Lecture4platetect CA

ISNS 4359 Earthquakes and
Volcanoes
(aka shake and bake)
Lecture 4: Plate Tectonics

Fall 2005


Plate Tectonics
Tectonic cycle:
• Melted asthenosphere flows upward as magma
• Cools to form new ocean floor (lithosphere)
• New oceanic lithosphere (slab) diverges from zone of formation
atop asthenosphere (seafloor spreading)
• When slab of oceanic lithosphere collides with another slab, older,
colder, denser slab subducts under younger, hotter, less dense slab
• Subducted slab is reabsorbed into the mantle
• Cycle takes as long as 250 million years, or more


Plate Tectonics
• Lithosphere of Earth is broken into plates
• Study of movement and interaction of plates:
Plate Tectonics
• Zones of plate-edge interactions are responsible for
most earthquakes, volcanoes and mountains
• Divergence zones
– Plates pull apart during seafloor spreading

• Transform faults
– Plates slide past one another

• Convergence zones
– Plates collide with one another


Plate Tectonics
Lithosphere of Earth is broken into plates separated by: divergence zones,
transform faults, convergence zones


Development of the Plate
Tectonics Concept
• 1620: Francis Bacon noted parallelism of Atlantic
coastlines of Africa and South America
• Late 1800s: Eduard Suess suggests ancient
supercontinent Gondwanaland (South America, Africa,
Antarctica, Australia, India and New Zealand)
• 1915: Alfred Wegener’s book supports theory of
continental drift – all the continents had once been
supercontinent Pangaea, and had since drifted apart
• Theory of continental drift was rejected (well, largely so
in the northern hemisphere, less so in southern) because
mechanism for movement of continents could not, at the
time, be visualized


Development of the Plate
Tectonics Concept
• 20th century: study of ocean floors provided wealth
of new data and breakthroughs in understanding

– Lithosphere moves laterally
– Continents are set within oceanic crust and ride along
plates

• Theory of plate tectonics was developed and widely
accepted


Magnetization of Volcanic
Rocks

• Magnetic patterns of ocean floor first observed
in mid 20th century – very important to theory of
plate tectonics
• Why does the ocean floor have a magnetic pattern?
– When lava cools to below 550oC (Curie point), atoms in
iron-bearing minerals line up in direction (polarity) of
Earth’s magnetic field
• Polarity of Earth’s magnetic field can be either to the north or to
the south and depends on time in Earth’s history


Magnetization of Volcanic
Rocks

• Successive lava flows stack up one on top of
another, each lava flow recording the Earth’s
polarity at the time at which it formed
• Each lava flow can also be dated using radioactive
elements in the rock to give its age

Magnetization of Volcanic
Rocks

• Magnetic patterns of ocean floor
• What does magnetic polarity of lava flows tell us?
– Plotting the polarity of different lava flows against their
ages gives us a record of the Earth’s polarity at different
times in the past
– Timing of polarity reversals (north to south; south to
north) seems random
– Reversals probably caused by changes in the flow of ironrich liquid in the Earth’s outer core


Earth’s Magnetic Field
• Earth’s magnetic field acts like giant bar magnet,
with north end near the North Pole and south end
near the South Pole
• Magnetic field axis is now tilted 11 o from
vertical (tilt has varied with time) so that
magnetic poles do not coincide with geographic
poles (but are always near each other)
• Inclination of magnetic lines can also be used to
determine at what latitude the rock formed
• Magnetic field is caused by dynamo in outer
core:
– Movements of iron-rich fluid create electric
currents that generate magnetic field

Magnetization Patterns on the
Seafloors

• Atlantic Ocean floor is striped by parallel bands of
magnetized rock with alternating polarities
• Stripes are parallel to mid ocean ridges, and pattern of
stripes is symmetrical across mid ocean ridges (pattern
on one side of ridge has mirror opposite on other side)
• Pattern of alternating polarity stripes is same as pattern of
length of time between successive reversals of Earth’s
magnetic field


Magnetization Patterns on the
Seafloors
• Magma is injected into the ocean ridges to cool
and form new rock imprinted with the Earth’s
magnetic field
• Seafloor is then pulled away from ocean ridge like
two large conveyor belts going in opposite
directions – seafloor spreading


Other Evidence of Plate
Tectonics

• Earthquake epicenters outline plate boundaries
– Map of earthquake epicenters around the world shows not
random pattern, but lines of earthquake activity that define the

edges of the tectonic plates


Other Evidence of Plate
Tectonics

• Oceanic mountain ranges and deep trenches
– Ocean bottom is mostly about 3.7 km deep, with two
areas of exception:
– Continuous mountain ranges extend more than 65,000 km
along the ocean floors
• Volcanic mountains that form at spreading centers, where plates
pull apart and magma rises to fill the gaps

– Narrow trenches extend to depths of more than 11 km
• Tops of subducting plates turning downward to enter the mantle


Other Evidence of Plate
Tectonics

• Deep earthquakes

– Most earthquakes occur at depths less than 25 km
– Next to deep-ocean trenches, earthquakes occur along
inclined planes to depths up to 700 km
– These earthquakes are occurring in subducting plates


•

Other Evidence of Plate
Tectonics
Ages from ocean basins
– The oldest rocks on ocean floor are about 200
million years old (less than 5% of Earth’s
4.5 billion year age)
– Ocean basins are young features – continually
being formed (at mid ocean ridges) and
destroyed (at subduction zones)
– Hot spots in the mantle cause volcanoes on
the plate above, which form in a line as the
plate moves over the hot spot in the mantle,
getting older in the direction of plate movement
– Sediment on the seafloor is very thin at mid
ocean ridges (where seafloor is very young) and
thicker near trenches (where seafloor is oldest)


Other Evidence of Plate
Tectonics

• Systematic increases in seafloor depth
– Ocean floor depths increase systematically with seafloor
age, moving away from the mid ocean ridges
– As oceanic crust gets older, it cools and becomes denser,
therefore sinking a little lower into the mantle
– Weight of sediments on plate also cause it to sink a little
into mantle

•

Other Evidence of Plate
Tectonics
The Fit of the Continents
– If continents on either side of the Atlantic used to be
adjacent, their outlines should match up
– Outlines of continents at the 1,800 m water depth line
match up very well
• 1,800 m water depth line marks boundary between lower-density
continental rocks and higher-density oceanic rocks


•

Other Evidence of Plate
Tectonics
Changing Positions of the Continents
– 220 million years ago, supercontinent Pangaea covered
40% of Earth (60% was Panthalassa, massive ocean)


•

Other Evidence of Plate
Tectonics
Changing Positions of the Continents
– 180 million years ago: Pangaea had broken up into Laurasia
and Gondwanaland

– 135 million years ago: north Atlantic Ocean was opening; India
was moving toward Asia
– 65 million years ago: south Atlantic Ocean was opening; Africa
and Europe had collided
– Present: India has collided with Asia; Eurasia and North
America are separate; Australia and Antarctica are far apart


The Grand Unifying Theory
• Tectonic cycle:
cycle
– Rising hot rock in the mantle melts and rises to surface as
liquid magma
– Buildup of magma causes overlying lithosphere to uplift
and fracture; fractured lithosphere is then pulled outward
and downward by gravity, aided by convection in mantle
– Asthenosphere melts and rises to fill fractures, creating
new oceanic lithosphere
– New oceanic lithosphere becomes colder and denser as it
gets older and farther from the ridge where it formed
– Eventually oceanic lithosphere collides with another
plate; whichever is colder and denser will be forced
underneath and pulled back down into the mantle


The Grand Unifying Theory
Tectonic cycle


Plate Tectonics and

Earthquakes

Most earthquakes can be explained by plate tectonics:
• Divergent plate boundaries
– Divergent motion and high temperatures cause rocks to fail
easily in tension
– Earthquakes are small and generally non-threatening

• Transform plate boundaries
– Plates slide past each other in horizontal movement, retarded at
irregularities in plate boundaries
– Energy required to move plates is released as large earthquakes

• Convergent plate boundaries
– Great amounts of energy are required to pull a plate back into the
mantle or push continents together
– Largest earthquakes are generated at convergent boundaries


Plate Tectonics and
Earthquakes
Examine example of Pacific plate:
• Created at spreading centers on eastern and southern edges,
producing small earthquakes
• Slides past other plates on transform faults (Queen Charlotte fault,
Canada; San Andreas fault, California; Alpine fault, New Zealand),
generating large earthquakes
• Subducts along northern and western edges, generating enormous
earthquakes

Spreading Centers and
Earthquakes
• Iceland:
Iceland
– Volcanic island fed by hot spot along the mid-Atlantic
ridge
– Swarms of moderate earthquakes too small to destroy
buildings or kill people