Transcript Plate Tectonics - Helena High School

Plate Tectonics
Earth’s Interior
Alfred Wegener (1880-1930)
German astronomer/meteorologist
 Worked in Greenland on polar air
circulation
 Died on expedition in 1930
 Proposed Continental Drift in 1912

Theory of Continental Drift
• Continental Drift
-Earth’s continents had
once been joined as a
single landmass
• Pangaea “all lands”
- Ancient landmass made up
of all continents forming a
supercontinent.
- Began to break apart about
200 million years ago (mya)
Pangaea Ultima?
Evidence from Rock
Formations

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Rock formations (ex.
mountain ranges)
fractured as the
continents separated.
Same rocks are found
in the Appalachians
and also in Greenland
and Europe.
Evidence from Fossils
Wegener found similar fossils of different
land animals and plants on separated
continents.
Ancient Climatic Evidence
Sedimentary Rock Coal
• Coal forms from dead
swamp plants.
• Coal was found in
Antarctica, therefore
Antarctica must have been
closer to the equator at one
time.
Ancient Climatic Evidence
Cont.
Glacial Deposits
-290 mya glacial deposits found
in Africa, India, Australia, and
South America.
-These continents were once
located on the South Pole.
Continental Drift was
rejected.
People rejected the
hypothesis because:
1- they believed
continents and ocean
basins were permanent,
fixed features of Earth’s
surface
2- Wegener could
not explain what forces
could cause a continent
to move without
shattering
Sea-Floor Spreading
Isochron Map

a line map that connects points of the same age.
Check this out on Google EARTH.
Theory of Plate Tectonics
Evidence
1. Rock Age
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Young Rocks –near ocean ridges.
Older Rocks –near deep-sea trenches
Ridges –the age of the oceanic crust increases
with distance from a ridge.
Seafloor Age – oldest part = 180 million years
old. Oldest Continental crust = 3.8 billion years
old.
Earth’s Magnetic Field
Our magnetic field is called the magnetosphere. It stretches
out through the atmosphere and acts as a protective barrier
to deadly, high-energy solar radiation.
Movement of the liquid outer core of the Earth
generates a strong magnetic field that surrounds
the planet. This causes the Earth to act much like
a large magnet, with the poles of the magnet
located near the poles of the Earth.
Theory of Plate Tectonics
Evidence.
1960s Technology:
• Magnetometer –makes a
map from detecting small
changes in magnetic fields.
•Geologists mapped
volcanoes and earthquakes
along plate boundaries.
• Sonar – used to map out
the seafloor
Theory of Plate Tectonics
Evidence
2. Paleomagnetism is the past
formation of rocks containing iron-bearing
minerals which provide a record of
earth’s magnetic field.
Basaltic rocks are found in the
ocean crust and are rich in iron and when
they cool the iron-bearing minerals orient
parallel to Earth’s magnetic field.
Magnetism & Paleomagnetism support sea-floor
spreading and Theory of Plate Tectonics.
Geomagnetic Time
Scale

Magnetic Reversal
a change in Earth’s magnetic field.

Normal Polarity
a magnetic field that has the same orientation as Earth’s
present field

Reversed Polarity
a magnetic field that is
opposite to the present field
Major plates of the world.
Mid-ocean Ridge – crust is
formed.
Subduction Zone – crust is
destroyed.
Slab-pull and Convection
Currents cause plate motion.
Plate Boundaries
Plate Boundaries
Three Major Types
1. Convergent
•plates come together.
2. Divergent
•plates move away from
one another.
3. Transform
•plates move horizontally
past one another
- places where tectonic plates
interact with one another.
Divergent Boundary
Divergent - Iceland Rift
Convergent OceanicContinental Boundary
Oceanic-Continental
Cascade Range
Convergent Oceanicoceanic Boundary
Convergent ContinentContinent Boundary
Continental-Continental
Himalayas
Transform Boundary
Transform – San Andreas
Fault
Know this drawing!