Transcript Introduction to Plate Tectonics - EHS

Plate Tectonics
–
New Evidence
Alfred Wegener & Continental Drift
• No mechanism to move the continents
from one place to another
• Scientists did not know much about the
ocean floor
• dismissed as being eccentric,
preposterous, and improbable
Early Studies
• 1855, U.S. Navy Lieutenant
Matthew Maury
– a bathymetric chart
– revealed the first evidence of
underwater mountains in the central
Atlantic
– This was later confirmed by survey
ships laying the trans-Atlantic telegraph
cable.
• World War I (1914-18)
– echo-sounding devices (primitive sonar
systems) measure ocean depth by
recording the time it took for a sound
signal from the ship to bounce off the
ocean floor and return.
– revealed that the ocean floor was much
more rugged than previously thought.
– demonstrated the continuity and
roughness of the submarine mountain
chain in the central Atlantic
World War II
Sea-Floor Topograpy
• Ships and Submarines used SONAR to map the ocean-floor topography
– Submarines used topography to hide from ships and had to avoid
under-sea topography
– Ships used SONAR to find submarines
• 1950s -- oceanographic surveys
– a great mountain range on the ocean floor, the global mid-ocean ridge,
encircled the Earth.
• more than 50,000 kilometers (km) long and up to 1000 km across
• zig-zags between the continents
• Rising about 4,500 meters(m) above the sea floor,
– Taller than all mountains in the US except for Mount McKinley (Denali)
at 6,194 m
• The most prominent topographic feature on the surface of our planet.
– Deep Sea trenches -- incredibly deep
Age of the Ocean Floor
• 1947 -- seismologists on the U.S. research ship Atlantis
– Sediment layer on the floor of the Atlantic was much
thinner than thought.
– Presumed age of the oceans = 4 billion years: the
sediment layer should have been very thick.
Youngest rocks at Ridge
Oldest furthest away from Ridge
• 1968 -- a research vessel, the Glomar
Challenger, designed specifically for marine
geology studies
– Deep-Sea Drilling Project -samples from ocean floor.
– Criss-crossed the Mid-Atlantic Ridge between South
America and Africa
– the ages of the samples
• paleontologic
• isotopic dating studies
Earth’s Magnetic Field & Polar Reversals
• Early 1900’s – Paleomagnetists (people
who study the Earth's ancient magnetic
field)
– Bernard Brunhes in France (in 1906)
– Motonari Matuyama in Japan (in the 1920s)
– rocks generally belong to two groups
according to their magnetic properties
• Normal: the magnetic minerals in the rock
having the same polarity as that of the Earth's
present magnetic field (North is North)
• Reversed: the magnetic minerals in the rock
having the opposite polarity as that of the
Earth's present magnetic field (North is South)
How can this happen?
•
Currents in the liquid outer core cause the Earth’s
magnetic field
•
•
•
Convection currents of liquid iron and nickel generate a
“dynamo” , an electrical current.
The Earth is a giant electro-magnet!
Grains of magnetite behave like little magnets
•
Align with the orientation of the Earth's magnetic field.
• Magma cools to form solid rock
• Alignment of the magnetite grains is "locked in"
• The grains “record” the Earth's magnetic orientation or
polarity (normal or reversed) at the time of cooling.
How did they figure this out?
• World War II – magnetometers used to detect
submarines
• 1950s, scientists used magnetometers to study the
ocean floor
– Odd magnetic variations across the ocean floor
– Ocean floor rock, basalt, contains magnetite (a magnetic iron
mineral)
– Magnetite can distort compass readings
– The magnetic variations are not random or isolated!
• Magnetic striping = a zebra-like pattern
– alternating stripes of normal & reversed rock
– laid out in rows on either side of the mid-ocean ridge
New oceanic crust forms continuously
at the crest of the mid-ocean ridge
• As you move away from
the ridge
– Crust is cooler
– Crust is older
• Crust moves away from
the ridge crest with seafloor
spreading
– a. the spreading ridge about
5 million years ago
– b. about 2 to 3 million years
ago
– c. now
Sea-Floor Spreading:
Putting it all together
• Evidence
– Sea-Floor Topography
– Young crust at ridges, old crust far from ridges
– Magnetic Striping
• 1961 – New Questions:
– Are mid-ocean ridges structurally weak zones
where the ocean floor was being ripped in two
lengthwise along the ridge crest?
– Could new magma from deep within the Earth
rises easily through these weak zones and erupts
along the crest of the ridges to create new
oceanic crust?
Sea-Floor Spreading: Putting it all together
• Harry H. Hess & Robert S. Dietz = seafloor spreading.
– Crust is added at the ridges
• new oceanic crust continuously spread away from
the ridges in a conveyor belt-like motion
– Crust is destroyed at the trenches
• After millions of years, oceanic crust descends into
very deep, narrow canyons along the rim of the
Pacific Ocean basin.
– the Atlantic Ocean was expanding while the Pacific
Ocean was shrinking.
– The ocean basins were perpetually being "recycled,"
with the creation of new crust and the destruction of
old oceanic lithosphere occurring simultaneously
Sea-Floor Spreading: Putting it all together
• Thus, Hess' ideas neatly explained
– why the Earth does not get bigger with sea floor
spreading
– why there is so little sediment accumulation on the
ocean floor
– why oceanic rocks are much younger than
continental rocks.
• The oceanic crust -– a natural "tape recording" of the history of the
reversals in the Earth's magnetic field.
The Theory of Plate Tectonics
• The outer layer of the Earth is broken up into
rigid plates
• These plates are made of the lithosphere
– Crust
– Upper most part of the mantle “stuck” to the bottom of
the crust
• Plates move around on the hot, weak, mobile
asthenosphere
– Weak zone in the mantle below the lithosphere
The Plates
• Rigid - movement doesn’t happen within a plate
• Plates move relative to each other resulting in
– Earthquakes
– Volcanoes
– Mountains
• The intensity of earthquakes, types of
volcanoes, and types of mounains are
dependent on how the plates interact
– 3 different plate boundaries
Divergent Boundaries
• Plates move away from
each other
• Mostly found at midocean ridges
• Crust is made as plates
move away from each
other and magma oozes
up at the ridge
• Earthquakes are common
but shallow
• Volcanic Activity is
prevalent
• Examples: Mid-Atlantic
Rise and the Red Sea
Transform Boundary
• Plates move past each
other
• Accommodate the
different types of
movement on a sphere
• Earthquakes very
common and can be
severe
• Volcanoes absent
• Examples: San Andreas
Fault and Mendicino
Boundary
Convergent Boundary
• Plates move toward each other
• Three different types
– Ocean-Continent
– Ocean-Ocean
– Continent-Continent
Ocean-Continent Convergent
• The more dense oceanic lithosphere is subducted under
the less dense continental lithosphere forming a trench
• Continental Volcanic Arcs result as the water in the
subducted oceanic lithosphere is released and melting
occurs in the mantle below the continent.
• Earthquakes are severe and common.
• Examples: Cascade and Andes Mountains
Ocean-Ocean Convergence
• The older, colder more dense oceanic lithosphere is subducted
beneath the younger, warmer less dense segment.
• Deep trenches form at edge of subduction zone
• Volcanic Island Arcs form as water is released into the mantle
causing melting
• Earthquakes are common and severe. Often result in Tsunamis.
Continent-Continent
Convergence
• Continental lithosphere
cannot be subducted
because it is not dense
enough.
• As the continents are
forced together, they form
huge mountains
• Earthquakes are severe
and common.
• Volcanism is rare.
• Examples: Himalaya,
Alps, and Appalachia
What Drives the Plates?
• http://www.visionle
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