Download theory of plate tectonics

Plate Tectonics
continental drift = at one time earth had one giant
landmass that split apart and drifted to their present
positions
1. Eduard Suess (Austrian) hypothesized that present
southern continents had once been joined as a single
landmass; named it “Gondwanaland”
2. first scientific hypothesis proposed by Alfred Wegner
(1912)
3. continents have moved
horizontally to their current
locations
4. Pangaea = giant landmass when continents were joined
together
5. Panthalassa = huge ocean that surround Pangaea
6. rejected because he could not explain
a) what forces could move continents and
b) how continents could move without shattering
EVIDENCE FOR CONTINENTAL DRIFT
1. similarities in coastlines of continents
2. fossil clues
a. similar fossils found on different continents
b. similar fossils found in areas with different climates
Ex: Mesosarus = South America and Africa
Glossopteris = Africa, Australia, India, South
America, and Antarctica
3. geologic evidence
a. age and type of rocks in coastal regions of widely
separated areas matched closely
b. similar rock structures are found on different
continents
c. mountain chains that ended at coastline of one
continent seemed to continue on landmasses across
ocean
Ex: Appalachian Mountains of
US similar to mountains in
Greenland and western Europe
4. climate clues
a. layers of debris from glaciers in southern Africa,
South America, India, & Australia
b. coal deposits in United States, Europe, Antarctica, and
Siberia indicate tropical or subtropical swamps
covered land once
5. puzzle like fit of continents
6. sea-floor spreading = states that new ocean crust is
formed at ocean ridges and destroyed at deepsea trenches
a. Mid-Atlantic Ridge = undersea mountain range with a
steep, narrow valley running down its center
b. mid-ocean ridges = system of undersea mountain
ranges that wind around earth
Mid-ocean ridge
c. ocean floor was very young compared to age of
continental rocks
1) oldest seafloor rock = 150 million years
2) oldest continental rock = 4 billion years
3) composed of bands with youngest rocks at mid-ocean
ridge
4) older rocks are pushed away from ridge toward deepsea trench
d. Harry Hess = proposed that the ocean floor was moving
away from both sides of ridge
1) as it moved away, rising magma replaced it creating
new ocean as it cooled
2) if ocean floor was moving, the continents might also
be moving
e. movement named sea-floor spreading by Robert Dietz
f. observation of ocean-floor sediments shows that
thickness of sediment layer increases with distance from
ocean ridge
7. paleomagnetism = study of magnetic record in rocks
a. iron-bearing minerals in rock become permanently
magnetically oriented as rock hardens and points to
north
b. rocks with magnetic orientation pointing south
indicate a reversal in earth’s magnetic field
c. magnetic reversal = change in Earth’s magnetic field
1) normal polarity = rocks with magnetic fields
pointing north
[same orientation as Earth’s present field]
2) reverse polarity = rocks with magnetic fields
pointing south [opposite to present field]
3) found that rocks show many magnetic reversals
d. magnetic patterns on ocean floor
1) showed alternating band of normal and reversed
magnetism
2) stripped patterns of magnetism on one side of ridge
were mirrors images of stripped patterns on other side
3) isochron maps = used to show age of ocean floor
based on magnetic fields
4) isochron = line on map that connects points that have
same age
e. magnetometer = detects,
measures, and records
presence of weak
magnetic fields
THEORY OF PLATE TECTONICS
1. tectonics = study of formation of features in earth’s crust
2. plate tectonics = theory that combines ideas of
continental drift and seafloor spreading to explain
formation of crust, its movements, collisions, and
destructions
3. types of earth’s crust: oceanic crust and continental crust
4. lithosphere = oceanic and continental crust and rigid
upper mantle
a. thin outer shell of earth
b. less dense than material below which causes
movement of plates = broken into sections
1) have identified 30 so far
2) interact together to create major surface features
a) move toward each other and collide
b) moving apart
c) slide past one another
c. composed of granite, basalt or peridotite, solid
5. asthenosphere = layer of solid rock that slowly flows
when under pressure
a. has plasticity = characteristics of a solid, but also
flows like a liquid
b. plates of lithosphere float on top
c. bottom part of upper mantle; made up of peridotite
plate boundaries = areas where two plates interact
1. can be in middle of ocean floor, around edges of
continents, or within continents
2. not always easy to identify
TYPES OF BOUNDARIES
1. divergent boundary = where two plates are moving or
pulling apart
a. occurs at mid-ocean ridges (on ocean floor)
1) rift valley = narrow valley formed as plates separate
2) can form at mid-ocean ridges or where continents are
separated by plate movement
3) those that form on continents could eventually form
new ocean basin
b. also called constructive boundaries
c. involved in process of
seafloor spreading and
formation of new ocean crust
d. usually associated with
volcanic and earthquake activity
2. convergent boundary = direct collision of one plate with
another
a. also called destructive boundaries
b. three types
1) oceanic crust plate collides with continental crust
plate
a) subduction zone = region where one plate moves
under another plate
b) ocean plate is subducted below continental plate
c) creates deep-sea trenches, form volcanic
mountains, earthquakes common
d) series of volcanoes form
along edge of continental plate
Ex: Peru-Chile Trench
2) two continental crust plates collide
a) edges are crumpled and uplifted
b) produce large mountain ranges
c) earthquakes common
3) two oceanic crust plates collide
a) subduction zones present which form deep-sea
trenches
b) formation of island arcs = chain of volcanic islands
c) earthquake and volcanic activity common
Ex: Mariana Trench and Mariana Islands
3. transform fault boundary = where two plates grind past
one another
a. occur when two plates move in opposite directions or
same direction at different rates
b. plates move past one another horizontally
c. scrape together and move in series of sudden spurts of
activity separated by periods of little or no motion
d. crust is only deformed or fractured, most occur in
oceans
e. earthquakes commonly occur
Ex: San Andreas Fault
(formed on continent)
Causes of Plate Motion
1. convection = transfer of heat through the movement
of heated fluid material
2. convection currents = cycle of heating, rising,
cooling, and sinking
3. processes in cycle of heating and cooling carrying
plates with it
a. material in lower asthenosphere become hot and
will be less dense than material above it
b. material will rise until reaches base of lithosphere
where it cools
c. when it cools it becomes more dense and starts to
sink moving horizontally carrying plates
4. subduction zones involve combination of
mechanisms
a. sinking region of mantle convection pulls ocean
plate down into subduction zone
b. weight of subducting plate helps pull trailing
lithosphere into subduction zone = slab pull
theory of suspect terranes = suggests that continents are
actually a patchwork of terranes
terranes = pieces of lithosphere with its own distinct
geologic history
1. identifying characteristics
a. contains rock and fossils that differ from rock and
fossils of neighboring terranes
b. there are major faults at boundaries of terranes
c. magnetic properties do
not match those of
neighboring terranes
2. blocks of terranes are carried along on ocean floor by
seafloor spreading toward subduction zone
3. terranes are scraped off descending ocean floor
4. can form mountains or add to surface area of continent