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.
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
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.
A cross section of
Earth showing the
These layers can be
described by their
chemical and physical
Layered Earth - Chemical Properties
Chemical Properties of Earth’s Layers
Layer Chemical Properties
Continental Crust Composed primarily of granite (Silicate)
density = 2.7 g/cm3
Oceanic Crust Composed primarily of basalt (MgO; CaO)
density = 2.9 g/cm3
Mantle Composed of silicon, oxygen, iron and magnesium
density = 4.5 g/cm3
Core 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.
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.
Physical Properties of Earth’s Layers
Layer Physical Properties
Lithosphere The cool, rigid outer layer
Asthenosphere Hot, partially melted layer which flows slowly
Mantle Denser and more slowly flowing than the
Outer Core Dense, viscous liquid layer, extremely hot
Inner Core Solid, very dense and extremely hot
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
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
describes the way the two
types of crust are
supported on the mantle.
Isostatic Equilibrium - an example
This figure shows how
the continental crust
adjusts itself to maintain
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
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
So how do we know all that?
• We place seismographs around the world.
• We wait for an earthquake and see where
and how long it takes for the pressure to
reach various areas.
• There are two types of pressure waves.
• S-waves cannot pass
through liquid. Only
• P-waves can pass
through both liquid
Evidence of Earth’s Layers
Earthquake seismic 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.
Wegener’s Theory of
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 Pangaea. His
evidence was based on
similarities of fossils, and large
areas of rock, on either side of
Wegener’s Theory of Continental
• 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).
• 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 (Hugo Benioff).
• Study of seismic waves.
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 Mid-
Atlantic Ridge conforms to
the shape of the continents.
The inset shows the center
of the Mid-Atlantic Ridge.
The Theory of Plate Tectonics
The ideas of continental drift and seafloor spreading were tied together in
the theory of plate tectonics by John Tuzo Wilson. Main points of the
• 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 mid-ocean 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.
Please answer briefly.
1. What are the layers of earth as defined by
composition and density?
2. What are they when defined by physical
3. What are the different types of plate boundaries?
4. What do the 2 types of seismic waves help us
5. Which ocean has a spreading ocean ridge?
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.
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.
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.
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. (Only crust material)
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. (Crust
and some mantle)
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 mid-
ocean 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!
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!
splitting and rifting.
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
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.
Fate of Oceans.
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.
Confirmation 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 Pangaea. In this position
the paleomagnetic fields in rocks on the
different continents all point to a single
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.
Hot Spots, continued
Islands and the
example of a
produced by a
how the age of
changes in a
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).
Confirmation of Plate Tectonics
Atolls Ring of coral reefs. Proof of sinking. Corals go very deep.
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.