Plate Tectonics: Earth's Moving Surface
Plate tectonics is the theory explaining that Earth's outer shell is broken into giant pieces (plates) that slowly move, causing earthquakes, volcanoes, mountain building, and continental drift.
The Basic Idea
- Crust: Thin outer shell (5-70 km)
- Lithosphere: Crust plus upper mantle (rigid)
- Asthenosphere: Partially molten, flows slowly
- Mantle and Core: Deeper layers
The lithosphere is broken into about 15 major plates that "float" on the asthenosphere.
- Plates move 1-10 cm per year (fingernail growth speed)
- Driven by convection in the mantle
- Continents ride on the plates
- Has been happening for billions of years
Types of Plate Boundaries
- Plates move apart
- New crust forms from rising magma
- Mid-ocean ridges
- Examples: Mid-Atlantic Ridge, East African Rift
- Plates move together
- One may subduct (dive under)
- Creates trenches, volcanoes, mountains
- Examples: Himalayas, Andes, Japan Trench
- Plates slide past each other
- No creation or destruction of crust
- Causes earthquakes
- Examples: San Andreas Fault
Evidence for Plate Tectonics
- Africa and South America fit together
- Alfred Wegener noticed this (1912)
- Initially rejected, now confirmed
- Same fossils on now-separated continents
- Mesosaurus in both South America and Africa
- Couldn't have swum across ocean
- Same rock types across ocean
- Mountain chains continue across continents
- Young rock at mid-ocean ridges
- Older rock farther away
- Magnetic stripes record pole reversals
What Plate Tectonics Causes
- Most occur at plate boundaries
- Transform: sliding friction
- Subduction: descending plate
- Divergent: rifting
- Subduction zones: melting creates magma
- Hot spots: plumes from deep mantle
- Rift zones: magma fills gaps
- Collision: Himalayas (India into Asia)
- Volcanic: Andes, Cascades
- Rift: East African mountains
- Form when continents rift
- Atlantic: still growing
- Pacific: shrinking (subduction around edges)
Deep Time
- Pangaea (300-200 mya): All continents together
- Before Pangaea: Other configurations
- Future: Continents will recombine
- Possibly started 3+ billion years ago
- May have begun differently
- Essential for Earth's habitability
Why It Matters
- Recycles crust and nutrients
- Regulates long-term climate
- Creates habitable conditions
- Shapes all of Earth's surface
Without it, Earth might be like Mars—geologically dead.