Plate Tectonics Notes
What is Pangaea?
• Scientific evidence suggests that all seven of the continents on Earth today
used to be connected in a single land mass called Pangaea.
• Through the process of continental drift, Pangaea broke up, and the
individual continents moved away from their placements within Pangaea.
Evidence for Pangaea
• Alfred Wegener had a theory: Continental Drift.
• He saw that the continents fit together like a jigsaw puzzle.
• He then found evidence to support his theory.
• Evidence for Pangaea
• First: Climate clues. He found evidence for glaciers covering the parts of the continents that were normally too hot
for ice.
• Second: Fossil Clues. He found that fossils seemed to jump from continent to continent. Which could only be
explained if they were once connected.
• Third: Rock Clues. He found mountain ranges on different continents with the same age and formation.
What is Sea Floor Spreading?
Sea Floor Spreading
• Plate tectonics is a relatively new scientific concept, combining the
earlier theories of continental drift and sea-floor spreading.
• Sea-floor spreading is the movement of the Earth's crust away from
the mid-ocean ridges.
• During the process of sea-floor spreading, hot rock rises up from the
mantle and spreads out on the surface to form the ocean floor.
• As the ocean floor spreads, it pushes the plates around, which in turn move the continents to new locations.
• A mid ocean ridge is an underwater mountain range. Created by sea floor spreading.
Sea floor Spreading Evidence
• Normal Polarity – when the compass points North.
• Reversed Polarity – when the compass points South.
• Moving away from the mid ocean ridges, there is evidence that shows the changing of polarity.
• This shows that the plates are moving away from each other.
• Since they are pushing away from each other, they must be taking the continents with them.
• The map below shows the locations of the Earth's mid-ocean ridges, which are the sites of sea-floor spreading.
So what is a Plate Tectonic?
Glad You Asked!
• Imagine the “shell” of earth. It is cracked like a boiled egg.
• These pieces of the lithosphere float on the asthenosphere, and they move and
bump into each other.
Plate Tectonics
• The scientific theory that explains the movements of the continents is called plate tectonics.
• The word "plate" is used to describe large, brittle blocks of the Earth's surface which appear to move upon the
Earth's mantle as large pieces.
• The word "tectonics" comes from the Greek word that means "to build."
• At the places where two plates meet, constructive (building) or destructive (tearing down) processes may take
• Some of those processes will be discussed in these notes.
What are the Types of Boundaries?
Plate Boundaries
• Convergent – when the plates are pushing into each other.
• Divergent – when the plates are moving away from each other.
• Transform – when the plate are moving past each other and scraping.
• Subduction: at a convergent boundary, the less dense plate goes under
the more dense plate.
What causes the movement?
• Remember that convection is the flow of heated material.
• Hot material flows up, and cooler material flows down.
• Same thing happens in the asthenosphere.
How do the plates move?
• Basal drag – the convection current drags the plate.
• Ridge Push – at the mid ocean ridge, it will push up and across, moving the plate.
• Slab Pull – As a slab sinks, it will be pulled down by the convection current.