GED Science
Plate Tectonics & Earth's Structure
Earth's rigid outer shell is cracked into plates that drift on the softer, hotter rock beneath. Nearly everything dramatic — earthquakes, volcanoes, mountain ranges, ocean trenches — happens along the cracks, and WHICH drama depends on whether the plates collide, separate, or grind sideways.
The words first
- crust / mantle / core
- the thin rocky skin / the thick hot middle / the metal center
- plate
- a rigid slab of crust riding on the mantle
- plate boundary
- where two plates meet — the action zone
- subduction
- a denser ocean plate diving beneath a lighter one
The method
THE THREE BOUNDARY TYPES
→│← convergent plates collide
two continents → crumple into mountains (Himalayas)
ocean vs land → ocean plate subducts →
trench + volcanoes + big quakes
←│→ divergent plates separate
magma rises to fill the gap → new crust,
mid-ocean ridges, rift valleys
⇅ transform plates grind sideways
no crust made or destroyed — just earthquakes
(San Andreas fault)
WHY PLATES MOVE
heat from the core churns the mantle in slow convection
currents — the plates are passengers.
EVIDENCE: matching coastlines and fossils across the Atlantic,
quakes and volcanoes mapping neatly onto plate edges
Worked all the way through
The problem
Japan sits where an ocean plate collides with a continental plate. Explain its frequent earthquakes and its volcanoes with one mechanism.
- Step 1 — Name the boundary type.Ocean plate meets continent and collides: convergent, with subduction.
- Step 2 — Get the earthquakes out of it.The diving plate sticks and slips against the plate above — each slip is an earthquake.
- Step 3 — Get the volcanoes out of it.As the plate sinks deeper it triggers melting; the magma rises through the plate above and erupts as a volcano chain.
- Step 4 — Tie it in one sentence.One subducting plate produces both: slipping makes quakes, melting makes volcanoes.
On the board
ocean + continent, colliding
convergent boundary
ocean plate subducts
denser, so it dives
stick-slip on the way down
→ earthquakes
deep melting → rising magma
→ volcanoes
one mechanism, both hazards
Answer
Subduction — the diving plate slips (earthquakes) and melts (volcanoes)
A second one, in a different form
The Atlantic Ocean grows a few centimeters wider every year. What boundary runs down its middle?
ocean widening = plates separating
divergent boundary
magma fills the gap → new seafloor
the mid-Atlantic ridge
Answer
A divergent boundary — the Mid-Atlantic Ridge, making new crust
The usual mistake
Confusing weathering (breaks rock in place) with erosion (moves the pieces).
Check yourself
Name the boundary type FIRST — collide, separate, or slide — and the features follow automatically. An answer pairing features with the wrong boundary (volcano chains at a transform fault) is the standard planted error.
Practice it
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