German scientist Alfred Wegener formed this idea of Continental Drift. He argued that todays continents once formed a single landmass, which he named Pangaea (Greek for "all land"). It broke into pieces due to the weaknesses in the earth's crust as they were made up of less dense materials, which drifted centimeter by centimeter over millions of years until they arrived at where they are now. Figure 1.5 shows how the Pangaea split up into plates and drifted over the millions of years.
(Fig 1.5) The plates drifted through the years to where they are now.
It is possible for the plates to move due to the convection currents in the mantle. The movement of these currents is similar to the movement of convection currents in a beaker of boiling water.
(Fig 1.6) Convection currents and plate movements.
(Fig 1.7) Magma heats up and rises.
Water that is heated expands and rises to the surface of the beaker. Similarly, the magma nearer the core expands and rises.
(Fig 1.8) Magma spreads out, plates move apart.
Water that has risen spreads out across the surface, cools and sinks to the bottom. Similarly, the magma that has risen spreads out beneath the plates. As the magma spreads out,
the plates are dragged along and they move away from each other. This is the process of sea-floor spreading.
(Fig 1.9) Magma sinks, plates dragged towards each other.
When the magma sinks, the plates are dragged towards each other. This is the process of subduction. The repeated heating and rising of the magma sets up continuous convection currents in
the mantle, causing the plates to move.