A Hydrothermal eruption is an eruption of steam and rocks whose energy derives solely from the internal dynamics of a geothermal system, i.e. no magma is involved. Such eruptions are common in active geothermal fields and can be locally very destructive, causing loss of life and damage to structures. They have been known to eject clasts up to 2-3 m in diameter up to several hundred meters from the vent, and may produce craters and deposits known as hydrothermal eruption breccias. They form when water flashes to steam which erupts, cracking (brecciating) rocks and ejecting them at the surface. Such eruptions are usually triggered by local depressurization, e.g. due to earthquakes or the sudden release of overburden from landsliding. They can also be triggered if the system becomes sealed or covered, allowing internal pressures to increase and become greater than that of the surrounding rock. This sealing can occur naturally (e.g. if silica precipitates from geothermal waters to form an impermeable deposit) or by man (e.g. if asphalt or concrete slabs are placed over geothermal features). A hydrothermal eruption may last for several days, and continue until the steam forms too slowly to provide sufficient lifting power to eject rocks from the crater, although steam may continue to be discharged for up to several years after the hydrothermal eruption has ended. Hydrothermal eruptions may cause major changes to the hydrology of the geothermal field