Layers Defined by Composition

8.4 Earth’s Layered Structure

Earth’s interior consists of three major zones defined by their chemical composition—the crust, mantle, and core.

• Thin, rocky outer layer

Crust

• Varies in thickness

- ~7 km under oceans

- Continental crust averages 8–40 km- Exceeds 70 km in mountainous regions

Layers Defined by Composition

• Continental crust Average density is about 2.7 g/cm3Up to 4 billion years old• Oceanic crustDensity about 3.0 g/cm3Younger than the continental crust

(180 million years or less)

**When these two collide, what do you think will happen???

Earth’s Layered Structure

The Mantle• 82% of Earth’s volume• Melted rock, flows in convection current

The Core• Inner – behaves like a solid (under great pressure)• Outer – liquid layer (so hot)• Composition is iron and nickel

Density = 11 g/cm3 - similar to metallic meteorites.

A CLOSER LOOK INSIDE EARTH

Layers Defined by Physical Properties

Lithosphere

*Crust and uppermost mantle (about 100 km)

*Cool, rigid, solid

Asthenosphere

*Beneath the lithosphere*Upper mantle

*To a depth of about 660 km

*Soft, weak layer that is easily deformed

Moho Discontinuity

• Separates crust from underlying mantle

Shadow Zone • Absence of P waves because of bending of waves

• Can be explained if Earth contains a core

composed of materials unlike the overlying mantle

Discovering Earth’s Layers

Layers of the Earth - Review

An Idea Before Its Time

9.1 Continental Drift

Wegener’s continental drift hypothesis- continents had once been joined to form a single supercontinent.

• Wegener proposed that the supercontinent, Pangaea, began to break apart 200 million years ago and form the present landmasses.

Breakup of Pangaea

4 Pieces of Evidence

9.1 Continental Drift

1. The Continental Puzzle- pieces fit together, South America and Africa

2. Matching Fossils – same dinosaur and plant fossils found on different land masses

9.1 Continental Drift

3. Rock Types and Mountain Ranges

- Several mountain belts that end at one coastline, only to reappear on a landmass across the ocean.

Present Day Map Pangea

Glacier Evidence

4. Ancient Climates & Glaciers

Rejecting the Hypothesis

9.1 Continental Drift

A New Theory Emerges Wegener could not provide an explanation of

exactly what made the continents move.

New technology led to findings which then led to a new theory called plate tectonics.

Wegener’s theory was ignored until the mid-1960’s, when structures on the ocean floor gave evidence of a mechanism for the movement of continents.

DO NOT COPY – JUST READ!

Earth’s Major Roles

9.2 Plate Tectonics

The uppermantle, along with the crust, behaves as a solid layer- known as the lithosphere.

• A plate is one of numerous solid sections of the lithosphere that move as a unit over the asthenosphere – molten, melted rock in the mantle

Types of Plate Boundaries

9.2 Plate Tectonics

Divergent boundaries (spreading centers) where two plates move in different directions.

Convergent boundaries two plates come closer

Transform fault boundaries two plates travel past each other

Three Types of Plate Boundaries

Different directions Coming closer Travel Past

Plate Tectonics

Divergent Boundaries

9.3 Actions at Plate Boundaries

Oceanic Ridges and Seafloor Spreading

1. Oceanic ridges are small mountain ranges on the ocean floor built from magma coming up in the gap of the boundary.

2. Seafloor spreading produces new oceanic lithosphere.

Divergent Boundaries

9.3 Actions at Plate Boundaries

Continental Rifts3. On land, a divergent plate boundary creates a

rift valley.

African Rift Valley

Spreading Center

Divergent Boundaries

Examples: Mid-Atlantic Ridge, East African Rift Valley, Rhine Valley in Northern Europe

Seafloor Spreading Clip

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

• Examples: Andes, Cascades, and the Sierra Nevada's.

Oceanic-Continental Convergent Boundary

Subduction Zone

Features formed: Trench, Continental Volcanic Arc

Convergent Boundaries

9.3 Actions at Plate Boundaries

• Two oceanic slabs converge and one descends beneath the other- forming a volcano on the ocean floor

Oceanic-Oceanic

.• Volcanic island arcs

form as volcanoes emerge from the sea.

Examples: Aleutian Islands in Alaska

Oceanic-Oceanic Convergent Boundary

Convergent Boundaries

9.3 Actions at Plate Boundaries

• When two continents collide forming mountain ranges (very little subduction occurs here)

Continental-Continental

Examples: Himalayas, Alps, Appalachians, Ural Mts.

Collision of India and Asia

Transform Fault Boundaries

9.3 Actions at Plate Boundaries

Plates travel past each other without destroying the lithosphere.

Transform faults

• Usually occur at mid-ocean ridges.

• Example: San Andreas Fault

Activity at boundary will result in an earthquake!

Evidence for Plate Tectonics

9.4 Testing Plate Tectonics

Paleomagnetism-magnetization that can be used to determine the location of the magnetic poles at the time the rock was formed – reversed many times in history

• Normal polarity —when rocks show the same magnetism as the present magnetic field

• Reverse polarity —when rocks show the opposite magnetism as the present magnetic field

9.4 Evidence for Plate Tectonics

1. The discovery of strips of alternating polarity is the strongest evidence of seafloor spreading.

9.4 Evidence for Plate Tectonics

2. Earthquake Patterns

• Scientists found a close link between deep-focus earthquakes and ocean trenches. (subduction zones)

• The absence of deep-focus earthquakes along the oceanic ridge system (divergent boundaries) was shown to be consistent with the new theory.

9.4 Evidence for Plate Tectonics

3. Ocean Drilling• The ages of seafloor sediment confirmed the

seafloor spreading hypothesis is correct

• The youngest oceanic crust is at the divergent plate boundary, and the oldest crust is farthest away (on land)

9.4 Evidence for Plate Tectonics

4. 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.

How Hawaii formed supports the evidence behind plate tectonics…

Causes of Plate Motion

9.5 Mechanisms of Plate Motion

Convection occurring in the mantle is the driving force for plate movement.

• Convective flow is the motion of matter resulting from changes in temperature.