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The Changing Face of
the Planet
Earth’s Interior
Information gathered about the Earth’s interior
comes from direct evidence from rock samples
and indirect evidence from seismic waves
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Rocks from inside the Earth, as well as those ejected during
volcanic eruptions, give geologists clues about Earth’s interior
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Earthquakes produce shock waves that travel through the Earth
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Changes occur in the movement of seismic waves due to
differences in the structure and makeup of the Earth’s interior
Scientist have determined what the interior looks like by
monitoring the path and speed of seismic waves
The Earth’s Interior
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The four layers of
the Earth are the
crust, mantle, inner
core and outer core
These layers vary
greatly in size,
composition,
temperature, and
pressure
The Earth’s Crust
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The crust is the Earth’s outermost layer
Made up of three types of solid rock
Thickness varies with type
 Oceanic crust (makes up the ocean floor)
 Less than 10 Km thick
 Consists mostly of basalt
 Continental crust (makes up the landmasses)
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Average thickness of about 32 Km
Made mostly of granite
The Earth’s Mantle
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Earth’s mantle is made up of rock that
is very hot, but solid
Divided into layers based on the physical characteristics
of those layers
Extends to a depth of about 3000 Km below the surface
Contains about 80% of the volume of the Earth and 68%
of its mass
Made mostly of the elements silicon, oxygen, iron, and
magnesium
Density increases with depth
Divisions of the Earth’s Mantle
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Mohorovicic discontinuity (Moho)- Boundary
separating the crust from the mantle
Lithosphere Topmost solid part of the Earth
 Composed of the crust and part of the upper mantle
 Broken into large sections called plates
Asthenosphere
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Located directly beneath the lithosphere
A hot weak zone, capable of gradual flow
Rock in this portion of the mantle can flow
like a thick liquid (Has plasticity)
Lower mantle
Zone of solid rock, located directly beneath
the asthenosphere, and extends to the core
The Earth’s Core
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The Earth’s core is
subdivided into two
layers, and inner
and outer core
Both layers are
composed mostly
of the metals iron
and nickel
The Outer and Inner Cores
Outer Core- Layer of the molten metal
surrounding the inner core
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High temperatures keep the iron and nickel in the
outer core molten
Inner Core- Dense ball of solid metal found
at the Earth’s center
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Intense pressure causes the particles of iron and
nickel to remain solid
The inner core rotates within the outer core
The Core & Earth’s Magnetic Field
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Scientists think that convection
currents in the liquid outer core
create Earth’s magnetic field
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The magnetic field helps
protect our planet from
the Sun’s solar winds
The Earth’s Changing Interior
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It is believed that the Earth was not originally layered, the
divisions we see today formed slowly over time
Shortly after the Earth was formed, the decay of radioactive
elements, along with heat released by colliding particles,
produced melting in the planet’s interior
Melting allowed the heavier elements (iron & nickel) to sink
toward the center, while lighter, rocky components floated
upward
Still occurs today on a smaller scale
Plate Tectonics
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Theory which links the concepts of Continental
Drift and Sea-floor Spreading to explain how the
Earth has evolved over time
Helps to explain the formation, movements,
collisions, and destruction of Earth’s outer layers
Helps people understand the geologic past and
predict its future
Evidence for Plate Tectonics
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Continental Drift
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Location of volcanoes,
earthquake belts and mountains
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Sea-floor Spreading
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Paleomagnetism
Continental Drift
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Proposed in 1910 by
Alfred Wegener
States that the continents
were once joined together
as a super-continent
called Pangaea and have
since drifted apart
Since Wegener could not
explain why the continents
would move, his theory
was originally rejected
Evidence for Continental Drift
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Shape of the continents
Similar fossil deposits on
continents thought to have
been joined
Rock formations that end
at the edges of continents
Glacial deposits (evidence
of past climates)
Distinctive rock types
Location of the world’s volcano,
earthquake belts, and mountain ranges
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Most volcanoes, earthquakes, and mountain ranges are found
along plate boundaries (places where one plate moves
relative to another)
Produced by stresses that build up along the boundaries
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As stresses become too
great, fractures form and
earthquakes occur
Fractures allow magma
from the asthenosphere to
reach the surface, forming
volcanoes
Bending and folding of the Earth’s
crust can create mountain ranges
Sea-floor Spreading
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Sea-floor spreading- Process in
which old ocean floor is pushed away
from a mid-ocean ridge by the
formation of new ocean floor
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As the ocean floor spreads, landmasses
on either side move apart
Occurs at divergent boundaries (also
called spreading centers)
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Younger rocks are found closer to the
spreading center
The further you go from the spreading
center, the older the rocks become
The same pattern of rocks are found on
both sides of the center
Paleomagnetism
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Paleomagnetism- Study of the
alignment of magnetic particles
in ancient rocks
Provides proof for sea-floor
spreading and a means of
determining how the continents
have moved
When magma cools, grains of
iron line up with the magnetic
pole (like little compasses)
Polarity reversals occur in
parallel bands on opposite sides
of the mid-ocean ridges
During the past 4 million years,
the magnetic poles have
reversed themselves 9 times
Theory of Plate Tectonics
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States that the topmost solid
part of the Earth is divided into
rigid plates that move resulting
in earthquakes, volcanoes,
mountains, and the
redistribution of landmasses
Lithospheric plates are made
of a thin layer of crust above a
thick layer of rigid mantle rock
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Usually contain both oceanic
and continental crust
Seven major plates, each
named after its surface
features
Plates move at different
speeds and in different
directions
Earth’s Tectonic Plates
Plate Boundaries
There are three basic types of plate boundaries
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Divergent- moving apart
Convergent- moving together
Transform fault- sliding past each other
Divergent Boundaries
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Plates move apart
(diverge)
Also called spreading
centers or constructive
boundaries
New rocks are formed as
older rocks are pushed
aside (Lithosphere is
created)
Examples: Mid-Atlantic
Ridge, East Pacific Rise,
and the Great Rift Valley
in Africa
Convergent Boundaries
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Occur where two
plates move towards
each other
Also called destructive
boundaries
Lithosphere is
destroyed
There are three types
Types of Convergent Boundaries
Convergent boundary where
two continental plates collide
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Forms a single larger continent
(India & Asia)
Causes the lithosphere at the
boundary to be pushed up,
forming a mountain range
Ex.: Himalayas, Urals, &
Appalachian Mtns.
Types of Convergent Boundaries
Convergent boundary where two oceanic plates
collide
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One plate subducts (goes under) the other plate
Also called a subduction zone
Forms a chain of volcanic islands on the overriding plate
and a deep sea trench where the plates meet
Ex.: The Mariana Islands and the
Mariana Trench are formed
where the Pacific Plate subducts
under the Philippine Plate
Types of Convergent Boundaries
Convergent boundary where an oceanic and a continental plate
collides
 The oceanic plate subducts under the continental plate
 Forms a chain of volcanic mountains on the continental
plate and a deep sea trench along the edge of the
continent
 Ex.: Along the west coast of
South America, the Nazca Plate
subducts under the South
American Plate, forming the
Andes Mtns. And the Peru-Chile
Trench
Transform Fault Boundaries
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Also known as strike-slip or sliding boundaries
Plates grind together as they try to slip past each other horizontally
causing stress to build up
Earthquakes occur when the stress is released
Examples:
 The San Andreas Fault in California,
is a result of the North American Plate
and the Pacific Plate trying to slide past
each other
 Transform fault boundaries connect
portions of the mid-ocean ridge system
that are moving at different rates
Why the Plates Move
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Convection currents within the asthenosphere are
thought to be the driving force behind plate movement
Convection current- the movement of material caused
by differences in temperature
Hot magma rises to the surface, pushing the older rocks
aside and driving the plates apart (occurs at divergent
boundaries)
Cooler, denser currents sink
back into the mantle, pushing
the plates together (occurs at
convergent boundaries)