1. Plate Tectonics
Chapter 6

2. Inside the Earth Section 1 The Earth is not a solid rock
Two ways scientists think about Earth’s layers –
by their composition = different mixtures of elements
by their physical properties = temperature, density, & ability to flow.

3. The Composition of the Earth
Section 1
The Composition of the Earth
3 layers of the Earth: crust, mantle, & core
The lightest materials make up the outermost layer, & the densest materials make up the inner layers.
This is because lighter materials tend to float up, while heavier materials sink.

4. The Composition of the Earth: Cont.
Section 1
The Composition of the Earth: Cont.
The Crust – less than 1% of the Earth’s mass
Crust = the outermost & thinnest layer
Two types of crust
Continental crust – has a composition similar to granite, has an average thickness of 30 km (mountains can be as much as 100 km thick)
Oceanic crust – has a composition similar to basalt, generally between 5 & 8 km thick
Oceanic crust is denser than continental crust
We know the most about this layer because it’s the one we live on.

5. The Composition of the Earth: Cont.
Section 1
The Composition of the Earth: Cont.
The Mantle – 67% of the Earth’s mass
Mantle = middle layer & extremely thick
No one has ever seen what the mantle really looks like. It’s too far to drill for a sample
In some places the rock has been pushed up to he surface by tectonic forces, allowing scientists to observe the rock directly.
Through underwater volcanoes scientists have learned that the mantles' composition is similar to that of the mineral olivine
2,900 km thick & contains most of the Earth’s mass
Scientist infer what the composition & other characteristics of the mantle are from observations they make on the Earth’s surface.
which has large amounts of iron & magnesium compared with other common minerals.

6. The Composition of the Earth: Cont.
Section 1
The Composition of the Earth: Cont.
The Core – 33% of the Earth’s mass
Core = extends from the bottom of the mantle to the center of the Earth
By studying the different layers that make up the Earth, geologists can get an idea of which elements each is made of.
They think that the Earth’s core is made mostly of iron, with smaller amounts of nickel & possibly some sulfur & oxygen.
Diameter of the core is about 6,856 km

7. The Structure of the Earth
Section 1
The Structure of the Earth
Earth is divided into 5 main physical layers
Lithosphere – “rock sphere” – The outermost rigid layer of the Earth. The lithosphere is made of two parts – the crust & the rigid upper part of the mantle. Unlike the other physical layers of the Earth, the lithosphere is not a single, solid layer. Instead, it is made up of pieces called tectonic plates.
thick

8. The Structure of the Earth: Cont.
Section 1
The Structure of the Earth: Cont.
Asthenosphere – “weak sphere” – is a soft layer of the mantle on which pieces of the lithosphere move. It is made of solid rock that flows very slowly, like putty. Low-strength rocks like those in the asthenosphere tend to lose their shape when stressed. Like warm tar, they flow very slowly – at about the same rate your fingernails grow.
250 km

9. The Structure of the Earth: Cont.
Section 1
The Structure of the Earth: Cont.
Mesosphere – “middle sphere” – it extends from the bottom of the asthenosphere down to the Earth’s core. Outer Core – the liquid layer of the Earth’s core that lies beneath the mantle & surrounds the inner core. Inner Core – the solid, dense center of our planet that extends from the bottom of the outer core to the center of the Earth, 6,378 km beneath the surface.
2,550 km
2,200 km
1,228 km

10. Tectonic Plates Section 1
Tectonic Plates = pieces of the lithosphere the move around on top of the asthenosphere.
A Giant Jigsaw Puzzle
But what exactly does a tectonic plate look like? How big are tectonic plates? How & why do they move around?
Notice
That each tectonic plate fits the other tectonic plates that surround it.
Not all tectonic plates are the same size
Don’t contain the same things (some have continents while others only oceanic curst while others have both)

11. Tectonic Plates: Cont. Section 1 A Tectonic Plate Close-up
Thickest part of the plate is under continental mountains
Thinnest part are in the oceanic crust (valleys especially)
Tip of the Iceberg
Why doesn’t the thick mountains of the continental crust sink into the mantle?
Because though it’s thicker it’s also less dense which means that it floats on top of the mantle like ice on top of water.

12. Mapping the Earth’s Interior
Section 1
Mapping the Earth’s Interior
If we have never even drilled through the Earth’s crust then how do we know so much about the layers under it?
Earthquakes cause seismic waves. Seismic waves are vibrations that travel through the Earth. Depending on the density & strength of the material they pass through, seismic waves travel at different speeds. For example, a seismic wave traveling through solid rock will go faster than a seismic wave traveling through a liquid.
When an earthquake occurs, seismographs measure the difference in the arrival times of seismic waves & record them. Seismologists can the use these measurements to calculate the density & thickness of each physical layer of the Earth.

13. Inside the Earth: REVIEW
Section 1
Inside the Earth: REVIEW
What is the difference between continental & oceanic crust?
How is the lithosphere different from the asthenosphere?
How do scientists know about the structure of the Earth’s interior? Explain.
Explain the difference between crust & the lithosphere.

14. Restless Continents Section 2
You can take the contents and fit them together like a jigsaw puzzle.
Is this just a coincidence?
Is it possible that the continents were actually together sometime in the past?

15. Wegener’s Theory of Continental Drift
Section 2
Wegener’s Theory of Continental Drift
In the early 1900s Alfred Wegener wrote about his theory of continental drift.
Continental Drift = the theory that continents can drift apart from one another and have done so in the past.
This theory seemed to explain a lot of puzzling observations, including the very good fit of some of the continents.
Why fossils of the same plant & animal species are found on both sides of the Atlantic Ocean
Similar types of rock & evidence of the same ancient climatic conditions were found on several continents.
Evidence left by ancient glaciers (glaciers cut grooves in the ground that indicate the direction they traveled.) When you look at the placement of todays continents, these glacial activities do not seem to be related. But when you bring all of these continental pieces back to their originals arrangement, the glacial grooves fit together!

16. The Breakup of Pangaea Section 2
245 Million years ago – Pangaea – “all earth” – existed when some of the earliest dinosaurs were roaming the Earth. It was surrounded by a sea called Panthalassa – “all sea”
180 million years ago – Gradually Pangaea broke into two big pieces. The northern piece is called Laurasia. The southern piece is called Gondwana.
65 million years ago – By the time the dinosaurs became extinct, Laurasia & Gondwana had split into smaller pieces.

17. Sea-Floor Spreading Section 2
Mid-Atlantic Ridge – chain of submerged mountains running through the center of the Atlantic Ocean There is a worldwide system of ocean ridges. Mid-ocean ridges are places where sea-floor spreading takes place.
Sea-Floor Spreading = the process by which new oceanic lithosphere is created as older materials are pulled away. As tectonic plates move away from each other, the sea floor spreads apart & magma rises to fill the gap.
When Wegener put forth his theory of continental drift many scientists would not accept his theory. What force in nature, they wondered could move entire continents? He couldn’t answer that question. It wasn’t until years later that new evidence provided a clue.
The newest crust is in the center of the ocean. The oldest crust in the Atlantic Ocean is found along the edges of the continents. It dates back to the time of the dinosaurs.

18. Magnetic Reversals Section 2
Magnetic reversal – when the Earth’s magnetic poles change places.
Throughout Earth’s history, the north & south magnetic poles have changed places many times.
How do we know this?
The molten rocks at the mid-ocean ridges contain grains of magnetic minerals. These minerals grains act like compasses. They align with the magnetic field of the Earth. Once the molten rock cools, the record of these compasses is literally set in stone.
This record is then carried slowly away from the spreading center as the sea floor spreading occurs.

19. Restless Continents: REVIEW
Section 2
Restless Continents: REVIEW
List three puzzling occurrences that the theory of continental drift helped to explain, & describe how it explained them.
Explain why Wegener’s theory of continental drift was not accepted at first.
Explain how the processes of sea-floor spreading & magnetic reversal produce brands of oceanic crust that have different magnetic polarities.

20. The Theory of Plate Tectonics
Section 3
The Theory of Plate Tectonics
The proof of sea-floor spreading supported Wegener’s original idea that the continents move.
But because both oceanic & continental crust appear to move, a new theory was devised to explain both continental drift & sea-floor spreading – the theory of plate tectonics.
Plate Tectonics = the theory that the Earth’s lithosphere is divided into tectonic plates that move on top of the asthenosphere.
What causes tectonic plates to move?

21. Possible Causes of Tectonic Plate Motion
Section 3
Possible Causes of Tectonic Plate Motion
An incredible amount of energy is needed to move something as massive as a tectonic plate.
Scientists still don’t know why tectonic plates move as they do but they have 3 theories.
Ridge Push – at mid-ocean ridges, the oceanic lithosphere is higher than it is where it sinks beneath continental lithosphere. Ridge push is the process by which an oceanic plate slides down the slope of the lithosphere-asthenosphere boundary.

22. Possible Causes of Tectonic Plate Motion: Cont.
Section 3
Possible Causes of Tectonic Plate Motion: Cont.
Convention – in the process of convection, hot material from deep within the Earth rises while cooler material near the surface sinks. When the warmer material cools, it becomes denser & begins to sink back down. The motion of convecting mantle material drags tectonic plates sideways. Slab Pull – when an oceanic plate collides with a continental plate, the oceanic plate is forced down into the asthenosphere. Because it is denser than material in the asthenosphere, the edge of the oceanic plate sinks & pulls the rest of the tectonic plate with it in a process called slab pull.

23. Tectonic Plate Boundaries
Section 3
Tectonic Plate Boundaries
All tectonic plates have boundaries with other tectonic plates. These boundaries are divided into 3 categories. They collide, separate, or slide past each other.
Convergent Boundaries
Convergent Boundaries = when two tectonic plates push into one another.
What happens at a convergent boundary depends on what kind of crust – continental or oceanic – the leading edge of each tectonic plate has. There are 3 kinds

24. Tectonic Plate Boundaries: Cont.
Section 3
Tectonic Plate Boundaries: Cont.
Continental/continental collisions – they buckle & thicken, pushing the continental crust upward. This creates some of the world’s tallest mountain ranges, such as the Himalayas.
Continental/oceanic collisions – the oceanic plate slides under the continental plate. This happens because the oceanic plates are more dense than continental plates. subduction zone – the region where oceanic plates sink into the asthenosphere.
Oceanic/oceanic collisions – one of the oceanic plates slides under the other.

25. Tectonic Plate Boundaries: Cont.
Section 3
Tectonic Plate Boundaries: Cont.
Divergent Boundaries
Divergent Boundaries = when two tectonic plates move away from one another. (where new lithosphere forms)
Moving apart – At a divergent boundary, two tectonic plates move apart from one another. As they move apart, magma rises to fill the gap. At a mid-ocean ridge, the rising magma cools to form new oceanic lithosphere. Divergent boundaries can also be found in the continents.

26. Tectonic Plate Boundaries: Cont.
Section 3
Tectonic Plate Boundaries: Cont.
Transform Boundaries
Transform Boundaries = when two tectonic plates slide past each other horizontally (San Andreas Fault)
Sliding past – at a transform boundary, two tectonic plates slide past one another. Because tectonic plates are not smooth, they grind & jerk as they slide.

28. Tracking Tectonic Plate Motion
Section 3
Tracking Tectonic Plate Motion
How fast do tectonic plates move?
It depends on many factors
Type of tectonic plate
The shape of the tectonic plate
The way it interacts with the tectonic plates that surround it.
Tectonic movements are generally so slow & gradual that you can’t see or feel them – they are measured in cm per year.
Global Positioning System is used to track the movement of the tectonic plates.
The exception to this is earthquakes. They can suddenly shift several meters.

29. The Theory of Plate Tectonics: REVIEW
Section 3
The Theory of Plate Tectonics: REVIEW
List & describe three possible driving forces of tectonic motion.
How do the three types of convergent boundaries differ from one another?
Explain how scientists measure the rate at which tectonic plates move.
When convection takes place in the mantle, why does cooler material sink, while warmer material rise?

30. Deforming the Earth’s Crust
Section 4
Deforming the Earth’s Crust
Stress = the amount of force per unit area that is put on a given material.
The conditions under which a rock is stressed determine its behavior.

31. Rocks Get Stressed Section 4
Deformation – when a rock changes its shape due to stress
Compression = stress that occurs when an object is squeezed (as when two tectonic plates collide)
Tension = stress that occurs when forces act to stretch an object. (happens at a divergent boundary)
Rocky mountains & the Cascade Range are two examples of compression at the convergent plate boundary.

32. Section 4
Folding
Folding = occurs when rock layers bend due to stress in the Earth’s crust.
It is assumed that all sedimentary rock layers start out as horizontal layers.
Anticlines – a ridge-shaped fold of stratified rock in which the strata slope downward from the crest
Synclines – a trough or fold of stratified rock in which the strata slope upward from the axis.
Monocline – a single bend in rock.

33. Section 4
Faulting
While some rocks bend (fold) when stress is applied, other other conditions rock layers break.
Fault = the surface along which rocks break & slide past each other
Fault blocks – the blocks of crust on each side of the fault.
Hanging wall – top of the wall is larger than the base (hangs over)
Foot wall – the bottom of the wall is larger than the base.
Check out the picture on page 153

34. Faulting: Cont. Section 4 Normal Faults
Normal Fault = movement causes the hanging wall to move down relative to the footwall. Normal faults usually occur when tectonic forces cause tension that pulls rocks apart.
Reverse Faults
Reverse fault = movement causes the hanging wall to move up relative to the footwall. Reverse faults usually happen when tectonic forces cause compression that pushes rocks together.
153

35. Faulting: Cont. Section 4 Telling the Difference
Look at the strata to see which is a normal fault & which is a reverse fault.
Strike-slip Faults
Strike-slip Faults = occurs when opposite forces cause rock to break & move horizontally. If you were standing on one side of a strike-slip fault looking across the fault when it moved, the ground on the other side would appear to move to your left or right.
San Andreas fault is a strike-slip fault.

36. Plate Tectonics & Mountain Building
Section 4
Plate Tectonics & Mountain Building
The three most common types of mountains
Folded Mountains
Form when rock layers are squeezed together & pushed upward.
Fault-block Mountains
Form when this faulting causes large blocks of Earth’s crust to drop down relative to other blocks
Volcanic Mountains
Form when molten rock erupts onto the Earth’s surface.
Appalachian – folded mountains – were once huge but have been wore down by weathering & erosion.
Tetons – fault-block mountains – jagged peaks
Located at convergent boundaries – Ring of Fire – form from new material being added to the Earth’s surface.

37. Deforming the Earth’s Crust: REVIEW
Section 4
Deforming the Earth’s Crust: REVIEW
What is the difference between an anticline & a syncline?
What is the difference between a normal fault & reverse fault?
Name & describe the type of tectonic stress that forms folded mountains.
Name & describe the types of tectonic stress that forms fault-block mountains.
If a fault occurs in an area where rock layers have been folded, which typed of fault is it likely to be? Why?