Plate tectonics explained
Geography · Key Stage 3 · The School
What moves the plates
Heat from the Earth's interior drives slow convection in the mantle, and the plates ride on it. Two other forces matter: RIDGE PUSH, where new crust at a mid-ocean ridge slides away downhill, and SLAB PULL, where a dense sinking plate drags the rest behind it. Slab pull is now thought to be the strongest of the three.
Three kinds of boundary
CONSTRUCTIVE: plates move apart, magma rises, new crust forms — Iceland and the Mid-Atlantic Ridge. DESTRUCTIVE: plates move together, the denser oceanic plate subducts, producing deep trenches, violent volcanoes and major earthquakes — Japan, the Andes. CONSERVATIVE: plates slide past each other, no new crust and no volcano, but severe earthquakes — the San Andreas Fault.
Reading a boundary
Given a boundary type you can predict the landforms and the hazards, which is exactly what the exam asks. Constructive: shield volcanoes, gentle eruptions, small quakes. Destructive: composite volcanoes, explosive eruptions, deep-focus quakes, fold mountains. Conservative: earthquakes only.