The earth seems stable to us, but it is a layered, constantly moving system. Tectonic plates, fragments of the earth's crust, move continuously, at a rate of 1–18 cm per year.
Where tectonic plates diverge, rifts occur. Mid-ocean ridges form where upwelling magma breaks through the crust at rift zones. Subduction occurs where dense, thin oceanic plates collide with less dense, brittle, continental crust. Subducted material often melts and rises, producing volcanoes.
Rocks are classified by mineral content, grain size, and origin. Three general classes of rock are igneous, metamorphic, and sedimentary. Each can be transformed into the others by processes such as weathering, sedimentation, heat and pressure, or melting.
Economically important concentrations of minerals occur in many parts of the world. Ores often occur as a result of hydrothermal (hot water) activity belowground.
Major mining methods include placer mining (of stream sediments), underground mining, open-pit mining, and strip mining. Strip mining is especially important for coal production.
Water contamination is one of the main environmental costs of mining. Pollutants include acids, cyanide, mercury, heavy metals, and sediment. Air pollution, especially acidic fumes from smelting ores, is also an important consequence of mining and mineral processing.
Worldwide, only a small percentage of minerals are recycled. Recycling saves energy and reduces environmental damage caused by mining and processing ore. Material substitution, such as replacing copper telephone lines with fiber-optic lines, also reduces resources consumption.
Earthquakes, tsunamis, landslides, and volcanoes are among the geologic hazards that take many lives. Big earthquakes are the most deadly geologic hazards, especially where building construction is poor.
