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Science 10
Plate Tectonics
UNIT 1
PLATE TECTONICS
PART A
THE DYNAMIC EARTH
Table of Contents:
Description
Complete ?
Structure of the Earth
Direct and Indirect Observations
Continental Drift
The Theory of Plate Tectonics
Sea-Floor Spreading
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UNIT 1
WHAT IS THE STRUCTURE OF THE EARTH
 crust: thin, solid outer layer of the Earth
 mantle (MAN-tul): thick layer of rock below the crust
 lithosphere (Lith-us-fear): the crust and the solid part of the upper mantle
 asthenosphere (az-THEEN-us-fear): semi-liquid upper part of the mantle
 outer core: outer layer of the core, made of liquid iron and nickel
 inner core: inner layer of the core, made of very dense solid iron
 diameter: the distance across a circle or sphere through its center.)
LAYERS OF THE EARTH
When you were a child, did you ever plan to dig a hole through the earth to the
other side? What did you think the inside of the earth is like?
The inside of the earth can be compared to a hard-boiled egg.
If you cut a hard-boiled egg with the shell on, you can see that
the egg has three different layers. They are the shell, the
white and the yolk.
The earth also has three main layers. These layers are the
crust, the mantle, and the core.
Budau Family
Jacob Budau's &
Anna Verviets
Parents
Born in Prussia
THE CRUST
The crust is the solid, outer layer of the earth.
 The top of the crust is made up of loose rocks and soil. Under the rocks and
soil, the crust is rigid, solid rock. We live on the crust.
There are two types of crust.
 Ocean or oceanic crust is covered by our planet’s oceans.
 Continental crust is the land on which we live.
 On average, continental crust is less dense than oceanic crust.
The crust is thick in some places and thin in others.
 Beneath the oceans, the crust is between 5 km and 10 km thick.
 Beneath the continents the crust is between 32 km and 70 km thick.
 Compared to the whole Earth, the crust is very thin. The whole earth is
about 12,700 km in diameter.
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UNIT 1
THE MANTLE
The layer of the earth beneath the crust is the mantle.
 The mantle is about 2800 km thick.
 More than two-thirds of the mass of the earth is in the mantle.
The mantle has two main parts: the Upper Mantle and the Lower Mantle.
 The outer part of the upper mantle is solid rock.
 The inner part of the upper mantle is semi-solid, and is called the
ASTHENOSPHERE.
 The lower mantle is dense liquid.
The crust and the solid outer part of the upper mantle are collectively called
the LITHOSPHERE.
 The lithosphere is a solid layer. It is cold and brittle and can fracture during
an earthquake.
 The lithosphere is divided into pieces called tectonic plates.
Below the solid lithosphere is the semi-solid ASTHENOSPHERE.
 This part of the upper mantle is hot, semi-solid and can bend like plastic.
The tectonic plates of the lithosphere float on the asthenosphere, much like
broken ice fragments float on the surface of water.
 Because the asthenosphere is too hot to fracture, earthquakes cannot occur
in the asthenosphere. The asthenosphere does, however, bend under
pressure.
As you can see in this
diagram, the lithosphere is
made up of the crust and
outer part of the upper
mantle. Below the lithosphere
is the asthenosphere. The
lithosphere (which is broken
into tectonic plates) floats on
the asthenosphere.
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THE CORE
The two innermost layers of the Earth make up the CORE.
 The core is about 3500 km thick.
The core has two parts.
 The outer core is a liquid layer. It contains melted iron and nickel. It is
about 2200 km thick.
 The inner core is a solid layer. It contains solid iron and nickel. It is about
1300 km thick.
 The core is the densest part of the Earth.
The layers of the Earth.
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UNIT 1
WHAT IS THE STRUCTURE OF THE EARTH? – questions
Complete each statement using a word or words from the list below. Write your
answers in the spaces provided. Some words may be used more than once.
Some words may not be used at all. 10 marks
asthenosphere
inner core
oceanic
mantle
crust
oceans
outer core
lithosphere
upper
lower
continental
continents
1. Starting with the top layer, the main layers of the Earth are the ___________, the
_____________, the __________________, and the ________________.
2. The top of the ______________________ is made up of loose rocks and soil.
3. The thickest crust is found beneath the ______________________.
4. More than two-thirds of the Earth's mass is in the _____________.
5. The _________________ contains the crust and the solid outer part of the upper
mantle.
6. The semi-solid part of the upper mantle is called the ________________.
7. The layer that has melted iron and nickel is the _____________________.
8. The ____________________ is made up of solid iron and nickel.
9. The layer of the Earth between the core and crust is the ________________.
10. The densest type of crust is the __________ crust .
Match the thickness with the correct earth layer by writing the correct letter on
the line in front of the thickness of each layer. 4 marks
____ 1300 km thick
a. the outer core
____ 2800 km thick
b. the crust
____ 5 - 70 km thick
c. the mantle
____ 2200 km thick
d. the inner core
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UNIT 1
WHAT ARE DIRECT AND INDIRECT OBSERVATIONS?
Until about 100 years ago, all that was known about the structure of the Earth was
that it is spherical and has a diameter of about 12 700 km at the equator.
Scientists now know a lot more about the structure of our Earth. They have made
their discoveries through both direct and indirect observations.
DIRECT OBSERVATIONS
Direct observations are those that you can see directly. They are the most
accurate type of observation. Examples of direct observations include:
Lava Samples: Volcanoes provide us with information about the Earth’s interior
without actually going there! Scientists can sample the lava to determine all sorts
of data.
Core Samples: A special drill is used to bring up tubular layers of rock, called core
samples. Scientists can use these to “see” the layers of rock deep below the
Earth’s surface.
Sampling Lava
Core Samples
Remote Sensing: Another type of direct observation that allows scientists to learn
about the Earth’s structure is remote sensing. Remote sensing simply means to get
information from a distance. Remote sensing techniques include satellite and aerial
photos, the use of GPS (Global Positioning System) receivers, and infrared and
radar images.
A satellite image of the Lower Mainland
and Southern Vancouver Island
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INDIRECT OBSERVATIONS
Not all areas of the Earth can be observed directly. In many cases, scientists use
observations made at the surface of the Earth to make suggestions about its inner
structures and the processes occurring underground.
Movement inside the Earth causes shock waves called seismic waves. Earthquakes
create seismic waves that cause destruction, but seismic waves also tell scientists
about the inner structure of the Earth. You will learn more about the different
types of seismic waves later in this unit.
EVIDENCE OF A DYNAMIC EARTH
Today, we know that the surface of the Earth is continually moving. New parts of
the lithosphere are formed and older sections are recycled.
Most scientists think that the lithosphere (the crust of the Earth and solid part of
the upper mantle) is broken into pieces called tectonic or crustal plates.
Below the tectonic plates is the asthenosphere, which is made up of hot rock that
flows like thick liquid. The tectonic plates float on the asthenosphere like broken
ice fragments float on a lake.
Plate tectonics is the theory that explains this plate movement and its
consequences. But this theory took a long time to develop and much evidence was
gathered along the way before this theory was accepted. The first step in this
process was the theory of Continental Drift. You will learn about this theory next.
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UNIT 1
WHAT ARE DIRECT AND INDIRECT OBSERVATIONS? questions
1. Give three specific different direct observation techniques that scientists use to
learn about the structure of the Earth.
__________________________________________
__________________________________________
__________________________________________
2. What do scientists study to learn about the inner structure of the Earth?
__________________________________________
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WHAT IS THE THEORY OF CONTINENTAL DRIFT?
Most scientists think that millions of years ago there was one giant continent. A
continent is a giant landmass. This giant continent was surrounded by one giant
ocean. About 180 million years ago, the continent began to break apart. The
pieces of the continent slowly drifted apart. They became today's seven
continents.
A German scientist named ALFRED WEGENER was the first to propose the idea
that our present continents were once part of a giant landmass that split apart.
Wegener called the giant landmass PANGAEA (pan-JEE-uh). He called his idea
CONTINENTAL DRIFT.
EVIDENCE TO SUPPORT CONTINENTAL DRIFT
Fit of the Continents: In the early 1900's,
Wegener noticed that the continents seemed
to fit together. If you look at the coastlines
of South America and Africa on a map, you will
notice that the coastlines seem to fit together
like pieces of a jigsaw puzzle. Other places can
also be found that might once have fitted
together. The fit of the continents is one
clue that supports continental drift.
All continents were once joined together in
a supercontinent called Pangaea.
Matching of Mountain Chains: The mountain ranges on different continents seem
to match. The Appalachian mountain range along the eastern United States and
Canada is similar to one in Greenland and northern Europe. When these landmasses
are placed in their pre-drift locations, as the supercontinent Pangaea, the
mountains fit together as one continuous chain.
Age and Kind of Rocks: The age and kind of rocks along the edge of one
continent match rocks along the edge of other continents. They match at the same
locations as the continents would fit together in a model of Pangaea.
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Fossil Evidence: Fossils of the same ancient plants and animals are found today on
widely separated continents. For
example, Wegener discovered
that Mesosaurus fossils were
found in Africa and in South
America. Mesosaurus was a
reptile that lived in fresh water.
How could it swim across the
salty Atlantic Ocean? Wegener
concluded that the animal must
have lived on one landmass. When
the landmass broke apart, some
of the animals were trapped on
each part.
Fossils of different animals and plants are found today on
widely separated continents, in the same locations where the
continents would fit together in a model of Pangaea.
Paleoglaciation: Evidence suggests that vast ice sheets existed in South America,
Africa, India, Australia, and Antarctica about 250 million years ago. However, many
of these areas are tropical today. The current position of the continents would
lead one to believe that glaciers must have existed in equatorial regions at the
same time as tropical climates existed in equatorial regions. This, of course, does
not make sense.
Today, scientists realize that the areas containing these ancient glaciated
landscapes were joined together into the single supercontinent of Pangaea that was
located far south of their present positions. This paleoglaciation evidence is
further evidence of continental drift. “Paleo” means ancient.
Current position of the continents, showing
areas of ancient glaciers.
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Position of glaciers prior to the continents
drifting apart.
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Crustal Movement at Mid-Ocean Ridges: There is evidence that the crust is
moving apart at ocean floor locations called mid-ocean ridges. If the ocean crust is
moving, then the continents must be moving as well. The next section will explain
this concept in more detail.
CHALLENGES TO CONTINENTAL DRIFT THEORY
Wegner’s theory faced a lot of opposition in the early 1900s. Unfortunately, he
could not provide strong enough evidence for how the continents moved over the
surface of the Earth. While many science groups were opposed to these new ideas,
around the time of World War II, evidence was gathered that allowed his theory
to be accepted, advanced and refined into a newer theory. In the next section, you
will learn about this newer theory called the Theory of Plate Tectonics.
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UNIT 1
WHAT IS THE THEORY OF CONTINENTAL DRIFT?
questions
Multiple Choice
1. Scientists believe that there was one giant continent
A.
B.
C.
D.
hundreds of years ago
thousands of years ago
millions of years ago
billions of years ago
2. The giant continent was called
A.
B.
C.
D.
Wegener
Huge
Pangaea
Eurasia
3. This continent began to break apart
A.
B.
C.
D.
180 years ago
180 thousand years ago
180 million years ago
180 billion years ago
4. The scientist who came up with the theory of continental drift was named
A.
B.
C.
D.
Alfred Wegener
Alfred Smith
John Pangaea
Harold Eurasia
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5. Which of the following are pieces of evidence for continental drift? Circle only the
letters of correct answers. Hint: there is more than one to circle!
A.
There are landslides on the coasts of the continents
B.
The coastlines of some continents seem fit together like a jigsaw puzzle
C.
Fossils of the same ancient plants and animals are found today on widely
separated continents
D.
Similar rocks are found along the coastlines of the continents just where they
seem to fit together like a jigsaw puzzle
E.
Some mountain ranges on different continents seem to match along the
coastlines
F.
There are mountains on all of the continents
G.
There is evidence of ancient glaciation just where the continents seem to fit
together like a jigsaw puzzle
H.
The crust is spreading apart at the mid-ocean ridges
I.
Scientists can see the continents drifting apart
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UNIT 1
WHAT IS THE THEORY OF PLATE TECTONICS?
Introduction:
During World War II, sound waves were bounced off objects to measure how deep
they were. This was very useful in detecting enemy submarines. But it had another
benefit, too. Scientists determined that the ocean floor was not flat and
featureless, but covered with trenches, crevasses, mountain ridges and volcanoes!
After the war when trans-Atlantic telephone cables were being laid, engineers
found an undersea mountain range. Oceanographers determined that this range ran
the entire length of the ocean, right up the middle. Named the Mid-Atlantic
Ridge, scientists found it had a deep, wide canyon running the length in its centre.
This deep crack is called a rift valley. Other mid-ocean ridges were also
discovered, as well as deep ocean trenches along the edge of some continents.
As scientists gathered more information about Earth, they began to notice that
most earthquakes, volcanoes, mountain ranges and ocean trenches are located along
certain boundaries.
Volcanoes and earthquakes are found along ocean ridges and trenches.
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UNIT 1
This discovery led to an expansion of the Continental Drift Theory, to the present
theory called the THEORY OF PLATE TECTONICS. The Theory of Plate
tectonics states that the lithosphere is divided into 12 large sections called
tectonic plates (or just “plates”), and about 20 smaller ones as seen below.
The Earth is divided into tectonic plates. It is at these plate boundaries that the lithosphere is moving and
causing earthquakes, volcanoes, ocean trenches, and mountain building.
The THEORY OF PLATE TECTONICS states that the lithosphere is moving at
the plate boundaries. This movement is causing earthquakes, volcanoes, ocean
trenches, and mountain building.
Each plate moves in a different direction so some plates are moving away from
each other, some are moving towards each other, and others are moving past each
other in opposite directions.
These plates meet at three types of plate boundaries which are classified (and
named) by the relative direction of plate movement. The three types of plate
boundaries are: divergent plate boundaries, convergent plate boundaries, and
transform plate boundaries.
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The three main types of plate boundaries are shown on maps with different map
symbols as follows:
 divergent plate boundaries are marked by double parallel lines,
 convergent plate boundaries are marked by sawtooth lines, and
 transform plate boundaries are marked by single lines.
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UNIT 1
WHAT IS THE THEORY OF PLATE TECTONICS? questions
1. The Continental Drift Theory is ________ The Theory of Plate Tectonics.
A.
B.
C.
newer than
older than
the same age as
2. Why are earthquakes and volcanic activity common along plate boundaries?
A.
B.
C.
D.
no one knows why
because plate movement occurs there
because plate movement does not occur there
None of the above. Earthquakes and volcanoes are not common along plate
boundaries.
Completion
3. Draw the tectonic map symbol you would see at a
a. Divergent plate boundary
b. Transform fault boundary
c. Convergent plate boundary
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UNIT 1
WHAT ARE THE THREE PLATE BOUNDARIES?
As mentioned in the last section, there are three types of plate boundaries.
The three types of plate boundaries are:

DIVERGENT plate boundaries,

CONVERGENT plate boundaries and

TRANSFORM plate boundaries.
In the following pages, you will learn more about how the plates are moving and
about the landscape features created by their movement.
.
DIVERGENT PLATE BOUNDARIES
The boundary between two plates that are moving apart is
called a divergent plate boundary or a spreading center
because the plates are diverging, or spreading apart.
As the plates spread apart, the gap between them is
filled with magma that oozes up from the hot mantle. The
molten rock cools slowly to make new ocean floor.
About 5 cm of new crust is created each year at a divergent plate boundary.
At the same time, the rising magma and
heat causes the edges of the diverging
plates to bulge upward. This creates a
mountain called a RIDGE. The mountains
running down the center of the oceans are
called mid-ocean ridges.
ridge
rift valley
Between the ridges, huge blocks of the
crust may collapse. This creates a valley
called a RIFT VALLEY.
Ridges and rift valleys form at divergent plate
boundaries.
The mid-ocean ridges of the world are connected and form the longest mountain
range in the world. Some of its peaks are 3048 m above the ocean floor. In a few
places, the peaks rise above the surface of the ocean. These peaks form islands.
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Iceland is a mountain peak of the Mid-Atlantic Ridge. The Mid-Atlantic Ridge
runs down the middle of the Atlantic Ocean.
Divergent plate boundaries can also occur on continents.
Perhaps the most famous continental rift valley is the Great Rift Valley found in
Africa. It is 4800 km long and runs down the eastern side of the whole continent.
It contains some of the deepest lakes and highest mountains on the continent.
CONVERGENT PLATE BOUNDARIES
While plates are moving apart in one part of the Earth’s
crust, other plates are moving toward each other.
Another name for “moving toward each other” is
“converge”. The boundary between plates that are moving
toward each other is called a convergent plate boundary.
When two tectonic plates converge, the plate with the
higher density usually subducts (slides under) the other. This collision can create a
variety of landscape features.
The features that form when plates collide at a convergent plate boundary depend
on the types of plates involved. Three types of collision are possible. Collisions
can take place between:
- an oceanic and a continental plate,
- two oceanic plates or
- two continental plates.
Collision Of An Oceanic And A Continental Plate:

When an oceanic and a continental plate collide, the oceanic plate will
subduct (slide under) the continental plate. This happens because the
oceanic plate is denser than the continental plate.

The subducting crust is drawn into the hot mantle, where it melts... it
is recycled to become new mantle.

The region where one plate descends into the asthenosphere is called a
SUBDUCTION ZONE.

As the oceanic plate slides beneath the overriding plate, the oceanic plate
bends and produces an oceanic trench and a mountain chain (that includes
volcanoes) parallel to the trench.
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You can see from this diagram that an oceanic trench is formed when one plate subducts under
another. The area where the subduction takes place is called a subduction zone. Subduction
takes place at convergent plate boundaries. Crust melts as it sinks into the hot mantle.
Collision Of Two Oceanic Plates:

If two oceanic plates collide, the denser one will subduct under the lessdense one.

The subducting crust is drawn into the hot mantle, where it melts ... it
is recycled to become new mantle.

As in any subduction zone, an oceanic trench is created between the plates.
Mountains and volcanoes are also created parallel to the trench, which may
become islands if they are tall enough. The Philippines were created in this
way.
Figure to the left:
A trench is formed between the plates
at a subduction zone.
Mountains and volcanoes are also
formed on the overriding plate (the
one on the top) as it pushes over the
subducting plate.
Collision Of Two Continental Plates:

If two continental plates collide, neither plate can slide beneath the other
because continental crust is light and cannot sink into the mantle. The plates
will buckle upwards and fracture instead. This will create mountains.

An example of this is where India collided with Asia, forming the Himalayan
Mountains.
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One further result of subduction is that some of the Earth’s crust gets
recycled as it sinks into the hot mantle. However, some of the Earth’s crust
never subducts. As a result, not all of the Earth’s crust is the same age. Let
me explain.

Continental crust is less dense than the ocean crust and so it does not
subduct. It is forever pushed around the surface of the Earth and
consequently contains the oldest rocks on Earth, up to 4 billion years old.

Ocean crust does subduct at convergent plate boundaries. When it
subducts, heat and pressure cause it to melt into the mantle (usually at
depths of about 700km). It is continually being recycled and so it is much
younger than continental crust.
TRANSFORM PLATE BOUNDARIES
At a transform plate boundary, plates slide past each
other in opposite directions. Neither plate rides up over
the other, nor does either plate subduct (slide under)
the other. Crust is neither created nor destroyed at
transform fault boundaries. Transform plate boundaries
are also called strike-slip faults. Earthquakes often
occur when these plates slide. The most famous of this
type of boundary is the San Andreas Fault in California.
Cracks in the Earth’s surface can often be seen at transform plate boundaries.
SUMMARY
In summary, there are 3 types of plate boundaries:

Transform plate boundaries (also called transform faults or strikeslip faults) are where plates slide past each other

Divergent plate boundaries (also called spreading centres) are
where plates are moving apart from each other.

Convergent plate boundaries are where plates are moving toward
each other.
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WHAT ARE THE THREE PLATE BOUNDARIES? questions
Multiple Choice
1. Divergent boundaries are also called
A.
B.
C.
D.
widening boundaries
converging boundaries
spreading centers
oozing boundaries
2. The plates at a divergent boundary are
A.
B.
C.
D.
moving apart from each other
moving towards each other
sliding past each other
not moving at all
3. The plates at a convergent boundary are
A.
B.
C.
D.
moving apart from each other
moving towards each other
sliding past each other
not moving at all
4. The plates at a transform fault boundary are
A.
B.
C.
D.
moving apart from each other
moving towards each other
sliding past each other
not moving at all
5. Another name for transform fault boundary is
A.
B.
C.
D.
sliding boundary
side-to-side boundary
strike-slip fault
sliding fault
6. New ocean floor crust is being produced at
A.
B.
C.
D
convergent boundaries
divergent boundaries
transform fault boundaries
None of the above: no new ocean floor is being produced today.
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7. Crust is being destroyed at
A.
B.
C.
D.
convergent boundaries
divergent boundaries
transform fault boundaries
None of the above. Crust is not being destroyed today.
8. When an oceanic and a continental plate collide,
A.
B.
C.
D.
the continental plate will slide under the oceanic plate
both plates will buckle upward
the oceanic plate will slide under the continental plate
both plates will move downwards
9. When two oceanic plates collide,
A.
B.
C.
D.
the heavier plate will slide under the lighter plate
the lighter plate will slide under the heavier plate
both plates will buckle upward
both plates will descend into the asthenosphere
10. When two continental plates collide,
A.
B.
C.
D.
one continental plate will slide under the other
both plates will buckle upward
both plates will move downwards
None of the above. Two continental plates never collide.
11. The place where one plate slides under another and descends into the
asthenosphere is called a
A.
B.
C.
D.
subduction zone
divergent zone
transform fault zone
None of the above. One plate does not slide under another plate.
12. Oceanic trenches are created between the plates at
A.
B.
C.
D.
E.
convergent boundaries
divergent boundaries
transform fault boundaries
All of the above.
None of the above. Oceanic trenches are not being created today.
13. Mid-ocean ridges are created between the plates at
A.
B.
C.
D.
E.
convergent boundaries
divergent boundaries
transform fault boundaries
All of the above.
None of the above. Mid-0cean ridges are not being created today.
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14. Oceanic trenches form when
A.
B.
C.
D.
a heavy plate subducts under a lighter plate
lighter plate subducts under a heavier plate
two light plates subduct together
two heavy plates subduct together
15. The collision of a continental and an oceanic plate will result in the formation of
(choose all correct answers … there are more than one)
A.
B.
C.
D.
a trench
a subduction zone.
Mountains
Volcanoes
16. The collision of two continental plates will result in the formation of
A.
B.
C.
a trench
a subduction zone
mountains
17. Rift Valleys form at
A.
B.
C.
D.
divergent plate boundaries
convergent plate boundaries
transform plate boundaries
subduction zones
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WHAT’S THE EVIDENCE FOR SEA-FLOOR SPREADING?
As mentioned previously, plates are spreading apart at divergent plate boundaries.
The plates spread apart because molten rock rises up through the cracks in the
rift valley of the mid-ocean ridge. As the magma cools, it forms new crust on both
sides of the rift valley. This causes the sea floor to spread apart as the new crust
is formed and pushes away the older crust on both sides of the rift valley. This
process is sometimes called SEA-FLOOR SPREADING.
A RIFT VALLEY is a deep crack running down the centre of
mid-ocean ridges.
Scientists have compiled the following evidence to prove sea-floor spreading.
Paleomagnetism and Geomagnetic Reversals:
The Earth is like a giant magnet. It has a magnetic field similar to that produced
by a bar magnet. Minerals that contain iron, such as magnetite, have magnetic
properties. When lava containing these minerals cools, the iron-rich grains become
magnetized in the direction of the existing magnetic field. If the rock is moved,
the magnetized mineral grains will retain their original alignment. The iron particles
in rocks forming today point north because that is the direction of the current
magnetic field. The iron particles in rocks that formed millions of years ago point
in the direction of the magnetic pole at the time and place of their formation.
They are said to possess PALEOMAGNETISM.
If the Earth’s magnetic field has not changed through its history, the magnetic
particles in rocks should point north. However, scientists have discovered rocks
with magnetic particles that point south. This suggests that the Earth’s magnetic
field has changed through its history. North became south; and south became
north. Scientists call this phenomenon GEOMAGNETIC REVERSAL.
The Learning Centres
page 25 of 43
Science 10
Plate Tectonics
UNIT 1
Scientists have used their knowledge of paleomagnetism and geomagnetic reversals
to prove the sea floor is spreading at divergent plate boundaries.
Using a special instrument called
a magnetometer, they found an
interesting pattern as they
moved outward from either side
of the ridges.
First there is a strip of rock
with magnetic particles pointing
north. Then there is a strip of
rock with magnetic particles
pointing south . Then north and
then south as you move outward
from the ridge. These strips run
parallel to the mid-ocean ridges.
They form a mirror image
pattern on the ocean floor at
the mid-ocean ridges.
Why?
The plates are spreading apart at the mid-ocean ridges. As they move apart, new
ocean crust is constantly being created as magma moves upward and cools on both
sides of the mid-ocean ridge. During the periods when the magnetic pole is in the
north, the magnetic particles point north. During the periods when the magnetic
pole is in the south, the magnetic particles point south. This pattern shows that
new crust is being created on either side of the mid-ocean ridges and that the
crust is moving.
Deep-Sea Drilling:
Further evidence for sea-floor spreading has come from deep-sea drilling. Deepsea drills have been used to bring up samples of oceanic crust. Scientists have
used radioactive dating techniques to determine that these samples of oceanic
crust are younger than samples of continental crust. Also, the crust near a midocean ridge is younger than the crust farther away from the mid-ocean ridge. The
youngest crust is in the center of the ridge.
The Learning Centres
page 26 of 43
Science 10
Plate Tectonics
UNIT 1
WHAT’S THE EVIDENCE FOR SEA-FLOOR SPREADING?
- questions
Multiple Choice
1. A rift valley is
A.
B.
C.
D.
a shallow crack running down the center of a mid-ocean ridge
a very long mountain chain
a valley found in Iceland
a deep crack running down the center of a mid-ocean ridge
2. The sea-floor is
A.
B.
C.
D.
spreading apart at the mid-ocean ridges
moving together at the mid-ocean ridges
not moving at all
spreading apart only near Iceland
3. Where does sea-floor spreading occur?
A.
B.
C.
D.
strike-slip faults
subduction zones
mid-ocean ridges
deep ocean trenches
4. When molten rock containing iron-rich minerals cool, the grains will point in the
direction of the Earth's magnetic field. This fact is called
A.
B.
C.
D.
sea floor spreading
paleoglaciation
geomagnetic reversal
paleomagnetism
5. Throughout history, the Earth’s magnetic field has changed from north to south
and visa versa. This fact is called
A.
B.
C.
D.
sea floor spreading
paleoglaciation
geomagnetic reversal
paleomagnetism
The Learning Centres
page 27 of 43
Science 10
Plate Tectonics
UNIT 1
6. Which of the following diagrams provide evidence for sea-floor spreading?
A. I only
B. I and II only
C. II and III only
D. IV only
7. The crust near the mid-ocean ridges is
A. older than the crust farther out from the mid-ocean ridges.
B. the same age as the crust farther out from the mid-ocean ridges.
C. younger than the crust farther out from the mid-ocean ridges.
The Learning Centres
page 28 of 43
Science 10
Plate Tectonics
UNIT 1
PART B
PLATE TECTONICS:
CAUSE AND EFFECT
Table of Contents:
Description
Complete ?
Causes of Plate Movement
Effects of Plate Tectonics – Landscape Features
Effects of Plate Tectonics – Geologic Events
Effects of Plate Tectonics – Seismic Waves
The Learning Centres
page 29 of 43
Science 10
Plate Tectonics
UNIT 1
WHAT ARE THE CAUSES OF PLATE MOVEMENT?
Scientists believe that there are three forces that move tectonic plates:
convection currents in the mantle, ridge push, and slab pull.
Mantle convection, ridge push, and slab pull combine to move tectonic plates.
Mantle Convection
Convection is the vertical movement of a gas or a liquid caused by differences in
temperature. When hot liquid iron and nickel rises in the mantle, it cools, and then
sinks in another place. This up and down movement forms a CONVECTION
CURRENT. You have probably seen a convection current in a pot of soup on the
stove. Particles in the soup rise and then sink and then rise and then sink again in a
circular motion.
When hot mantle rises in one place, cools, and then sinks in another place, this
convection current is known as MANTLE CONVECTION. This current in the
asthenosphere is one of the forces that move the tectonic plates, and the
continents of the lithosphere move with them.
The Learning Centres
page 30 of 43
Science 10
Plate Tectonics
UNIT 1
Ridge Push
Where the hot mantle rises up at a divergent boundary, it heats the crust above it
until the crust expands, becomes less dense, and floats higher. This action makes a
ridge, and the crust is pulled thinner; kind of like pushing up on a slab of modeling
clay with your finger. Cracks are formed in this thinner area, and magma comes to
the surface through the cracks. As this magma cools in the cracks, it wedges the
plates apart. As this new sea floor cools further, its density increases and it sinks
down. As it sinks, it pushes the plate away from the mid-ocean ridge. This push
from the ridge is called RIDGE PUSH.
Slab Pull
At a subduction zone, a more dense plate collides with a less dense plate, and the
more dense plate subducts under the less dense plate. As the edge of the plate
descends into the mantle, gravity and convection currents pull the rest of the plate
with it. This is called slab pull.
Summary of the Causes of Plate Movement:
 Mantle convection currents help to move the plates along.
 The edge of a tectonic plate is moved away from the spreading ocean
ridge by ridge push.
 Slab pull at the edge of a subducting plate acts to pull the rest of the
plate down into the mantle.
Source of the Heat
The source of the heat that melts the outer core & mantle and that causes the
convection currents is the decay of radioactive minerals deep within the Earth.
When radioactive substances decay, they release heat. You will learn more about
this process in the radioactivity unit later in this course.
The Learning Centres
page 31 of 43
Science 10
Plate Tectonics
UNIT 1
WHAT ARE THE CAUSES OF PLATE MOVEMENT? questions
Multiple Choice
1. A convection current is the movement of a gas or a liquid caused by differences in
A.
B.
C.
D.
air pressure
temperature
color
opinion
2. The mantle rock close to the core is
A.
B.
C.
D.
cool
cold
warm
hot
3. Which of the following does NOT contribute to the motion of tectonic plate?
A.
B.
C.
D.
Slab pull
Ridge push
Centrifugal force
Mantle convection
4. Convection currents that help move the lithospheric plates occur in the
A.
B.
C.
D.
mantle
crust
inner core
outer core
5. Why does the sea floor bulge up and make a ridge at a spreading centre?
A.
B.
C.
D.
Heated crust is more dense, so it floats highest.
Magma piles up as it rises from the mantle
Heated crust is less dense, so it floats higher.
Friction curls up the edges of the crust as it moves.
The Learning Centres
page 32 of 43
Science 10
Plate Tectonics
UNIT 1
WHAT ARE THE EFFECTS OF PLATE TECTONICS?VOLCANOES and EARTHQUAKES
The interaction of tectonic plates also causes geologic events like volcanoes and
earthquakes which, in turn, further alter the landscape of the Earth.
VOLCANOES
A volcano forms at a crack in the lithosphere where magma (molten rock below the
surface) and gases reach the Earth’s surface. Once the magma reaches the
surface, whether underwater or on a continent, it is called lava. Magma forms deep
underground in the asthenosphere when rocks melt. This happens due to one of
three events:
 a drop in pressure, perhaps from a crack in the crust
 a change in composition of the rock, perhaps due to a subduction zone
 an increase in temperature, perhaps due to a hot spot or convection
current upflow
Volcanoes form at different locations for different reasons.
Volcanoes form at divergent plate boundaries.
When tectonic plates diverge, it relieves
pressure on the mantle below and the magma
flows upward to the surface, forming a volcano.
Krafla Volcano, Iceland,
on the Mid-Atlantic Ridge
Volcanoes also form at convergent plate boundaries.
These are the most common type of volcano found around the Earth. At
convergent plate boundaries, the crust melts and turns to magma as it is
pulled into the hot mantle. The magma moves upwards, following cracks
formed in the subduction zone. The magma reaches the surface of the
overriding plate about 100 - 300 km from the ocean trench formed from the
subduction. This creates a row of volcanoes approximately parallel to where
the plates meet.
The Learning Centres
page 33 of 43
Science 10
Plate Tectonics
If the plate boundary is an oceanic –
continental convergent boundary, a
chain of inland volcanoes called a
volcanic belt is created. The Cascade
Mountains are such a volcanic belt,
stretching from southern British
Columbia to northern California. The
last to erupt was Mount St. Helen’s in
1980. Mount Baker, easily seen from
Surrey, is a dormant volcano that lets
off a bit of steam once in a while.
Mount Garibaldi is a volcano of this
range found near Whistler, B.C.
UNIT 1
Mount
Baker
Mt. Baker is a volcano in this chain whose magma
is produced when the Juan de Fuca Plate subducts
under the North American Plate.
Subduction zone volcanoes are also
formed parallel to an oceanic – oceanic
convergent plate boundary. In this case,
the chain of volcanoes created is called a
volcanic island arc. An example of this
occurrence is the Aleutian Islands which
stretch across the North Pacific Ocean
between Alaska and Siberia, Russia.
Notice how the volcano forms away from the
trench, on the overriding plate.
The Learning Centres
page 34 of 43
Science 10
Plate Tectonics
UNIT 1
Volcanoes also occur over hot spots.
HOT SPOTS are small regions of very hot mantle that are thought to be
created by excess radioactivity near the Earth’s core. This heat creates a
very hot column of rising mantle which causes the lithosphere to thin out and
crack as it bulges up. Eventually, magma bursts through the weakened
lithosphere over the hot spot, forming a volcano. Hot spot volcanoes can
form in oceanic crust and continental crust.
The hot spot stays in one place while the lithosphere moves over it.
As the tectonic plate continues to move, the location on the plate overtop of
the hot spot changes. The original volcano moves away from the hot spot and
a new volcano is formed.
Over time, a chain of volcanoes is formed, stretching away from the hot
spot in the direction of the plate movement. If the hot spot is beneath
the ocean, the volcanoes form a chain of islands. The Hawaiian Islands are an
example of a hot spot volcanic island chain.
Volcanic Island Chains can form from hot spots deep within the
asthenosphere.
If the hot spot is beneath a continent, a chain of land volcanoes is created
like the Anahim Belt located in southwestern B.C..
Yellowstone Park in the continental USA is also located over a hot spot.
The Learning Centres
page 35 of 43
Science 10
Plate Tectonics
UNIT 1
EARTHQUAKES
As convection currents, ridge push, and slab pull try to move tectonic plates,
friction between the plates resists the movement. When the forces trying to move
the plate become stronger than the force of friction resisting the movement, the
plates move suddenly and release a massive amount of energy. This sudden, strong
movement in the Earth’s crust is called an earthquake. Small movements of the
crust you may or may not feel are called tremors. There are more than six million
tremors on Earth each year.
The actual location inside the Earth where an earthquake starts is called the focus.
The place on the surface of the Earth directly above the focus is called the
epicenter. Earthquakes can occur anywhere on Earth. However, 95% occur at
tectonic boundaries and about 80% of all earthquakes occur in a ring around the
Pacific Ocean commonly known as the Pacific Ring of Fire.
The “Pacific Ring of Fire”
Scientists categorize earthquakes according to how far beneath the Earth’s
surface the focus occurs. There are three categories:

shallow-focus earthquakes, where the focus is up to 70 km deep and occurs
in the crust;
The Learning Centres
page 36 of 43
Science 10
Plate Tectonics
UNIT 1

intermediate-focus earthquakes, where the focus is 70-300 km deep and
occurs in the subduction zone; and

deep-focus earthquakes where the focus is 300-700 km deep and occurs in
the mantle.
Earthquake Categories
Earthquakes occur at all types of plate boundaries.; the plates could be sliding past
each other along a transform fault, they could be pulled apart at a divergent
boundary, or they could be subducting at a convergent boundary.
hotspot
This diagram shows possible locations of volcanoes and earthquakes at different types of plate boundaries
and hot spots.
A fault is a displacement of the lithosphere – whether vertically, horizontally, or
both – created by the movement of tectonic plates.
As the rocks move during an earthquake, they release mechanical energy in the
form of vibrations. These vibrations are called seismic waves, or earthquake
waves.
The Learning Centres
page 37 of 43
Science 10
Plate Tectonics
UNIT 1
WHAT ARE THE EFFECTS OF PLATE TECTONICS –
LANDSCAPE FEATURES?- GEOLOGIC EVENTS - questions
Multiple Choice
1. Volcanoes can be found
A.
B.
C.
D.
at convergent plate boundaries
at divergent plate boundaries
at hot spot locations
all of the above
2. Molten rock that reaches the Earth’s surface is called
A.
B.
C.
D.
magma
lava
molten rock
rock
3. Molten rock that under the Earth’s surface is called
A.
B.
C.
D.
magma
lava
molten rock
rock
4. Small movements of the crust that you may or may not feel are called
A.
B.
C.
D.
shakes
waves
tremors
earthquakes
5. Sudden, strong movements of the Earth's crust are called
A.
B.
C.
D.
shakes
waves
tremors
earthquakes
The Learning Centres
page 38 of 43
Science 10
Plate Tectonics
UNIT 1
6. The place inside the Earth where the earthquake starts is called the
A.
B.
C.
D.
focus
epicenter
seismic wave
starting point
7. The place on the Earth’s surface above where the earthquake starts is called the
A.
B.
C.
D.
focus
epicenter
seismic wave
starting point
8. Earthquakes are caused mostly by
A.
B.
C.
D.
the movement of tectonic plates
huge explosions
strong winds
landslides
9. Vibrations caused by the movement of rocks during an earthquake are called
A.
B.
C.
D.
faults
seismographs
seismic waves
epicenters
10. Hot spots
A.
B.
C.
D.
move across tectonic plates
remain stationary while a tectonic plate moves over top of it
occur only beneath oceans
occur only on continents
Short Answer
1. In your own words, define what a fault is. ______________________________
___________________________________________________________________
___________________________________________________________________
2. Magma is formed when one of three things happens to melt the rock in the
asthenosphere inside the Earth. What are these three things?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
The Learning Centres
page 39 of 43
Science 10
Plate Tectonics
UNIT 1
WHAT ARE SEISMIC WAVES?
There are two main types of seismic waves; body waves and surface waves.
BODY WAVES are waves that travel through the underground.
There are two types of body waves caused by earthquakes: primary and
secondary.
Primary waves, or P-waves, are compression
waves; they cause the ground to compress and
stretch like a spring along the path of the
wave. P-waves can travel through solids,
liquids and gases, and are the fastest moving
of the seismic waves, moving at about 6 km/s
through the Earth’s crust. They can pass
through all the different layers of the Earth.
P-waves move like a “slinky”; they cause the
ground to compress and stretch like a spring
along the path of the wave.
Secondary waves, or S-waves, are also known
as shear waves. S-waves move more slowly than
P-waves, about 3.5 km/s, and can only travel
through solid rock They cannot travel through
the Earth’s liquid core. They cause the ground
to compress and stretch perpendicular to the
direction of the wave. S-waves usually cause
more structural damage than P-waves do
S-waves move like a rope being shaken upand-down; they cause the ground to compress because S-waves are larger.
and stretch perpendicular to the direction of
the wave.
SURFACE WAVES travel along the surface of
the earth. The type of surface wave caused by
earthquakes is called an L-wave.
L-waves cause the most destruction. L-waves
roll along the Earth’s surface like ripples in a
pond. These waves travel more slowly than
both P- and S-waves, and only travel a few
hundred kilometres from the epicenter.
The Learning Centres
L-waves are a surface wave that rolls along
the Earth’s surface like ripples on a pond.
page 40 of 43
Science 10
Plate Tectonics
UNIT 1
Measuring Earthquakes
A seismograph is an instrument that detects and measures
earthquakes. A seismograph can even measure very small
tremors that people cannot feel. It makes a record of the
movements in the Earth’s crust on a piece of paper. The
record is called a seismogram. It provides information about
when the earthquake occurred, how long it lasted, and the
amount of ground shaking. It is a trace on a piece of paper
that looks like wavy lines. The higher the wavy lines are on the seismogram, the
stronger the earthquake.
These traces show a series of
seismograms for an earthquake felt on
Vancouver Island and the Lower
Mainland in 2001. It is easy to see
that the closer the seismograph was
to the earthquake, the larger the
trace of P- and S-waves.
Scientists use the time difference in
arrival between the P- and S-waves to
determine how far away the
earthquake was.
The most common earthquake scale used is the Richter Scale, which is a measure
of the magnitude or energy released during an earthquake. Magnitude is a number
that rates the strength (or energy) of an earthquake. Higher magnitudes indicate
larger earthquakes. With each 1-step increase in magnitude, the energy released
by the earthquake is approximately 32 times larger. Thus, an earthquake of
magnitude 4.0 is about 1024 (32x32) times greater in energy than an earthquake
of magnitude 2.0.
The Learning Centres
page 41 of 43
Science 10
Plate Tectonics
UNIT 1
WHAT ARE SEISMIC WAVES? - questions
Multiple Choice
1.
_______________ are seismic waves that travel through solids, liquids, and gases.
A.
B.
C.
D.
2.
______________ are seismic waves that travel through solids only.
A.
B.
C.
D.
3.
S-waves
P-waves
L-waves
G-waves
S-waves
P-waves
L-waves
G-waves
The first seismic waves to be recorded by a seismograph are the
A.
B.
C.
D.
S-waves
P-waves
L-waves
G-waves
(Hint: the fastest waves will arrive at the seismograph first).
4.
The last seismic waves to be recorded by a seismograph are the
A.
B.
C.
D.
5.
S-waves
P-waves
L-waves
G-waves
The slowest moving waves are the
A.
B.
C.
D.
S-waves
P-waves
L-waves
G-waves
The Learning Centres
page 42 of 43
Science 10
6.
S-waves
P-waves
L-waves
G-waves
The instrument used to detect and measure earthquakes is called a
A.
B.
C.
D.
9.
S-waves
P-waves
L-waves
G-waves
The waves known as surface waves are the
A.
B.
C.
D.
8.
UNIT 1
The earthquake waves that cause the most damage are the
A.
B.
C.
D.
7.
Plate Tectonics
seismologist
voltmeter
earthquake meter
seismograph
The scale used to measure the magnitude and energy of an earthquake is the
A.
B.
C.
D.
temperature scale
bathroom scale
Richter scale
seismograph scale
Match the wave name with the direction of wave movement.
9. ____ P-waves
rolls like ripples on a pond
10. ____ S-waves
compress and stretch perpendicular to the wave direction
11. ____ L-waves
compress & stretch like a spring
The Learning Centres
page 43 of 43