Download Unit 11: Plate Tectonics

Unit 11: Plate Tectonics
A . Alfred Wegner
1. Continental drift hypothesis
a. single supercontinent called Pangaea
b. 200 million years ago Pangaea (all land) began to break up and started drifting to their
present positions
2. Evidences
a. Matching fossils - Mesosaurus fossils limited to South America and southern Africa
b. Rock Types and Structures - several mountain belts that end at one coastline, reappear on a
landmass across the ocean
3. Hypothesis rejected
a. Wegener could not describe a mechanism that was capable of moving the continents across
the globe
B. Theory of Plate Tectonics
1. The theory that proposes that Earth’s outer shell consists of
individual plates that interact in various ways and
thereby produce earthquakes, volcanoes, mountains,
and the crust itself
2. Lithosphere – rigid outer layer of Earth, including the crust
and upper mantle
a. divided into segments called plates seven major
plates
b. largest = Pacific plate
3. Move about 5 cm/yr (about as fast as your fingernail)
Types of Plate Boundaries
C. Divergent boundaries - when two plates move
apart
1. results in upwelling of material from the
mantle to create new seafloor
2. Oceanic ridge – seafloor that is
elevated along well developed
divergent boundaries
3. Rift valleys – deep faulted structures
found along the axes of some
segments
4. Seafloor spreading – The process in
which the ocean floor is extended
when two plates move apart, forming
a crack where magma can rise to the
surface, cooling and forming new
crust.
Ex: East African Rift valley
D. Convergent boundaries - when two plates move together
1. results in a subduction zone - when one oceanic plate is forced down into the mantle beneath a
second plate
2. creates an ocean trench
3. 3 types of convergent boundaries
a. Oceanic-Continental - Ocean plate (more dense) sinks down under continental plate
i. Can eventually cause volcanic eruptions
ii. Ex. Andes
b. Oceanic- Oceanic - One descends
beneath the other
i. Can cause volcanoes to form
on ocean floor
ii. Activity will eventually build a
chain of volcanic
structures that become
islands
called volcanic island arc
iii. Ex. Aleutian Islands off the
shore of Alaska
c. Continental-Continental - Plates collide
i. Causes formation of complex mountains
ii. Ex. Himalayas
iii. Urbanizing mountainsides leads to rosion, landslides, and water pollution
E. Transform fault boundaries - two plates grind past each other without the production or destruction of
lithosphere
1. can cause earthquake activity
2. Ex. San Andreas fault zone
F. Evidence for Plate Tectonics
1. Paleomagnetism - the study of the Earth’s magnetic field in the past – used as evidence for seafloor
spreading
a. Normal Polarity – when rocks show the same magnetism as the present magnetic field
b. Reverse polarity – rocks that show the opposite magnetism
c. the discovery of strips of alternating polarity, while lie as mirror images across the ocean
ridges, is among the strongest evidence of seafloor spreading
2. Earthquake patterns
3. Ocean drilling
4. Hot spots - a concentration of heat in the mantle capable of producing magma, which rises to Earth’s
surface
a. pacific plate moves over a hot spot, producing the Hawaiian Islands
b. supports the idea that the plates move over Earth’s surface
G. Mechanisms of Plate Motion
1. Convection – warm, less dense material rises and cooler,
denser material sinks
2. Convection Currents – transfer of thermal energy from warmer
regions of magma below the crust to cooler regions
a. convection occurring in the mantle is the basic
driving force for plate movement
3. Slab-Pull - occurs because old oceanic crust, sinks into the
asthenosphere and pulls the trailing lithosphere
along
a. thought to be the primary downward arm of
convective flow in the mantle
4. Ridge-Push - results from the elevated position of the
oceanic ridge system
a. causes oceanic lithosphere to slide down the sides f the oceanic ridge
b. less important than slab-pull
5. Mantle convection - mantle plumes are masses of hotter-than-normal mantle material that ascend
toward the surface
a. the unequal distribution of heat within Earth causes the thermal convection in the mantle
that ultimately drives plate motion