Dr.

Md Masidul Haque 31/12/24

The vast world ocean

Marine Geology vMain ocean basins
• Pacific Ocean - the largest (181 X 106 km2 ) and deepest (11 Km), the
mean depth is 3700 m. Is surrounded by linear mountain chains,
trenches and island arc systems. Contains large numbers of volcanic
island and extensive marginal basins. It touches the west coast
border of the Americas along with East Asia and Australia.

• Atlantic Ocean – about half the size of the Pacific Ocean (94 x 106 km),
Morphology of the Ocean about 500 km wide, marked by north – south extension and link two
polar oceans. It serves as an avenue for cold waters produced in the
polar oceans. Slightly shallow with a mean of 400 m depth to a
maximum 4 km.

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The vast world ocean The oceans of Earth

vFour main ocean basins
• Indian Ocean – The Indian Ocean is the third largest ocean surrounding
a densely populated region. It contains an additional 20% of the water
on Earth’s surface.
It borders India in the North, East Africa, Australia, and the Southern
Ocean. Because of the higher water temperature, it has limited marine
life.

• Antarctic ocean – is 20 million square kilometres in size. This means

that it covers about 4% of the earths surface. It extends out to 60
degrees South latitude.
• Arctic Ocean – about 7 percent the size of the Pacific

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Mapping the ocean floor Topography

v Bathymetry – measurement of ocean depths and the shape
or topography of the ocean floor • Is the study of Earth's surface shape and features.
v Echo sounder (also referred to as sonar) • Ocean topography is the study of the ocean floor and the features of
• Invented in the 1920s which.
• Primary instrument for measuring depth • The ocean floor divided into Three major regions:
• Reflects sound from ocean floor

Ocean Floor

Continental Margin Ocean Basin Mid Ocean Ridge

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Three major topographic units of the Ocean floor

1. Continental margins:
continental margin lie between continents and ocean basins and include all the
features between them. Represent 20% of ocean area.
Continental margin
• Passive margins or Atlantic type margin; consists of:
continental shelf, continental slope & continental rise
Active or Pacific Passive or Atlantic
• Active margins or Pacific type margin; consists of:
shelf, slope & deep trench
Chilean type Island –arc (marina) type

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2. Ocean basin floor

v Abyssal plains Main features of Ocean Floor
• Likely the most level places on Earth
• Sites of thick accumulations of sediment
• Found in all oceans
v Seamounts and guyots
• Isolated volcanic peaks
• Many form near oceanic ridges

3. Mid-ocean ridge
• Mid-ocean ridge Characterized by:
• An elevated position
• Extensive faulting
• Numerous volcanic structures that have developed on
newly formed crust

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Major topographic divisions of the Features of the Sea Floor

North Atlantic Ocean • Passive continental margins have a continental shelf,
continental slope, and continental rise descending to the
abyssal plain
• Active continental margins have continental shelves and
slopes, but the slope extends down into a deep oceanic trench
• A mid-oceanic ridge system encircles the globe, typically
running down the center of oceans
• Numerous conical seamounts rise from the deep ocean floor

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I - Continental margins
v Passive (Atlantic type/ aseismic) continental margins
• Found along most coastal areas that surround the Atlantic Ocean
• Not associated with plate boundaries
• Experience little volcanism and
• Few earthquakes
• Wider sandy beaches

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Features of a passive continental margin Passive continental margins;

Comprises three features:

Continental
margin

Continental Continental Continental

shelf slope Rise

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Passive continental margins Continental shelf

• Features comprising a passive continental margin • The continental shelf is the extended perimeter of each continent and
1. Continental shelf associated coastal plain, and was part of the continent during the glacial
• Flooded extension of the continent
• Contains oil and important mineral deposits periods, but is undersea during interglacial periods.
• Continental shelves – • The shelf break area marks the increase of slope at the outer margin
Shelf break
•gently dipping (0.1°)
seaward-sloping
• shallow submarine
platforms at the edges of
continents
• Range in width from a
few km to >500 km
- Typically covered with young
sediments

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Passive continental margins

•Features comprising a passive continental margin Continental slope
2. Continental slope • The descending slope which connects the sea floor to the
•Marks the seaward edge of the continental shelf Continental shelf. This is still considered to be part of the Continent.
o Marks the seaward edge of the continental shelf
•Relatively steep structure
o Relatively steep structure
•Submarine canyons and turbidity currents
o Submarine canyons and turbidity currents Found in
•relatively steep slopes (typically 4-5°, but locally much regions where trenches are absent
steeper) that extend down from the edge of the o Continental slope merges into a more gradual incline –
continental shelf to the abyssal plain the continental rise
o Thick accumulation of sediment

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Continental margins Continental rise

• A gentle slope with a generally smooth surface, built up by the shedding
of sediments from the continental block, and located between the
3. Continental rise continental slope and the abyssal plain.
• Found in regions where trenches are absent
• Continental slope merges into a more gradual incline –
the continental rise
• Thick accumulation of sediment

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Turbidity currents
Active continental margins
• Continental slope descends abruptly into a deep-ocean trench
• Located primarily around the Pacific Ocean
• Accumulations of deformed sediment and scraps of ocean
crust form accretionary wedges
• Some subduction zones have little or no accumulation of
sediments (narrow beaches)

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An active continental margin Active continental margins

v Deep-ocean trenches
• Long, relatively narrow features
• Deepest parts of ocean
• Most are located in the Pacific Ocean
• Sites where moving lithospheric plates plunge into the
mantle
• Associated with volcanic activity
• Volcanic islands arcs (Japan)
• Continental volcanic arcs (Andes, Cascades mts)

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Trench Mariana Trench

• The oceanic trenches are hemispheric-scale (one
hemisphere to another) long but narrow • Is the deepest part
topographic depressions of the sea floor. They are of the world's
also the deepest parts of the ocean floor. oceans, and the
deepest location on
the surface of the
Earth's crust. It has
a maximum depth
of 10,911meters, or
11 kilometers.

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II - Ocean basin floor Ocean basin floor

v Abyssal plains vSeamounts and guyots
• Likely the most level places on Earth vA seamount is a mountain rising from the ocean seafloor that does
• Sites of thick accumulations of sediment not reach to the water's surface (sea level), and thus is not an island.
• Abyssal plains are flat or very gently sloping areas of the deep ocean These are typically formed from extinct volcanoes, that rise abruptly.
basin floor. Found in all oceans • May emerge as an island
• They generally lie between the foot of a continental rise and a mid- vMay sink and form flat-topped seamounts called guyots or
oceanic ridge tablemounts
• Isolated volcanic peaks
• Many form near
• oceanic ridges

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Guyot III - Mid-ocean ridge

• An elevated position
• A guyot also known as a tablemount, is a flat-topped seamount. Their
flatness is due to erosion by waves, winds, and atmospheric • Extensive faulting
processes. • Numerous volcanic structures that have developed on newly formed
crust
• ridge system is the longest topographic feature on Earth’s surface
• Over 70,000 kilometers (43,000 miles) in length
• Twenty-three percent of Earth’s surface
• Winds through all major oceans
• Along the axis of some segments are deep down faulted structures
called rift valleys

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Mid-Ocean ridge Mid-ocean ridge

• A mid-ocean ridge is an underwater mountain range, typically • Consist of layer upon layer of basaltic rocks that have been faulted
having a valley known as a rift running along its spine, formed by
plate tectonics. It is usually an oceanic spreading center, which and uplifted
is responsible for seafloor spreading.
• Mid-Atlantic Ridge has been studied more thoroughly than any
other ridge system
• A long mountain range on the ocean floor, extending almost
continuously through the North and South Atlantic Oceans, the
Indian Ocean, and the South Pacific Ocean.

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Mid-ocean ridge
• A deep rift valley is located at its center, from which magma flows and
forms new oceanic crust.

• As the magma cools and hardens it becomes part of the mountain

range.
• The mid-ocean ridge is approximately 1,500 km (930 mi) wide, 1 to 3
km (0.62 to 1.86 mi) high, and over 84,000 km (52, 080 mi) long.

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Sea Mount Oceanic Island

• A seamount is a mountain rising from the ocean seafloor that does • One type of oceanic island is found in a volcanic island arc. These
not reach to the water's surface (sea level), and thus is not an island. islands arise from volcanoes where the subduction of one plate under
These are typically formed from extinct volcanoes, that rise abruptly. another is occurring.

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Atoll Volcanic fissure

• An atoll is an island of coral that encircles a lagoon partially or • A linear volcanic vent through which lava erupts, usually without any
completely. explosive activity. The vent is usually a few meters wide and may be
many kilometers long

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Hydrothermal Vent
• A hydrothermal vent is a fissure in a
planet's surface from which
geothermal heated water issues.
Hydrothermal vents are commonly
found near volcanically active places,
areas where tectonic plates are
moving apart, ocean basins, and
hotspots.

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Economic Significance of Continental Margin Economic Significance of Continental Margin

The continental shelf is the most economically valuable part of the ocean. It Such resources include fishing grounds, oil and gas accumulations, sand,
often is the most productive portion of the continental margin, as well as the gravel, and some heavy minerals in the shallower areas of the margin. Metallic
most studied portion, due to its relatively shallow, accessible depths. minerals resources are thought to also be associated with certain active margins,
and of great value.
Due to the rise of offshore drilling, mining and the limitations of fisheries off
the continental shelf, the United Nations Convention on "Law of the Sea" was
established. The edge of the continental margin is one criterion for the boundary
of the internationally recognized claims to underwater resources by countries in
the definition of the "continental shelf" by the United Nations Convention on
the Law of the Sea (although in the UN definition the "legal continental shelf"
may extend beyond the geomorphological continental shelf and vice versa).

What's the difference between an active and passive continental margin?

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Relevant Terminologies:-
Seafloor Spreading
ü Accretionary Wedge
ü Crustal Accretion Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is
ü Driving Forces: Slab Pull, Ridge Push formed through volcanic activity and then gradually moves away from the ridge.
ü Earthquake
ü Geohazards
ü Island Arc Volcanism, Volcanic Arcs
ü Magmatism at Convergent Plate Boundaries
ü Morphology Across Convergent Plate Boundaries
ü Ocean Margin Systems
ü Orogeny
ü Seamounts
ü Seismogenic Zone
ü Subduction
ü Subduction Erosion
ü Wadati-Benioff Zone
ü Wilson Cycle-Marine Geosciences

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Our Earth is a warm planet sailing through cold space. Understanding of Seafloor Spreading
Much of the rocky interior (the mantle) of our planet is hot
Before understanding this activity (seafloor spreading), we should be familiar with:
enough to flow, like a candy bar kept too long in one’s
pocket. The surface of the Earth, however, is chilled by the ü Types of boundaries between lithospheric plates;
cold of space, and so the familiar rocks of the Earth’s ü Features of the ocean floor;
surface are hard and brittle. The cold outer layer of our ü The concept of sea-floor spreading; and
planet, which holds together as a rigid shell, is not made of ü The nature of the Earth's magnetic field and the fact that it has reversed its polarity many times in
one solid piece. Instead this shell is broken into many the past.
separate pieces, or tectonic plates, that slide around atop the
mobile interior.
The Earth's layers
The tectonic plates are in motion. They are driven by the flowing mantle below and their motions are
The Earth is a layered planet consisting of crust, mantle and core (Fig.).
controlled by a complex puzzle of plate collisions around the globe. There are three types of plate-plate
interactions based upon relative motion: convergent, where plates collide, divergent, where plates The outer 100 km or so is a rigid layer called the lithosphere, which is made up of the crust and
separate, and transform motion, where plates simply slide past each other. Seafloor Spreading is the uppermost mantle. The lithosphere is broken into a number of large and small plates that move over
usual process at work at divergent plate boundaries, leading to the creation of new ocean floor. As two the asthenosphere, a plastic layer in the upper mantle.
tectonic plates slowly separate, molten material rises up from within the mantle to fill the opening. In Earthquakes and volcanoes are concentrated at the boundaries between lithospheric plates. It is thought
this way the rugged volcanic landscape of a mid-ocean ridge is created along the plate boundary. that plate movement is caused by convection currents in the mantle (Fig.), although the exact
mechanism is not known. Lithosphere plates are moving at rates of a few cm per year.

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Types of Plate Boundaries The Ocean Floor

There are three types of boundaries between lithospheric plates (Fig.): A map of the ocean floor shows a variety of topographic features: flat plains, long mountain chains, and
Convergent boundary — Plates converge, or come together. If a plate of oceanic lithosphere collides deep trenches.
with thicker and less dense continental lithosphere, the denser oceanic plate will dive beneath the Mid-ocean ridges are part of chain of mountains some 84,000 km long. The Mid-Atlantic Ridge is the
continent in a subduction zone. longest mountain chain on Earth.
Divergent boundary — two plates diverge, or move apart and new crust or lithosphere is formed. These ridges are spreading centers or divergent plate boundaries where the upwelling of magma from
Transform fault boundary — plates slide past one another with no creation or destruction of the mantle creates new ocean floor.
lithosphere.
Deep-sea trenches
The deep-ocean trenches are long, narrow basins which extend 8-11 km below sea level. Trenches
develop adjacent to subduction zones, where oceanic lithosphere slides back into the mantle.

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Continental drift
Continental drift is the theory that the Earth's continents have moved over geologic
time relative to each other, thus appearing to have "drifted" across the ocean bed.
The idea that continents move is an old one; Alfred Wegener, a German meteorologist,
proposed the hypothesis of continental drift in the early 1900's. Wegener used
several lines of evidence to support his idea that the continents were once joined
together in a supercontintent called Pangaea and have since moved away from one
another:
(1) the similarity in shape of the continents, as if they once fit together like the
pieces of a jigsaw puzzle;
(2) the presence of fossils such as Glossopteris, a fossil fern whose spores could
not cross wide oceans, on the now widely-separated continents of Africa,
Australia, and India;
(3) the presence of glacial deposits on continents now found near the equator; and
(4) the similarity of rock sequences on different continents.

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Sea-floor spreading
In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in
which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. On each
side of the ridge, sea floor moves from the ridge towards the deep-sea trenches, where it is subducted
and recycled back into the mantle.
Ocean floor moves like a conveyor belt carrying continents with it.
Evidence for Sea-Floor Spreading New ocean floor forms along cracks in the ocean crust as molten material erupts from the mantle
Several types of evidence supported Hess’s theory of sea-floor spreading: eruptions of molten spreading out and pushing older rocks to the sides of the crack. New ocean floor is continually added
material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves. This by the process of sea-floor spreading.
evidence led scientists to look again at Wegener’s hypothesis of continental drift.
Evidence from Molten Material – Rocks shaped like pillows - show that molten material has erupted
Evidence From Molten Material again and again from cracks along the mid-ocean ridge and cooled quickly
In the 1960s, scientists found evidence that new material is indeed erupting along mid-ocean ridges.
The scientists dived to the ocean floor in Alvin, a small submarine built to withstand the crushing
pressures four kilometers down in the ocean.
In a ridge’s central valley, Alvin’s crew found strange rocks shaped like pillows or like toothpaste
squeezed from a tube. Such rocks form only when molten material hardens quickly after erupting under
water. These rocks showed that molten material has erupted again and again along the mid-ocean ridge.

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Evidence From Magnetic Stripes Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized
“stripes.” These stripes hold a record of reversals in Earth’s magnetic field. The rock of the ocean floor
When scientists studied patterns in the rocks of the ocean floor, they found more support for sea-floor
contains iron. The rock began as molten material that cooled and hardened. As the rock cooled, the iron
spreading. You read earlier that Earth behaves like a giant magnet, with a north pole and a south pole.
bits inside lined up in the direction of Earth’s magnetic poles. This locked the iron bits in place, giving
Surprisingly, Earth’s magnetic poles have reversed themselves many times during Earth’s history. The
last reversal happened 780,000 years ago. If the magnetic poles suddenly reversed themselves today, the rocks a permanent “magnetic memory.”
you would find that your compass needle points south. Evidence From Drilling Samples
At present, the lines of force of the Earth's magnetic field are arranged as shown in Figure 4; the present The final proof of sea-floor spreading came from rock samples obtained by drilling into the ocean floor.
orientation of the Earth's magnetic field is referred to as normal polarity. The Glomar Challenger, a drilling ship built in 1968, gathered the samples. The Glomar
In the early 1960s, geophysicists discovered that the Earth's magnetic field periodically reverses; i.e. the Challenger sent drilling pipes through water six kilometers deep to drill holes in the ocean floor.
north magnetic pole becomes the south pole and vice versa. Hence, the Earth has experienced periods Core samples from the ocean floor show that older rocks are found farther from the ridge; youngest
of reversed polarity alternating with times (like now) of normal polarity. Although the magnetic field rocks are in the center of the ridge
reverses at these times, the physical Earth does not move or change its direction of rotation.
What causes the rock of the ocean floor to have a pattern of magnetic stripes?
Basaltic lavas contain iron-bearing minerals such as magnetite which act like compasses. That is, as
these iron-rich minerals cool below their Curie point, they become magnetized in the direction of the Ø differing amounts of molten material erupting along the mid-ocean ridges
surrounding magnetic field. Studies of ancient magnetism (paleomagnetism) recorded in rocks of Ø different ages of the rocks
different ages provide a record of when the Earth's magnetic field reversed its polarity.
Ø the reversals of the Earth’s magnetic poles
Ø underwater earthquakes

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