Download Notes - Plate Tectonics

Journal Question: If your finger nails grow at
about a two inches per year, how long would
it take for them to grow to be a mile? (hints:
12 inches in a foot, 5,280 feet in a mile)
Answer: A nail will grow 1 foot in 6
years. 6 years multiplied by 5,280
feet is how long it will take to grow a
mile…
6 x 5,280 = 31,680 years to grow a
mile
CONTINENTAL DRIFT & PANGAEA
In 1912, Alfred Wegener proposed
Pangaea- long time ago all continents were
one land mass
His evidence:
1. Continent’s fit together like a puzzle
2. rocks and fossils, mountain
chains, and glacial marks on
different continents matched
Continental Drift –Wegener’s theory that
the continents were once joined together
in a supercontinent call Pangaea that has
since moved apart
His Theory of Continental Drift was dismissed because he
could not explain how or why the plates moved. As a
German meteorologist, he lacked credentials, and he
couldn’t explain how huge continents could move.
2
Compositional Properties:
1. Crust – solid, rocky outermost surface
2. Mantle – plastic-like (deforms) Mg & Si
3. Outer Core – hot, liquid Fe & Ni
4. Inner Core – hot (hotter than the
surface of the sun), solid Fe & Ni
Physical Properties:
1. Lithosphere – rigid, crust & upper mantle
2. Asthenosphere – asphalt-like upper
mantle (flows)
In the early ’60’s, Harry Hess and J. Wilson, proposed a mechanism for
continental movement, which eventually became known as Seafloor
Spreading
1969 - The Glomar Challenger
drilled a series of holes in the
Mid-Atlantic ridge.
What they found revolutionized
geology and oceanography!
NOAA
Continental rocks date the Earth at about 4.6 billion years old. Since the
ocean floor is lower in the lithosphere, scientists expected to find older
rocks at those depths.
Continental geology’s Law of Superposition states that oldest rocks
are laid down first and should be found horizontally lowest in a bed
unless uplifted.
Instead, they found no rock older than 200 million years old and most were
much younger. How could the ocean floor be younger than the continents
riding on it?
The Deep Sea Drilling Project
showed that rocks became older
as they moved away from the Mid
Ocean Ridge (MOR).
Oceanic geology showed rock
layers are created vertically - not
according to the Law of
Superposition.
Theory of Seafloor Spreading
1960s – Seafloor Spreading – theory proposed that new
ocean crust forms at ocean floor as plates move apart, and
causes continents to move.
-Powered by the convection currents of magma in the mantle
7
Seafloor Spreading
Subduction Zone – areas where Lithospheric Plate descends
into Asthenosphere. (think about it: if plates are spreading in
one area, they must be coming together in another)
The MOR seemed to be a parallel distance from continents on either side.
Core samples revealed matching magnetic stripes in rocks on both sides of
MOR.
In 1977, Project FAMOUS, using
Alvin, photographed magma
squeezing out of the MOR and
creating new ocean floor.
Scientists, Vine and Matthews, now
had the data for a cohesive theory
on plate tectonics.
The sea floor was spreading - moving the continents with it.
The theory of sea floor spreading explained continental drift.
Wegener was right.
PLATE TECTONICS - TO BUILD
Earth’s lithospheric crust:
- is divided into large
and small plates
- makes up the ocean
crust AND the continental
crust, along with the upper
mantle
- these plates move
THEORY OF PLATE TECTONICS
– Earth’s crust and upper mantle
is broken up into lithospheric
plates that move relative to each
other
The oceanic crust averages 5 km (3 mi) thick
The continental crust averages 20-50 km (12-30 mi) thick
Both are relatively thin
By looking at seismic and volcanic activity around the world,
scientists can identify the plate boundaries.
Tectonic Plate Boundaries and Movement
These are the major plates: North American, South American,
Pacific, Eurasian, African, Nazca, Indo-Australian, and Antarctic
Tectonic Plate Boundaries and Movement
13
Evidence for Plate Movement
1. Age of Sea Floor Crust – The crust near a MOR is the
youngest and gets older as you move farther away from
the ridge
- the oldest seafloor is about 260 million years
(the oldest continental crust is 4.4 billion years old)
14
EVIDENCE FOR PLATE MOVEMENT – SEA FLOOR CRUST
Oldest continental crust is 4.4 billion years old
Oldest oceanic crust is 260 million years old and gets older away from MOR
Why the difference?
New seafloor is created
at MORs and destroyed
at trenches
Evidence for Plate Movement
2. Magnetic Strips of Rock– The positions of the north pole
switches every 5 million years or so (recently every 200-300K
years).
EVIDENCE FOR PLATE TECTONICS – MAGNETIC PATTERNS
Pattern of reverse & normal polarity in ocean floor rocks
As seafloor cools – the
iron minerals in the
magma align with the
current magnetic field
Bands of seafloor rock
alternating between
normal and reverse
polarity parallel MORs
and are mirror images
of each other
Evidence for Plate Movement
3. Hot Spots – Areas in Earth’s mantle that are hotter – can
lead to chain of islands; as plate moves over stationary hot
spot, volcanic islands can be formed (ex: Hawaii)
The position of the hot
spot in the mantle doesn’t
change.
EMPEROR SEAMOUNT CHAIN –
YELLOW ARROWS SHOW PLATE MOVEMENT
Types of Plate Boundaries
Description
Where two
plates move
apart
Where two
plates are
pushed
together
Where two
plates move
past one
another
Example?
Oceanic Ridges
Continental Rifts
Oceanic Trenches
Continental Mountains
San Andreas Fault
in California
TYPES OF PLATE BOUNDARIES
1. Oceanic-Oceanic Divergent Boundary - meaning to move apart
a. both plates are composed of basalt - the primary type of
ocean floor rock (iron, magnesium and silicon )
b. both plates have the same higher
density rock
c. the result is a spreading center
where new ocean floor is created
ex: Mid-Ocean Ridge (MOR)
This same ridge is 40,000 miles long
and is found in many places in
addition to the middle of the Atlantic
Ocean.
2. Continental-Continental Divergent - meaning to move apart
a. both plates are granitic - a
type of igneous rock
b. plates have the same lower
density rocks
c. The result is that continent is pulled apart and a rift
valley forms – eventually becoming a new ocean
ex: East Africa Rift Zone
NASA
3. Continental-Continental Convergent Boundary - meaning to
come together
a. both plates are granitic - a
type of igneous rock (silicon
and aluminum)
b. plates have the same lower
density rocks – so neither plate
will subduct
c. The result is that edges are forced up into mountains.
ex: Himalayas, Atlas Mountains, Appalachians
NASA
4. Oceanic-Continental or Oceanic-Oceanic Convergent Boundary a subduction zone where one plate overrides and the other is
forced down into the mantle
a. lighter continental plate or oceanic plate overrides
the denser oceanic plate
b. oceanic plate edge is subducted down into the asthenosphere
and remelted
c. The result is a trench
ex: Peru/Chile Trench
USGS
Chile Trench