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The Origin of Ocean Basin: Plate Tectonics
Plate tectonics is the concept that the outer part of the earth is split
up into a set of rigid, moving plates. These plates move because of
slow convecting currents of hot rock inside the earth.
The Earth’s Structure
To understand plate tectonic theory, we need to know something
about the structure of the earth.
Lithosphere - the outer
rigid shell of the
earth’s structure.
Mantle - layer or shell
of the earth below the
lithosphere and above
the core.
Core - the central
portion of the earth. It
The outer core is
molten, while the
inner core, even
though just as hot, is a
solid because of the
increased pressure.
Summary of Important Concepts
• Earth is composed of layers. These layers have different chemical
and physical properties. The main layers are the core, mantle, and
two types of crust (oceanic crust and continental crust), and also
the lithosphere and asthenosphere.
• Elevations of different parts of the earth’s crust are controlled by
isostatic equilibrium: the concept that the oceanic crust and the
continental crust float buoyantly in the denser mantle beneath.
• Earth’s internal layers are studied by observing how earthquake
waves change as they pass through the earth.
• In plate tectonic theory the Earth’s outer rigid surface -- the
lithosphere -- is divided into moving segments called plates. These
plates move away from one another, move toward each other, or
slide side-by side past each other.
Summary of Important Concepts, continued
• The theory of plate tectonics explains many important
features of Earth’s surface, such as:
- mid-ocean ridges and the earthquakes and volcanic activity
there
- ocean trenches and the earthquake and volcanic activity
there
- the young age of the ocean floor
• One force that drives plate motion is heat-driven convection
currents in the mantle. The heat is generated by the decay of
radioactive elements within Earth.
• Many separate pieces of evidence demonstrate that plate
tectonic theory is correct.
A Layered Earth - Density
Density is a key concept for understanding the structure of Earth.
Density measures the mass per unit volume of a substance.
Density =
Mass
Volume
Density is commonly expressed as grams per cubic centimeter.
Water has a density of 1 g/cm3
Continental crust has a density of 2.7 g/cm3
Oceanic crust has a density of 2.9 g/cm3
The mantle has a density of 3.3 to 4.5 g/cm3
The fact that the mantle is denser than either type of crust is
important: The less dense oceanic and continental crust float
buoyantly in the mantle. This is the concept of isostatic
equilibrium (more on this is explained below).
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Layered Earth
A cross section of
Earth showing the
internal layers. These
layers can be
described by their
chemical and physical
characteristics.
You should learn the
difference between
the core, mantle, and
crusts (oceanic &
continental), and the
differences between
the lithosphere and
the asthenosphere
(explained below).
Layered Earth - Chemical Properties
Layer
Continental Crust
Oceanic Crust
Mantle
Core
Chemical Properties of Earth’s Layers
Chemical Properties
Composed primarily of granite
density = 2.7 g/cm3
Composed primarily of basalt
density = 2.9 g/cm3
Composed of silicon, oxygen, iron and magnesium
density = 4.5 g/cm3
Composed mainly of iron
Density = 13 g/cm3
Note that Earth is density stratified, that is, each deeper
layer is denser than the layer above.
*****************************
The layers listed above are distinguished based on chemical
composition and density. Another important layered aspect of the
Earth is layers distinguished based on their physical properties,
in particular whether they are stiff and rigid, versus able to flow
slowly. (Next slide.)
Layered Earth - Physical Properties
The outer ~100 kilometers of the earth includes both the crust and
the upper part of the mantle. In this region the rock is cool, and
therefore rigid (stiff and not easily deformed). This layer is called the
lithosphere (crust plus uppermost mantle).
In contrast, below this layer for several hundred kilometers within the
mantle is a layer in which the rock is so hot that it flows slowly. This
layer is called the asthenosphere.
Layer
Lithosphere
Asthenosphere
Mantle
Outer Core
Inner Core
Physical Properties of Earth’s Layers
Physical Properties
The cool, rigid outer layer
Hot, partially melted layer which flows slowly
Denser and more slowly flowing than the
asthenosphere
Dense, viscous liquid layer, extremely hot
Solid, very dense and extremely hot
Keep these differences between the chemical layers and the
physical layers in mind in the following sections.
Layered Earth - Isostatic Equilibrium
Think for a moment about the crust. Why doesn’t it sink into the mantle?
Why do the continents stick up above the ocean surface? How are features
such as mountains supported?
The concept of buoyancy is illustrated by a ship. The ship sinks until it
displaces a volume of water equal to the weight of the ship and its contents.
Earth’s continental and
oceanic crust are
supported on the denser
underlying mantle in a
similar manner. Both
types of crust “float” in
the mantle. Instead of
buoyancy, the term
isostatic equilibrium
describes the way the
two types of crust are
supported on the
mantle.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Isostatic Equilibrium - an example
This figure shows how
the continental crust
adjusts itself to maintain
isostatic equilibrium.
A great weight, like the
formation of a glacial ice
cap, will cause the crust
to slowly sag down into
the mantle. After the ice
melts, the crust will
gradually rise back up.
Several places on earth
are presently rising
upward this way, because
Ice Age ice caps have
been melting.
Layered Earth - Internal Heat
Where does the heat within Earth’s layers come from?
Heat from within Earth keeps the asthenosphere flowing.
This allows the lithosphere to keep moving. The source of
this heat is radioactive decay, given off when the nuclei of
unstable forms of elements break apart.
This heat causes the
rock of the mantle to flow
very slowly by
convection. Hotter areas
of the mantle (shown
here in RED) are less
dense, and so rise
upward, while cooler
areas of the mantle
(BLUE) are more dense,
and sink downward.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Evidence of Earth’s Layers
What evidence supports the idea that Earth has layers?
The behavior of seismic waves generated by earthquakes give scientists
some of the best evidence about the internal layers of Earth. Earthquake
waves bend, bounce off different layers, and change speed and direction as
they pass through the earth. These changes reveal the layers you have been
learning about.
For example: S waves (above-left) cannot penetrate Earth’s liquid core, and P
waves (above-right) are bent as they pass through the liquid outer core.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Wegener’s Theory
of Continental
Drift
Alfred Wegener gathered
evidence in the early
1900’s that the
continents on either side
of the Atlantic Ocean
were once joined to form
a single large continent
he called Pangea. His
evidence was based on
similarities of fossils,
and large areas of rock,
on either side of the
Atlantic.
Animation
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Fossil Evidence
Fossil remains of the same organisms can be found
on different continents.
Lungfish
Remains of the modern-day lungfish are present in fossil
records stretching back 300 million years.
These fish have lungs that allow them to survive dry
periods by forming a “cocoon” in the mud and breathing
quietly until the next rains.
There are species of lungfish in Africa, South America,
and Australia. The distribution of lungfish reflects
their origins in Gondwanaland before continental
drift separated these continents.
Alfred Wegener’s theory of continental drift was out of favor
with the scientific community for decades. Eventually new
technology provided evidence to support his idea.
(Unfortunately this evidence did not come along until after his
death -- sorry Alfred!!)
- Radiometric dating of rocks revealed that the oceanic
crust is surprisingly young compared to the continents.
Oceanic crust is not more than about 200 million years old
anywhere.
- Echo sounders revealed the shape of the Mid-Atlantic
Ridge.
- Seismographs revealed that volcanoes and earthquakes
occur mostly in narrow belts. (See next slide.)
Earthquakes show where plate boundaries are located, providing
important evidence for movements of the earth’s plates. Notice on
this figure that earthquakes occur in narrow zones on the earth.
These areas correspond to the edges of tectonic plates. As the
plates move against each other, they make earthquakes!
Seafloor Spreading - A Key Idea
• An idea proposed by Harry Hess and Robert
Dietz in 1960 explained the development of the
seafloor at the Mid-Atlantic Ridge
• Rising convection currents in the mantle force
the sea floor apart at the ridge, causing it to
grow and spread: a process called sea floor
spreading.
• As the sea floor spreads, the continents on
either side drift apart
Thus the MidAtlantic Ridge
conforms to
the shape of
the
continents.
The inset
shows the
center of the
Mid-Atlantic
Ridge.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Apa mekanisme yang menggerakkan sfl: Teori
Covenction Cells
• Pemanasan material dalam bumi melebur
menimbulkan suatu aliran. Jika aliran
lapisan bergerak ke atas mencapai litosfer,
aliran membelok dibawah lapisan tersebut
dan selanjutnya mengalami pendinginan.
• Material dingin menjadi pekat dan menurun
ke arah pusat bumi. Selanjutnya material
terangkat kembali ke atas karena proses
pemanasan.
• Terbentuk gunung api (menembus litosfer)
• Terbentuk gerakan lateral (bergerak
dibawah litosfer)
Creating new ocean crust
Theory of Plate
Tectonics begins
to be accepted in
the 1960s
3-3
Destructive margins
Subduction zones
Constructive margins
Midocean ridges
Driving Mechanisms for Plate Motions
Sea Floor Spreading
Rising convection currents of hot rock in the mantle cause new
oceanic crust to form and spread apart at mid-ocean ridges.
Rate of Seafloor Spreading
For reasons yet unknown, the polarity of the earth’s magnetic
field reverses periodically.
These reversals are easily
identified in the magnetic
orientations of the basalt
layer of the ocean floor.
The length of time between
reversals enables
oceanographers to estimate
the rate of seafloor
spreading at about 1-4 cm
per year.
The Young Ocean Basins
Sea floor spreading and subduction together explain the
young age of the oceanic crust.
New oceanic crust forms by sea floor spreading at midocean ridges. This crust is pushed away from the ridges by
continuing sea floor spreading. Eventually the oceanic crust
subducts below another plate at an oceanic trench and gets
melted back into the mantle.
Oceanic crust is continually created at mid-ocean ridges,
and continually destroyed at oceanic trenches!
Age of the Ocean Floor
A special research ship, the
Glomar Challenger, was built
that was able to conduct
drilling experiments out in the
deep sea on the abyssal plain on
both sides of the Mid-Atlantic
Ridge.
The youngest rocks were at
the base of the ridge, and
the rocks became older as
they moved away from the
ridge, consistent with
seafloor spreading.
The Theory of Plate Tectonics
The ideas of continental drift and seafloor spreading were tied
together in the theory of plate tectonics. Main points of the theory:
• Earth’s outer layer is divided into moving lithospheric plates.
• The plates move apart at mid-ocean ridges, in a process called
sea floor spreading. Magma rising and solidifying at the
ridge forms new oceanic crust. This crust spreads away from
the ridge to make room for more magma to rise up and form
more crust. This process causes many earthquakes at midocean ridges.
• The plates come together at oceanic trenches, where one plate
dives down beneath another one and gets melted back into the
mantle: a process is called subduction. This process causes
many earthquakes and volcanoes near oceanic trenches.
• These plates move because of convection in the underlying
asthenosphere, and also the downward pull of the subducting
plate.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
The Major Lithospheric Plates
The major lithospheric plates and their direction of relative movement
are shown here. The boundaries between plates correspond to most of
the earth’s earthquakes and volcanoes.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Batas Lempeng
• Lempeng bagian litosfer
berupa kerak benua
atau laut atau sebagiansebagian.
• Batas lempeng trenches,
ridges dan faults.
• Arah gerakan; menjauh
(ridges); menyatu
(trenches); sejajar
mendahului (faults).
• Wilayah mid-ocean
ridges dan rise bergerak
lateral pada faults
dikenal dengan
transform faults
Plate Boundaries
Convergent Plate Boundaries - plates come together; further classified
as:
Oceanic crust subducting under continental crust - for example, the west
coast of South America.
Oceanic crust subducting under oceanic crust - occurring in the northern
Pacific and much of the western Pacific.
Continental crust colliding with continental crust – one example is the
Himalayas
Compression
at convergent
boundaries
produces
buckling and
shortening.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Subduction
The oceanic trench marks the location where one plate bends down
and descends into the mantle beneath the other plate. Notice the
earthquakes, and the formation of magma (and therefore volcanoes)
resulting from the melting of the subducting plate.
Subduction
As a plate travels toward a subduction
zone, it may be carrying seamounts,
islands, or even small continents. These
objects may not be subducted, but rather
scraped off and attached to the other
plate! These scraped off pieces are called
terranes.
Sometimes a large slice of the oceanic
crust or lithosphere can be scraped off
and attached to the other plate. This type
of terrane is called an ophiolite.
Formation of the Rocky Mountains
Plate Boundaries
The lithospheric plates can either move apart from one another,
toward one another, or slide side-by-side past one another.
Divergent plate boundaries – plate move apart, further classified
as:
Divergent oceanic crust – for example, the Mid-Atlantic Ridge, and
other oceanic ridges, where sea floor spreading is occurring.
Divergent continental crust - for example, the Rift Valley of East
Africa. This is an area where the continental crust is pulling apart,
and may eventually form a new ocean basin!
Extension of
divergent
boundaries causes
splitting and
rifting.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
The
Wilson
Cycle
Plate Boundaries
Transform plate boundaries - plates move side-by-side past one
another, for example, the San Andreas fault.
Side-by-side motion at transform boundaries causes shearing.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Transform Fault
Conservative margins
Transform faults
Confirmation of Plate Tectonics
Paleomagnetism: strips of alternating magnetic polarity at spreading regions.
The patterns of paleomagnetism support plate tectonic theory. The
molten rocks at spreading centers takes on the polarity of the planet
while it cools. When Earth’s polarity reverses (as it does periodically
through time), the magnetic polarity of newly formed rock reverses too.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Confirmation of Plate Tectonics
Evidence supporting the theory of
plate tectonics:
Apparent Polar wandering: plate
movement causes the apparent
position of the ancient magnetic poles
to appear to be in different places,
unless the continents are all put back
together in the configuration of
Pangea. In this position the
paleomagnetic fields in rocks on the
different continents all point to a single
magnetic pole!
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Confirmation of Plate Tectonics
Hot Spots: Surface expression of plumes of magma.
As a plate passes over a stationary hot spot (a stationary area of
rising hot mantle rock), volcanic islands are formed in sequence.
The volcanoes get progressively older in the direction of plate
movement.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Hot Spots, continued
The Hawaiian
Islands and the
Emperor
Seamounts are
a classic
example of a
line of
volcanoes
produced by a
plate moving
over a
stationary
hotspot. Notice
how the age of
the volcanoes
changes in a
regular way
along the line.
Confirmation of Plate Tectonics
Guyots were once volcanic peaks above sea level. They were eroded
flat by wave action, and then gradually sank beneath the ocean
surface as the plate below grew cooler and denser, sinking slowly
into the mantle (another example of isostatic equilibrium).
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.