Field notes: Earth’s moving surface

Earth’s rigid outer shell, the lithosphere, is divided into large tectonic plates. They move a few centimetres each year over the hotter, softer mantle. Heat inside Earth helps drive slow movement in the mantle, which can push and pull the plates. At a convergent boundary, plates move towards one another. One plate may sink beneath another (subduction), helping magma form; collisions can also build mountains. At a divergent boundary, plates move apart and magma may rise to create new crust. At a transform boundary, plates slide past each other. Friction can lock their edges, allowing stress to build until rocks suddenly slip and release energy as an earthquake. Many earthquakes and volcanoes occur near plate boundaries, but not every boundary produces both. Australia sits within the Indo-Australian Plate, while active boundaries around the wider region help explain earthquakes and volcanoes in places such as Indonesia and New Zealand. Plate movement is slow, but over millions of years it reshapes Earth’s surface.