CH. 8 – PLATE TECTONICS

Continental Drift

1915:

Alfred Wegener

- hypothesized the supercontinent Pangaea (“all land”)

Plate Tectonics

WWII New technology = echosounder-used to map ocean floor

topography

Sea-floor spreading

a) Ocean crust splits apart at mid-ocean ridges (MOR)

- pushes continents away from each other

Sea-floor spreading

b) Ocean crust pulled down into mantle at deep-sea trenches

New idea: ocean crust is moving!

Evidence for sea-floor spreading (late 1960’s - 1970’s)

1) Deep-sea Drilling Projects

Glomar Challenger

Discovered:

a) Ocean crust is __________

Deep-sea drilling projects

b) Sediment thickness varies:

- thicker further away from MOR

c) Sediment age:

- increases with distance from MOR

2) Submersible Expeditions

1974 FAMOUS expedition

(French-American Mid-Ocean Undersea Study)

Submersible Expeditions

Evidence for mafic eruptions: - hydrothermal vents (~350o C)- pillow lavas

Problem:

How does crust move over solid rock of mantle?

3) Clues: seismic waves

Lithosphere – rigid rx of crust and uppermost mantle

Asthenosphere – weaker rocks (partially molten) in upper mantle

Plate Tectonics Theory

Plate – rigid slab of lithosphere

7 major plates on Earth

Each plate moves independently

Same plate

Locations do not change position relative to one another

Ex: NYC & Peoria always same distance apart

Different plates

Locations do change position relative to one another

Ex: NYC & London getting further apart

Plate Boundaries

Identified by geologic activity:1) Earthquakes2) Volcanism

- mid-ocean ridges - circum-Pacific belt

EQ Foci

Volcanic Activity

Plate Boundaries

3) Mountain Ranges- indicates plate collisions

4) Deep-Sea TrenchesSubduction = one plate “dives”

beneath another

Subduction

Marked by Benioff Zone

- line of shallow- to deep-focus EQ’s along a deep-sea trench

Deep-Sea Trenches

NOTE: Most trenches located in Pacific Ocean

Atlantic Ocean is predominately a spreading center

Plates converge on Pacific Ocean side

Types of Plate Boundaries

1) Divergent – plates pull away from each other

Characteristics:a) shallow-focus EQ’sb) mafic magmac) rift valleys

Divergent Plate Boundaries

b) mid-ocean ridges

Ex: Iceland

b) continental rift valleys

Ex: East African Rift Valley

Red Sea

Types of Plate Boundaries

2) Convergent – plates collide

3 subtypes:

a) Ocean-Continental

- ocean plate subducted beneath continental plate

Ocean-Continent Convergent

Ocean-Continental Convergent

Characteristics:

Deep-sea trenches parallel continent

Benioff Zone

Ocean-Continental Convergent

Volcanic arcs on continental margin - source of magma is melting plate- composite cones

Ex: Andes Mtns., S. America Peru-Chile Trench

Cascade Mtns., N. America

Types of Convergent Boundaries

b) Ocean-ocean

Denser ocean plate is subducted

Ocean-Ocean Convergent

Characteristics:

Deep-sea trench

Benioff Zone

Volcanic island arcs (composite cones)

Ocean-Ocean Convergent

Ex: Japanese Islands

Aleutian Islands

Caribbean Islands

Indonesia

Types of Convergent Boundaries

c) Continent to Continent

Characteristics:

Shallow-focus EQ’s only

No volcanism

Complex folded mountains

Continent-Continent Convergent

Ex: Himalayan Mtns. (active)

Appalachian Mtns. (inactive)

Types of Plate Boundaries

3) Transform- plates slide laterally

Characteristics:- shallow-focus EQ’s only- crust neither created nor

destroyed

Transform Boundary

Ex: San Andreas Fault

North American Plate meets Pacific Plate

Hot Spot Volcanism (handout)

Associated with intraplate volcanism

Hot spots are stationary w/in mantle (“mantle plume”)

Exam 3

EQ & Plate Tectonic Study Guide

Map on back of PT Study Guide

Know: plate boundaries

characteristics

locations