the Dynamic Earth
and Plate Tectonics
Early Ideas on a moving Earth
• Alfred Wegener proposed idea: “Continental
Drift”
– Stated that all continents used to be joined in
Pangaea (meaning “all land”) that broke up into
smaller continents, “drifted” to current locations
What patterns do we see in continent shapes?
Continental Drift - Wegener’s hypothesis of
continent movement after Pangaea broke apart
Early evidence #1:Coastlines on early maps
seem to fit like puzzle pieces
Why are there fossils of cynognathus on both
Africa and South America and no where else?
“Continental Drift”, Early evidence #2
Fossils of the same organism were on opposite
land masses (puzzle pieces), known to have
been joined at one time
Why do rocks from Rhode Island and Scotland
match?
“Continental Drift”, early evidence #3
Mountains and rocks continued on opposite
puzzle pieces
Why did southern India used to have glaciers?
“Continental Drift”, early evidence #4
Misplaced
glacial remnants
and tropical
swamps can be
explained if
continents were
once joined
Newer evidence
• Proof of seafloor spreading at the MidAtlantic Ridge (1947)
– Rock at the ridge was dated much younger than
continental rock
– Paleomagnetism also apparent
• As magma solidifies, iron crystals align with
Earth’s magnetic field like a compass.
Geologists have known that this happens
periodically throughout history, as magnetic
poles flip flop.
Modern Theory of Plate Tectonics
• Earth is made up of 9 primary plates and
several microplates made up of continental
and oceanic crust.
– Upper lithosphere is crusty and moves about on
plasticy asthenosphere.
– Asthenosphere moves most likely due to
convection in the mantle due to heat from core
and radioactivity from mantle
• Where plates meet is called a plate boundary
– Types: Convergent, divergent, transform
PLATE BOUNDARIES
1. Convergent Boundary
•Ocean – Continent
•Ocean – Ocean
•Continent – Continent
Continent – Continent convergence
Ocean – Continent Convergence
2. Divergent Boundary
3. Transform
Boundary
Where in the
World do we
find those
boundaries?