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I have a QUESTION for YOU!
How do the continents
fit together?
Continental
Drift
Objectives
At the end of this unit you will be able to…
• Describe the hypothesis of continental
drift.
• Evaluate evidence in support of continental
drift.
• Identify the main objections to Wegener’s
hypothesis of continental drift.
Essential Questions
At the end of this unit, you will be able
to answer the following questions….
• What is the hypothesis of continental
drift?
• What evidence supports continental drift?
How did Wegener’s opponents counter his
evidence?
• What were the two main objections to
Wegener’s continental drift hypothesis?
Key Concepts
The continental drift hypothesis proposed by Alfred
Wegener states that Earth’s continents were once joined
to form a single supercontinent.
Wegener called the supercontinent Pangaea and
hypothesized that it began breaking apart around 200
mya during the Mesozoic Era.
Wegener and others who advocated the continental drift
hypothesis had substantial evidence to support their
claim.
Evidence #1 - The topographic fit of South America and
Africa suggests that the continents were once joined
together.
Evidence #2 - Several fossil organisms (plant and animal)
are found on different landmasses (continents)
separated by vast oceans.
Key Concepts
Evidence #3 – Rocks types and geologic features
such as mountain belts that end at one coastline,
reappear on a landmass across the ocean.
Evidence #4 - Dramatic global climate changes
were preserved in rocks and evidenced in glacial
deposits.
Wegener’s opponents rejected his hypothesis
because:
He could not explain how the continents
moved.
He could not explain how the continents could
move without shattering.
Continental Drift – The hypothesis
that states that the continents were
once joined to form a single
supercontinent.
Pangaea – The single supercontinent
proposed by Alfred Wegener.
Early Observations
• Late 1500’s - Abraham Ortelius, a Dutch
mapmaker, noticed the apparent fit of
continents on either side of the Atlantic
Ocean.
• Late 1800’s - Eduard Suess, an Austrian
geologist, hypothesized that the present
southern continents had once been
joined as a single landmass that he
named Gondwanaland.
Abraham Ortelius
One of the first to notice the topographic fit of the continents
Alfred Wegener
The first time that the idea of moving
continents was proposed as a serious scientific
hypothesis was in 1912 by a German scientist
named Alfred Wegener.
Pangaea
• Wegener’s hypothesis,
continental drift,
proposed that Earth’s
continents had once
been joined as a single
landmass.
• This single landmass
was named Pangaea, a
Greek word meaning
“all the earth”.
Continental Drift
• Wegener proposed that Pangaea began to
break apart about 200 million years ago
(mya), during the early part of the
Mesozoic Era, and that the continents had
continued to slowly move to their present
positions.
• Wegener collected and organized rock,
fossil, and climate data to support his
hypothesis.
Evidence: The Continental Puzzle
Similarities between the coastlines on
the opposite sides of the South Atlantic
(topographic fit).
This apparent jigsaw-like fit is no
coincidence – the two continents split apart
from one landmass during the Cretaceous
period, and oceanic crust developed in
between them.
Topographic Fit
Evidence: Matching Fossils
• Similar fossils of several different animals and
plants that once lived on land had been found on
widely separated continents.
• Mesosaurus, a small, freshwater reptile, are
limited to eastern South America and southern
Africa (areas which are separated by large oceans
of saltwater).
• Glossopteris, a seed fern that resembled low
shrubs, have been found on many continents,
indicating that the areas had a single climate that
was close to the equator.
Fossil of a
Mesosaurus
Artist rendition
of a Mesosaurus
Evidence: Matching Fossils
Opponents of
continental drift
account for the
existence of
similar fossils on
widely separated
continents through
migration across
land bridges and
ocean rafting.
Evidence: Rock Types and Structures
• Wegener reasoned that large geologic structures,
such as mountain ranges would have fractured as
the continents separated.
• Using this reasoning, Wegener hypothesized that
there should be areas of similar rock types on
opposite sides of the Atlantic Ocean.
• Similar groups of rocks were observed in the United
States, Greenland, and Europe which supported
Wegener’s idea (Mountain ranges, such as the
Appalachian mountains in the United States, shared
similar features with rocks in Greenland and Europe).
The image at left shows the
Appalachian and Caledonian
Mountains in their current
locations.
The image at right shows
where the Mountains would
have occurred if Pangaea
had existed as Wegener
believed.
Evidence: Ancient Climates
• Various sedimentary rocks offer evidence
of vast climate changes on some
continents.
• Coal deposits in Antarctica, made by the
remains of temperate climate trees,
suggested that it must have been closer
to the equator.
Evidence: Ancient Climates
• Glacial deposits and striations left on
bedrock found in Africa, India, Australia,
and South America suggested that these
areas had once been covered by thick ice
caps.
• Wegener proposed that thethese
continents were once located near the
south pole before Pangaea began to
fracture.
The distribution of
glacial features can best
be explained if the continents
were once a part of pangaea
(A) Pangaea as it was
thought to be 300 million
years ago. Glaciers cover
much of Antarctica and
parts of South America,
Africa, India, and
Australia.
(B) The current locations of
the continents with the
evidence of glaciers
shown.
Does it make sense to see glaciers in India but not northern Africa
assuming the continents have always been were they are now? Or
does Pangaea make more sense in light of glaciers from 300 million
years ago?
A Rejected Hypothesis
 Wegener’s hypothesis had two
major flaws:
• Could not satisfactorily explain
what was causing the continents
to move. Wegener suggested tidal forces
which was countered by physicist Harold Jeffreys
that tidal forces large enough to move
continents would stop Earth’s rotation.
• Or how continents could move
without shattering.
 Evidence to support Wegener’s hypothesis
would later be found on the seafloor and a
new theory would emerge.
Show What You Know!
1. What is continental drift?
2. Who developed the theory of continental
drift?
3. What is Pangaea?
4. Name four types of evidence for continental
drift.
5. Why was the continental drift theory not
generally accepted by the scientific world?
Answer the Essential Questions
• What is the hypothesis of continental
drift?
• What evidence supports continental
drift? How did Wegener’s opponents
counter his evidence?
• What were the two main objections to
Wegener’s continental drift
hypothesis?
Seafloor Spreading Activity
I have a QUESTION
for YOU!
How do the features of
the oceanic crust compare
to the features of the
continental crust?
Seafloor Spreading
Objectives
At the end of this unit you will be able to…
• Summarize the evidence that led to
the discovery of sea floor spreading.
• Explain the significance of magnetic
patterns on the seafloor.
• Explain the process of seafloor
spreading.
Essential Questions
• What is seafloor spreading?
• What is paleomagnetism and how
does it relate to seafloor spreading?
• How does seafloor spreading help
support Wegener’s hypothesis of
continental drift?
Seafloor Spreading – The process by
which plate tectonics produces new
oceanic lithosphere. Harry Hess’ theory
that new ocean crust is formed at midocean ridges and destroyed at deep-sea
trenches.
Oceanic Ridge – An elevated area of
the ocean floor. Oceanic ridges are the
longest physical features on the surface
of Earth. Formed by seafloor spreading.
Trench – An elongated depression in
the seafloor produced by the bending of
ocean crust during subduction.
Paleomagnetism - The study of
changes in Earth’s magnetic field, as
shown by patterns of magnetism in
rocks that have formed over time.
Key Concepts
The ocean floor has huge underwater
mountain chains called oceanic ridges and
deep underwater valleys called trenches.
Oceanic ridges are called constructive margins
because new crust is “constructed” here.
Seafloor spreading is the process by which
new oceanic crust is made (constructed).
Ocean crust is destroyed at deep-sea trenches
(destructive margins).
Key Concepts
Oceanic crust near ocean ridges is younger
than the crust farther from ocean ridges.
Oceanic crust in the deeper parts of the ocean
at the trenches is the oldest.
The oldest oceanic crust is geologically
younger than the youngest continental crust.
Earthquakes and volcanoes are common at
the oceanic ridges (also called constructive
margins).
Key Concepts
The thickness of ocean-floor sediments
increases with distance from an ocean ridge.
Strips of rock of alternating polarity
(magnetism), lie as mirror images across
ocean ridges.
Paleomagnetism is evidence of seafloor
spreading because the magnetic properties of
the rocks indicate Earth’s magnetic polarity at
the time the rocks were created.
Continental Drift
Continental drift – i.e., the continents once formed a
giant landmass called Pangaea that began breaking
apart 200 mya - was proposed by Alfred Wegener in
1912.
Wegener’s hypothesis was rejected by the scientific
community because he could not provide a plausible
mechanism for the movement of the continents.
A Theory Resurrected
It is important to understand that, until the
mid-1900’s, most people thought that the
ocean floor was flat.
Many also thought the ocean crust was
unchanging and was generally much older
than continental crust.
A Theory Resurrected
Advances in technology
during the 1940’s &
1950’s, such as SONAR,
allowed scientists to
“see” that the ocean
floor was not flat, and
was actually much
younger than the
continental crust.
Reflected
Sound Waves
Sound
Waves
A Theory Resurrected
As a result of new information, Wegener’s
hypothesis was resurrected by several scientists,
most notably Harry Hess, a professor from
Princeton University.
Hess believed that the ocean floor acted as a
conveyor belt and the continents simply travelled
across the globe on top.
A Theory Resurrected
In 1960 Hess wrote, "The History of Ocean Basins,"
in which he claimed that the oceans were not
permanent.
He stated that Mid-Ocean ridges are cracks in
Earth's crust where magma continually wells up
and spreads outward.
He also described how new crust is created on
either side of the ridges (causing the sea-floor to
spread) at a rate of one-half inch per year.
A Theory Resurrected
Hess proposed that, at that rate of spread, the Atlantic
Ocean would have been created during the past 200
million years - just as Wegner suggested!
Hess further explained:
• While new ocean crust is being formed at mid-ocean
ridges, old ocean crust is being destroyed at deep ocean
trenches called subduction zones.
• He also proposed that mantle convection causes the
movement of the crust.
Finally there was a mechanism for the movement of
continents - Seafloor Spreading!
Seafloor Spreading
• The theory of seafloor spreading states that new ocean
crust is formed at ocean ridges (constructive margins)
and destroyed at deep-sea trenches (destructive
margins) through a process called subduction.
• Magma, because it is hotter and less dense than
surrounding oceanic rock, is forced toward the crust along
an ocean ridge and fills the gap that is created.
• When the magma hardens, a small amount of new ocean
floor is added to Earth’s surface. As spreading along a
ridge continues, more magma is forced upward and
hardens.
Magnetic Polarity
Evidence for Seafloor Spreading
Several types of evidence supported
Hess’s theory of sea-floor spreading:
• Ocean floor topography
• Eruptions of molten material
• Ages of the rocks themselves
• Magnetic stripes in the rock of the
ocean floor
Ocean Floor Topography
• The maps made from the data collected by
sonar surprised many scientists.
• Vast underwater mountain chains called
ocean ridges were discovered.
• Earthquakes and volcanism are common
along the ridges.
Ocean Floor Topography
• Maps generated with sonar data also revealed
deep-sea trenches.
• A deep-sea trench is a narrow, elongated
depression in the seafloor with very steep sides.
• Trenches can be thousands of kilometers long and
extremely deep. (ex. Mariana Trench)
Ocean Rocks and Sediment
Scientists also collected samples of deep-sea
sediments and the underlying crust.
Analysis of the rocks and sediments produced
two important discoveries:
• Ages of ocean rocks vary in different places
and the variations change in a predictable
way.
• The depth of sediments deposited on the
ocean floor varied based on the distance
from a mid-ocean ridge.
Ocean Rocks and Sediment
Conclusions from the analysis of rocks and sediment:
• Rock samples taken from areas near ocean ridges
were younger than samples taken from near deep-sea
trenches.
• The ages of rock consistently increases with distance
from a ridge.
• Scientists also discovered that the oldest part of the
seafloor is geologically younger than continental crust.
• The thickest sediments are farthest from the oceanic
ridges. This is because older lithosphere has had more
time to accumulate sediment.
Type of
Crust
Average
Thickness
Average Age
Major
Component
Continental
Crust
20-80
kilometers
3 billion years
Granite
10 kilometers
Hundreds of
millions of years
Basalt
Oceanic Crust
Earth’s Magnetic Field
• The outer core is the source
of Earth’s geomagnetic field.
• The core is an electricallyconducting ocean of iron in
constant motion.
• The outer core has
"hurricanes“ powered by the
Coriolis forces of Earth's
rotation. These motions
generate our planet's
magnetism.
Earth’s Magnetic Field
• Throughout the history of Earth, there have
been reversals in the polarity of Earth’s
magnetic field. This last happened about
700,000 years ago and has happened about
300 times in the last 200 million years.
• A magnetic field that has the same
orientation as Earth’s present field is said to
have normal polarity. (+ reading).
• A magnetic field that is opposite to the
present field has a reversed polarity. (reading).
Paleomagnetism
• Prior to their studies of the ocean floor,
scientists knew that continental rocks
containing iron-bearing minerals,
particularly magnetite, provided a record
of Earth’s magnetic field.
• As basaltic lava cools, the iron-bearing
minerals become oriented parallel to
earth’s magnetic field. These rocks act
much like a compass.
Magnetometer
A magnetometer is
a device that can
detect small
changes in magnetic
fields.
Magnetometers
towed by ships
record the
magnetic field
strength in the
rocks that make
up the ocean
floor.
Paleomagnetism
• As scientists collected more data about the
ocean floor, they noticed a series of
magnetic stripes positive and negative
stripes that were parallel to ocean ridges.
• The magnetic pattern on one side of the
ridge is a mirror image of the pattern on
the other side of the ridge.
The last couple of million years worth' of reversals. Each polarity
interval, or 'chron', is named after either a famous palaeomagician
(Brunhes, Matuyama) or the location where it was first identified
(Olduvai). As seen below, reversals do not occur on a regular
interval.
Plate Tectonics
By 1968, a new theory developed – Plate Tectonics.
The theory of plate tectonics proposes that
Earth’s lithosphere consists of individual plates
that move as a result of mantle convection.
Plate tectonics explains how:
•
•
•
•
•
•
Continents are moved
Volcanoes are formed
Natural resources are recycled
Mountains are built
Rocks are made
Earthquakes happen
Plate Tectonics
~The Unifying Theory~
The theory of plate tectonics had a major
impact on Earth sciences. It represents a
scientific revolution as significant to geology as
relativity was to physics and evolution was to
biology.
In other words, plate tectonics is the unifying
theory for Earth science because it helps to
relate and explain many seemingly unrelated
phenomena.
Show What You Know!
1. What is seafloor spreading?
2. What is an oceanic ridge?
3. What two things are common at oceanic
ridges?
4. What is a constructive margin?
5. What is a destructive margin?
6. Which is older – continental or oceanic crust?
7. Where do you find the youngest oceanic crust?
The oldest?
8. How does paleomagnetism provide evidence
of seafloor spreading?
Answer the Essential Questions
• What is seafloor spreading?
• What is paleomagnetism and how
does it relate to seafloor spreading?
• How does seafloor spreading help
support Wegener’s hypothesis of
continental drift?
Seafloor Spreading Activity
Label your model with the following terms:
•
•
•
•
•
•
•
•
•
•
Asthenosphere
Convection Current
Mid-Ocean Ridge
Trench
North America
Europe
Old Crust
New Crust
Divergent Boundary
Convergent Boundary
I have a QUESTION for YOU!
What is the theory
that expanded on
continental drift?
Theory of
Plate Tectonics
Plate Tectonics
Objectives
At the end of this unit you will be able to…
• Explain the theory of plate tectonics, including
the evidence supporting the theory.
• Compare and contrast the three types of plate
boundaries and the features/formations
associated with each boundary.
• Summarize what is meant by plate-mantle
convection and explain the primary driving
forces of plate motion.
Essential Questions
• What is the theory of plate tectonics?
• What are lithospheric plates?
• What are the forces that drive plate
tectonics?
• What are the three types of plate
boundaries?
• What features are associated with each
type of plate boundary?
Plate Tectonics – The theory that
proposes that Earth’s outer shell consists of
individual plates that interact in various ways
and thereby produce earthquakes, volcanoes,
mountains and the crust itself.
placa tectonica
Lithospheric Plate – The lithosphere
(crust and upper mantle) is divided into
segments called plates. Also known as
tectonic or lithospheric plates.
placa de litosfera
Convection – The transfer of heat by the
movement of a substance (fluid). In mantle
convection, hot material at the deepest part of the
mantle rises and then cools, and sinks again,
repeating the cycle over and over. When
convection currents flow in the mantle they also
move the crust.
convección
Divergent Plate Boundary – Occurs
when two plates move apart. Most are
found on the seafloor. Also called a
spreading center or constructive
boundary.
Formations include:
• Mid-ocean ridge
• Rift valley.
divergente límite de placa
Convergent Plate Boundary – Form
where two plates move together. Also called
destructive boundary.
There are three types of convergent boundaries:
oceanic-oceanic, oceanic-continental and
continental-continental.
Formations include:
• Mountains
• Ocean trench
• Continental volcanic arc
• Volcanic island arc
convergente límite de placa
Transform Fault Boundary – A margin
where two plates grind past each other
without the destruction or production of
lithosphere. Also called a conservative
boundary. Example: San Andreas fault
falla (geológica) de transformación
Rift Valley – Deep, faulted structures
found along some segments of an oceanic
ridge or on land. A constructive plate
margin. Continental rift valleys are currently
forming in Africa and Iceland.
garganta de fisura; valle de fisura media; zona de grieta volcánica
Subduction Zone – A destructive plate
margin where oceanic crust is being
pushed down (subducted) into the mantle
beneath a second plate. Occurs at a
convergent plate boundary.
zona de subducción
Convergent Boundary: Oceanic - Oceanic
Convergent Boundary: Oceanic - Continental
Trench – A steep-walled valley on the
ocean floor, adjacent to a continental
margin. Produced by the descending
plate during subduction.
trinchera; fosa (océanica, submarina)
Continental Volcanic Arc – Mountains
formed in part by volcanic activity caused
by the subduction of oceanic lithosphere
beneath a continental lithosphere.
Volcanic Island Arc – A chain of
volcanic islands generally located a few
hundred kilometers away from a trench.
Form where subduction of one oceanic
slab beneath another is occurring.
Key Concepts
The theory of plate tectonics states that the
rigid lithosphere is divided into plates that
float on a plastic-like asthenosphere.
There are seven major lithospheric plates
and they are in constant motion with respect
to one another.
Convection currents in the mantle are
responsible for the movement of the
lithospheric plates.
Convection is the transfer of heat through
the movement of a substance (fluid).
Key Concepts
Heat, generated from the radioactive
decay of elements deep in the interior of
the Earth, creates magma (molten rock)
in the asthenosphere.
Convection currents are created in which
hot mantle rocks (magma) rise, and cold,
dense oceanic lithosphere sinks.
Convection in the mantle is thus due to
the unequal distribution of heat within
Earth.
Key Concepts
Ridge-push causes oceanic lithosphere to
slide down the sides of the ocean ridge.
Ridge-push occurs at a mid-ocean ridge.
Slab-pull is the primary downward arm
of convective flow in the mantle.
Slab-pull occurs at a subduction zone.
Key Concepts
Tectonic plates interact at places called
plate boundaries.
There are three main types of plate
boundaries: divergent, convergent and
transform fault.
At divergent boundaries, two tectonic
plates are moving away from each other.
Key Concepts
Divergent boundaries are also called
constructive margins because new crust is
created here.
Seafloor spreading occurs at a divergent plate
boundary. Mid-ocean ridges and rift valleys are
features found at divergent boundaries.
The youngest oceanic crust is at the crest of the
mid-ocean ridges. The oldest oceanic crust is at
the continental margins (trench).
Key Concepts
Convergent plate boundaries occur where two tectonic
plates are moving toward each other.
Convergent plate boundaries are also called destructive
margins because crust is destroyed here.
There are three types of convergent plate boundaries:
oceanic-oceanic; oceanic-continental, and; continentalcontinental.
Mountains, trenches, continental volcanic arcs and volcanic
island arcs are created at convergent plate boundaries.
Key Concepts
A subduction zone occurs when an oceanic
plate is forced down into the mantle beneath a
second plate (oceanic or continental).
At a transform fault boundary, plates grind past
each other without destroying the lithosphere.
Transform fault boundaries are also called
conservative margins.
Earthquakes occur at transform fault
boundaries.
Hot spot evidence supports the theory that the
plates move over Earth’s surface.
Plate Tectonics
• The theory of plate tectonics states that
Earth’s crust and rigid upper mantle are
broken into enormous slabs called plates.
• There are a seven major tectonic
(lithospheric) plates and several smaller
ones.
• Tectonic plates move in different directions
and at different rates.
Plate
Covers
Type
Africa
Continental
Antarctic
Antarctica
Continental
Australian
Australia
Continental
Indian Sub-continent and the
Indian Ocean
Continental
Europe and Asia
Continental
North America and part of
Siberia
Continental
South
American
South America
Continental
Pacific
Pacific Ocean
Oceanic
African
Indian
Eurasian
North American
Plate Boundaries
Tectonic plates interact at places called
plate boundaries. There are three types of
plate boundaries
Divergent Boundary
• Two tectonic plates are moving away from each
other. Also called a constructive plate boundary.
• Most are found on the sea floor and are associated
with mid-ocean ridges. Form rift valleys on
continents and in the ocean. Examples: East African
Rift Valley and the Mid-Atlantic Ridge.
• Can cause the growth of oceans, i.e., Atlantic Ocean
is growing at a rate of 2 – 3 cm/yr.
Iceland
Iceland
Iceland
East Africa Rift Valley
Divergent Boundary
• The plates are moving apart.
• They are mostly found on the sea floor, where they form
ocean ridges.
• High heat flow, volcanism, and earthquakes are
associated with these boundaries.
• Occurs only at oceanic-oceanic (mid-ocean ridges) or
continental-continental (East Africa rift valley)
East Africa Rift Valley
Convergent Boundary
• Two tectonic plates are moving toward each other. Also
called a destructive plate boundary.
• There are three types of convergent boundaries, classified
according to the type of crust involved:
• Continental-Continental - Mountains (Himalayas)
• Oceanic-Oceanic – Volcanic island arcs and ocean trenches
(Mariana Trench in the South Pacific)
• Oceanic-Continental - Volcanic Mountain Ranges (Andes
Mountains in western South America & Cascades in North
America)
Oceanic-Oceanic Boundary
• An oceanic plate descends beneath another
oceanic plate in a process called subduction,
which forms a deep-sea trench.
• Forms an arc of volcanic islands that parallel the
trench.
Aleutian Islands
Oceanic–Continental Boundary
• Oceanic crust subducts under the continental
crust.
• Produces a trench and continental volcanic
mountain ranges. Examples include the Andes
and the Cascades.
Continental–Continental Boundary
• Two continental plates collide.
• The colliding edges of the continents are crumpled
and uplifted to form mountain ranges. Examples
include the Himalayan Mountains, Appalachian
Mountains and the Alps.
Transform Boundary
• Two plates slide horizontally past each other. Also called
a conservative plate boundary.
• At this boundary crust is deformed or fractured and
characterized by long faults.
• Movement along the faults results in earthquakes.
• Oceanic-continental (San Andreas Fault) and oceanicoceanic (offsets along mid-ocean ridges).
San Andreas Fault
East of San Luis Obispo
A
E
B
A) Divergent Boundary:
Convergent Boundaries
B) Ocean – Ocean:
C) Ocean – Continental:
D) Continental – Continental:
E) Transform Fault Boundary:
C
Mid-ocean Ridge and Rift Valley
Volcanic Island Arc and Ocean Trench
Continental Volcanic Arc and Ocean Trench
Mountains
Earthquakes
What Causes Plate Movement?
• What actually causes tectonic plates to move is
not well understood.
• The most widely accepted theory is mantle
convection.
• Convection currents occur in the asthenosphere
due to the transfer of heat between Earth’s
hotter interior and its cooler exterior.
When the convection currents flow in the asthenosphere
they also move the crust. The crust gets a free ride with
these currents, like the wooden blocks in this illustration.
On Earth, convection
not only occurs in the
mantle, but also in the
atmosphere and the
oceans.
Push and Pull
• The formation of an oceanic ridge pushes an
oceanic plate toward the trench formed at
the subduction zone in a process called ridge
push.
• The weight of a subducting plate pulls the
lithosphere into the subduction zone in a
process called slab pull.
Show What You Know!
1. What is the theory of plate tectonics?
2. What is a plate boundary?
3. What are the three types of boundaries?
4. Describe a divergent boundary. What features occur at
a divergent plate boundary?
5. Describe a convergent plate boundary. What types of
convergent plate boundaries are there and what
features occur at each type?
6. What causes plate movement? What is responsible for
the “push” in plate movement? The “pull”?
H
I
C
B
A
B
D
G
A
E
F
Answer the Essential Questions
• What is the theory of plate tectonics?
• What are lithospheric plates?
• What are the forces that drive plate
tectonics?
• What are the three types of plate
boundaries?
• What features are associated with each
type of plate boundary?