Download 2PlateTectonicsPowerPoint4

Plate Tectonics Notes
Plate Tectonic Theory
• In the mid 1900’s scientists proved that
tectonic plates in Earth’s Lithosphere move.
• Explanations about how the plates moved
developed.
• The Theory of Plate Tectonics evolved it was
built on Alfred Wegener’s ideas about
Continental Drift.
Theory of Plate Tectonics
Based on: Evidence from Sea Floor
•
•
•
•
•
Mid-Ocean Ridge
Sea Floor Spreading
Age of the Sea Floor
Ocean Trenches
Magnetic Reversal
Mid-Ocean Ridges
– 1950s-scientists began mapping the sea
floor.
– Huge underwater mountain ranges, called
Mid-Ocean ridges were discovered.
– Ridges appeared in every ocean, circling
Earth. “like the seams on a baseball.”
Sea Floor Spreading
– Ridges form cracks in the Oceanic Crust.
– Molten rock rises through these cracks.
– Molten material cools and forms new crust.
– Old ocean floor crust is pulled away-to
make room for newly formed floor.
– Sea floor slowly spreads apart.
Sea Floor Spreading
Age of Rock in Oceanic Crust
– Glomar Challenger-research ship took core
samples of ocean floor.
– Samples close
to ridge are the youngest.
– Samples furthest from ridge are the oldest.
– Oldest ocean floor only 160 - 180 million years
old. Compared to the older Continental crust that
is, 4.6 billion years old.
Ocean Trenches
– Why doesn’t the Earth get bigger-if the sea floor
has been spreading for millions of years?
– Scientists discovered trenches on ocean floor.
– At these trenches, old ocean floor sinks back
down into the Asthenosphere.
– Old crust is destroyed at same rate it is made.
Old Ocean Floor Sinks Down into Ocean Trenches
Back to the Asthenosphere Where it Melts
Earth’s Magnetic Field
Evidence of Earth’s Magnetic
Properties
• Earth has a magnetic north and south
pole.
• Throughout Earth’s history, these
magnetic poles have reversed.
• Evidence of this reversal has been
found in the rock on the ocean floor.
• Last happened 780,000 years ago.
How Magnetic Stripes Form
• Rock of ocean floor, contains “iron” particles.
• When molten material cooled, iron particles
line-up in the direction of the Earth’s magnet
field.
• Magnetic stripes that formed in the rock of the
ocean floor show the direction of Earth’s
magnetic field at the time the rock hardened.
Formation of Magnetic Stripes
on Ocean Floor
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.