Download Plate tectonics

Unit 3: The Dynamic Earth
Mr. Ross Brown
Brooklyn School for Law and
Technology
How have the continents drifted?
• 26 October 2015
• Do now: How is continental drift like a jigsaw
puzzle?
Unit highlights
• Continental drift and how there was once a
single landmass
• Seafloor spread as evidence of drift
• Plate tectonics
• Geologic activity at plate boundaries
• Lithospheric plates and convection
Coastlines lead to a hypothesis
What is Continental Drift?
• Mapmakers and explorers noticed similar
shorelines on either side of the Atlantic Ocean
• Could the continents have once fit together?
• 1912- Alfred Wegener proposed the theory of
Continental Drift; the continents had moved!
• This suggested a single landmass: Pangea
• Surrounded by super ocean Panthalassa
What is Pangea?
• From late Paleozoic to early Mesozoic eras
• Assembled from earlier continental units
around 300MYA and began to break apart
around 175MYA
Fossil evidence for Pangea and drift
• Similar fossils found on different continents
• Mesosaurus, alive 270MYA, found on Africa’s
west coast and S. America’s east
Who was Mesosaurus?
What other evidence supports the
theory of Pangea?
• Geological evidence
– Mountain ranges spanning continents
• Climatic evidence
– Glacial debris found across S Hemisphere
landmasses that are today warmer
Was the theory of Pangea fully
accepted?
• What was the force that made the continents
drift apart?
What if continents hadn’t drifted?
What if continents hadn’t drifted?
Homework #6
26 October 2015
1. What observations first led to Wegener’s
hypothesis of continental drift?
2. What types of evidence supported
Wegener’s hypothesis?
What force makes the continents
drift?
• 27 October 2015
• Do now: Based on what we discussed
yesterday, what is the force that moved the
continents apart from Pangea, and still moves
them today?
What is the Mid-Atlantic Ridge?
• Undersea mountain range, with a steep valley
running down its center.
• Part of an 80,000km system of mid-ocean
ridges
• In 1940s scientists found that none of the rock
there was older than from 175MYA
• Continental rock is from up to 4BYA!
What are Mid-Ocean Ridges
What is seafloor spreading?
How did the theory of seafloor
spreading develop?
• What if the valley at the center of the ridge
was a rift, or break?
• Magma wells up through this break
• Possible if ocean floor moved away from ridge
• If seafloor moved, then perhaps continents
moved, too?!
Homework #6, continued
27 October 2015
1. Describe the process of seafloor spreading.
How did our understanding of
plate movements develop?
• 30 October 2015
• Do now: How is ‘plate tectonics’ different from
‘continental drift?’
Plate tectonics
• The theory that Earth's outer shell is divided
into several plates that glide over a plastic
inner layer above the core. The plates act like
a hard and rigid shell compared to Earth's
interior.
How did seafloor spread refine the
theory of plate tectonics?
• Moved beyond notion of “continental drift”
• Plate tectonics: the study of the formation of
the earth’s crust
• Lithosphere: the rocky stuff. Crust and rigid
mantle
• Asthenosphere: plastic rock on which
lithosphere floats
Earth’s plates
What happens where plates meet?
• Divergent Boundaries: plates move away from
each other, forming rifts or mid-ocean ridges
• Convergent Boundaries: plates collide
– Subduction: Oceanic pushed under continental
– Obduction: Continental pushed under oceanic
– Orogenic: both push upwards
• Transform Boundaries: Plates grind past each
other
Where plates meet
What happens where plates meet?
How did the continents form?
• 2 November 2015
• Do now: Why are there seashells on the tops
of mountains?
How does subduction increase
continental crust?
• As oceanic crust is subducted, some
lithosphere is “scraped off” and accumulates
on the continent. This is a terrane.
What is a terrane?
• Pieces of lithosphere with unique geological
history
How is each terrane unique?
• A terrane contains rocks and fossils that are
unique from its neighbors
• There are major faults at the boundaries of
the terrane
• The magnetic properties of the terrane do not
match its neighbors