Download File - Mr. Snelgrove

Unit 4
THE FORCES WITHIN EARTH
Theory of Continental Drift
 A German scientist, Alfred Wegener, propose that
continents had once been together. His publication was
called, “The Origin of Continents and Oceans.”
 Wegener called his idea, “Continental Drift”.
Alfred Wegener - 1912
• Proponent of the Continental
Drift Theory/Hypothesis
• German meteorologist
Old Idea
• There was a commonly held belief that
continents simply ‘drifted’ through the
ocean floor pushing up material in front,
forming mountain belts.
• THINK OF A BOAT MOVING
THROUGH WATER PUSHING
WATER UP IN FRONT OF IT!
Theory of Continental Drift
 Hundreds of millions of years ago, a supercontinent called
“Pangaea” existed. At approximately 200 million years ago,
this landmass split into smaller pieces and drifted apart
pushing up folded mountains until they reached their
present positions.
Continental Drift - EVIDENCE
1) Fit of the Continents:
Atlantic Continental coastlines
appear to fit together like a
jigsaw puzzle. For example,
South America and Africa.
Continental Drift - EVIDENCE
2) Fossil Correlation:
Fossils of similar organisms
found in matching fossil beds
on different continents. E.G.
South America and Africa.
E.g. Fossil fern plant
(Glossopteris) and an aquatic
reptile (Mesosaurus).
Wegener’s Fossil Correlation
Continental Drift - EVIDENCE
3) Rock Types and Structures:
Type of rock on neighboring continents, shield
areas, line up.
E.G. Shield area of South America and shield
area of Africa line up.
On neighboring continents, structural
similarities such as folded mountains line up .
E.g. The Appalachian Mountains and
Caledonian Mountains.
Continuous
Mountains
Similar
Rock
Types
Continental Drift - Evidence
4) Ancient Climates: Paleoclimates
Glacial deposits found in South
America, Africa, India, and
Australia line up.
Coal deposits that form in
tropical swamps line up on
different continents.
Glacial striations in South
America and Africa line up
when continents are reunited.
Glacial Striations
SO WHAT WAS WRONG WITH THE
THEORY/HYPOTHESIS OF CONTINENTAL DRIFT?
Main criticism centered around how the continents move
(i.e. the mechanism).
Wegener suggested that Earth’s rotation and lunar
gravitational forces (i.e. moon’s tidal influence) caused
continents to drift (in a westward motion). He also
suggested that large, sturdier continents broke through the
oceanic crust (like an ice breaker).
BUT IS THIS REALISTIC?
BOO WEGENER!
Early Contributions To Plate Tectonics
Alexander DuToit:
 Mapped the fit of the continents more accurately
by using the continental shelves instead of the
actual coastlines. E.g. Africa and South America.
Also proposed the formation of Laurasia and
Gondwanaland. (Two pieces from the split of
Pangaea).
Early Contributions To Plate Tectonics
 The question Wegener failed to answer was explained by a
scientist named Arthur Holmes.
 Holmes suggested that continents and the ocean floor move
due to forces in the asthenosphere (i.e. upper mantle). NO
SUCH THING AS DRIFITNG!
 Mantle material moves up at ridges and move away in
opposite directions moving the ocean floors and continents.
WHAT IS BEING TALKED ABOUT HERE? Convection
Currents.
Convection Current
Convection Current
Early Contributions To Plate Tectonics
Harry Hess and Robert Deitz:
Through observations of the ocean floor, these two
guys proposed the “Sea Floor Spreading” hypothesis.
Convection
Current
The sea-floor can be no greater than 200 million years old.
Early Contributions To Plate Tectonics
Convection and Seafloor Spreading
 Convection currents in the asthenosphere,
proposed by Arthur Holmes, and the seafloor
spreading idea, propose by Harry Hess and Robert
Deitz, provides evidence for a mobile Earth.
 By combining the seafloor spreading theory with
continental drift ( which can now be better
explained by convection currents) and earthquake
information, the new Theory of Plate Tectonics
arose, which could better explain crustal
movements.
J. Tuzo Wilson – CANADIAN!
 Mapped earthquakes and volcanoes over
several years.
He put forth the idea that “Earth consisted
of several fragments called plates. Not just
one rigid, solid layer! There are 21 known
plates!
Huge contribution to the proposed Plate
Tectonic Theory.
Besides the idea of plates, Wilson also discovered stationary
hotspots, which explained volcanism within plates. He also
discovered transform faults along divergent plate boundaries.
(WILL DEAL WITH THESE LATER).
Plate Tectonic Theory
Theory of Plate Tectonics States:
 “Earth’s crust is divided into approximately twenty (21)
rigid slabs called tectonic plates.”
 These tectonic plates are in continuous slow motion
relative to each other.
 As the plates move (thanks to the convection currents in the
asthenosphere) the continents are carried along on top like a
conveyor belt. (DRAW DIAGRAM ON WHITEBOARD)

When plates collide at plate margins or when continents
collide with other continents, mountain ranges are forced
up.
• Tectonic Plate is a massive, irregularly shaped slab of
solid rock. (i.e. slabs of the lithosphere).
• See Figure 19.17 on Pages 528-529. Shows the plates
being moved by convection currents.
The edges of these plates are called boundaries.
• Plates are composed of oceanic crust only or oceanic
and continental crust.
• Oceanic crust is mostly basaltic in composition. Made
from basalt (lava) and gabbro (magma).
• Continental crust is largely granitic in composition. Made
from rhyolite (lava) and granite (magma).
• Plate boundaries can be mapped by tracing the ocean
ridges, ocean trenches and fault zones.
Plate Boundary Type
• It is determined by the motion of the plates.
 Because oceanic lithosphere is created at ridges and
destroyed at subduction zones (trenches), the oceanic
basins are continuously being recycled and are relatively
young.( ~200mya)
Destroyed
Created
Sample Problem
Explain why it is impossible for oceanic crust to be older than
200 million years.
Answer:
A geologic process called sea floor spreading causes the
ocean floor to move.Ocean floor is created at oceanic ridges
and is destroyed or consumed within Earth at subduction
zones. This process of recycling the ocean floor occurs
within a span of 180 - 200 million years.
Ocean Depths
 During World War II, geologists carried out studies of the
sea floor. Two important findings include:
1) Oceanic Ridges - long sinuous ridges that occupy the
middle of the Atlantic Ocean. They also exist in the Indian
Ocean and the eastern part of the Pacific Ocean.
2) Oceanic Trenches - deep trenches along the margins of
continents, particularly surrounding the Pacific Ocean (i.e.
the Pacific Ring Of Fire).
Three Types of Plate Boundaries Include:
1) Divergent Boundary
 Two plates move apart. Upwelling molten material from the mantle
creates new ocean floor.
 Features on the ocean floor are called ridges. This is where new
ocean crust (ocean floor) is created.
 Tensional forces cause the two plates to move apart.
* Look at directions
on convection
currents.
 Basaltic magma
Ocean
Moho
Ocean Crust
Magma
Continental Crust
Mid-Ocean Ridges: ALSO CALLED CONSTRUCTIVE MARGINS
SINCE NEW OCEAN CRUST IS BEING CREATED.
Ocean floor features:
Trenches
Sea mounts
Mountain chains
Fracture zones
Sediments on the Ocean Floor:
Three Types of Plate Boundaries Include:
2) Convergent Boundary
 Two plates move together. One slab of the lithosphere (i.e. one plate)
is consumed into the mantle. It descends beneath the overriding plate.
This is called subduction.
Ocean
Crust
 Ocean trenches are formed.
Old crust is destroyed at
these boundaries. Some can
be really deep.
 Compressional forces cause
the plates to move together.
* Look at directions
on convection
currents.
Trench
Volcanic Island Arc
Harry’s Idea
Pacific Ocean Floor: Look at the Aleutian Trench
WHY IS THE EARTH NOT GETTING ANY BIGGER?
Ridge volcanism at divergent plate boundaries is
responsible for the construction of new oceanic crust.
BUT
Subduction zones at convergent plate boundaries are
responsible for the destruction of old oceanic crust.
RECYCLING OF CRUST!
Three Types of Plate Boundaries Include:
3) Transform Boundary (Discovered By J. Tuzo Wilson)
 Two plates move past each other (grind) in opposite
directions. Lithosphere (or crust) is not created or
destroyed at these boundaries.
 No vertical movement. For example, San Andreas Fault in
California and Alpine Fault in New Zealand .
 Shearing forces cause the plates to move past one another.
These forces are caused by the directions of the convection
currents.
Birds Eye View
or
Top View
J. Tuzo Wilson’s transform faults!
Ridges are not totally linear!
Transform faults often link divergent and convergent plate boundaries!
Divergant Boundaries and Rift Valleys
Textbook – Figure 19.19 – Page 533
Upwelling of molten material from the mantle
creates tensional forces.
This tension pulls the continents apart causing
the central region to drop down (normal
faulting) forming a Rift Valley (top diagram).
Continents split!
Tensional forces cause the plates to continually
move apart forming new ocean crust and a new
ocean when the crust thins enough (middle
diagram).
A rift valley has evolved into a divergent plate
boundary when ridge volcanism and seafloor
spreading is occurring. Two segments move
apart continually in opposite directions.
E.g. East African Rift Valley
Evidence For Plate Tectonics
Six Pieces of Evidence:
1) Earthquakes and Volcanoes
2) Paleomagnetism
3)
4)
5)
6)
Polar wandering
Magnetic Reversals and Seafloor Spreading
Ocean Drilling
Hot Spots
Reference:
Tarbuck and Lutgens
1) Earthquakes and Volcanoes
 Earthquakes and volcanoes occur in rather limited belts.
These belts mark the locations of Plate Boundaries.
 The largest active belt of
earthquakes and volcanoes
in the world is “The
Pacific Ring of Fire”.
Mt.
Fuji
DOTS = LOCATIONS OF EARTHQUAKES AND VOLCANOES
Hugo Benioff suggested that this Earthquake pattern shows a plate
subducting (sinking) into the mantle. Wadati-Benioff Zone
Intermediate -Focus
Shallow-Focus
Deep-Focus
Draw Map View Of
This On Whiteboard!
2) Paleomagnetism (Fossil Magnetism)
 Iron-rich minerals become magnetized in the direction of the magnetic
field at the time when the rock solidified.
If the rocks move or if the magnetic poles change, the rocks retain
“remember” the location of the magnetic poles at the time they formed.
When the shield rocks on the continents (e.g. North America and
Europe) were tested their ancient magnetism no longer lines up with
the modern magnetic poles. Therefore, the continents (and the plates
they are on) must have moved.
Draw Map View Of This
On Whiteboard!
Shields of N.A. and Europe
on opposite sides of the
Mid-Atlantic Ridge!
3) Polar Wandering
 A plot of this magnetism showed that the magnetic pole appeared
to change position considerably over the past 500 million years.
 Either the magnetic pole had moved with time, known as polar
wandering, or the rocks had moved, continental drift and/or
plate tectonics.
 Plate Tectonic theory is believed to be the best explanation (Not
Continental Drift).
4) Magnetic Reversals and Seafloor Spreading
 The earth’s magnetic field reverses polarity (south becomes north
and north becomes south). Basaltic lavas solidifying during a time
of reverse polarity would display opposite magnetism as rocks
forming today.
 Rocks with magnetism the same as our present magnetic field is
said to have normal polarity, while rocks with opposite polarity is
said to have reverse polarity.
 Can be seen in:
1) successive lava flows on volcanoes; and
2) the basaltic rock making up the ocean floor (seafloor spreading).
Magnetic Reversals and Seafloor Spreading
 At oceanic ridges, the plates move apart and new basaltic rock
is added to each plate. The magnetism of these basaltic rocks
appear to alternate to produce identical magnetic patterns on
both sides of oceanic ridges. This is strong evidence to support
seafloor spreading and therefore, plate tectonics.
Normal Polarity
Convection Currents
Reverse polarity
Asthenosphere
Showing seafloor spreading at a ridge (divergent plate boundary). You
can also see both normal and reverse polarity (same on both sides).
5) Ocean Drilling
 Deep Sea Drilling Project collected convincing evidence for the
theory of Plate Tectonics.
 The age of the sediment increased with increasing distance
from the ridge (on both sides).
 The age of the basaltic rocks increased with increasing distance from the
ridge (on both sides).
Ocean Drilling – Evidence of Seafloor Spreading
(1) Seafloor Sediment and (2) Age
 Sediment thickness increases in both directions away from
the ridge.
 The age of the oceanic crust (rocks) increases in both
directions away from the ridge.
Younger
Older
Map showing
ages of
oceanic crust.
6) Hot Spots (i.e. intraplate volcanism)
 In the Pacific Ocean, a chain of volcanoes and seamounts extend
from the Hawaiian Islands to the Aleutian trench.
 A plume of magma exists beneath Hawaii, and the Pacific plate
moves over this stationary magma chamber (i.e. hot spot). Each
time the plate moves, the magma burns through to create a new
volcanic island. This confirms that the tectonic plates do move.
 Dates of the seamounts and
islands shows that the age
increases the farther away you
go from Hawaii (and the hot
spot).
First proposed by J. Tuzo
Wilson
Yellowstone National Park is
another example of this!
Older
Asthenosphere
Hot
Spot
Plate Boundaries - Review
1) Divergent Boundary
Ridge volcanism
Seafloor Spreading
Tensional Forces
Mid-Oceanic Ridges
Shallow-focus Earthquakes
2) Convergent Boundary
Subduction volcanism
Compressional Forces
Trenches
Earthquakes
Three types: O-O, O-C, and C-C.
3) Transform Boundary
No volcanoes
Shallow-focus Earthquakes
Three Types of Convergent Plate Boundaries:
1) Ocean – Ocean Collisions
 Compressional forces cause plates to move together (both of equal
density), one slab of lithosphere is subducted into the mantle initiating
volcanic activity which creates volcanoes. (i.e. Island Arcs) Curved
arc on the surface!
 Ocean trenches are formed at these boundaries.
 Subducting plate is basaltic.
Subducting sediments are
granitic. Mix the two and the
result is andesitic magmas and
lavas (diorite and andesite).
SHIELD VOLCANOES
 Examples: Japan arc,
Japan, Mariana Arc,
Philippines, Aleutian Arc,
Aleutian Islands.
2) Ocean – Continent Collisions
 Compressional forces cause an ocean plate and a continent plate to
move together. The more dense ocean plate sinks into the
asthenosphere. This region called a subduction zone.
 Lithosphere is destroyed.
 At depths, the oceanic plate melts producing magmas, which rise
and melts its way up through the continental crust. The basaltic
mantle and basaltic oceanic crust melts together with continental
rocks (granitic) to produce granitic magma and lava (granite and
rhyolite).
Some magma may reach the surface and erupt through composite
volcanoes as violent volcanic eruptions.
A volcanic arc is created on the surface of the continent.
 Examples include Rocky Mountains and Andes Mountains.
Ocean – Continent Convergent Boundary
 If the subduction occurs beneath continental crust, a
continental volcanic arc is produced. Composite volcanoes
that are granitic in composition (granite and rhyolite).
3) Continent – Continent Collisions
 Compressional forces cause two continental plates to move together.
Because of the low density of continental crust neither plate will
subduct and the two plates ram into one another forming mountains.
 Such a collision occurred when India collided with Asia forming the
Himalayas. An ocean existed between two continents and both had
continental shelves. The compressed shelves create the mountains
once the oceanic crust has been subducted and the process of
subduction ceases! The Appalachians in Newfoundland is another
example.
Mountain Building
 The processes that produce a mountain system is orogenesis.
Mountain systems show evidence of great forces.
 The building up of mountains is directly related to plate tectonics
and in particular, Convergent Plate Boundaries (two types of
collisions relate to mountain building).
Mountains and Convergent Plate Boundaries
1) Ocean – Continent Convergent Boundary
Mountain
Chain
Over-riding
Plate
Mountains and Convergent Plate Boundaries
1) Ocean – Continent Convergent Boundary
 The Andes Mountains in South America and the Rocky
Mountains in Western Canada are examples.
Andes
Mountains
Pacific
Plate
South
American
Plate
Mountains and Convergent
Plate Boundaries
2) Continent – Continent Convergent Boundary
Mountains and Convergent
Plate Boundaries
2) Continent – Continent Convergent Boundary
Himalayan Mountains
Plate Tectonics and Newfoundland
~ 600 million years ago, North America was part of a much
larger supercontinent (Rodina). This supercontinent broke
apart. A mid-ocean ridge developed along the break and the
Iapetus Ocean began to open.
 ~540 million years ago, the convection currents shifted
and subduction began. As the continents “drifted” or
pushed together, the ocean floor was squeezed and then
pushed upward to form the Appalachian Mountains.
Subduction and abduction occurred! Volcanoes were
also occurring! Once the Iapetus Ocean had been
subducted and abducted, a new supercontinent formed
called Pangaea.
Iapetus Ocean
North
American
Plate
African
Plate
Plate Tectonics and Newfoundland
The Geological Formation of Newfoundland
 This mountain range
now exists throughout
central and western
Newfoundland;
however, it has been
weathering and
eroding for quite some
time.
 ~225 million years ago, the forces in Earth’s
mantle again reversed and slowly began to pull
the continents apart again (divergent plate
boundary again). The Atlantic Ocean began to
open and is still opening (to the east of the island).
New Divergent Boundary
North Americam Plate
African Plate
 During this process a small bit of Africa got left behind!
The eastern part of Newfoundland was once a part of
the African plate.
North Americam Plate
African Plate Left Behind
New Divergent Boundary
African Plate
Plate Tectonics and Newfoundland
The Geological Layout of Newfoundland
 Humber (Western) Zone
Has been a part of the North American
plate for at least the last billion years.
 Central (Zone) Mobile Belt
Remnants of island arcs and the
ancient Iapetus ocean crust.
 Avalon (Eastern) Zone
Once part of the African plate, which
remained attached as Pangaea split
200 million years ago.
Humber
Zone
Central
Mobile
Belt
Avalon
Zone
Sample Problem
Use the diagram and your knowledge of the
theory of Plate Tectonics to explain how the
three geologic zones of the island portion of
Newfoundland and Labrador were formed.
Cape Ray/Baie
Verte Line Fault
HermitageDover Fault
A
B
Answer:
It is thought that the geology of the Island of Newfoundland
resulted long ago when the Iapetus Ocean was closing. The North
American plate collided with the African plate and as a result a
portion of the Iapetus Ocean floor was sandwiched in between.
Zone “A” is referred to as the Western Zone and was part of the
North American plate. Zone “C” is referred to as the Eastern
Zone and is thought to be part of the African plate. Zone “B” is
referred to as the Central Mobile Belt (or Central Zone) and is
thought to be once part of the ancient Iapetus Ocean.
C
Crustal Deformation
 Deformation- a general term that refers to all changes in the
original form and /or size of a rock body. It may also produce
changes in the location and orientation of rocks. MOST
CRUSTAL DEFORMATION OCCURS ALONG PLATE
BOUNDARIES!
 Force - that which tends to put a stationary object in motion or
change the motion of moving bodies.
Stress- the amount of force applied to a given area.
Three types of forces/stresses that produce crustal deformation:
1.) Compressional -
A force or pressure that attempts to flatten
or squeeze a material. ( reduces volume).
Stresses or forces that shorten a rock body.
2.) Tensional -
A stretching force on an object. Stresses or
forces that elongate a rock body.
3.) Shear-
Any external force acting perpendicular to the material.
Stresses that cause two separate rock bodies to slide past
one another.
Crustal Deformation
Three types of deformation include:
1.) Elastic-
A temporary shape change that is self-reversing
after the force is removed. The object returns
to its original shape.
2.) Brittle-
Deformation that results in fracturing or breaking
and is permanent.
3.) Ductile-
When rocks bend or flow, like clay, it is
permanent. (solid state flow)
Crustal Deformation
Factors affecting deformation:
1.) Temperature
The colder the Earth materials, the more
brittle the deformation will be. The warmer the
Earth materials, the more elastic or ductile the
deformation will be.
2.) Confining Pressure
Will only result in elastic or ductile
deformation. Brittle deformation
typically occurs on the surface with no
pressure from above.
3.) Rock Type
All rock types can be deformed, but
sedimentary rocks could be more
easily deformed since they are softer.
4.) Time
Elastic or ductile deformation occurs over a
long period of time. Rapid deformation (short
time) tends to cause brittle deformation.
Faults
Fault A break or crack in Earth’s crust (rock layers) along
which subsequent motion occurs.
Rapid, continual tensional (or compressional) forces, usually at shallower
depths, result in brittle deformation (faulting).
Three parts of a Fault include:
1) Hanging Wall - The top part of the rock above the fault plane/line.
2) Foot Wall – The bottom part of the rock below the fault plane/line.
3) Fault Plane - The surface that separates the two moving pieces.
Fault Plane
HW
FW
You could draw a vertical
line down through the
rock blocks. You will “hit”
the HW first and you will
“hit” the FW second.
Two Major Types of Faults:
1) Dip-Slip
-
Normal (i.e. tensional forces)
Horst and graben (i.e. tensional forces)
Reverse (i.e. compressional forces)
Thrust (i.e. compressional forces)
2) Strike-Slip (i.e. transform)
- Left-lateral (shear forces)
- Right-lateral (shear forces)
Dip-Sip Faults
1) Normal Fault (dip-slip)
 Caused by tensional forces.
 Hanging wall drops in relation to the foot wall.
Hanging Wall
2) Reverse Fault (dip-slip)
 Caused by compressional forces.
Foot Wall
 Hanging wall moves upward in relation to the foot wall.
Dip-Slip Faults
Horst (dip-slip)
 An uplifted block of crust
Graben
bounded by two normal faults.
 Caused by tensional forces.
Graben (dip-slip)
 A valley formed by the
downward displacement of a
block of crust bounded by two
faults.
 Caused by tensional forces.
Horst
Dip-Slip Faults
Hanging Wall
3) Thrust Fault (dip-slip)
 Caused by Compressional forces.
 Hanging wall moves upwards over foot wall.
 Low angle reverse fault. Less than 45 degrees
Foot Wall
Strike-Slip Faults
Transform Fault (strike-slip)
 Caused by shearing forces.
 Two plates slide side by side.
 No vertical movement.
Place yourself
on a fault block
and ask
yourself which
way the
opposite block
is moving. Left
or right?
Folds
The bending of rock layers into a wave like pattern due to slow,
continual, compressional forces, usually at depth. (i.e. ductile
deformation).
Folds
Parts of a fold include:
1) Anticline
 Caused by compressional forces.
 Crust moves upward forming a hill.
 Referred to as an up-fold.
Anticline
2) Syncline
 Caused by compressional forces.
 Crust moves downward forming a valley.
 Referred to as a down-fold.
Syncline
Draw a horizontal line.
Is an A formed?