220
Amanda W.J. DEMOPOULOS et al.
PHYSICAL CHARACTERISTICS OF THE DEEP
INDIAN OCEAN
The Indian Ocean, including adjacent seas (e.g., the
Arabian Sea, the Bay of Bengal, and the Southern
Ocean), covers 73 426 000 km
2 , roughly one-fifth of
the total world oceanic area. It has an average depth
of 3890 m, which is approximately equivalent to the
average world-ocean depth.
Morphology
The morphological features of the Indian Ocean are
similar to those of the Atlantic Ocean (see Chapter 6),
and include mid-ocean ridges, abyssal plains, and few
deep-sea trenches. The Indian Ocean has very few
seamounts and islands (see Rogers, 1994), but contains
numerous submarine plateaus and rises (Fig. 7.1).
Two types of continental margins are present in this
basin: divergent and convergent. Divergent margins
are the most common, and are characterized by wide
continental shelves, broad continental rises, and little
seismic activity. Such margins are found along East
Africa, the Arabian Peninsula, much of the Indian
subcontinent, and Western Australia. The Indian Ocean
contains only one convergent margin in its northeast
corner, the Java (Sunda) Trench, where oceanic crust
is subducted beneath a continental plate (Fig. 7.1). The
7500 m deep Java Trench is part of the Indonesian Arc,
which contains 14% of the world’s active volcanoes.
Because of large riverine inputs of terrigenous
sediment, particularly from the Indus and Ganges
Rivers, onto gradually sloping divergent margins, the
Indian Ocean has vast continental rises and abyssal
plains (Kennett, 1982). The rises are gradually sloping
plains of terrigenous sediment several kilometers thick.
Beyond the continental rises lie level abyssal plains; the
abyssal plain south of the Bay of Bengal is the flattest
large area of the earth’s surface (Tomczak and Godfrey,
1994). Much of this plain has arisen from a turbidity
flow down the northern slopes of the Bay of Bengal,
extending 3000 km southwards into the deep sea of the
Bay.
Like the Atlantic Ocean, the Indian Ocean is
subdivided into a number of major basins by long
sections of mid-ocean ridge (Fig. 7.1). In the Indian
Ocean, some of these ridges (e.g., the Ninety-East
Ridge, the Mascarene Ridge and the Chagos-Laccadive
Ridge) are aseismic; they do not appear to be sites
of active seafloor spreading. Active ridges include the
Carlsberg Ridge and the Mid-, Southwest and Southeast
Indian Ridges, the last two of which extend beyond the
limits of the Indian Ocean, connecting with the world
Mid-Ocean Ridge system. The abyssal Indian Ocean
is divided into several smaller basins by meridional
ridges. The West Australian Basin and the Mid-Indian
Basin are separated by the Ninety-East Ridge whilst
to the west of the Mid-Indian Ridge are a series of
basins including the Somali, Mascarene, Madagascar
and Natal Basins. The Carlsberg Ridge lies north of
the Arabian Basin (Fig. 7.1).
Surface circulation
Three important factors make the circulation and
hydrology of the Indian Ocean different from those of
any other ocean: the closure of the Indian Ocean in the
northern subtropics; the seasonally-reversing Monsoon
Gyre; and the blocking effects of the equatorial currents
to the spread of water masses along the thermocline
(Fig. 7.2). Owing to seasonal heating and cooling of
the vast Asian landmass, winds vary seasonally north
of the equator, resulting in the Indian Ocean monsoons.
From November to March the Northeast Monsoon is
accompanied by the northeast trades, which are reinforced by the rapid winter cooling of air over Asia. As
a result, the westward-flowing North Equatorial Current
from 8ºN to the equator is prominent in January
through March, generating a small anticyclonic gyre
north of the equator (Fig. 7.2). Very little upwelling
occurs during the Northeast Monsoon, and hydrological
effects are generally superficial (Wyrtki, 1973). From
April to September, the Asian landmass warms faster
than the ocean, drawing moist air ashore from over the
ocean, and creating the Southwest Monsoon. During
this period, eastward surface currents north of the
equator combine with the Equatorial Countercurrent,
establishing the Southwest Monsoon Current between
15ºN and 7ºS (Fig. 7.2) and a strong westward-flowing
South Equatorial Current around 5ºS. This reversal
of surface currents gives rise to the greatest seasonal
variation in hydrography of any ocean basin (Burkill
et al., 1993). Strong upwelling occurs off the Somali
and Oman coasts, resulting in substantial increases in
surface production.
The South Equatorial Current forms a marked
hydrographic boundary between the monsoon-driven
circulation in the north and the Southern Hemispheric
Sub-tropical Anticyclonic Gyre to the south. The
circulation pattern to the south of 5ºS is analogous to
Amanda W.J. DEMOPOULOS et al.
PHYSICAL CHARACTERISTICS OF THE DEEP
INDIAN OCEAN
The Indian Ocean, including adjacent seas (e.g., the
Arabian Sea, the Bay of Bengal, and the Southern
Ocean), covers 73 426 000 km
2 , roughly one-fifth of
the total world oceanic area. It has an average depth
of 3890 m, which is approximately equivalent to the
average world-ocean depth.
Morphology
The morphological features of the Indian Ocean are
similar to those of the Atlantic Ocean (see Chapter 6),
and include mid-ocean ridges, abyssal plains, and few
deep-sea trenches. The Indian Ocean has very few
seamounts and islands (see Rogers, 1994), but contains
numerous submarine plateaus and rises (Fig. 7.1).
Two types of continental margins are present in this
basin: divergent and convergent. Divergent margins
are the most common, and are characterized by wide
continental shelves, broad continental rises, and little
seismic activity. Such margins are found along East
Africa, the Arabian Peninsula, much of the Indian
subcontinent, and Western Australia. The Indian Ocean
contains only one convergent margin in its northeast
corner, the Java (Sunda) Trench, where oceanic crust
is subducted beneath a continental plate (Fig. 7.1). The
7500 m deep Java Trench is part of the Indonesian Arc,
which contains 14% of the world’s active volcanoes.
Because of large riverine inputs of terrigenous
sediment, particularly from the Indus and Ganges
Rivers, onto gradually sloping divergent margins, the
Indian Ocean has vast continental rises and abyssal
plains (Kennett, 1982). The rises are gradually sloping
plains of terrigenous sediment several kilometers thick.
Beyond the continental rises lie level abyssal plains; the
abyssal plain south of the Bay of Bengal is the flattest
large area of the earth’s surface (Tomczak and Godfrey,
1994). Much of this plain has arisen from a turbidity
flow down the northern slopes of the Bay of Bengal,
extending 3000 km southwards into the deep sea of the
Bay.
Like the Atlantic Ocean, the Indian Ocean is
subdivided into a number of major basins by long
sections of mid-ocean ridge (Fig. 7.1). In the Indian
Ocean, some of these ridges (e.g., the Ninety-East
Ridge, the Mascarene Ridge and the Chagos-Laccadive
Ridge) are aseismic; they do not appear to be sites
of active seafloor spreading. Active ridges include the
Carlsberg Ridge and the Mid-, Southwest and Southeast
Indian Ridges, the last two of which extend beyond the
limits of the Indian Ocean, connecting with the world
Mid-Ocean Ridge system. The abyssal Indian Ocean
is divided into several smaller basins by meridional
ridges. The West Australian Basin and the Mid-Indian
Basin are separated by the Ninety-East Ridge whilst
to the west of the Mid-Indian Ridge are a series of
basins including the Somali, Mascarene, Madagascar
and Natal Basins. The Carlsberg Ridge lies north of
the Arabian Basin (Fig. 7.1).
Surface circulation
Three important factors make the circulation and
hydrology of the Indian Ocean different from those of
any other ocean: the closure of the Indian Ocean in the
northern subtropics; the seasonally-reversing Monsoon
Gyre; and the blocking effects of the equatorial currents
to the spread of water masses along the thermocline
(Fig. 7.2). Owing to seasonal heating and cooling of
the vast Asian landmass, winds vary seasonally north
of the equator, resulting in the Indian Ocean monsoons.
From November to March the Northeast Monsoon is
accompanied by the northeast trades, which are reinforced by the rapid winter cooling of air over Asia. As
a result, the westward-flowing North Equatorial Current
from 8ºN to the equator is prominent in January
through March, generating a small anticyclonic gyre
north of the equator (Fig. 7.2). Very little upwelling
occurs during the Northeast Monsoon, and hydrological
effects are generally superficial (Wyrtki, 1973). From
April to September, the Asian landmass warms faster
than the ocean, drawing moist air ashore from over the
ocean, and creating the Southwest Monsoon. During
this period, eastward surface currents north of the
equator combine with the Equatorial Countercurrent,
establishing the Southwest Monsoon Current between
15ºN and 7ºS (Fig. 7.2) and a strong westward-flowing
South Equatorial Current around 5ºS. This reversal
of surface currents gives rise to the greatest seasonal
variation in hydrography of any ocean basin (Burkill
et al., 1993). Strong upwelling occurs off the Somali
and Oman coasts, resulting in substantial increases in
surface production.
The South Equatorial Current forms a marked
hydrographic boundary between the monsoon-driven
circulation in the north and the Southern Hemispheric
Sub-tropical Anticyclonic Gyre to the south. The
circulation pattern to the south of 5ºS is analogous to