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Evidence for Plate Tectonics
 Remember that Wegener proposed the theory of Continental drift in the early 1900’s.
 Evidence for continental drift:
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Continental jigsaw fit
Matching geological structures and rocks
Matching fossils
Paleoglaciation
Coal deposits
 In 1960 (30 years after Wegener’s death), scientists proposed a mechanism to explain these
observations called Plate Tectonic theory – plates can include continents, oceans or both.
 The scientist that is credited for explaining the evidence for plate tectonics is a Canadian
Geologist J. Tuzo Wilson.
 Evidence for plate tectonic theory:
1.
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Earthquake and volcano zones
Mountain formations
Sea floor spreading
Ridge sea floor age and sediment thickness
Magnetic field reversal - Paleomagnetism
Evidence for Plate Tectonics
1.
Earthquake and volcano zones
Notes:
notice the distribution of Earthquakes and volcanoes along plate boundaries.
Ring of Fire
Evidence for Plate Tectonics
2. Mountain Range formations – like the Himalayas
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The Himalayas - referred to as the "roof of the world“
the highest peaks on earth - Mount Everest at 8,848 meters above sea level.
Notes:
Rocks on Mount Everest contain limestone, which forms at the bottom of warm, shallow seas
These rocks contain fossilized marine creatures like plankton, clams and fish.
Evidence for Plate Tectonics
The Burgess Shale - Yoho National Park on the BC side of the Rockies.
The town of Field, British Columbia is located between Lake Louise, AB and Golden
Described as "the world's most significant fossil discovery"
The Burgess Shale was discovered in 1909 and although it's high on a mountain now, at
one time it was underwater on the continental edge of North America.
The fossils embedded in the rock show a huge variety of species
To protect from over-collection, the area became a World Heritage Site in 1981.
Evidence for Plate Tectonics
3. Sea floor spreading - the mid Atlantic ridge forms a long mountain range running north
to south down the length of the Atlantic ocean.
Notes:
Along spreading centers in the sea floor, melted rock rises through cracks, cools, and forms new
crust that builds up mid-ocean ridges.
Evidence for Plate Tectonics
4. Ridge sea floor age and thickness
Notes:
Youngest rocks found closest to the ridge.
The further from the ridge centre the older the rocks and the thicker the sediments (silt and
organic debris) deposited on the ocean floor.
Evidence for Plate Tectonics
5. Magnetic field reversal - Paleomagnetism
The Earth as a magnet
Animation
Notes:
The Earth is like a large magnet. Notice the normal magnetic field can be reversed –
called reverse polarity.
Evidence for Plate Tectonics
5. Magnetic field reversal - Paleomagnetism
Seafloor rock contains the magnetic mineral magnetite. When magma from the ridges rises and
cools, it crystallizes, locking the magnetite crystals into alignment with the earth's magnetic
field like the needle of a compass
The Earth’s magnetic field fluctuates in intensity and occasionally even reverses itself.
Notice the strongly patterned magnetic field
reversals recorded on the seafloor provided the
necessary proof of seafloor spreading.
Specifically, they proved that new crust was
continuously being generated at the mid-ocean ridges,
where magma cooled and magnetite crystals "locked in"
according to the orientation of the Earth's magnetic
field at that time.
Polarity of the Ocean Crust
Evidence for Plate Tectonics
5. Magnetic field reversal - Paleomagnetism
D = strong magnetic field at ridge (new rock being formed) – location of ridge
C and E same distance on either side of D and same magnetic field strength – rock
formed at same time on either side of ridge
B and F same distance on either side of D and same magnetic field strength – rock
formed at same time on either side of ridge
Notice the magnetic field reversals