Download Plate Tectonics “The Grand Unifying Theory”

Plate Tectonics
“The Grand Unifying Theory”
Tectonics
• Study of the origin and arrangement of
broad structural features of Earth’s surface.
Plate Tectonics
• Idea that the Earth’s surface is divided into
a few large, thick plates that move slowly
and change in size through time.
• There are eight major plates and twelve
smaller plates.
Plate Tectonics Model
• Combined two pre-existing ideas:
– Continental Drift
• Continents move freely over Earth’s surface
changing positions through time
– Seafloor Spreading
• Seafloor forms at crest of mid-ocean ridges, then
moves symmetrically away from ridge toward
oceanic trenches.
Continental Drift
• Proposed by Alfred Wegener in early
1900’s.
• Evidence cited included:
– Striking fit between east coast of South
America and west coast of Africa.
– Similarities between continents, including:
• Ancient climate and glaciation 150 million years
ago on land areas presently within 30° of the
equator.
• Fossils of reptiles (like Mesosaurus) and plants
• Rock strata and structures including mountain belts.
Wegener’s Model
• 200 million years ago
– Supercontinent of Pangea consisting of
modern-day North America, Eurasia, South
America, Africa, India, Antarctic, Australia
separated into two parts.
• Laurasia - consisting of N. A. and Eurasia.
• Gondwanaland - consisting of S. A., Africa, India,
Antarctic and Australia.
Wegener’s Model
• 135 million years ago
– Africa and South America began splitting apart
• 120 million years ago
– India split from Gondwanaland
• 65 million years ago
– North America split from Eurasia
• 45 million years ago
– India collided with Eurasia
Mechanism of Continental Drift
• Larger sturdier continents “plow” though
oceanic crust.
• Driven by combination of:
– centrifugal forces of Earth’s rotation, and
– gravitational forces like tidal drag of moon and
sun.
• But, oceanic crust is too strong to be broken
by proposed forces.
• Hence, model was not widely accepted.
Seafloor Spreading
• Harry Hess (1962) proposed seafloor was
moving as well as continents.
• Evidence cited included:
– Topography of ocean floors (mid-ocean ridges,
transform faults, ocean trenchs and guyots)
– Marine magnetic anomalies/reversals
– Paleomagnetism
– Location and depth of earthquakes (Benioff
zone) and style of volcanism
– Age of seafloor rocks and sediments
– Hotspot traces and ocean island chains
Hess’s Model
• Seafloor is moving like a conveyer belt
from mid-ocean ridges toward ocean
trenches.
• Defined:
– Spreading center - ridge crest, with seafloor
moving away from it.
– Subduction - sliding of seafloor beneath a
continent or island arc.
• Spreading rates range from 1 to 16 cm/year
Mechanism of Seafloor Spreading
• Driven by deep-mantle convection.
• Convection refers to slow circulation of a
substance driven by differences in
temperature (heat) and density within that
substance.
• Hot, less dense magma and rock rises at
mid-ocean ridges and cool, denser rock
sinks at the trenches.
Topography of Seafloor
• Extensive studies of the topography of the
seafloor were initiated during WWII.
• Important findings included existence of:
–
–
–
–
Mid-ocean ridge system
Transform faults
Deep ocean trenches
Guyots
Mid-ocean Ridge Systems
• Continuous marine mountain chain that
encircles the globe.
• Total length exceeds 80,000 km.
• Ridge rises an average of 3 km above
surrounding seafloor.
• Rift valleys, 1-2 km deep, split the ridge
crests.
Transform Faults
• Hundreds of fractures cut across rift valley
and mid-ocean ridges.
• Fractures extend through entire thickness of
lithosphere.
• Offset ridge by <1 km to 100’s of km.
• Location of shallow, low magnitude
earthquakes.
Deep Ocean Trenches
• Long, narrow, steep-sided depressions.
• Depths of 8 to 11 km.
• Dip of up to 15°
Guyots
• Flat-topped seamounts.
• Seamounts are submarine mountains of
volcanic origin that rise 1 km or more above
surrounding seafloor
• Hess proposed that guyots formed as ocean
islands, eroded flat by wave action, slowly
subsided as the oceanic plate cooled and
moved away from the ridge.
Marine Magnetics
• Mid-1960’s, magnetic surveys of seafloor
indicated magnetic anomalies arranged in
bands parallel to the rift valley of mid-ocean
ridges.
• Alternating positive (normal) and negative
(reverse) magnetic anomalies form stripelike pattern parallel to ridge crest.
Vine-Matthews Hypothesis
• Pattern of magnetic anomalies is
symmetrical about ridge crest.
• Same pattern of magnetic anomalies exists
over different parts of mid-ocean ridges.
• Pattern of magnetic anomalies at sea match
pattern of magnetic reversals established
from studies of continental lava flows.
Vine-Matthews Hypothesis
• Proposed origin of magnetic anomalies
– (see fig. 19.16 of textbook):
• During time of normal magnetism, series of basalt
dikes intrude at ridge crest and become normally
magnetized.
• Dike zone is torn in half and moves away from ridge
valley as a new group of reverse magnetized dikes
form at ridge crest.
• Process continues through time producing a
symmetrical pattern of normal and reverse
magnetized rocks about the ridge crest.
Paleomagnetism
– Study of ancient positions of the continents
relative to magnetic poles.
– Iron-rich minerals like magnetite can act as
“fossil compass”.
– When lava cools through Curie point, magnetite
crystals acquire the direction of earth’s
magnetic field at that time.
Paleomagnetism
– In 1950’s, it was discovered that magnetic
alignment of lava flows of different ages varied
widely, but in a systematic fashion.
– Best explanation for apparent “polar
wandering” is that the plates have changed
locations through time.
Earthquakes and Volcanism
–Pattern of earthquake and volcanic activity is
strongly correlated with boundaries of plates.
–Earthquake depths and magnitudes, and the style
of volcanism also correlated with type of plate
boundary.
Boundary
Earthquakes
Volcanism
Divergent
Transform
Convergent
shallow, low magnitude
shallow, low-high magnitude
shallow-deep, low-high magnitude
(Benioff zone)
passive basaltic volcanism
no volcanism
violent andesitic volcanism
Age of Seafloor & Sediments
• Seafloor Drilling (ODP)
• Dating of microfosssils in sediments
indicates youngest oceanic crust is at ridge
crest and oldest is at ocean trenches.
• No sediment >160 million years old has
been found in ocean basins.
• Dates of sediments and rocks match
estimates from magnetic reversal patterns.
Hotspot Traces and Ocean
Island Chains
• Hotspots are narrow columns of hot mantle
magma/rocks associated with deep plumes.
• As seafloor moves over a hotspot, a chain of
ocean islands and/or seamounts forms.
• Dating of Hawaiian islands - Emperor
Seamount chain showed systematic increase
in ages going away from current volcanism.
• Show direction and rate of plate motions.
Plate Tectonics
• Definitions
– Plate Tectonics - theory that the Earth’s surface
is divided into a few large, thick plates that are
slowly moving and changing in size.
– Plates are segments of the lithosphere made of
rigid, strong rock that move as a unit over the
ductile asthenosphere.
Plate Boundaries
• Tectonic activity is concentrated at plate
boundaries where plates interact with each
other.
• There are three types of boundaries based
upon relative motion of the plates.
Divergent - plates are moving apart.
Convergent- plates are moving toward each
other.
Transform - plates are moving horizontally past
each other.
Divergent Plate Boundaries
• Marked by:
–
–
–
–
–
Rift valleys
Shallow focus earthquakes
Normal faulting
High heat flow
Passive basaltic volcanism
Transform Plate Boundaries
• Marked by:
– Shallow focus earthquakes (some high
magnitude)
– Strike-slip faulting
– Absence of volcanism
Convergent Plate Boundaries
• Cause subduction or continental collision.
• Marked by:
– Deep trenches
– Shallow to deep focus earthquakes (Benioff
zone)
– Reverse faulting
– low heat flow
– Violent andesitic volcanism
– Young mountain ranges or island arcs
Ocean-Ocean Convergent
Boundary
• Deep oceanic trench
– Accretionary wedge
• Volcanic island arc
– Violent andesitic volcanism
• Benioff zone of earthquakes
– Reverse faulting (compression) along slab
edges
– Normal faulting (tension) within slab
• Example: West Aleutian Islands
Ocean-Continent Convergent
Boundary
• Oceanic trench
– Accretionary wedge
• Magmatic arc
• Benioff zone
• Young mountain belt on edge of continent
– Uplift by crustal thickening
– Backarc thrust (reverse) faults
• Example: Andes mountain chain
Continent-Continent Convergent
Boundary
• Suture zone
– Shallow, large magnitude earthquakes
• Thrust belts and subsiding basins
• Crustal thickening by:
– Shallow underthrusting of one continent
– Accretion of original island arc
– Stacking of thrust sheets
• Mountain belt in interior of continent
• Example: Himalaya Mountains
Characteristics and Examples of Plate Boundaries
Type of
Boundary
Divergent
Convergent
Transform
Type of
Plate
Geologic
Features
Geologic
Events
Modern
Examples
Ocean-Ocean
Mid-ocean ridge
Mid-Atlantic ridge
Continent-Continent
Rift Valley
Sea-floor spreading
shallow earthquakes
basaltic volcanism
normal faulting
Continent torn apart
shallow earthquakes
basaltic volcanism
normal faulting
Ocean-Ocean
Island arcs and
ocean trenches
West Aleutians
Ocean-Continent
Mountains and
ocean trenches
Continent-Continent
Mountains
Subduction
deep earthquakes
andesitic volcanism
reverse faullting
Subduction
deep earthquakes
andesitic volcanism
reverse faulting
deep earthquakes
reverse faulting
Ocean-Ocean
Offset of mid-ocean
ridge axis
Small mountain
ranges
Shallow earthquake
strike-slip faulting
Shallow earthquakes
Strike-slip faults
East Pacific Rise
Continent-Continent
East African Rift
Andes
Himalayas
San Andreas fault
Plate Motion
• Caused by:
– Ridge push
• Plates cool, thicken, and subside while moving away
from ridge axis (plate slides downhill).
– Slab pull
• Dense cool slab sinking at steep angle through hot
mantle “pulls” the slab along.
– Trench suction
• When plates fall into mantle at angles steeper than
their dip, trench literally “sucks-down” slab.
Summary
• Plate Tectonics explains:
– Distribution and composition of volcanoes.
– Distribution and relative magnitude of
earthquakes.
– Occurrence of young mountain belts.
– Seafloor features
• Mid-ocean ridges, ocean trenches, fracture zones,
seamounts, etc.