Download Coastal geomorphic features around Indian Ocean

Indian Journal of Geo-Marine Sciences
Vol. 39(4), December 2010, pp. 557-561
Coastal geomorphic features around Indian Ocean
P Nag
National Atlas and Thematic Mapping Organisation Bidhan Nagar, Kolkata -700 064, India
[E-mail: [email protected]]
Received 20 August 2010; revised 21 December 2010
Coastal areas are important globally. These areas have also drawn attention of geomorphologists for in-depth studies.
Several institutions have made contributions towards analysis, identifications of different geomorphic features and their
mapping. Attempts have been made to map oceans and associated coastal geomorphic features. In this paper an effort has
been made for the Indian Ocean. There are straits, channels, continental sub-marine margins and continental shelf which
form different components of the coastal ocean. They are shaped by land erosion, sub aerial deposition, volcanic,
diastrophic, marine erosion and deposition and organisms. The interactions of these features are so variable that the coastal
features are almost infinitely complex. The coasts of Indian Ocean realm fall into more than one category. Therefore, a
matrix reference can be used with definite colour band to depict each single or compound feature.
[Keywords: Coastal features, Shepard's classification, Erosion, Marine deposition]
Introduction
According to United Nations estimate, 66 per cent
of the world population lives within few kilometers
of the coast. Consequently, food production,
communication, settlements and even recreation
facilities are concentrated along the coast. As such,
coastal areas, all over the world draw attention of the
geomorphologists for in-depth study of various
coastal features of their interest. There are different
considerations for demarcating the coastal area. As
such, some schools of thought define that the coastal
zone may extend landward and seaward keeping in
view the influence of sea on land and land on sea
respectively.
The Indian Ocean Atlas1,2 depicts the physiographic profiles of the Indian Ocean and the adjoining
land area. The contours indicate the asymmetrical
distribution of the physical features in land part and
the bathymetry backed by recent geophysical
investigations indicate the most complex nature of the
Indian Ocean with regard to its history of formation
and thereby its physiography. In this context it is to be
remembered that this ocean is nearly land-locked
from three sides and open to the Southern Ocean.
Again this Southern Ocean is itself considered a
special subdivision of each adjoining major ocean
of the world (Fig. 1).
Materials and Methods
Sources of Information and Techniques
For the preparation of the map shown in Fig. 1,
a scale of 1:25,000,000 is used. Isobaths and contours
are given in kilometers. Bathymetric data are
compiled from charts No.7070 to 7073 published by
the National Hydrographic Office (NHO), Dehra Dun,
and sheet No.1-9 and map of Indonesia, showing
bottom topography of the Indian Ocean and also sheet
1-4 showing topography of the continental slope of
Antarctica in Geological–Geophysical Atlas of the
Indian Ocean3. The charts published by NHO are at a
scale of 1:5,000,000. Land topography including that
of Antarctica has been compiled from the maps and
atlases published by Survey of India, National Atlas
& Thematic Mapping Organisation and other
agencies. Geophysical names for physical feature
are more or less followed as generally used in
English, though there are exceptions also viz. Massif
Di Ivakoanv is the French term of Massif Ivakoanv,
Bab-el–Mandeb is the Arabian term used for the strait
of Mandeb, and Nafud is an Arabian term to designate
desert and Dasht-e-Kebir is the Persian term of Kebir
desert. The horizontal cross profile along 20˚S
parallel has been drawn to show a representation
of the bathy-orographical features of the study area.
The inset map at a scale 1:100,000,000 show some
selected geo-features viz. the epicentres of
earthquakes, active volcanoes, sea mounts, guyots and
canyons.
In the map, the important physical features of land
part are mountains and hills, plateaus and rift valleys,
and river valleys and coastal plains. The important
physical features of the ocean part are seas, bays,
gulfs, straits, channels, banks, submarine ridges,
plateaus, basins, abyssal plains, etc. Deep blue to light
INDIAN J. MAR. SCI., VOL. 39, No. 4, DECEMBER 2010
558
Ocean with the South China Sea. It is approximately
805 km long and varies in width from 64 km in the
south to 257 km in the north. The strait assumed its
present configuration as a result of the post-glacial
rise of sea level, which drowned the Sunda strait.
Strait of Hormuz—The Strait of Hormuz, the
entrance to the Persian Gulf from the Indian Ocean,
is a narrow, strategically important stretch of ocean
between the Gulf of Oman in the southeast and the
Persian Gulf in the southwest. On the north coast of
the strait is Iran (Persia) and on the south coast is the
United Arab Emirates and Musandam, an exclave of
Oman. The strait at its narrowest is 34 km having two
1.6 km wide channels for marine traffic separated by
a 3.2 km wide buffer zone. This is the only sea
passage to the open ocean for large areas of the
petroleum exporting Persian Gulf states.
Continental sub-marine margins
This part of the ocean has direct links with the
adjoining continents. The shelf regions, the
continental slope and the continental foot are the
three distinguished physiographic zones of continental
sub-marine margins.
Continental shelf
Fig. 1―Indian Ocean – Physical.
shades indicate variations in depth whereas the
variation of land elevation from plains to mountains
is shown by green to brown. The prominent features
are written on the body of the map. Contours are
drawn in area concerned.
Results and Discussion
Straits and Channels
There are a large number of straits and channels in
the Indian Ocean. The straits which are narrow stretch
of sea connecting two extensive areas of sea have
been formed by fracture across an isthmus or by the
sea overflowing land, which had subsided or eroded.
On the other hand, the channels as applicable for the
ocean features are relatively narrow stretch of sea
between two landmasses, connecting two or more
extensive areas of sea. The straits and channels of
Indian Ocean along with their strategic location have
been enlisted.
Malacca Strait—The Malacca Strait (also called
the Straits of Malacca, and in Malay Salet Melaka),
a shallow passage between the Malay peninsula and
the Indonesian island of Sumatra, connects Indian
This region directly adjoins the land and is made up
of shallow water fringe zones with average depth of
less then 0.2 km and a relatively gentle slope towards
the ocean. The surface topography of the shelf includes
old shorelines, flooded terraces, submarine extension of
river valleys and other forms of the land relief. The
width of the shelf varies from a few kilometers to
several hundred kilometers. The Indian Ocean except
for the relatively wide shelf zones in the eastern sector
of the Arabian Sea, the eastern sector of the Bay of
Bengal and the northwest coast of Australia, the
continental shelf of much of the Indian Ocean and
adjacent seas is narrow and in places precipitous. The
shelf area of Indian Ocean is about 3,141,000 sq. km as
against 11,150,000 sq. km for the Pacific and
14,180,000 sq. km for the Atlantic Ocean.
Along the eastern coastline of Africa in general, the
shelf is narrow, less than 4 km from shore, except
southern tip of Africa adjoining Cape Agulhas and
area adjoining Baria. From Mombassa to Cape
Guardafui along the Somali coast, the extremely
narrow shelf is suggestive of a fault coast. However,
east of 51˚E off the African coast, the continental
shelf is well developed and forms a platform.
The continental shelf of Red Sea region widens in
the central and southern sector. In some areas in south
sector the shelf extends for more then 100 km off
NAG: COASTAL GEOMORPHIC FEATURES AROUND INDIAN OCEAN
shore. The shelf is approximately 37 km across the
Gulf of Aden at the entrance of the Red Sea. East of
Aden, the shelf becomes narrow. There are a couple
of places along Yemen and Oman coasts where there
is no shelf area at all. At the same time along the
Arabian coast between 56˚E and 58˚E the shelf forms
the platform supporting the Kuria-Muria Islands.
The whole of Persian Gulf is under shelf region
and less than 40 m in depth. The Gulf of Oman has a
broad mud covered shelf on its head. Thereafter as
one proceeds eastward, the shelf is narrow along the
coast of southern Iran (Makran coast) and Baluchistan
coast (west of Karachi). The continental shelf along
the greater part of the coast of western Pakistan is
about 40 km wide except across the Indus delta,
where it becomes around 100 km wide. In some
places east of Karachi, the shelf is 180 km or more in
width. The continental shelf of western coast of India
is comparatively wide between 20 and 160 km except
near Bengal coast at the mouth of River Ganga, where
it is as much as 160 km wide .The Andhra shelf has
an average width of about 43 km.
The continental shelf along the entire coastline of
Bangladesh is broad varying between 125-160 km and
the shelf area is approximately 170 sq. km. The
continental shelf of Myanmar is in continuity with that
of Bangladesh and is comparatively wide, ranging from
70 to 180 km. Thailand has two distinct parts of the
coastlines. In Indian Ocean area, the coastline is small
facing the Andaman Sea, where the continental self is
rather narrow. The second part is Gulf of Thailand,
which is actually a part of the South China Sea.
Malaysia and Singapore have a wide continental
shelf all around the peninsula. The continental shelf of
the western part of the islands of Sumatra, Java, Bali,
Lomba of Indonesia have a narrow continental shelf
whereas the continental shelf of the eastern part of
these islands including Borneo have very wide
continental shelf known as Sunda shelf. In fact the
Sunda shelf covers the area between the three large
islands namely Sumatra, Borneo and Java and
mainland Asia, including the southern part of South
China Sea, the Gulf of Thailand and the Java Sea as
well as the shallow part of the Strait of Malacca,
previously called the Southeast Asiatic shelf.
The northwestern Australia has a vast continental
shelf and was previously known as Great Australian
Bank of East (to match the Great Asiatic Bank–Sunda
shelf). This Great Australian Bank has been divided
into three parts viz. Rowley Shelf (115˚ to 122˚E),
Sahul Shelf (122˚ to 131˚E) and Arafura shelf. The
559
first two have areas of about 3,00,000 km² while the
third one is 9,30,000 km². In Western Australia,
around Shark bay, the continental shelf is moderately
wide but the other areas are comparatively narrow.
The shelf area of Antarctica coast adjoining Indian
Ocean is also narrow.
Coastal land form features
As an integral part of the earth system, the
geomorphologic processes at work on coastal
landforms are influenced by number of environmental
factors such as geological, climatic, biotic, tidal and
other oceanographic factors including salinity and sea
level changes. These factors vary from one sector of
the coast to another. The variation is zonal in terms of
climatic regions and irregular in terms of geological
outcrops. Thus, there are rocky coasts, beaches, river
mouths, fiords, estuaries, lagoons, barrier islands and
inter-tidal wetlands. In and around them, a number of
specific biological communities have developed
including inter-tidal and marsh communities,
mangroves, sea grass, deep beds and coral reefs.
The coastal area of the Indian Ocean is adorned
with a large variety of features, which have been
shaped by different agents in different environmental
conditions. Broadly, the Indian Ocean coasts are
shaped by the actions of seven features namely (Fig. 2):
.
Fig. 2―Indian Ocean - Coastal Features.
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INDIAN J. MAR. SCI., VOL. 39, No. 4, DECEMBER 2010
(1) Land Erosion
(2) Sub aerial deposition
(3) Volcanic
(4) Diastrophic
(5) Marine erosion
(6) Marine deposition
(7) Organisms.
The interactions of these features are so variable
that the coastal features are almost infinitely complex.
Many times the coasts of Indian Ocean realm fall into
more than one category. Therefore, a matrix reference
can be used with definite colour band for each single
or compound feature (Fig. 3). Thereafter, where third
feature comes into action, a colour pattern has to be
superimposed as shown in the Indian Ocean Atlas
prepared by the National Atlas & Thematic Mapping
Organisation. For example, the coasts of extreme
southern India are primarily formed by sub-aerial
deposition as well as marine deposition.
Simultaneously, marine erosion is also an action to
that very area in shaping the coasts. Now, to depict
the coastal features of that area, the first two actions
are shown by a particular colour band, and for the
third feature colour pattern (horizontal black lines)
has been superimposed. Similarly, predominantly
three features of which two features namely volcanic
and marine erosion are shown by colour band while
for the third feature i.e. ‘marine deposition’ by black
dot pattern is superimposed on the colour band form
the coast from Karachi to Gwadar in Pakistan.
The Tanzanian coast is shaped by five predominant
features namely marine erosion, marine deposition,
organism, and diastrophic and sub-aerial deposition.
This particular combination can be shown by a
complete separate colour band and pattern. Another
category, which has recently been receiving closer
attention, is the ‘Frozen Coast’; it is well developed in
Antarctica where ice shelves terminate in cliffs up to
35 m high. In summer, icebergs calves from these and
in winter the adjacent sea freezes. Such condition in
Antarctica is represented by ‘frozen coast’.
Studies on coastal classification
One of the early attempts at coastal classification
was put forward by Suess4. He distinguished Atlantic
coasts which run transverse to the general trend of
geological structures from Pacific coasts, which run
parallel to structural trends. The former is
characterised by the Atlantic coast of Europe and the
latter by the Pacific coast of North America.
In another method of classification Gulliver5
distinguished the coasts formed by submergence from
the coasts formed by emergence. The distinction
made by him was developed into a genetic
classification of shorelines by Johnson6, who proposed three categories, namely:
(1) Shorelines of submergence
(2) Shorelines of emergence and
(3) Neutral shoreline formed neither due to
submergence nor emergence but by deposit
or tectonic movement e.g. delta shorelines,
alluvial plain shorelines, outwash plain
shorelines, volcano shorelines and fault
shorelines.
Cotton7 suggested a scheme, which is most straight
forward. He made a fundamental distinction between
coasts of stable and mobile regions. Tectonic
movement during quaternary period has not affected
stable regions, while mobile regions have been
subject to quaternary period’s earth movement, which
may still be continuing.
Vatentin8 made a fundamental distinction between
advancing and retreating coasts, noting that advance
may be due to coastal emergence or progradation by
deposition while retreat is due to coastal submergence
and/or retrogation by erosion.
The coastal features of Indian Ocean realm can
however primarily be classified based on Shepard's
method and with a combination of various processes.
The following features came into force in shaping the
coasts of Indian Ocean realm:
a
b
c
d
e
Fig. 3―Combination of Coastal Features.
Sub aerial deposition+marine erosion.
Sub aerial deposition+Marine deposition.
Sub
aerial
deposition+Marine
decomposition + marine erosion.
Sub aerial deposition+Marine erosion
+coast built by organisms.
Sub aerial deposition+Marine deposition
+land erosion.
NAG: COASTAL GEOMORPHIC FEATURES AROUND INDIAN OCEAN
f
Sub aerial deposition+coast built by
organisms.
g
Sub aerial deposition+land erosion.
h
Sub aerial deposition+volcanic.
i
Sub aerial deposition.
j
Diastrophic+marine erosion+marine deposition
k
Diastrophic+marine deposition.
l
Diastrophic+marine
deposition+coasts
built by organisms.
m Diastrophic+coasts built by organism
+ volcanic.
n
Diastrophic.
o
Volcanic+marine erosion+marine deposition.
p
Volcanic+diastrophic.
q
Volcanic.
r
Marine erosion.
s
Marine deposition+marine erosion.
t
Marine deposition+marine erosion +land
erosion.
u
Marine deposition
v
Coast built by organisms+marine erosion.
w Coast built by organisms+marine erosion
+marine deposition.
x
Coasts built by organisms.
y
Marine erosion+marine deposition +coast
built by organisms+diastrophic+sub aerial
deposition.
On the whole, it is observed (rank—wise) that
marine deposition, organism, sub aerial deposition,
marine erosion, diastrophic, volcanic and land
erosional factors are active in 43, 40, 34, 27, 7, 5 and
2 areas respectively in shaping the Indian Ocean
coasts. It is also observed that organisms, marine
561
deposition, sub aerial deposition, marine erosion,
volcanic and diastrophic factors have been
predominantly responsible in shaping the coasts in
17, 14, 6, 4, 4 and 2 areas respectively in Indian
Ocean realm.
Conclusion
The study of the coastal areas is exciting and also
a challenging field. In such a study one might observe
changes in the coastal features even within a single
day. A point-to-point description of coastal features
and its components have been given. But it should be
noted that this picture is a generalized one. Exact
cause of action and the coverage may be drawn by
more detailed micro-level studies. However, this map
would be a useful in the quest for the factors
responsible in shaping the coasts of Indian Ocean
realm.
References
1 Nag P, (Ed.), Indian Ocean Atlas, (National Atlas & Thematic
Mapping Organisation, Kolkata) 1998.
2 Nag P, (Ed.), Geography of the Indian Ocean, (National Atlas
& Thematic Mapping Organisation, Kolkata) 2006.
3 Academy of Sciences of the USSR, International Indian Ocean
Expedition (1960-1965), Geological-Geophysical Atlas of the
Indian Ocean (Moscow) 1975.
4 Suess E, The Face of the Earth, (Carleton Press, Oxford) 1906.
5 Gulliver F P, Shoreline topography, (Proceedings of the
American Academy of Arts & Science) 1899, Vol. 34, pp. 151258.
6 Johnson D W, Shore Process and Shoreline Development,
(Wiley, New York) 1919.
7 Cotton C A, Criteria for the classification of coats,
(Proceedings of the 17th International Geographical Congress,
Washington) 1952, pp. 315-19.
8 Vatentin, Harmut, Die Kusten der Erde, (Justus Perthes Gotha,
Berlin) 1952.