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Plate Tectonics
A.Structure of Earth’s Interior• Earth’s interior consists of three
major zones defined by their chemical composition—the Crust, Mantle, and Core.
• Determined by sending seismic waves through the earth
I. EARTH
Seismic Waves Paths Through the Earth
1. CRUST• Thin, rocky outer layer• Varies in thickness
- Roughly 7 km in oceanic regions- Continental crust averages 8–40 km- Exceeds 70 km in mountainous regions
I. EARTH
A. Structure of Earth’s Interior
A. Structure of Earth’s Interior
I. EARTH
1. Crust• Continental crust
- Upper crust composed of granitic rocks
- Lower crust is more akin to basalt- Average density is about 2.7 g/cm3
- Up to 4 billion years old
A. Structure of Earth’s Interior
I. EARTH
1. Crust
• Oceanic crust - Basaltic composition - Density about 3.0 g/cm3 - Younger (180 million years or
less) than the continental crust
A. Structure of Earth’s Interior
I. EARTH
2. Mantle
• Below crust to a depth of 2900 kilometers
• Composition of the uppermost mantle is the igneous rock peridotite (changes at greater depths).
A. Structure of Earth’s Interior
I. EARTH
Lithosphere
• Crust and uppermost mantle (about 100 km thick)
• Cool, rigid, solid
2. Mantle
2. Mantle
I. EARTH
Asthenosphere
• Beneath the lithosphere• Upper mantle
• To a depth of about 660 kilometers• Soft, weak layer that is easily deformed
A. Structure of Earth’s Interior
2. Mantle
I. EARTH
Lower Mantle• 660–2900 km • More rigid layer
• Rocks are very hot and capable of gradual flow.
A. Structure of Earth’s Interior
I. EARTH
3. CORE• Below mantle
• Sphere with a radius of 3486 kilometers
• Composed of an iron-nickel alloy
• Average density of nearly 11 g/cm3
A. Structure of Earth’s Interior
3. CORE
I. EARTH
Inner Core
• Sphere with a radius of 1216 km• Behaves like a solid
A. Structure of Earth’s Interior
3. CORE
I. EARTH
Outer Core• Liquid layer • 2270 km thick• Convective flow of metallic iron within
generates Earth’s magnetic field
A. Structure of Earth’s Interior
Earth’s Layered Structure
4. Earth’s Composition
I. EARTH
Mantle
Crust• Early seismic data and drilling technology
indicate that the continental crust is mostly made of lighter, granitic rocks.
• Composition is more speculative.• Some of the lava that reaches Earth’s
surface comes from asthenosphere within.
A. Structure of Earth’s Interior
4. Earth’s Composition
I. EARTH
Core• Earth’s core is thought to be mainly
dense iron and nickel, similar to metallic meteorites. The surrounding mantle is believed to be composed of rocks similar to stony meteorites.
A. Structure of Earth’s Interior
4. Earth’s Layers
I. EARTH
• Velocity of seismic waves increases abruptly below 50 km of depth
• Separates crust from underlying mantle
Moho (Mohorovičić discontinuity)
A. Structure of Earth’s Interior
4. Earth’s Layers
I. EARTH
Shadow Zone
• Absence of P waves from about 105 degrees to 140 degrees around the globe from an earthquake
• Can be explained if Earth contains a core composed of materials unlike the overlying mantle
A. Structure of Earth’s Interior
Earth’s Interior Showing P and S Wave Paths
I. EARTH
B. DensityThe relationship of mass and volume in an objectMass per unit volume
OrThe concentration of material in a given amount of spaceSI Unit: g/ml or g/cm3
Density =Mass
Volume
I. EARTH
C. Metric System1.BASE UNITS•Gram (g)•Liter (l)•Meter (m)2. PREFIXES•Kilo (k) = 1,000•Hecto (h) = 100•Deca (da) = 10
•King Hector Died, (base) drinking chocolate milk
}1• milli (m) = 0.001• centi (c) = 0.01• deci (d) = 0.1
A. An Idea Before Its Time
II. Theory of Continental Drift
1. Wegener’s continental drift hypothesis stated that the continents had once been joined to form a single supercontinent.a. Wegener proposed the supercontinent,
Pangaeab. It began to break apart 200 million years
ago and form the present landmasses.
Breakup of Pangaea
A. An Idea Before Its Time
II. Theory of Continental Drift
2. Evidencea. The Continental Puzzleb. Matching Fossils
- Fossil evidence for continental drift includes several fossil organisms found on different landmasses.
A. An Idea Before Its Time
II. Theory of Continental Drift
2. Evidence
d. Ancient Climates
c. Rock Types and Structures- Rock evidence for continental exists in
the form of several mountain belts that end at one coastline, only to reappear on a landmass across the ocean.
Matching Mountain Ranges
Glacier Evidence
B. Rejecting the Hypothesis
II. Theory of Continental Drift
1. A New Theory Emerges• Wegener could not provide an
explanation of exactly what made the continents move. News technology lead to findings which then lead to a new theory called plate tectonics.
III. Plate Tectonics
According to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere.• A plate is one of numerous rigid
sections of the lithosphere that move as a unit over the material of the asthenosphere.
A. Causes of Plate Motion
III. Plate Tectonics
1. Types of Heat (energy) Transfera) Convection – transfer of heat through
a fluidb) Conduction – transferred from
molecule to molecule by direct contact (solids)
c) Radiation – transfer of heat by means of electromagnetic waves (no matter needed)
HEAT TRANSFER
A. Causes of Plate Motion
III. Plate Tectonics
2. Convectiona. Takes place in the asthenosphereb. Movement in the fluid caused by the
changes in densityc. Heating → molecules move further
apart → less dense → rise hits ceiling → molecules , closer together → more dense → cools → sinks → continues
Mantle Convection Models
Mantle Convection Models
III. Plate Tectonics
B. Plate Boundaries• Place where plates meet• faults form along these boundaries• Plate boundaries are where the plate
move and cause earthquakes• Ring of Fire
III. Plate Tectonics
B. Plate Boundaries
III. Plate Tectonics
1. Boundary Types a. Divergent boundaries (also called spreading
centers) are the place where two plates move apart. Tension Stress
b. Convergent boundaries form where two plates move together. Compression Stress
c. Transform fault boundaries are margins where two plates grind past each other without the production or destruction of the lithosphere. Shearing Stress
Three Types of Plate Boundaries
Stress at Plate Boundaries
2. Divergent Boundariesa. Oceanic Ridges and Seafloor Spreading
i. Oceanic ridges are continuous elevated zones on the floor of all major ocean basins. The rifts at the crest of ridges represent divergent plate boundaries.
III. Plate Tectonics
2. Divergent Boundariesa. Oceanic Ridges and Seafloor Spreading
ii. Rift valleys are deep faulted structures found along the axes of divergent plate boundaries. They can develop on the seafloor or on land.iii. Seafloor spreading produces new
oceanic lithosphere.
III. Plate Tectonics
Spreading Center
3. Divergent Boundaries
III. Plate Tectonics
a. Continental Riftsi. When spreading centers develop
within a continent, the landmass may split into two or more smaller segments, forming a rift.
East African Rift Valley
Convergent Boundaries
9.3 Actions at Plate Boundaries
A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate.
• Denser oceanic slab sinks into the asthenosphere.
Oceanic-Continental
• Pockets of magma develop and rise.• Continental volcanic arcs form in part by
volcanic activity caused by the subduction of oceanic lithosphere beneath a continent.
• Examples include the Andes, Cascades, and the Sierra Nevadas.
Oceanic-Continental Convergent Boundary
Convergent Boundaries
9.3 Actions at Plate Boundaries
• Two oceanic slabs converge and one descends beneath the other.
Oceanic-Oceanic
• This kind of boundary often forms volcanoes on the ocean floor.
• Volcanic island arcs form as volcanoes emerge from the sea.
• Examples include the Aleutian, Mariana, and Tonga islands.
Oceanic-Oceanic Convergent Boundary
Convergent Boundaries
9.3 Actions at Plate Boundaries
• When subducting plates contain continental material, two continents collide.
Continental-Continental
• This kind of boundary can produce new mountain ranges, such as the Himalayas.
Continental-Continental Convergent Boundary
Collision of India and Asia
Transform Fault Boundaries
9.3 Actions at Plate Boundaries
At a transform fault boundary, plates grind past each other without destroying the lithosphere.
Transform faults • Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel the direction of plate movement.
• They aid the movement of oceanic crustal material.
Transform Fault Boundary
Evidence for Plate Tectonics
9.4 Testing Plate Tectonics
Paleomagnetism is the natural remnant magnetism in rock bodies; this permanent magnetization acquired by rock can be used to determine the location of the magnetic poles at the time the rock became magnetized.• Normal polarity—when rocks show the same
magnetism as the present magnetism field
• Reverse polarity—when rocks show the opposite magnetism as the present magnetism field
Paleomagnetism Preserved in Lava Flows
Evidence for Plate Tectonics
9.4 Testing Plate Tectonics
The discovery of strips of alternating polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading.
Polarity of the Ocean Crust
Evidence for Plate Tectonics
9.4 Testing Plate Tectonics
Earthquake Patterns• Scientists found a close link between deep-focus
earthquakes and ocean trenches. • The absence of deep-focus earthquakes along
the oceanic ridge system was shown to be consistent with the new theory.
Evidence for Plate Tectonics
9.4 Testing Plate Tectonics
Ocean Drilling• The data on the ages of seafloor sediment
confirmed what the seafloor spreading hypothesis predicted.
• The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the continental margins.
Evidence for Plate Tectonics
9.4 Testing Plate Tectonics
Hot Spots• A hot spot is a concentration of heat in the
mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands.
• Hot spot evidence supports that the plates move over the Earth’s surface.
Hot Spot
Causes of Plate Motion
9.5 Mechanisms of Plate Motion
Slab-Pull and Ridge-Push
• Ridge-push causes oceanic lithosphere to slide down the sides of the oceanic ridge under the pull of gravity. It may contribute to plate motion.
• Slab-pull is a mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along. It is thought to be the primary downward arm of convective flow in the mantle.
Causes of Plate Motion
9.5 Mechanisms of Plate Motion
Mantle Convection
• The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion.
• Mantle plumes are masses of hotter-than-normal mantle material that ascend toward the surface, where they may lead to igneous activity.
Mantle Convection Models