Download Powerpoint Presentation Physical Geology, 10/e

ISNS 3359 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
iron-rich 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 11o 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)
Changing Positions of the Continents
180
million
years ago:
Pangaea
had
broken up
into
Laurasia
and
Gondwan
aland
–65
million
years ago:
south
Atlantic
Ocean
was
opening;
Africa
and
Europe
had
collided
–135
million
years ago:
north
Atlantic
Ocean
was
opening;
India was
moving
toward
Asia
–Present: India has collided with
Asia; Eurasia and North
America are separate; Australia
and Antarctica are far apart
The Grand Unifying Theory
• Tectonic 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:
– Volcanic island fed by hot spot along the mid-Atlantic
ridge
– Swarms of moderate earthquakes too small to destroy
buildings or kill people
Spreading Centers and Earthquakes
Red Sea and Gulf of Aden
• Young spreading center and new ocean basin
• Hot upper mantle under Africa melts and uplifts
crust, which gravity then pulls apart and
downward, creating pull-apart basins (or rift
valleys)
• As down-dropped pull-apart basins widen,
become flooded by ocean to form new arm
• At south end of Red Sea, three pull-apart basins
meet at triple junction
• Spreading has split Arabian plate from Africa
• East African Rift Valley may someday split
‘Somali’ plate from African plate
Convergent Zones and Earthquakes
• Largest earthquakes
• Three types of convergence:
– Ocean-ocean: older, denser oceanic plate is subducted
– Ocean-continent: oceanic plate is subducted
– Continent-continent: both plates are too buoyant to be
subducted; continental upheaval results
Subduction Zones
• Sites of great earthquakes
• Shallow earthquakes:
– Compressive movements of overriding plate and
subducting plate
– Pull-apart movements where subducting plate bends
downward
– Most damaging earthquakes
• Inclined plane of deep earthquakes, defining
descending slab of oceanic lithosphere
– Rigid interior of slab can stay cold enough to generate
earthquakes down to depths of 700 km
– Most seismic energy is dissipated before reaching surface
Seismic Gap Method
• If some segments of a fault have moved recently, it
is reasonable to expect that unmoved portions will
move next, to fill the gaps
• Yields expectations, not guarantees
• Segments may move in two or more earthquakes
before adjacent unmoved segments move once
Subduction Zones
• Tokyo, Japan, 1923 – one of world’s most deadly
disasters (probably about 144,000 people killed)
• Series of earthquakes, with principal one worst of
year globally
• Tsunami 11 m high hit city
• Fires raced through city for 2½ days, destroying
71% of Tokyo and all of Yokohama
– 38,000 people were killed by fire, crowded into a park
that was consumed by fire from three sides
Continent-Continent Collisions
• Collision of India into Asia
– India has moved 2,000 km north into Asia
from initial contact
– Pre-collision, Indian and Asian crusts
were 35 km thick
– Now crust under area of Tibetan plateau is
70 km thick and highest-standing
continental area on Earth
– India continues to move 5 cm/year into
Asia, along a 2,000 km front, affecting
India, Pakistan, Afghanistan, Tibetan
Plateau, eastern Russia, Mongolia and
China with great earthquakes, and
pushing parts of China to the east and
southeastern Asia farther to the southeast
Continent-Continent Collisions
• Shaanxi province, China, 1556
– Deadliest earthquake in history: about 830,000 people
killed
– Soft soil made caves practical homes for many
– Shaking caused ground and caves to collapse
• Tangshan, China, 1976
– Deadliest earthquake in recent history: more than 240,000
people killed (though on the web, estimates are up to
655,000)
– Dense mining city of 2 million, with most buildings of
mud-brick built under lenient building codes
– 93% of residential buildings collapsed
Transform Faults and Earthquakes
• Horizontal movements cause major earthquakes
• Turkey, 1999:
– Segment of North Anatolian fault ruptured for 120 km in
magnitude 7.4 earthquake near Izmit, followed weeks
later by rupture to the east in magnitude 7.1 earthquake
– Residential buildings on soft ground, adding sand to
concrete resulted in buildings collapsing during shaking
Transform Faults and Earthquakes
• Turkey, 1999:
– Turkey is pushed westward along the North Anatolian
fault, which runs for 1,400 km along the Black Sea
– Since 1939, the North Anatolian fault has ruptured in 11
earthquakes, from east end of fault to west
• Unique, semi-regular pattern
• Next event? Probably to west of Izmit, closer to Istanbul
• Probably within next 30 years
The Arabian Plate
Continent-continent collision earthquakes:
• Spreading in Red Sea and Gulf of Aden pushes Arabian
plate into Eurasia, uplifting mountains and creating
large earthquakes
The Arabian Plate
Continent-continent collision earthquakes:
• Armenia, 1988
– Magnitude 6.9 earthquake followed minutes later by
magnitude 5.9 aftershock
– 25,000 people killed, 31,000 injured and 500,000 homeless
– Comparison of similar-sized earthquakes:
• 1988 Armenian earthquake killed 25,000 of 700,000 residents
• 1989 Loma Prieta earthquake killed 25 of 1.5 million residents
– “Earthquakes don’t kill people, buildings do”
The Arabian Plate
Transform fault earthquakes:
• On western side, Arabian plate slides past African plate at
edge of Mediterranean Sea along Dead Sea fault zone
– Runs through Holy Land
– Steps in fault zone have created pull-apart basins that hold
Dead Sea, Sea of Galilee
– Dead Sea fault zone generates magnitude 6-7 earthquakes
about every 100-200 years