Plate tectonics: why continents move

Alfred Wegener proposed continental drift in 1912, pointing to the fit of coastlines, matching fossils across oceans, and glacial deposits in places now tropical. The idea was rejected for decades because he could not explain what moved the continents.

The mechanism arrived in the 1960s with seafloor spreading. Magnetic stripes on the ocean floor, symmetric about mid-ocean ridges and recording reversals of the earth's magnetic field, showed that new crust is manufactured at ridges and moves outward. That evidence settled the argument.

Heat from the earth's interior, partly leftover from formation and partly from radioactive decay, drives slow convection in the mantle. Plates ride on this, but the largest forces are more direct: ridge push, as new elevated crust slides away, and slab pull, as a dense sinking plate drags the rest behind it.

Speeds are around a few centimetres per year, roughly the rate fingernails grow. Small rates over geological time are enormous: a few centimetres a year is thousands of kilometres over a hundred million years.

• Divergent: plates pull apart, magma rises, new crust forms. Mid-Atlantic Ridge, and on land the East African Rift.
• Convergent: plates collide. Ocean meeting continent produces subduction, deep trenches and volcanic arcs. Continent meeting continent produces mountain ranges, such as the Himalayas.
• Transform: plates slide past each other, building no crust but storing strain that releases as earthquakes, as along the San Andreas fault.

Earthquakes occur when accumulated strain exceeds friction and rock slips suddenly. The focus is where slipping starts, the epicentre is the point on the surface directly above. Magnitude scales are logarithmic, so each whole step is roughly thirty times more energy released.

Volcanoes cluster at boundaries, but the type varies with magma chemistry. Subduction produces silica-rich, viscous magma that traps gas and erupts explosively. Divergent boundaries and hotspots produce runnier basaltic lava that flows rather than explodes, which is why Hawaiian eruptions look so different from those in the Pacific ring of fire.

Igneous rock forms from cooling magma, coarse-grained when it cools slowly underground and fine-grained when it cools quickly at the surface. Weathering and erosion break rock into sediment, which compacts and cements into sedimentary rock, often preserving layers and fossils.

Heat and pressure, without melting, transform any rock into metamorphic rock: limestone into marble, shale into slate. Melt any of them and the cycle restarts. The point is that rock types are stages in a process, not fixed categories.