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 - EVIDENCE1) Fit of the Continents:

Atlantic Continental coastlines appear to fit together like a jigsaw puzzle. For example, South America and Africa.

Continental Drift - EVIDENCE2) 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 - EVIDENCE3) 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.

Similar Rock Types

Continuous Mountains

Continental Drift - Evidence4) 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

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!

SO WHAT WAS WRONG WITH THE THEORY/HYPOTHESIS OF CONTINENTAL DRIFT?

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

Holmes suggested that continents and the ocean floor move due to forces in the asthenosphere (i.e. upper mantle). NO SUCH THING AS DRIFITNG!

The question Wegener failed to answer was explained by a scientist named Arthur Holmes.

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

Harry Hess and Robert Deitz:Through observations of the ocean floor, these two guys proposed the “Sea Floor Spreading” hypothesis.

The sea-floor can be no greater than 200 million years old.

Convection Current

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.

Early Contributions To Plate Tectonics

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.

When plates collide at plate margins or when continents collide with other continents, mountain ranges are forced up.

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)

• 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.

• Plate boundaries can be mapped by tracing the ocean ridges, ocean trenches and fault zones.

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. Madefrom basalt (lava) and gabbro (magma).

• Continental crust is largely granitic in composition. Madefrom rhyolite (lava) and granite (magma).

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)

CreatedDestroyed

Sample Problem

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.

Explain why it is impossible for oceanic crust to be older than 200 million years.

Ocean Depths During World War II, geologists carried out

studies of the sea floor. Two important findings include:

2) Oceanic Trenches - deep trenches along the margins of continents, particularly surrounding the Pacific Ocean (i.e. the Pacific Ring Of Fire).

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.

1) Divergent Boundary

Two plates move apart. Upwelling molten material from the mantle creates new ocean floor.

Three Types of Plate Boundaries Include:

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.

Ocean Ocean Crust

Moho Magma Continental Crust

Basaltic magma * Look at directions on convection currents.

Mid-Ocean Ridges: ALSO CALLED CONSTRUCTIVE MARGINS SINCE NEW OCEAN CRUST IS BEING CREATED.

Ocean floor features:

Trenches

Mountain chains

Sea mounts

Fracture zones

Sediments on the Ocean Floor:

2) Convergent Boundary

Three Types of Plate Boundaries Include:

Ocean trenches are formed. Old crust is destroyed at these boundaries. Some can be really deep.

Compressional forces cause the plates to move together.

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.

Volcanic Island ArcTrenchOcean Crust

* Look at directions on convection currents.

Plate

Melts!

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!

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.

Three Types of Plate Boundaries Include:

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

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

Textbook – Figure 19.19 – Page 533

Evidence For Plate Tectonics

1) Earthquakes and Volcanoes

3) Polar wandering

4) Magnetic Reversals and Seafloor Spreading

5) Ocean Drilling

6) Hot Spots

Six Pieces of Evidence:

Reference: Tarbuck and Lutgens

2) Paleomagnetism

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

Shallow-FocusIntermediate -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 SpreadingThe 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.

Convection Currents Asthenosphere

Normal Polarity Reverse polarity

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

The age of the sediment increased with increasing distance from the ridge (on both sides).

Deep Sea Drilling Project collected convincing evidence for the theory of Plate Tectonics.

The age of the basaltic rocks increased with increasing distance from the ridge (on both sides).

Ocean Drilling – Evidence of Seafloor Spreading

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.

YoungerOlder

(1) Seafloor Sediment and (2) Age

Map showing ages of oceanic crust.

6) Hot Spots (i.e. intraplate volcanism)

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.

In the Pacific Ocean, a chain of volcanoes and seamounts extend from the Hawaiian Islands to the Aleutian trench.

Dates of the seamounts and islands shows that the age increases the farther away you go from Hawaii (and the hot spot).

Older

Asthenosphere

Hot Spot

First proposed by J. Tuzo Wilson

Yellowstone National Park is another example of this!

Plate Boundaries - Review1) Divergent Boundary

Ridge volcanism Seafloor Spreading

Tensional Forces Mid-Oceanic Ridges

Shallow-focus Earthquakes2) 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

Ocean trenches are formed at these boundaries.

Examples: Japan arc, Japan, Mariana Arc, Philippines, Aleutian Arc, Aleutian Islands.

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!

Subducting plate is basaltic. Subducting sediments are granitic. Mix the two and the result is andesitic magmas and lavas (diorite and andesite).

SHIELD VOLCANOES

2) Ocean – Continent Collisions

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.

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.

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

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.

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.

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

Over-riding Plate

Mountain Chain

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

South American

Plate

Pacific Plate

Mountains and ConvergentPlate Boundaries

2) Continent – Continent Convergent Boundary

Mountains and ConvergentPlate 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.

North American Plate

African Plate

Iapetus Ocean

Plate Tectonics and Newfoundland

This mountain range now exists throughout central and western Newfoundland; however, it has been weathering and eroding for quite some time.

The Geological Formation of Newfoundland

~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).

North Americam Plate

African Plate

New Divergent Boundary

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

New Divergent BoundaryAfrican Plate Left Behind

Plate Tectonics and Newfoundland

Humber (Western) Zone Has been a part of the North American plate for at least the last billion years.

The Geological Layout of Newfoundland

Humber Zone

Avalon Zone

Central Mobile Belt

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.

Sample Problem

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.

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.

AB C

Cape Ray/Baie Verte Line Fault

Hermitage-Dover Fault

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 DeformationFactors 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.

FaultsA break or crack in Earth’s crust (rock layers) along which subsequent motion occurs.

Fault

Three parts of a Fault include:

1) Hanging Wall - The top part of the rock above the fault plane/line.

Fault Plane

3) Fault Plane - The surface that separates the two moving pieces.

2) Foot Wall – The bottom part of the rock below the fault plane/line.

Rapid, continual tensional (or compressional) forces, usually at shallower depths, result in brittle deformation (faulting).

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

Caused by tensional forces.

Hanging wall drops in relation to the foot wall.

1) Normal Fault (dip-slip)

Hanging Wall

Foot Wall

2) Reverse Fault (dip-slip)

Caused by compressional forces.

Hanging wall moves upward in relation to the foot wall.

Dip-Slip Faults

An uplifted block of crust bounded by two normal faults.

Caused by tensional forces.

A valley formed by the downward displacement of a block of crust bounded by two faults.

Caused by tensional forces.

Horst (dip-slip)

Horst

Graben (dip-slip)

Graben

Dip-Slip Faults

Caused by Compressional forces.

Hanging wall moves upwards over foot wall.

3) Thrust Fault (dip-slip)

Low angle reverse fault. Less than 45 degrees

Hanging Wall

Foot Wall

Strike-Slip Faults

Caused by shearing forces.

Two plates slide side by side.

No vertical movement.

Transform Fault (strike-slip) 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

Caused by compressional forces.

Crust moves downward forming a valley.

Referred to as a down-fold.

Caused by compressional forces.

Crust moves upward forming a hill.

Referred to as an up-fold.

Parts of a fold include:

1) Anticline

Anticline

2) Syncline

Syncline

Draw a horizontal line. Is an A formed?