The Continental Margins

Topic of the lesson
The Continental MarginsThe Continental Margins
By
Prof.A. Balasubramanian &
Dr. K.N. Somashekar
Centre for Advanced Studies in Earth Science
University of Mysore, India

 Introduction
 The Earth’s Crust
 The Asthenosphere
 The Age of Ocean Floor
 The Continental drift hypothesis
 The Concept of Plate Tectonics
 Types of Plate Movement
 Volcanism
 Conclusion
Table of ContentsTable of Contents

 After attending this lesson, the user should be able to
understand the importance of continental margins, in
the context of earth and oceanographic studies.
 The continental margin is the zone of the ocean floor
that separates the thin oceanic crust from thick
continental crust.
 The continental margins are the submerged
continental shelf and slope zones, forming the outer
edge of major landmasses.
Objectives

 The very important aspects like plate tectonics, sea-
floor spreading, subduction, tectonic deformation
generating earthquakes and magmatic evolution, the
kinds and characteristics of continental margins will
be known.
Objectives

 Oceanography is an interesting subject. Geological
oceanography deals with a lot of unique aspects of the
oceans including the ocean morphology and relief,
continental margins, tectonic processes acting on the
ocean bottoms, marine mineral resources, and the
deep sea deposits.
 The subject also focuses on the never ending dynamic
processes like ocean waters, ocean currents and their
impacts with reference to space and time.
Introduction
(…Contd)

 Understanding the tectonic disposition and
movement of crustal plates are an important part
while studying the earth and atmospheric sciences, in
general and oceanography, in particular.
 The continental margins and the deep ocean basins
are the two major aspects to be understood in this
subject. This lesson is on the characteristics of
continental margins.
Introduction

 Based on the seismic observations, the earth’s
lithosphere is made of three major layers as the crust,
mantle and the core.
 The Crust is the outermost solid layer of the earth.
 Although, the crust is composed of igneous,
sedimentary and metamorphic rocks, more than 95%
of the rocks are of igneous in origin.
The Earth’s Crust
(…Contd)

 The crust is not a layer of uniform thickness. It is
thinnest beneath the oceans and thickest under the
continents.
 These rock masses are extending 5 to 70 kilometres
downwards from the surface up to the Mohorovicic
discontinuity which is called as Moho.
 The crustal layer that is underlying the continents is
called as continental crust. The crustal layer found
under the oceans is called as oceanic crust.
The Earth’s Crust

 The Continental crust is thicker – about 35 to 70
km.
 This layer is usually older than the oceanic crust.
 Some rocks of these may date back to 4 billion
years.
The Continental Crust
(…Contd)

 The Continental crust is composed mostly of rocks
containing minerals such as feldspar and quartz,
which form less-dense, lighter-coloured, granitic
masses.
 This less dense mass floats on the mantle, in the
state of acquiring a balance, due to the concept of
Isostasy.
The Continental Crust

 The oceanic crust is about 7 to 10 km in
thickness.
 The oceanic crust is made up of rocks containing
minerals that are high in iron and magnesium.
 These masses are much dense and dark-coloured.
The Oceanic Crust
(…Contd)

 They are of basaltic in composition.
 The basaltic rocks of the oceanic crust are
relatively young.
 None of them is older than 200 million years.
The Oceanic Crust

 The soft layer which is underlying beneath the
oceanic and continental crust, is known as the
Asthenosphere.
 The Asthenosphere is composed of partially molten
material that is heated unevenly by the radioactive
decay of mass present below which include both the
mantle and the hot core.
The Asthenosphere
(…Contd)

 The earth’s temperature increases with reference to
depth.
 At about 100 km depth, inside the lithosphere, the
temperature reaches to a maximum of 1,400 degree
celcius.
 This temperature condition is enough to melt the
materials present in most of the rocks.
The Asthenosphere
(…Contd)

 As a result, the rock masses melted and are able to
flow slowly.
 On this zone, the rigid plates lying above are found to
be floating or drifting.
 The drifting of plates away from each other leads to
spreading of ocean floors and creation of rift valleys.
 In some areas, there is a collision and subduction of
merging plates.
The Asthenosphere

 The Age of the ocean floor is about 200 million
years, although age of the earth is estimated about
4600 million years.
 Such a geologically younger age is found due to
ocean floor spreading.
 The Mid oceanic ridges are continuously filled by
magma which cools and helps in expanding or
spreading of ocean floor.
The Age of Ocean Floor
(…Contd)

 Mantle convection current, on the other side, help
in dragging the ocean floor.
 As result of this, new crust forms along the ridges
and the old ones sink along the trenches.
The Age of Ocean Floor

 The continental margins are the submerged
continental shelf and slope zones, forming the
outer edge of a major landmass.
 The Continental margins represent the shallowest
parts of the oceans.
The Continental Margins
(…Contd)

 The differenc between the composition of the
continental and oceanic crusts are fundamental to
continental margin development.
 The difference in densities and isostatic balance
between the continental and oceanic blocks are
reflected on the continental slope.
(…Contd)

 The shallowest part of a continental margin,
extending seaward from the shore, is the
continental shelf.
 Seaward of the continental margin is the Ocean
Basin.

 It was Alfred Luthar Wegener (1880-1930), a German
Meteorologist, who proposed the theory of
continental drift.
 It was cited that the Earth’s continents had once been
united as a single landmass that broke apart and
moved as continental drift.
The Continental drift hypothesis
(…Contd)

 This super-continent is called as Pangaea, a Greek
word that means “all the earth”.
 This concept also suggested that the Pangaea began
to break apart about 200 million years ago.
 Since that time, the continents have continued to
slowly move and came to their present positions.
(…Contd)

 Wegener was the first to put forward his ideas.
 He assembled the continents just like a puzzle like
fit of continental coastlines on either side of the
Atlantic Ocean.
 For Wegener, these gigantic puzzle pieces which just
the beginning.
(…Contd)

 The hypothesis was supported by stratigraphy,
climatic conditions, geology, structure and fossil
data.
 The theory of continental drift was never accepted
by the scientific community and concept of plate
tectonics was introduced.

 The world’s widely accepted theory is the theory of
plate tectonics.
 It was formulated in the late 1960s.
 According to this concept, the Earth's crust and upper
mantle consist of moving plates lying above a weaker
semi-plastic layer called as the asthenosphere.
The Concept of Plate Tectonics
(…Contd)

 The theory revolutionized the geological sciences in
the 1960s by combining the earlier idea of
continental drift and the new concept of seafloor
spreading into a coherent whole.
 It explains how tectonic plates move and shape
Earth’s surface.
(…Contd)

 The lithosphere is made up of tectonic plates.
 These tectonic plates are able to move because the
earth’s lithosphere has a higher strength than the
underlying asthenosphere.
 Plate tectonics provides the framework for
interpreting the history and character of the
(…Contd)

 The plate movements and the basic difference in the
density of oceanic and continental crustal units
initiate the structural pattern of continental margins
and result in a tectonic classification of coastlines as
active (Pacific, leading edge) or passive (Atlantic,
trailing edge) margins, each of which have certain
fundamental characteristics.

 The cause of plate movement is not accessible to
anybody’s direct observation.
 The various features of plate movement, and the
increased heat flow along the mid-oceanic ridges are
consistent with the idea that plate movement is
caused by convection in the mantle.
The Convection Currents
(…Contd)

 The driving force behind the convection is heat
generated by radioactive decay in the earth.
 The heat released by this decay (radiogenic heat) is
transferred by convection or (slow movement of
hot, plastic rock) to the surface of the earth.
(…Contd)

 Scientists think that these convection currents are
set in motion by subducting slabs.
 The rising material in the convection current spreads
out as it reaches the upper mantle and causes both
upward and sideways forces.
 These forces lift and split the lithosphere at divergent
plate boundaries.
(…Contd)

 As the plates separate, material rising from the
mantle supplies the magma that hardens to form
new ocean crust.
 The downward part of convection current occurs
where a sinking force pulls tectonic plates
downward at convergent boundaries.

 The data collected from topographic, sedimentary,
and paleomagnetic research, prompted the scientists
to propose the concept of seafloor spreading.
 Seafloor spreading is the process by which new
oceanic crust is formed at the ocean ridges.
 The crust slowly moves away from the spreading
centre, until it is subducted and recycled at deep-sea
trenches.
The Sea Floor Spreading
(…Contd)

 Deep-seated magma is also expected to intrude into
the ocean floor along a ridge and fills the gap that is
created.
 When the molten material solidifies, it becomes new
oceanic crust.
 The continuous spreading and intrusion of magma
result in the addition of new oceanic crust.
(…Contd)

 Two halves of the oceanic crust spread apart slowly,
and move apart like a conveyor belt.
 The far edges of the oceanic crust sink beneath the
continental crust.
 As it descends, water and the minerals cause the
oceanic crust to melt, forming a new magma.
 This magma rises and forms a new part in the
continental crust.

 Continental shelves vary greatly in width, averaging
60 km wide.
 The average depth of the water above continental
shelves is about 130 m.
 Beyond the continental shelves; the sea-floor drops
away quickly to depths of several kilometres, with
slopes averaging nearly 100 m/km.
Spread of Continental Margins
(…Contd)

 These sloping regions are the continental slopes.
 The gently sloping zone with the accumulation of
deposits from turbidity currents that forms at
the base of the continental slope is called as
continental rise.
(…Contd)

 A continental rise can be of several kilometres in
thickness.
 The rise gradually becomes thinner and eventually
merges with the sediments of the seafloor beyond
the continental margins.

 Earth’s tectonic plates have been named based on the
overlying landmasses and oceans.
 There are about 15 major plates existing on the
earth’s surface. They are namely
Plates and Plate Boundaries
(…Contd)

The line where the two plates meet each other is called
as a plate boundary.
Plates and Plate Boundaries

 The plates are all moving in different directions and
at different speeds. The rate of movement is from 2
cm to 10 cm per year.
 During their movements, the plates may crash
together, pull apart, or sideswipe each other.
Types of Plate Movement
(…Contd)

 Most of the movement occurs along narrow zones
between plates where the results of plate-tectonic
forces are most evident.
 There are basically three different types of plate
boundaries exiting in the world as divergent,
convergent, and transform boundaries.
Types of Plate Movement

 Divergent boundaries are regions where two
tectonic plates are moving apart and are called
divergent boundaries.
 Most divergent boundaries are found along the
seafloor, where they form mid ocean ridges.
Divergent Plate Boundaries
(…Contd)

 The actual plate boundary is located in a fault-
bounded valley called as an oceanic rift, which forms
along a ridge.
 Almost all divergent boundaries are characterised by
the high heat flow, active volcanism, and continuing
earthquakes.
(…Contd)

 Although most divergent boundaries form ridges on
the ocean floor, some divergent boundaries exist on
the continents.
 When continental crust begins to separate, the
stretched crust forms a long, narrow depression
called a rift valley.
(…Contd)

 The rift valley which is under formation currently is
in East Africa.
 The rifting might eventually lead to the formation of
a new ocean basin.
 It is the divergent plate boundaries that split the
continents move them apart and create new

 The oceanic crusts that are generated at the
divergent plate boundaries are composed of form
kinds of geological layers.
 They are the deep marine sediments, pillow basalts,
sheeted dykes and the gabbroic layer.
Features of Divergent Plate Boundaries
(…Contd)

 It is also expected to generate basaltic magmatism
due to decompression melting of the mantle.
 Seawater is heated along with these sediments
which may create mud volcanoes.
Features of Divergent Plate Boundaries

 Convergent boundaries occur where two plates slide
towards each other to form either a subduction zone
or continental collision.
 At convergent boundaries, two tectonic plates are
expected to move toward each other.
Convergent Plate Boundaries
(…Contd)

 When two plates collide, the denser plate eventually
descends below the other, “less-dense plate”. This
process is called as subduction.
 This convergence can create the most remarkable
structural and topographic features on the earth’s
surface.
 Deep ocean trenches are the typical examples.
Convergent Plate Boundaries

 The subduction zones of these convergent
boundaries possess an outer swell, a trench fore
arc, magmatic arc and a back arc basin.
 Continental collision is characterised by the
horizontal compression which is reflected in folding
and thrust faulting.
Features of Convergent Boundaries
(…Contd)

 Magma is also generated at the subduction zones.
 Andesitic and silicic magmas processes also create
low temperature high pressure facies near the
trenches.
 Continents grow larger due to addition of crusts at
some parts.
Features of Convergent Boundaries

 There are three types of convergent boundaries, classified
according to the type of crust involved, and on the
differences in density of the crustal material. They are
1. The convergence of two oceanic plates (O-O Type)
2. The convergence of a continental plate and an oceanic
plate (C-O Type) and
3. The convergence of two continental plates (C-C Type).
Types of Convergent Plate Boundaries

 In the oceanic-oceanic convergent boundary, a
subduction zone is formed when one oceanic
plate, which is denser as a result of cooling,
descends below another oceanic plate.
 The process of subduction creates a deep-sea
trench.
Oceanic-Oceanic (O-O Type)
(…Contd)

 Some examples of trenches and island arcs are the
Marianas Trench and Marianas Islands in the West
Pacific Ocean and the Aleutian Trench and Aleutian
Islands in the North Pacific Ocean.
Oceanic-Oceanic (O-O Type)

 The Subduction zones are also found where an
oceanic plate converges with a continental plate.
 The denser oceanic plate is subducted below less
dense continental plate.
 Oceanic continental convergence also produces a
trench and volcanic arc.
Oceanic-Continental (O-C Type)
(…Contd)

 However, instead of forming an arc of volcanic
islands, oceanic-continental convergence results in a
chain of volcanoes along the edge of the continental
plate.
 The result of this type of subduction is a mountain
range with many volcanoes.
(…Contd)

 The Peru-Chile Trench and the Andes mountain
range, which are located along the western coast
of South America, formed in this way.

 This type of convergent boundary forms, when two
continental plates collide.
 Continental-continental boundaries form long after
an oceanic plate has converged with a continental
plate.
 Continents are often carried along attached to
oceanic crust.
Continental-Continental (C-C Type)
(…Contd)

 Over time, an oceanic plate can be completely
subducted, dragging an attached continent behind it
toward the subduction zone.
 As a result of its denser composition, oceanic crust
descends beneath the continental crust at the
subduction zone.
(…Contd)

 The Himalayan mountain range, dramatically
demonstrates one of the most visible and spectacular
consequences of plate tectonics.
 When two continents meet head-on, neither is
subducted because the continental rocks are relatively
light and, like two colliding icebergs, resist downward
motion.
 Instead, the crust tends to buckle and be pushed
upward or sideways.

 A region where two plates slide horizontally past
each other is a transform boundary.
 Transform boundaries are characterized by long
faults, sometimes hundreds of kilometres in length,
and by shallow earthquakes.
 Transform boundaries were named based on the
way the Earth’s crust changed, or transformed.
Transform Boundaries
(…Contd)

 Crust is only deformed or fractured somewhat along
transform boundaries.
 Sometimes transform boundaries occur on
continents.
 The San Andreas Fault is probably the best-known
example.
(…Contd)

 The San Andreas Fault system is part of a transform
boundary that separates south western California
from the rest of the state.
 Movements along this transform boundary create
situations that are responsible for most of the
earthquakes that strike California every year.

 Volcanoes are fuelled by magma. Magma is a
slushy mixture of molten rock, mineral crystals,
and gases.
 Once magma forms, it rises toward Earth’s
surface because it is less dense than the
surrounding mantle and crust.
Volcanism
(…Contd)

 Magma that reaches Earth’s surface is called the
lava.
 Volcanism describes all the processes associated
with the discharge of magma, hot fluids, and gases.
Volcanism
(…Contd)

 Approximately 20 volcanoes are erupting. In a given
year, volcanoes will erupt in about 60 different
places on Earth.
 The distribution of volcanoes on Earth’s surface is
not random.
 A map of active volcanoes reveals striking patterns
on Earth’s surface. Most volcanoes form as plate
boundaries.
Volcanism
(…Contd)

 The majority form at convergent and divergent
boundaries.
 Along these margins, magma rises toward Earth’s
surface.
 Only about 5 percent of magma erupts far from plate
boundaries.
Volcanism

 Tectonic plates collide at convergent boundaries,
which can form subduction zones - places where
slabs of oceanic crust descend into the mantle.
 An oceanic plate descends below another plate into
the mantle magma forms.
Convergent Volcanism
(…Contd)

 As it rises, the magma mixes with rock, minerals,
and sediment from the overlying plate.
 Most volcanoes located on land result from oceanic-
continental subduction.
 These volcanoes are characterized by explosive
eruptions.
Convergent Volcanism

 The volcanoes associated with convergent plate
boundaries form two major belts.
 The larger belt, the Circum-Pacific Belt, is also called
the Pacific Ring of Fire.
 The name Circum-Pacific gives a hint about the
location of the belt.
Major Volcanic Belts
(…Contd)

 The outline of the belt corresponds to the outline
of the Pacific Plate.
 The belt stretches along the western coasts of
North and South America, across the Aleutian
Islands, and down the eastern coast of Asia.
(…Contd)

 The smaller belt, which is called the Mediterranean
Belt, includes Mount Etna and Mount Vesuvius, two
volcanoes in Italy.
 Its general outlines correspond to the boundaries
between the Eurasian, African, and Arabian plates.

 At divergent plate boundaries tectonic plates move
apart and new ocean floor is produced as magma
rises to fill the gap.
 At ocean ridges, this lava takes the form of giant
pillows and is called pillow lava.
Divergent Volcanism
(…Contd)

 Unlike the explosive volcanoes, volcanism at
divergent boundaries tends to be non-explosive,
with effusions of large amounts of lava.
 About two-thirds of Earth’s volcanism occurs
underwater along divergent boundaries at ocean
ridges.
Divergent Volcanism

 Some volcanoes form far from plate boundaries
over hot spots.
 Scientists hypothesize that hot spots are unusually
hot regions of Earth’s mantle where high-
temperature plumes of magma rise to the surface.
Hot Spots
(…Contd)

 Hot spot volcanoes of Earth’s best-known volcanoes
formed as a result of hot spots under the ocean.
 For example, the Hawaiian Islands are located over a
plate of magma.
 As the rising magma melted through the crust, it
formed volcanoes.
Hot Spots
(…Contd)

 The hot spot formed by the magma plume remained
stationary while the Pacific Plate slowly moved
northwest.
 Over time, the hot spot has left a trail of volcanic islands
on the floor of the Pacific Ocean.
 The volcanoes on the oldest Hawaiian island, Kauai, are
inactive because the island no longer sits above the
stationary hot spot.
Hot Spots
(…Contd)

 Even older volcanoes to the northwest are no
longer above sea level.
 The world’s most active volcano, Kilauea, on the Big
Island of Hawaii, is currently located over the hot
spot.
Hot Spots
(…Contd)

 Another volcano, Loihi, is forming on the seafloor
southeast of the Big Island of Hawaii and might
eventually rise above the ocean surface to form a
new island.
Hot Spots
(…Contd)

 Hot spots and plate motion creates chains of
volcanoes that form over stationary hot spots
provide information about plate motions.
 The rate and direction of plate motion can be
calculated from the positions of these volcanoes.
Hot Spots
(…Contd)

 The Hawaiian Islands are at one end of the Hawaiian-
Emperor volcanic chain.
 The oldest seamount, Meiji, is at the other end of the
chain and is about 80 million years old, which indicates
that this hot spot has existed for at least that many years.
 The bend in the chain at Daikakuji Seamount records a
change in the direction of the Pacific Plate that occurred in
43 millions years.
Hot Spots

 When hot spots occur beneath continental crust,
they can lead to the formation of flood basalts.
 Flood basalts form when lava flows out of long
cracks in Earth’s crust. These cracks are called
fissures.
Flood Basalts
(…Contd)

 Over hundreds or even thousands of years, these
fissure eruptions can form flat plains called
plateaus.
 Columbia River Basalts volume of basalt erupted by
fissure eruptions is a good example.
Flood Basalts
(…Contd)

 However, the Columbia River Basalts are small in
comparison to the Deccan Traps of India.
 The volume of basalt in the Deccan Traps is
estimated to be about 5,12,000 cubic kilometers.
Flood Basalts

 Most earthquakes are caused by movements along
faults that some slippage along faults is relatively
smooth but other movements, modelled by the
sandpaper-covered blocks, show that irregular
surfaces in rocks can snag and lock.
Earthquake
(…Contd)

 As stress continues to build in these rocks, they
reach their elastic limit; undergo plastic deformation,
then break, and the vibrations from the energy that is
released produce an earthquake.
Earthquake

 Another type of earthquake hazard is a tsunami. It
is a large seismic ocean wave generated by vertical
motions of the seafloor during an earthquake.
 These motions displace the entire column of water
overlying the fault, creating bulges and depressions
in the water.
Tsunami
(…Contd)

 The disturbance then spreads out from the
epicentre in the form of extremely long waves.
 While these waves are in the open ocean, their
height is generally less than 1 m.
Tsunami
(…Contd)

 When the waves enter shallow water, however, they
can form huge breakers with heights occasionally
exceeding 30 m.
 These enormous wave heights, together with open-
ocean speeds between 500 and 800 km/h, make
tsunamis dangerous threats to coastal areas both
near to and far from the earthquake’s epicentre.
Tsunami
(…Contd)

 The Indian Ocean tsunami of 26th
December 2004,
originated with a maximum magnitude of 9.0.
 The 30-m-tall tsunami wave radiated across the
Indian Ocean and struck the coasts of Indonesia, Sri
Lanka, India, Thailand, Somalia, and several other
nations.
Tsunami

 The extensive work on the bathymetry of the ocean
basins carried out using echo-sounding devices, has
revealed many morphological features that were
previously unknown, such as oceanic ridges, abyssal
plains (and hills), seamounts, trenches, and
Conclusion
(…Contd)

 A continental margin that has a broad continental
shelf, a gentle continental slope, and a pronounced
continental rise is known as a passive continental
margin.
 This type of margin experiences little, if any,
volcanic or earthquake activity.
 The build-up of sediment is the primary activity
affecting a passive margin.
Conclusion
(…Contd)

 A continental margin that has a very narrow
continental shelf and a narrow and steep continental
slope is known as an active continental margin.
 Earthquakes and volcanic activities are prominent
around the active continental margins.
 Delineating their boundaries and monitoring their
conditions are necessary in order to forecast their
dynamic impacts.
Conclusion

The Continental Margins

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The Continental Margins