Field notes: a moving Earth

Earth’s rigid outer shell, made of the crust and uppermost mantle, is broken into tectonic plates that move slowly over the hotter, softer asthenosphere. Heat inside Earth helps drive movement in the mantle, and plates usually travel only a few centimetres each year. Where plates meet, they can move apart, collide, or slide past one another. At a boundary where plates move apart, magma may rise and cool to make new crust. Where plates collide, one plate may sink beneath another; this process is called subduction. At a sliding boundary, friction can hold rocks together until stress builds and they suddenly slip. That sudden movement releases seismic waves. The underground starting point is the focus; the point on the surface directly above it is the epicentre. Undersea earthquakes can sometimes trigger tsunamis. Volcanoes may form above subduction zones, where plates move apart, or above hotspots. Molten rock below the surface is magma; once it erupts onto the surface, it is called lava. These hazards can damage communities, but their effects can be reduced. Seismometers record ground shaking, satellites can track changes in the ground, and gas sensors can detect changes near volcanoes. Monitoring cannot reliably tell people the exact time an earthquake will happen, but it can support planning, warnings and safer building decisions.