous, Limestone, Radiolarian,
Foraminiferal oozes etc. The raw materials of these rocks accumulate
mainly on the sea floor. They can also accumulate in fresh water
environments and on land as well. An organic rock may be built up
directly from the remains of organisms, from the beginning as a solid
material as in the case of coral rocks and some algal limestones or the
deposition may be bio-chemical or bio-mechanical. They are said to be
bio-chemical when the vital activities of the organisms promote
chemical conditions which favour precipitation e.g “Bacterial iron ores”
and “Limestones”. Bio-mechanical rocks are formed due to the detrital
accumulation of organic materials e.g Crinoidal and Shelly Limestone
and some Coals.
Organically, Sedimentary rocks are further sub-divided into many types on
the basis of their chemical compositions:
1. Calcareous Rocks; these rocks contain large amounts of carbonates of
Calcium and Magnesium and are derived from the skeletons and
remains of those animals and plants which contain lime in abundance
e.g Calk and Limestone. Foraminifera, Corals, Crinoids and Crustacea are
the animal classes most concerned in limestone formation.
2. Carbonaceous Rocks; these rocks are dominated by carbon content and
represent the remains of vegetal matter. Coal is the most important rock
belonging to this category. Peat, Lignite, Anthracite, Cannel and
Boghead, all carbonaceous rocks, consist very largely of plant debris in
various stages of alteration.
3. Siliceous Rocks; These have very large amounts of silica content and are
formed due to accumulation and compaction of wastes derived from
sponges, radiolarian organisms and diatom plants. Radiolarian and
diatom oozes covering vast tracks of ocean floors are the common
examples of this type.
4. Ferruginous Rocks; These rocks contain large proportion of iron. Certain
bacteria and algae precipitate ferric oxide and ferrous sulphide from the
solution. The accumulation of the ferruginous casts of the bacteria along
with the granules which are formed due to precipitation, produce the
ferruginous material known as Bog Iron Ore.
5. Phosphatic Rocks; These deposits contain Phosphorous, Calcium
phosphate, present in the sea water, is utilised by certain organisms
especially fish, crustacean and some brachiopods, in building their shells
and skeletons. When these organisms die, their remains accumulate on
the sea floor and form phosphatic deposits. Phosphorites and Guano are
examples of organically formed phosphatic rocks.
IGNEOUS ROCKS
Igneous rocks are formed from the consolidation of molten material (magma).They are also known
as magnetic rocks. The nature of igneous rock in dependent on the type of magma and depth at
which it cools. The two types of magma are:
A) Basaltic magma which contains about 50% SiO2
B) Granitic Magma,which contains between 60% to 70% SiO2
They are source of different kinds of igneous rocks.The magma may cool beneath the surface forming intrusive
rocks or may erupt on the surface forming Extrusive rocks.
CLASSIFICATION OF IGNEOUS ROCKS
Igneous rocks can be classified on the basis of:
1.
2.
3.
4.
Mode of origin (depth of cooling)
Texture of the rock
Chemical composition
Mineralogical Composition
Mode of Origin: Magma may solidify at three levels:

Deep beneath the surface forming Plutonic rocks, and at shallow depths forming Hypabyssal
rocks, and on the surface after ejection of lava forming volcanic rocks.Masses of igneous
rocks called Plutons can have various forms-stocks,batholiths,dykes,laccoliths,phacoliths,sills
etc.

The Plutonics rocks are coarse, grained because they were consolidated under deep seated
conditions. Te volcanic rocks were formed under eruptive conditions and are fine grained or
glassy in appearance.

The Hypabyssal rocks are medium grained and occur on dykes and sills.
TEXTURE OF IGNEOUS ROCKS:
The texture of igneous rocks refers to the shape,size and arrangement of its constituent mineral
grains. The texture of an igneous rock is governed by its rate of cooling. The rate cooling, intern is
governed by the depth of cooling.At greater depth, the prevalence of higher temperature means
that the rate of cooling becomes slow. The rate of cooling is fastest on the surface under
atmospheric conditions and this cooling is medium paced at intermediate depths.
The major texture of igneous rocks are related to the size of crystals which in turn is related to the
rate and depth of cooling.
1. GLASSY TEXTURE: No grains are formed and are produced at rapid cooling on the surface.
Volcanic rocks have this texture.
2. APHANITIC (NOT “PHANEROUS” visible) has grains but are too small to be seen without a
microscope.Aphanitic texture is also produced by rapid cooling but not as rapid as that
which produces glassy texture.Such structures are typical of interior of lava flows.
3. VESICLES & VESICULAR TEXTURE: Vesicles are small cavities of spherical or ellipsoidal shape
and are found on glassy rocks as well Aphanitic rocks. Vesicular texture are produced by
gases trapped in the solidifying of igneous rocks.When the vesicles are filled with secondary
materials they produce a texture called Amygdaloidal Texture.
4. PHANERITIC TEXTURE: are visible without a microsope and the grains are of equal size and
form an interlocking mosaic.
5. PROPHYRITIC TEXTURE: Such textures are produced due to two different stages of cooling.
E.g. at initial stage of slow cooling (large crystal) followed by a period of rapid cooling
(smaller crystals) resulting into phenocrysts large crystals (phenocryts) set in a five grained
ground mass. Such textures occur in both aphanitic and phaneritic rocks.
6. PROCLASTIC TEXTURE: includes broken and fragmented rather than interlocking crystals.
Such texture is produced when explosive eruptions blow ash,earlier- formed minerals and
glass into the air as a mixture of hot fragments and glass.The fragments are fused together if
they hot during deposition and cemented together if they are deposited later during cooling
process.
MINEROLIGICAL & CHEMICAL COMPOSITION
CHEMICAL COMPOSITION: Igneous rocks vary in their decimal composition, between wide limits.
Rocks like granite contain 70% to 80% of silica and may contain very little quantities of iron,
magnesia and lime while, at other extreme ends , rocks like peridotite ,dunite etc often contain 35%
to 40% of silica and larger quantities of iron, magnesia and lime on the basis of their chemical
composition. Igneous rocks may be classified as:
1.
2.
3.
4.
Acidic Igneous Rocks
Intermediate rocks
Basic rocks
Ultra basic rocks
 The acidic rocks or over saturated rocks with free Quartz (SiO2 ,oxides of silicon) over
66%. Some common rocks are – granite,pegmatite,Dacite, Rhyolite
 Intermediate Rocks (SiO2 between 66% to 55%) typical types – Andesite, syenite,
Diorite,Phonolite etc
 Basic Rocks with Plagiolase and pyroxenes mainly (SiO2 55% to 44%) –typical types:
Basalt ,Dolerite, Gabbro, Anorthosite etc
 Ultra Basic igneous rocks with olivive,pyroxenes with little or no Feldspar (SiO2 less than
42%) Some common rocks of this group are: olivive,basalt,limburgite,dunite,perkinite
etc
MINEROLOGICAL COMPOSITE
Mineralogical composition is the most convenient basis of classification of igneous rocks
because minerals are easy to identify and more over rocks are aggregates of minerals
and a classification based on this criteria is more logical. The minerals commonly found
in igneous rocks may be classified into Felsic and Mafic varieties . Felsic minerals like
quartz, feldspar etc are light coloured with low melting point and sp.gravity while as
mafic minerals like pyroxene,olivive,biotite etc are dark coloured. Thus igneous rocks on
mineralogical basis are felsic group and mafic group.
Unit IInd
Semester
Ist
Materials of the earth’s crust
Topic: METAMORPHIC ROCKS
1. Introduction
2. Definition
3. Classification on the basis of:
TEXTURE & AGENTS
TEXTURE
Crystalloblastic
Porphroblastic
Palimpest or relict
Granoblastic
AGENTS
Heat
Pressure
Chemically Active Fluids
Mrs. Razia Bashir
Associate
Professor
Department of
Geography
Amar Singh
College
Srinagar
INTRODUCTION
Primary, secondary or other types of rocks may or may not undergo any physical or
chemical; change after their formation. If a rock has adjusted itself to the given conditions of
the surrounding environment, it may not undergo any change in its set up. This state of the
rock is called the equilibrium state with the physical and chemical conditions of the
surroundings. But once there is an effective change in any one or more of these surrounding
conditions viz. pressure, temperature and chemical environment, the equilibrium is disturbed
and the rock is unable to exist in its original form. Thus it must obey the changes in its
environment and consequently there is a change in the rock itself. The new formation will
tend to be in equilibrium with the new set of conditions. All those changes in the body of the
rock due to variations in the factors of pressure, temperature and chemical environment are
known as metamorphic changes and the process is called metamorphism.
DEFINITION
Metamorphic Rocks are defined as those rocks which have formed through the
operation of various types of metamorphic processes on the pre-existing primary (Igneous),
secondary (Sedimentary) or other rocks involving either textural and structural changes or
changes in the mineralogical composition or reconstruction in both the directions. They are
megascopically distinguished from other types of rocks by the development of features like
cleavage, foliation, schistosity and granulation and by the presence of such minerals which
are known to be of metamorphic origin. Some important examples of metamorphic rocks are:
Slate, Schist, Gneiss, Quartzite’s, Marble, Phyllite, Hornfels, etc. There are different types of
metamorphism like thermal, that is due to temperature and heat, second is metamorphism due
to pressure that is known as dynamic metamorphism or regional metamorphism.
CLASSIFICATION OF METAMORPHIC ROCKS
Metamorphic rocks have been variously classified on the basis of texture and
structure, degree of metamorphism, mineralogy, composition and mode of origin. However, a
common way of classifying these rocks is based on the presence or absence of the parallel
structures. Two main divisions are recognized on this basis:
(i)
(ii)
Rocks with a conspicuous parallel structure--- This group include the foliate and
banded rocks like slates, phullites, schists and gneisses of various types.
Rocks without a parallel structure--- This group include non-foliated rocks like
quartzite, hornfels, amphibolites and soapstone.
Classification on the basis of Texture and Agents
TEXTURE OF METAMORPHIC ROCKS
Texture of metamorphic rocks can be grouped under two headings:
(a) Crystalloblastic Textures: - which includes all those types that have been newly
imposed upon the rock during the process of metamorphism and are, therefore,
essentially the product of metamorphism.
(b) Palimpest or relict Textures:- that comprise the textures which were present in the
original rock and which have resisted the effects of metamorphism and thus are
retained by the rock.
Among the crystallobastic textures, porphyroblastic and granoblastic types are very
common.
In porphyroblastic type, the fine grained ground mass of the rock shows embedded in
it idioblastic crystals i.e. crystals with perfect outlines of stronger minerals.
In granoblastic texture the rock is composed of equidimensional grains of hard
minerals.
Palimpest textures are generally by using the word blasto- as prefix to the original texture
that has been retained by the rock. For example, an igneous rock with porphyritic texture
undergoes metamorphism whereby its mineral composition is changed but the same texture is
retained in the new rock. The porphyritic texture of the rock will now be described as
blastoporphyritic.
AGENDS OF METAMORPHISM
Following agents can bring about metamorphism in rocks:
(i)
(ii)
(iii)
Heat,
Pressure, and
Chemically active fluids.
(i)
Heat: Heat is the most important factor in metamorphism. As the rock temperature
(ii)
increases, minerals begin o change from the solid to liquid state, and the amount
of pore fluid in the rocks increases. Chemical reactions become more vigorous.
Crystal lattices are broken down and recreated using different combinations of the
same ions, resulting in the formation of different minerals. The common cause for
elevated temperature is the magmatic intrusions.
Pressure: Pressure, like temperature, also increases with the depth; high pressure
within the earth’s crust causes significant changes in the physical properties of
many rocks. Under high temperature and pressure, a rock becomes plastic and
may be deformed. The mineral grains are moved, rotated, fractured and sheared.
This reorientation of the constituent mineral grains caused by the stress produces a
new rock texture.
Pressure exerted on the rocks may be of two types; (a) uniform pressure,
resulting from the weight of the superincumbent load, and (b) directed pressure or
stress, resulting from horizontal earth movements. The former is also called
lithostatic or confining pressure.
(iii)
High (confining/dynamic) pressure tends to reduce the space occupied by the
mineral components and thus can produce new minerals with closer atomic
packing resulting in higher coordination number the constituents e. g. slates.
Directed pressure operates during folding movements that accompany
mountain building. It plays an important role in causing metamorphism near the
earth’s surface (at shallow depths). With increase in depths, its role however
dwindles considerably and that of uniform pressure increases. Examples:
mylonites and fault breccia
Chemically Active Fluids: The environment of chemically active fluids is the most
important factor in metamorphism because the reactions can only take place
through partial or complete solution of the minerals. Most common actively fluid
is water contains ions in solution. Water is contained in the pore spaces of
virtually every rock. In addition, many minerals are hydrated and contain water in
their crystalline structures. When deep burial occurs, water is forced out from
mineral structure and is then available to aid in chemic al reactions. Dehydration
of minerals due to heat also releases water. Water that surrounds the crystals acts
as a catalyst by aiding ion migration. Ion exchange among the minerals results in
the formation of completely new minerals.
Suggestive Readings:
1. Physical Geography by Richard H. Bryant, Eleventh Impression 2002 published by
Rupa & Co.
2. Engineering and General Geology by Parbin Singh, Reprinted in September 1980
published by Dewan Sushil Kumar Kataria.
3. Physical geography by Prof. Majid Hussain
4. Physical Geography by Savinder Singh
Weathering
Temperature, air, water etc. soften the rocks of earth’s surface.
After sometime the outer surface of the rock is covered by the soften rock
mixture. It happens in two phases – (i) Disintegration through physical
action & (ii) Decomposition through chemical action which soften the
outer hard cover of the rock. The process of forming softened mixture
over a hard rock is called weathering.
Weathering is the mechanical fracturing and chemical
decomposition of rocks, in situ, by natural agents at the
surface of the earth (SPARKS)
The hard and solid rock on which weathering takes place is called Bed
rock. The softened and loose rock mixture formed on the bed rock due
to weathering is called Regolith.
Types of Weathering
According to physical and chemical processes, weathering can be
divided into two types
1. Physical weathering
2. Chemical weathering
The importance of physical and chemical weathering depend upon the
nature of climate. Physical weathering becomes important in arid
climate while chemical weathering is important in hot and humid
climate.
1. Physical weathering:
Though the main factor of physical weathering is temperature
change, yet other factors like pressure release, freeze, gravity and
biological action help a lot in this process.
i) Temperature Change: The rocks are bad conductors of heat and
expend on heating in the day but contract on cooling in the night.
A pressure develop on the rocks due to continuous expansion and
contraction. It produces cracks and joints on the rocks. For
example granite rock with a diameter of 30.48 meters when heated
and temperature raised by 65.5 degree Celsius, its diameter is
increased by 2.54 cm.
Most of the sedimentary and igneous rocks are composed of
different minerals. Their expansion is different on account of
different specific heats, colours etc. Due to the different expansion
of the constituents of rocks, small fissures, fractures etc. are
produced into rocks which help in the disintegration of the rocks.
ii) Pressure release: Many igneous and metamorphic rocks
crystalline deep in the interior under the combined influence of
high pressure and temperature. When the rocks above them are
eroded away, the rocks exposed are released from the pressure.
Due to pressure release, cracks are produced in the rocks and
exfoliation (peeling off thin flakes from the rock) starts.
iii) Freeze thaw: in cold countries, water in the day enters the cracks,
fissures and holes in the rocks. The water freeze in the night and its
volume increase by 1/11 times. This exert a huge pressure on rocks
and weaken them.
iv) Gravity: Many rocks which have large joints disintegrate on
account of gravity. Though the effect of gravity is weak, yet its
action in breaking down the cracking rocks is not insignificant.
v) Biological action: Vegetation and animals are helpful in the
disintegration of rocks. The roots of plants and trees enter the
cracks and fissures of the rocks and loosen the rocks from inside.
Animals also go on wearing away rocks.
2. Chemical weathering: The part of the atmosphere in contact
with the earth’s surface contain excess of oxygen, carbon dioxide
and water vapour. Oxygen and carbon dioxide become very active
in the presence of water and water vapour. The following are the
main processes which constitute chemical weathering:i) Oxidation: The oxygen of the atmosphere reacts with minerals in
the presence of water. For example the iron compound change
from ferrous to ferric state (red brown). Clay as long as is
submerged in water appear blue as it has iron in ferric state. When
the clay is taken out of water, the ferric iron of clay is converted
into ferric state and the clay becomes red-brown. When pyrites are
acted upon by water, sulphuric acid is produced which start
dissolving the pyrites.
ii) Carbonation: ordinary water cannot dissolve calcium carbonate
and magnesium carbonate of rocks but when carbon dioxide from
atmosphere or rain water comes in contact with them, they are
converted to bicarbonates of calcium and magnesium. The
limestone, marble and gypsum dissolve in water.
iii) Hydration: It is the process when minerals incorporate water
into their molecular structure. Hydration causes swelling thus
helps in the crumbling of coarse grained igneous rocks. When
calcium carbonate is hydrated it becomes gypsum. When carbon
dioxide is dissolved in water, the chemical action takes place at a
fast speed. Feldspar mineral through hydration is converted into
Kaolin.
iv) Desilication: The separation of silica from the rocks is called
desilication. The running water separates silica from granite. Due
to excess silica separation, the rock is readily disintegrated. Silica
in the form of Quartz is very hard. If there is silica in sedimentary
rocks, the latter becomes harder than even igneous rocks but these
rocks are readily disintegrated with the separation of silica.
v) Solution: The rain water is able to dissolve certain minerals and
leach the soil. Through this process many minerals are washed out
of the soil and rocks so that their chemical composition changes.
Soluble rock forming minerals like nitrates, sulphates, and
potassium etc. are affected by this process. So, these minerals are
easily leached out without leaving any residue in rainy climates
and accumulate in dry regions. Minerals like calcium carbonate
and calcium magnesium bicarbonate present in lime stones are
soluble in water containing carbonic acid (formed with the
addition of carbon dioxide in water), and are carried away in water
as solution. Carbon dioxide produced by decaying organic matter
along with soil water greatly aids in this reaction. Common salt
(sodium chloride) is also a rock forming mineral and is susceptible
to this process of solution.
Factors affecting weathering
The various factors which affect weathering are the nature of rock,
climate and time. Weathering is a complex process to understand
because many factors act in combination but it is necessary to
understand the nature of various factors.
i. Nature of rocks: the rock may be hard or soft, but we have
to see whether the rock is porous, soluble and traversed by
planes of weakness. Stability of the minerals that constitute
the rocks has also to be taken into account. The exposure of
rock also fall within the same category. The strength of a
particular rock structure, its durability and power of
resistance depend upon massiveness of rock, its
compactness, composition, joint system (whether wide
spaced or close spaced) and perviousness or permeability.
The weathering of dark colour rocks is faster than that of the
light coloured ones. Acidic rocks are less easily weathered
than basic rocks. The rocks with coarse grains are more easily
weathered than those with fine grains.
ii. Type of Climate: Climate has a close relation with
weathering. Physical weathering is important in cold and dry
climate while chemical weathering is important in warm and
iii.
humid climates. In equatorial regions there is high
temperature as well as high humidity. Chemical weathering
is definitely most important in this region. Rocks as deep as
62 meters from the surface have been found to be decaying.
The carbon dioxide from air and nitric acid from thunder of
clouds help in the chemical weathering of rocks.
Time: Weathering increases with time but weathering can
decrease if the regolith deposited on the rocks is not
removed. Davis thinks that the regolith deposited on bed
rocks acts as a security layer and almost stops the weathering
process. This conclusion may be correct for physical
weathering but the same cannot be said for chemical
weathering which is not stopped by regolith layer. Ordinary
water and acidic water reach the bed rock through regolith
and chemical weathering starts.
EARTH MOVEMENTS
The surface relief or topography results basically from the action of two forces
A. Endogenic, Endogenetic or Hypogene forces, which give rise to land upliftment, subsidence,
folding,fracturing and volcanic eruptions. In general, these forces are responsible for the
major structural units of the Earths surface, for example, mountains, plateaus and plains.
B. Exogenic or Exogenetic or Epigene forces, which give rise to destruction, carving, moulding
and smoothening of the major relief features. They produce the intricate details of the
surface topography, for example, waterfalls, hills, valleys, spurs, dunes, caves, stacks, etc.
The various types of Earth movements result from the work of Endogenetic forces.
Endogenetic Movements
Endogenetic movements fall into major categories
movements
Diastrophic movements and Sudden
Diastrophic Movements
Diastrophic movements are those, which result directly or indirectly, in relative or absolute
changes of position, level or altitude of the rocks forming the Earths crust. These diastrophic
movements lead to production of primary landforms. Primary landforms vary in size from
continents to miniature Earthquake fault scarps. These are three major classes of diatrophic
movements, all of which are interrelated.
a. Tectonic Movements
b. Isostatic Movements
c. Eustatic Movements
Tectonic Movements
Tectonic Movements include Epirogenic and orogenic movements.
Epirogenic movements
Epirogenic movements relate to the behavior of continental platform or stable blocks
involving broad gentle upwarping of relatively large crustal areas. These vertical movements,
caused by radial forces and are characterized by large-scale upliftment, subsidence or
submergence and emergence of land areas. The movements involved are so slow and
widespread that no obvious fracturing or folding is produced in the rock.
Every continent, including Antartica, offers unmistakable evidence of both downward and
upward movements since Pre-Cambrian. The proof of such movements is found in marine
sedimentary rocks of Palaeozoic,Mesozoic and Cenozoic age, which lie within continents
upon well eroded older rocks.
Orogenic Movements
Orogenic Movements relate to behavior of plate margins and involves intense folding,
thrusting, faulting and uplift of narrow belts. They are caused by tangential forces and
tensional forces.
COMPRESSIONAL FORCES
Folding
A sequence of fold structures may be formulated, which are related to the degree of
compressional forces involved.
a. An anticline is formed when the strata is bent upward into simple upfold.
b. A syncline results from the strata being bent downwards.
Types of folds
a. A monocline results when horizontally laid beds dip and thenflatten out producing
simple flexure.
b. Asymmetrical fold is one where one limb in a fold structure is steeper than the other.
c. An overturned fold is formed when one limb occupies the normal position while the
other bends more than 90 degrees.
d. An isoclinals fold results from the continued lateral compression upon an overfold
crowding it upon the adjacent overfold. Here both the limbs dip at equal angels in the
same direction.
e. A recumbent fold is literally a fold lying down, resulting from the continuation of
pressure. The axial plane and both limbs of a fold lie roughly horizontally.
f. Nappe results when the pressure exerted upon a recumbent is sufficiently great to cause
it to be torn from its roots and to be thrust forward.
TENSIONAL FORCES
Faulting
Block Mountains, Horst and Graben. Excessive stresses and strains produce fracturing and is
usually accompanied by dislocation. Great masses of the crust may be either teared above
or dropped below the general level of the land. Such relatively rapid movement produce
crustal blocks (horsts) on the margin of which is a fault scarp, rift valley or depression
(graben) bounded by faults. Horsts and grabens are found in association with one another.
The mountains, which are remnants of the Hercynian mountain building period, for example,
Central Plateau of France, Vosges, Black Forest and Bohemian massifs are all Block
Mountains ( in fact, they are relicsof old fold mountains which have been denuded and
shattered). The Rhine Rift Valley forms a graben. The Flinders Mountains and the Torrens
Valley in South Australia are other examples of uplifted and down thrown areas.
Isostatic Movements
Isostatic movements involves vertical movements under the action of floatation
displacement between rock layers of differing density and mobility, to achieve balanced
crustal columns of uniform mass above a level of compensation, in which topographic
elevation is inversely related to the underlying rock density. This isostatic movements is best
represented by the re-adjustments, which followed Pleistocene glaciations.
The formation of the great ice sheet over Scandinavia led to the land being depressed by the
enormous weight of accumulated ice. When the ice eventually melted, the land surface
begins to rise in response to the gradual easing of load. This upliftment is illustrated by the
occurance of a series of raised beaches. The uplift is continuing and can be illustrated by
changes which have gone along the Bothnian coast of Baltic, for example, the emergence
Alland Islands from the sea. More and more land is emerging from the sea, and more islands
that grow larger and larger. Because of this movement, it is possible that someday that
islands may be united in a solid neck of land linking Finland to Sweden, transforming the Gulf
of Bothnia in a broad land-locked sea. The finnish port of Vaasa has been in existence for
several hundred of years but the modern port lies six miles off the original harbor site, a fact
which illustrated how the continuing uplift along this coast has rendered some of the old
harbours quite useless.
Eustatic Movements
Eustatic movements involves world wide movement of sea level resulting from changes in
the total volume of liquid sea water in the capacity of the ocean basins. Eustatic movements
are caused by formation of mid-oceanic ridges or plate movement towards each other, thus,
enclosing basins.
EXOGENETIC AND ENDOGENETIC FORCES/PROCESSES
1.
2.
3.
4.
Introduction
Definition
Description
Types of forces and processes
Introduction
The state of forces effecting the crust of the earth or of the geological
processes is of paramount significance because these forces and resultant
movements are involved in the creation, destruction, recreation and
maintenance of geo-materials and numerous types relief features of varying
magnitude. These forces very often affect and change the earths surface. In fact
the change is the law of nature, the changes have taken place, they are taking
place and will take place. Changes are positive as well as negative. The
geological changes are of two types, I) long period change and II) short period
change.
Long period changes occur so slowly that man is unable to notice such
changes during his life period. On the other hand, short period changes take
place so suddenly that these are noticed within few seconds to few hours, for
example seismic events, volcanic eruptions etc.
The forces which effect the crest of the earth are divided into two broad
categories on the basis of their source of origin:
1. Endogenetic
2. Exogenetic
Definition and Description
Endogenic Forces
The forces coming from within the earth and causing horizontal and
vertical movements are known as endogenetic forces. It is these movements
which lead to land upliftment and subsidence, folding and faulting, earthquakes
and volcanism, etc. endogenetic movements are responsible for giving birth to
major relief features such as mountains, plateaus, plains, valleys, etc. These
endogenetic movements fall into two major categories, on the basis of intensity.
There are three main endogenous processes: folding, faulting and
volcanism. They take place mainly along the plate boundaries, which are the
zones that lay on the edges of plates. These zones are weak. Endogenous
processes cause many major landform features
Endogenic (internal origin) processes are driven by the internal heat of
the Earth, which in turn results from the radioactive decay of elements deep
beneath the surface. This heat bubbles upward providing a huge driving force
that bends, cracks, lifts, and moves Earth's rigid outer layer. Occasionally we
see this rising energy empty directly onto the surface in the form of molten lava.
Typically, endogenic forces are mountain building processes.
Exogenic forces
Exogenic forces (also known as exogenetic) refer to external processes
and phenomena that occur on or above the Earth 's surface. Comet and
meteoroid impacts, the tidal force of the Moon and radiation from the Sun are
all exogenic. Weathering effects and erosion are also exogenic processes.
An example of an exogenous process that is not as a result of bodies in
space is erosion. Erosion happens as a result of wind, water, ice, or people,
animals, or plants digging in the Earth. Some other examples of exogenous
process are rainfall, snowfall, hailstorm, erosion ,tsunamis, avalanches, winds,
wave currents etc.
Exogenic (external origin) processes are driven by the energy in sunlight.
Sunlight causes air to move, water to be lifted into mountains, and ocean waves
to rise. These moving fluids attack the solid surface, eroding it, carrying the
broken pieces far away, and depositing them to fill low places in the landscape.
In other words, exogenic forces are mountain destroying processes.
Types of forces and processes
Endogenetic
Diastrophic forces
1. Eperogenetic 2. Orogenetic
Exogenetic
Sudden forces
1. Volcanic Eruptions 2. Earthquakes
1. Eperogenetic forces
i) Upward Movement
ii) Downward Movements
a). Emergence
b). Submergence
2. Orogentic forces
Tensional forces
Compressional forces
Crustal fractures
i). Cracking ii). Faulting
Crustal Bending
a). Warping b). Folding
i. Upward
ii. Downward
An outline of the exogenetic processes are as follows:
Gradation
i). Aggradation
ii). Degradation
a. Running water
a. Weathering
b. Ground water
b. Mass wasting
c. Marine process
c. Erosion
d. Wind
1. Running water
e. Glaciers
2. Ground water
3. Marine processes (waves, currents,
tides, and tsunamis)
4. Wind (aeolian processes)
5. Glacier
6. Peri- glacial processes
Mrs Razia Bashir
Associate Professor
A.S College Srinagar
Land forms- Processes and formation (Fluvial, Aeolian and
Karst)
Introduction
The formation and structure of the Earth, looked at how our planet was formed.
It also looked at the Earth's structure. When different materials inside the Earth
move they might create earthquakes, volcanic eruptions or movements of the
tectonic plates. All these events create and shape the Earth's landforms. This
chapter looks at the different types of landforms and how they are formed.
Landforms and the landscape
Tectonic plates move and interact with each other constantly. Their activities
lead to earthquakes, volcanic eruptions and the splitting of the Earth's crust.
These events change the surface of the Earth by creating different landforms. A
landform is a term that describes the shape of a natural land feature. Landforms
are created by different forces of nature. For example, mountains, oceans,
valleys and deserts can be called landforms. A group of landforms in one area
makes up a landscape. The view from an aeroplane or from the top of a hill
gives a good picture of a landscape.
Types of landforms
There are many different types of landforms on the Earth. Some of them were
formed over millions of years and others were formed in a matter of hours. The
formation of a mountain range, for example, would usually take a few million
years. Events like earthquakes and volcanic eruptions can 'wipe off' landforms,
or form new ones in a matter of hours. Examples of some natural landforms are
mountains, oceans, rivers, hills, volcanoes, valleys, desserts, waterfalls, caves
and cliffs.
Mountains
A mountain is a raised part of the Earth's surface. Mountains can be formed in
different ways that involve internal (inside) or external (outside) natural forces.
The movement of tectonic plates is called plate tectonics. Plate tectonics is an
internal natural force because it happens inside the Earth. When tectonic plates
collide, they raise the Earth's crust. As mentioned before, tectonic plates move
very slowly, so it takes many millions of years to build a mountain. Mountains
can also be formed by external natural forces like rain, wind and frost in the
process of erosion.
Mountains with shapes that are sharp and jagged are called young mountains.
Mountains that have a smoother, more rounded look are called old mountains.
The South American mountain range, the Andes, is a young mountain range.
Old mountains look smoother because they have been shaped by natural
weathering over a longer period of time. The Himalayan Mountains, which are
an older type of mountain, are still 'growing' due to plate tectonics.
If they are given enough time, usually millions of years, all mountains crumble.
High, jagged peaks become low, rounded hills. Eventually, mountains wear
away, becoming soil and sand.
Valleys
A flat area of land between hills or mountains is called a valley. Valleys are
usually formed by river water. The speed at which a river deepens its valley
depends on the speed of the flow of the river water and the type of materials
from which the river bed (the bottom of the river) is made. Softer and lighter
materials are moved by water faster than hard and heavy ones. That means that
a river bed made from soft sediments can be changed or deepened faster than a
hard and rocky one.
Oceans
An ocean is a large body of salty water that surrounds a large land mass. After
studying different rock, scientists have established that the first ocean on the
Earth was formed about 4000 million years ago. Even though early Earth did
not have any water, it had the chemical elements that make up a water molecule.
Some scientists believe that the Earth's first rain was just cooled-down volcanic
steam. Rainwater started to collect in low-lying areas of the Earth's crust,
forming the first ocean. Another group of scientists believes that first water was
'delivered' on the Earth by massive ice-bearing comets.
The formation of the first ocean was the starting point for the evolution of life
on Earth. Oceans made the Earth's climate milder and more suitable for life. An
ancient ocean was the place where the first oxygen-producing algae were
formed. Today, more than two-thirds of the Earth's surface is covered with
water which is in constant motion. This motion of water currents plays a very
important role in shaping landforms
Deserts
A desert is an area that receives very little or no rain through the year. Deserts
usually form as a result of climate change. Deserts have very dry air and lots of
wind. Deserts can be hot or cold. During the daytime the temperature in hot
deserts is very high and at night it drops to a few degrees. A cold desert is a
desert that has snow in the winter. An example of a hot desert is the Sahara
desert. Sometimes people call Antarctica a frozen desert. It has not rained or
snowed in some places there for over 100 years. A cold desert never becomes
warm enough for plants to grow in it. Deserts cover about a fifth of the Earth's
land surface.
Landforms are defined as the geomorphologic units defined by its surface form
and location in the landscape. Landforms are typical elements of the
topography. The water body interfaces also called landforms. They are
categorized on the basis of elevation, slope, orientation, stratification, rock
exposure, and soil types as follows:
Aeolian landforms, Coastal and oceanic landforms, Erosion landforms, Fluvial
landforms, Mountain and glacial landforms, Slope landforms, Volcanic
landforms.
Aeolian landforms refer to the Landforms that are formed by the winds. There
are two types of the Aeolian Landforms viz. Erosional and Depositional.
Erosional landforms such as river valleys and coastal cliffs are formed when
forces such as wind and water wear away surfaces. Erosion often takes a
significant amount of time, but its effects are easily measured by examining
geological evidence such as rock layers.
Depositional landforms are formed when minerals and other substances are
deposited over time. In some cases, these landforms become sedimentary rocks
after the deposits are altered by forces such as chemicals, heat and pressure.
Examples of depositional landforms include deltas, flood plains and beaches.
Fluvial Landscapes The landforms which develop as a result of the water
action are known as Fluvial Landforms. Running water such as rivers are the
most important agent of erosion. Other agents such as Glaciers, Groundwater,
wind and sea water are locally dominant agents of erosion. The Fluvial
processes are most important of all the exogeneric processes as landforms
associated with them have overall dominance in the environment of terrestrial
life. These fluvial processes can be divided into three phases viz. erosion,
transportation and deposition.
Karst landscape: The landscape whose distinctive features are to a large
extent, produced by the dissolving power of ground water is referred to as karst
topography. The process by which karst scenery is formed in called
karstification. Since limestone is relatively common rock type, features created
by the action of ground water are found in many parts of the world. However, a
true karst landscape is rather uncommon because of want of the other special
circumstances required for its development.
There are the following main conditions required for the development of
a true karst;
1. Topography like rain water may easily and quickly infiltrate in to the
beds to form karst features.
2. The lime stones must be massive, thickly bedded hard tenacious, well
cemented and well jointed.
3. The limestone should be less porous otherwise the charged solutions will
get its entry into the pore-space, subjecting the whole rock to chemical
weathering and it will collapse
4. The chemically reactive water should infiltrate through the joints for
effective dissolution of the rock and to form wider and deeper holes.
5. The limestone region should be elevated with an outlet at a lower level
such as a deep stream valley or a tectonic depression for the active
movement of ground water so that the water saturated with dissolved
calcium carbonate can be quickly replaced by unsaturated water.
6. The climate must be hot and humid to favour the dissolution chemical
reaction. Karst features do not develop in arid climate. Some arid
regio0ns however have karst features but those were originated during a
period when climate was more humid.
7. The soluble rock should have substantial horizontal and vertical
dimension.
The following are the erosional features of karst region are:
i. Stylotites
ii. Sinkholes/ sinks/ doline
iii. Swallow holes / Embut
iv. Uvalas/ valley sinks
v. Poljes
vi. Karren/ lapis/ bogaz
vii. Galleries and Shafts
viii. Aven or Ponor
ix. Terra Rossa
x. Caves and caverns
xi. Natural bridges or karst bridges
The following are the main depositional features of Karst topography
i. Stalactites
ii. Stalagmites
The resultant features of stalagmites are:
a) Drip curtain or drape
b) Cave pillar
c) Flowstone
d) Geodes
Mrs Razia Bashir
Associate Professor
Department of Geography
Amar Singh College
Suggested readings
i. Physical geography by Moonis Raza
ii. Physical geography by Aijaz u din
iii. Physical geography by Majid Hussain
iv. Physical geography by Savinder Singh
v. Physical Geography by Richard H. Bryant
Fluvial Landscapes
The landforms which develop as a result of the water action are known as Fluvial
Landforms. Running water such as rivers are the most important agent of erosion.
Other agents such as Glaciers, Groundwater, wind and sea water are locally
dominant agents of erosion. The Fluvial processes are most important of all the
exogeneric processes as landforms associated with them have overall dominance in
the environment of terrestrial life. These fluvial processes can be divided into three
phases viz. erosion, transportation and deposition. Erosional Landforms The Erosion
can be normal erosion which takes place by the natural physical processes or the
Accelerated Erosion, which is produced by human interference. The Sheet Erosion
refers to the surface flow removing soil in thin layers. It can be accelerated in the
Steep slopes, where innumerable closely spaced channels are formed, which grows
larger form in gullies (steep-walled canyon like trench). The Erosion can be of
following types: Chemical erosion: Corrosion (Or solution) and carbonation.
Mechanical erosion. Impaction (effect of blow upon the river bed or banks by large
boulders). Cavitations (shattering and breaking up of the stream load through
collisions and mutual abrasion). Hydraulic action (lifting and quarrying effect of
rushing water). Corrosion or abrasion (stream uses its load to scrape away its bed,
particularly in steep confined sections of stream channels). Landforms made by
River Erosion V-shaped Valley Valley starts as small and narrow rills which gradually
develop into long and wide gullies. The gullies will further deeper widen and lengthen
to give rise to valleys which is V-shaped. The River valley is an important erosional
landform. They are formed in the youthful stage of fluvial cycle of erosion. The
vertical erosion or valley deepening causes the V-shaped valleys. Gorge & Canyons
The V-shaped valley can be a Gorge, where steep precipitous wall within which a
narrow river is confined (e.g. – Indus, Sutlej, Brahmaputra, Rhine, Zambezi). Thus,
we can say that Gorge is a V-shaped valley but its sides becomes so steep that they
look almost vertical. Or it can be a Canyon, which is basically a very deep and
extended gorge. The Grand Canyon in Arizona, United States of America is the
largest Canyon in the world. Meander The meanders or meandering rivers are the
low slope rivers which are not choked with the sediment and move back and forth in
a zig-zag order of loops. The meander has thus a serpentine path and it helps in
accommodating in extra volume of water. River Terraces River terraces are
abandoned floodplains that formed when a river flowed at a higher level than it does
today. Thus, these are the surfaces that mark an old valley floor or floodplain levels.
Peneplain When an extensive area has been eroded sufficiently to give the look of
almost a plain, it is called a Peneplain. Landforms made by River Deposition Alluvial
Fans When the velocity of the running water, as it comes out of hills and meets the
plain, decreases, it dumps the transported material at the foothills. The structure
made are called alluvial fans. The alluvial fans are formed due to accumulation of
materials in the form of fan and cones respectively at the base of foot hills Alluvial
cones are made of coarse materials than the alluvial fans. Natural leaves Narrow
belt of ridges of low height built by the deposition of sediments by the spill water of
the stream on its either bank. Food plain Surfaces on either side of a stream that is
frequently inundated. Crevasse splays Formed by breaching of leaves when water
escapes through a series of distributaries channels. Back swamps Plain area
adjoining a levee may contain marshes called back swamps. Yazoo streams
Distributions of rivers occupying lateral positions. Delta Delta is the
triangular deposition at the mouth of a river debouching in a lake or a sea. The
Factors that help in delta formation are as follows: Long courses of rivers. Medium
size sediments. Calm or sheltered sea. Suitable place (shallow sea and lake shores).
Large amount of sediments. Accelerated Stable condition of sea coast. On the basis
of shape delta can be divided into following categories such as arcuate, bird-foot,
Estuarine, Cuspate, Truncated etc. Arcuate (lobate form) Delta The Arcuate delta
resembles the fan and is convex towards the Sea. It is semicircular in shape and is
commonly found in semi-arid region; growing delta such as Nile, Niger, Ganga,
Indus, Mekong, Irrawaddy, Rhine, Volga, Danube, Rhone, Lena rivers. Bird-foot
Delta Birdfoot Delta is also known as a finger delta. In these deltas, the sediments
deposited are composed of those fine particles which are received from the
limestone rocks. The rivers with high velocity carry suspended finer load to greater
distance inside the oceanic water (such as Mississippi). Estuarine delta When a river
enters the sea through the single mouth or estuary, then the Estuarine Delta is
formed which is submerged under marine water. Examples are Narmada River,
Congo River, Amazon River and Hudson River. Cuspate Delta Cuspate delta are
pointed. They are shaped by regular, opposing, gentle water movement as seen at
the Tiber river. Oxbow lakes The Oxbow lakes are formed by the depositional and
erosional actions taking place simultaneously. Please note that excessive
meandering would result in Oxbow lakes. How Oxbow lakes are formed? On the
inside of the loop, the river travels more slowly leading to deposition of silt.
Meanwhile water on the outside edges tends to flow faster, which erodes the banks
making the meander even wider. Over time the loop of the meander widens until the
neck vanishes altogether. Then the meander is removed from the river’s current and
the horseshoe shaped oxbow lake is formed. Black Swamps When the water spills
out onto the flood plains, the heaviest material drops out first and finest material is
carried over a greater distance. This fine grained alluvium would hold much water
and would give rise to a wetland which is called Black swamps or simply swamps.
Landforms made by River Transportations The dissolved solids in the rivers travel
downstream and become a part of Ocean. The particles of clay, silt and fine grains
are carried in suspension. Whenever a soft rock obstructs the course of stream and
is eroded and sediments are scattered all around, it would be called Eddies. These
Eddies sometimes look like discs and so are called potholes. The large potholes are
called
Plungepools.
Glaciers and landforms
Glaciers are not landforms. The action of glaciers, however, creates landforms. It is a
process known as glaciation. Glacial ice is an active agent of erosion, which is the gradual
wearing away of Earth surfaces through the action of wind and water. Glaciers move, and as
they do, they scour the landscape, "carving" out landforms. They also deposit rocky material
they have picked up, creating even more features. The work of present-day glaciers,
however, is slow and confined to certain areas of the planet. Less obvious but far more
reaching has been the work of Ice Age glaciers. Many of the distinctive features of the
northern landscapes of North America and Europe were formed by glaciers that once
covered almost one-third of the planet's land surface. A glacier is a large body of ice that
formed on land from the compaction and recrystallization of snow, survives year to year, and
shows some sign of movement downhill due to gravity. Two types of glaciers exist: relatively
small glaciers that form in high elevations near the tops of mountains are called alpine or
mountain glaciers; glaciers that form over large areas of continents close to the poles (the
North and South Poles; the extreme northernmost and southernmost points on the globe)
are called continental glaciers or ice sheets. Two continental glaciers are found on Earth:
one covers 85 percent of Greenland in the Northern Hemisphere and the other covers more
than 95 percent of Antarctica in the Southern Hemisphere.
Valley glacier:
An alpine glacier flowing downward through a pre-existing stream valley.
Both types of glaciers create landforms through erosion. These erosional features may be as
large as the Great Lakes of North America or as small as scratches left in pebbles. As a
glacier moves, it scours away material underneath it, plucking up rocks, some of which may
be house-sized boulders. This material then becomes embedded in the ice at the base of a
glacier. As the glacier continues to move, the embedded material abrades or scrapes the
rock underneath. The slow scraping and grinding produces a fine-grained material known as
rock flour. It also produces long parallel scratches and grooves known as striations in the
underlying rocks. Because they are aligned parallel to the direction of ice flow, glacial
striations help geologists determine the flow path of former glaciers. Another small-scale
erosional feature is glacial polish. This is a smooth and shiny surface produced on rocks
underneath a glacier when material encased in the ice abrades the rocks like fine
sandpaper.
When a cirque glacier expands outward and flows downward through a stream valley that
already exists, it becomes a valley glacier. Through erosion, valley glaciers turn V-shaped
stream valleys into U-shaped glacial troughs. Smaller valley glaciers, known as tributary
glaciers, may form alongside a main valley glacier and eventually flow into it. The shallower
glacial troughs created by these glaciers are known as hanging valleys. A valley glacier that
flows out of a mountainous area onto a gentle slope or plain and spreads out over the
surrounding terrain is a piedmont glacier. A valley glacier may flow all the way to a coastline,
carving out a narrow glacial trough. If the glacier melts and the valley fills with seawater, it is
known as a fjord (pronounced fee-ORD). Although prominent along the west coast of
Norway, fjords are also found along the coasts of Alaska, British Columbia, Chile,
Greenland, New Zealand, and Scotland.
Since a glacier can carry rocks for great distances before depositing them, those rocks
generally differ from the surrounding native rocks in that area. In fact, because they are
derived from a very large area eroded by a glacier, glacial deposits contain the widest variety
of rock types. A glacially deposited large boulder that differs in composition from the rocks
around it is called an erratic.
A moraine deposited at the leading edge of a glacier, marking its farthest advance, is a
terminal or end moraine. Finally, a continuous layer of till deposited beneath a steadily
retreating glacier is a ground moraine.
Another common glacial landform is the drumlin. This tear-drop-shaped hill forms
underneath a glacier. The tail of the drumlin points in the direction of the ice movement.
Geologists are unsure exactly how drumlins form, whether a glacier scrapes up material
beneath it or deposits material it already carries or a combination of both. Drumlins may be
quite large, measuring up to 200 feet (60 meters) in height and 0.6 mile (1 kilometer) in
length.
As a glacier melts, it produces meltwater that flows on top, within, and underneath the
glacier through channels. This meltwater moves large quantities of sediment from the
glacier. At the leading edge of the glacier, also known as the terminus or glacier snout, the
meltwater emerges in large streams that carry it away from the glacier. The sediment in the
meltwater is then deposited, forming a broad, sweeping plain called an outwash plain. Since
the sediment was carried in water, it is deposited in a sorted manner, with the largest
particles first and the smallest particles last. If a glacier melts and retreats, curving, snakelike
ridges of sediment may mark the former locations of streams that existed under the glacier.
These long, twisting ridges are called eskers.
Two other features that result from the melting of glaciers are kames and kettles. As a
glacier begins to melt, a depression may form on its top surface, filling with water and
sediment. When the glacier finally melts away, the sediment is set down on the surface of
the ground, forming a steep-sided, conical mound or hill known as a kame. A kettle forms
when a large chunk of ice separates from the main glacier. Buried by glacial till, the ice then
melts, leaving a depression in the landscape. This eventually becomes filled with water,
forming a kettle lake.
Glacial formation
A glacier does not start out as a glacier. All that ice began to form when snow—delicate,
feathery crystals of ice—fell in areas above the snow line, the elevation above which snow
can form and remain all year. It takes snow on top of snow on top of more snow to create a
glacier; it also takes a long time. On average, 10 feet (3 meters) of snow will turn into 1 foot
(0.3 meter) of ice. In polar regions, where annual snowfall is generally very low because the
air is too cold to hold much moisture, it may take snow about 1,000 years to turn into ice.
In time, if snow does not melt but is buried beneath additional layers of snow, it will begin to
compress. This forces the snow crystals to recrystallize, forming grains similar in size and
shape to cane sugar. As new snow piles on top and the snow below becomes further
compressed, the grains grow larger and the air spaces between them become smaller. Over
a short period of time, perhaps the span of two winters, the compressed snow turns into a
granular material known as firn or névé (pronounced nay-VAY). The density (amount of
mass in a given volume) of regular snow is about 10 percent that of water. The density of firn
is about 50 percent that of water. Once the thickness of the overlying snow exceeds about
165 feet (50 meters), the firn turns into a solid mass of glacial ice.
Over a period of years, depending on the amount of snowfall and seasonal temperatures, a
glacier may gain more mass than it loses. If this occurs, the terminus of the glacier will likely
advance. If the opposite happens, with the glacier losing more mass than it gains, its
terminus will likely retreat. Thus, depending on the balance between accumulation and
ablation, a glacier may grow or shrink.
Compiled by Ummar Ahad