Plate Tectonics and Geophysical Systems

Geophysical hazards begin deep inside the Earth, where heat left over from the planet's formation and generated by radioactive decay drives the slow churning of the mantle. The outermost rigid shell of the Earth, the lithosphere, is broken into roughly fifteen major and minor plates that float on the weaker, partly molten asthenosphere below. These plates move only a few centimeters each year, about the speed a fingernail grows, yet over millions of years that motion rearranges continents, builds mountain ranges, and concentrates the stresses that release as earthquakes and eruptions. Understanding this movement is the foundation for understanding why geophysical hazards cluster where they do.

The engine behind plate movement is thermal convection. Hot, less dense material rises from near