Download Plate tectonics - Seattle Central College

Tsunami, earthquakes and volcanic eruptions
Tsunami, earthquakes and volcanic eruptions
Tsunami:
Wavelenths > 200 km
Very fast in open ocean
Destruction of Moawhitu
Brian Flintoff, New Zealand
Plate tectonics
Plate tectonics: terminology
• Continental drift
– continents move
• Plate tectonics
– thin, rigid blocks of lithosphere move
horizontally
– Interactions of plates build major features of
Earth’s crust
Continental Drift
Continental Drift: Evidence
• Alfred Wegener, 1912
• Suggests Pangaea, surrounded by
Panthalassa, 200 million years ago
1. Puzzle-like fit of
continents
1
Continental Drift: Evidence
Continental Drift: Evidence
1. Puzzle-like fit of
continents
1. Puzzle-like fit of
continents
2. Matching rocks and
mountain chains
– Structures
– Rock types
– Rock ages
Continental Drift: Evidence
Continental Drift: Evidence
3. Glacial ages
4. Distribution of
organisms
– Ancient glaciations
in modern tropical
regions
– Direction of glacial
flow
– Same land
animals, different
continents
Continental Drift: Evidence
Continental Drift: Evidence
5. Earth’s magnetic field
5. Earth’s magnetic field
Sources
Sources
1. conducting, liquid iron, outer core
(~90% of magnetic field strength)
1. conducting, liquid iron, outer core
(~90% of magnetic field strength)
2
Continental Drift: Evidence
Earth’s magnetic field
Declination (D)
5. Earth’s magnetic field
Sources
1. conducting, liquid iron, outer core
(~90% of magnetic field strength)
Earth’s magnetic field
Inclination (I)
Earth’s magnetic field
Though experiments:
1. If we take out a compass and align the needle to the north, we are pointing
towards _________ (geographic north, magnetic north).
2. At the north magnetic pole, the north end of the dip (or inclination) is
______(up,down); at the south magnetic pole, the dip is ______(up,
down).
3. At the magnetic poles, the dip angle is _______(0,90), the horizontal
intensity is ______(0,90).
4. At the magnetic equator the dip or inclination is _____(0,90).
5. The declination in Seattle is about 17 degrees (remember, declination is
positive east). If we take out a compass and align the needle on N
(north), what change do we need to make to point towards geographic
north?
Continental Drift: Evidence
Continental Drift: Evidence
5. Earth’s magnetic field
5. Earth’s magnetic field
In some minerals, the spin of unpaired
electrons will align with the Earth’s
magnetic field as it cools below a certain
temperature, recording declination for
millions of years
Magnetite
Fe3O4
Hematite
Fe2O3
3
Continental Drift: Evidence
Continental Drift: Evidence
5. Earth’s magnetic field: apparent polar wander
5. Earth’s magnetic field: apparent polar wander
. . . puzzle-like fit of continents, matching rock
sequences, glacial evidence, distribution of fossils,
apparent polar wander . . . we are convinced that
continents move . . .
• Earth formed 4.6 Byr
but oldest seafloor fossils are 180 Myr, oldest marine
fossils found on land are much older
• Oceans formed 4 Byr
but relatively little sediment accumulation on the seafloor
Seafloor spreading
• Harry Hess, Navy Captain
collects echo-sounding surveys
in between battles during WWII
– Flat topped mountains, 2 km deep
– Trenches, 7 miles deep
• 1953, physicists Ewing and
Heezen discover Great Global
Rift in Atlantic
The continents are moving,
but how?
Seafloor spreading
• Crust created at mid-ocean ridges
• Crust destroyed at oceanic trenches
4
Seafloor spreading: Evidence
Seafloor spreading: Evidence
1. Magnetic polarity reversals
1. Magnetic polarity reversals
- changes in Earth’s magnetic polarity are
recorded in the rocks as seafloor is created
Seafloor spreading: Evidence
2. Age of ocean floor: youngest at mid-ocean ridge
Animation: seafloor magnet
Seafloor spreading: Evidence
Seafloor spreading: Evidence
3. Heat flow: highest at mid-ocean ridge crest
4. Earthquakes: most occur along plate margins
5
Plate tectonics theory
Tectonic plates
Plate boundaries
• Rising heat from within the Earth pushes and
pulls tectonic plates
• As a result, continents move and seafloor
spreads from mid-ocean ridges
Animation: GlacialIsostasy
6
Plate boundaries
Plate boundaries: convergent
1. Divergent
• Plates move toward each other
• Crust is subducted
• Deep earthquakes
2. Convergent
3. Transform
Plate boundaries: convergent
Plate boundaries: convergent
Oceanic-continental plate convergence
– Oceanic plate subducted
– Oceanic trench
– Continental arc
7
Plate boundaries: convergent
Oceanic-oceanic plate convergence
– Oceanic plate subducted
– Oceanic trench
– Island Arc
Plate boundaries: convergent
Continental-continental convergence
– Uplifted mountain ranges
8
Plate boundaries: divergent
• Plates move apart, shallow earthquakes
Plate boundaries: divergent
Fast (10-20 cm/yr)
– Oceanic rise
– Gentle slopes
– Relatively more magma
Seamounts 200 m tall
Plate boundaries: divergent
Medium (5-10 cm/yr)
– Oceanic ridge
– Rift valley ~5 km wide
– Rift valley 50-200 m deep
Plate boundaries: divergent
Slow (1-3 cm/yr)
– Oceanic ridge
– 6 miles wide (10 km), 2 miles deep (3 km)
– Steep slopes
9
Plate boundaries: transform
• Segments of plates slide past each other
• Occur between two segments of mid-ocean ridge
• Shallow but strong earthquakes
Plate boundaries: transform
Oceanic transform fault
Plate boundaries: transform
Continental transform fault
10