Most geological processes, including the formation mineral bodies, are extremely slow, and even major tectonic movements are measured on the order of only centimetres per year. Few of these processes provide changes readily detectable in a decade. From time to time, however, they generate extreme natural events such as earthquakes, volcanic eruptions and tsunamis. These, with landslides and snow avalanches, constitute geological hazards which emphasize the natural variability in environmental systems, remind human communities of the great power of natural forces, and test the abilities of people and nations to respond to them.
The Mexico City earthquake of September 1985, killed approximately 5000 people, while the volcanic eruption that decimated Armero, Colombia (November 1985) claimed 25,000 lives.