Plate Tectonics
The Earth’s layers
• The layers of the Earth differ in thickness, composition,
and temperature.
• The layers are the crust, the mantle, and the inner and
outer cores.
• The crust and the top portion of the upper mantle
(which is solid rock) make up the lithosphere.
• The rest of the upper mantle is made of partly melted
rock. This layer is called the asthenosphere.
• The lower mantle is solid rock.
• All of the Earth’s liquid and solid water – oceans, lakes,
rivers, glaciers, underground water – make up the
hydrosphere.
The crust
• The first layer of
the Earth is called
the crust. It is a
thin layer of solid
rock that makes up
the outermost part
of our planet.
• The thickness of
the crust varies
from place to
place, but it is
generally very thin.
• The Earth’s crust
makes up the
surface of the Earth.
The surface includes
all the continents
and the ocean floor.
• Geological features
are physical
structures on the
Earth’s surface.
• These include hills,
rivers, lakes, deserts,
plateaus, mountains,
caves and valleys.
Mantle
• The next layer of
the Earth is called
the mantle. The
mantle is made of
a thick, solid, rocky
substance that
represents about
85% of the total
weight and mass
of the Earth.
The core
• The middle of
the Earth is
called the core.
• It is made up of
2 parts: the
outer core and
the inner core.
• The outer core
is made of liquid
metals.
The core
*The inner core is
at the very center
of the Earth.
*It is made up of
made up of
solid metals.
The Theory of Continental Drift
• German geologist Alfred Wegener noticed that
the Earth’s continents looked like pieces of a
jigsaw puzzle.
• Wegener thought that millions of years ago the
Earth’s continents were joined together.
• As time passed, some force pulled the pieces
apart.
• The continents slowly moved to the positions
they are in today.
• Scientists have found evidence that supports
Wegener’s theory.
• Mountains on the east coast of South America had the
same types of rocks as the mountains on the west coast
of Africa. The rocks were also the same age.
• Plant and animal fossils provided additional evidence.
• A fossil of a freshwater reptile called Mesosaurus has
been found in very old rocks in South America and in
Africa. Mesosaurus could not have swum the long
distance of the Atlantic Ocean’s salt water.
• Scientists have
concluded that South
America and Africa
were once part of a
supercontinent called
Pangea.
• Over millions of years
the continents drifted
apart and water filled
the spaces between
them.
Why is the ocean floor moving?
• The ocean floor between South America and
Africa is spreading at a rate of about 4
centimeters every year.
•In the 1950s, scientists
mapping the ocean floor
discovered that there
was an underwater
mountain chain in the
middle of the Atlantic
Ocean. On both sides of
the mountain chain, the
ocean floor was moving.
• Scientists have developed the theory of plate
tectonics to explain how forces deep within Earth
can cause ocean floors to spread and continents to
move.
• This theory describes the lithosphere as being
made of huge plates of solid rock.
• The Earth’s continents rest on these plates.
• The almost-melted rock of the asthenospere acts
as a slippery surface on which the plates can
move.
• In the middle of the ocean where the plates are
moving apart, magma, is pushed up from the
mantle toward the surface.
• Magma is hot, melted rock.
• The upward movement of magma causes tension on the
plates.
• This moves the ocean floor apart and separates the plates.
• As the ocean floor grows wider, the continents move
farther apart.
• This is called sea floor spreading.
Sea-floor spreading is a type of divergent boundary. A
divergent boundary is a boundary where plates are
moving away from each other.
• Divergent boundaries
can also occur on land.
• Tension makes the
crust longer and
thinner. When the
force exceeds the
strength of the crust, a
fault occurs.
• A fault is a break or
crack in the rock of the
lithosphere along
which movements
take place.
The Great Rift Valley in
Africa is a type of
divergent boundary.
The San Andreas Fault between the North
American and Pacific Plates is the most noticeable
land transform boundary.
•A transform
boundary occurs
when plates slide
past each other.
•Many powerful
earthquakes
occur along this
fault, as well as
many that are not
felt at all.
• The forces that move continents can also change
the continents’ shapes.
• A force called shearing works like a pair of
scissors, causing rocks to break along transform
boundaries.
Transform Boundaries
•Eventually this rock may
pile up and form narrow
ridges and valleys.
•Earthquakes occur along
these boundaries as strain
on the rock build up and
energy is released.
•Most transform
boundaries are located on
the ocean floor
Convergent Boundaries occur at locations where
plates collide. Depending on what types of plates
collide at these boundaries, different things occur.
If both colliding plates are continental plates, the
pressure lifts and crumples the plates, forming
mountains.
The Himalayan Mountains were formed as a result
of continental convergent boundaries.
Continental plates are light and buoyant. Oceanic
plates are more dense. When these two types of
plates collide, the oceanic plate gives way to the
continental plate, causing a subduction zone.
Volcanoes tend to erupt at subduction zones. The Ring
of Fire is a circle of volcanoes that surrounds the Pacific
Ocean. The Ring of Fire follows the boundaries of the
plates that meet around the Pacific Ocean.
If both colliding plates are oceanic plates, the older
of the ocean plates will give way to the newer plate.
This causes a trench in the ocean floor.
Faults
•
•
•
•
Forces cause different kinds of faults.
Shearing forms strike-slip faults.
Tension produces normal faults.
Compression produces reverse
faults.
Strike-Slip Fault
• The San Andreas Fault is an example of a strike-slip fault.
• This type of fault is produced due to shearing along
transform boundaries.
• The plates slide past each other without moving up or
down.
Strike – Slip Fault
Strike – Slip Fault
Strike – Slip Fault
Reverse Fault
• A reverse fault is
produced due to
compression.
• This type of fault often
occurs at convergent
boundaries.
• As the plates push
together, rock above the
fault surface moves
upward.
• The Himalayas in central
Asia are forming at a
reverse fault.
Reverse Fault
The
Himalayan
Mountains
Reverse Fault
Reverse Fault
Normal Fault
• A normal fault is produced due to tension and often occurs
at divergent boundaries.
• As the plates pull apart, rock above the fault surface moves
down.
• The Sierra Nevada Mountain Range in California was
formed at a normal fault.
Normal Fault
Sierra Nevada Mountains
Normal Fault
Normal Fault
Mountains
• Mountains form where plates converge.
• Folded Mountains are made up of rock layers
that are folded together by compression.
Great Smoky
Mountains
Folded Mountains
• Millions of years ago, the eastern edge of the North
American Plate collided with the African Plate.
•Over millions of
years, the Great
Smoky
Mountains
formed as
horizontal layers
of the rocks were
bent or folded.
Folded Mountains
• The Himalayas are another example of folded
mountains.
•They began
forming millions of
years ago when the
Indian and Asian
Plates collided.
•As the plates
continue to push
into each other, the
Himalayas grow
about 5 millimeters
taller every year.
Fault-Block Mountains
•At other times, when plates push against each other, the
rocks break and forms faults.
•Mountains made by huge, tilted blocks of rock are called
fault-block mountains.
Sierra
Nevada
Mountains
Fault-Block Mountains
•The Sierra Nevada Mountains and the Teton Range are
examples of fault-block mountains.
Teton
Range