Don't dance on a volcano. - French Proverb

A volcano is a mountain that opens downward to a reservoir of molten rock below the surface of the earth.When pressure from gases within the molten rock becomes too great, an eruption occurs. Eruptions can be quiet or explosive. There may be lava flows, flattened landscapes, poisonous gases, and flying rock and ash that can sometimes travel hundreds of miles downwind. Active volcanoes in the U.S. are found mainly in Hawaii, Alaska and the Pacific Northwest. The danger area around a volcano covers approximately a 20-mile radius however some danger may exist 100 miles or more from a volcano.

Because of their intense heat, lava flows are great fire hazards. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way.

Fresh volcanic ash, made of pulverized rock, can be abrasive, acidic, gritty, gassy and odorous. While not immediately dangerous to most adults, the acidic gas and ash can cause lung damage to small infants, to older adults and to those suffering from severe respiratory illnesses. Volcanic ash also can damage machinery, including engines and electrical equipment. Ash accumulations mixed with water become heavy and can collapse roofs. Volcanic ash can affect people hundreds of miles away from the cone of a volcano.

Sideways directed volcanic explosions, known as "lateral blasts," can shoot large pieces of rock at very high speeds for several miles. These explosions can kill by impact, burial or heat. They have been known to knock down entire forests.


  • Ashfall - Volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered.
  • Lahar - A flowing mixture of water and rock debris, sometimes referred to as a debris flow (originating at a volcano) or mudflow.
  • Pyroclastic flow - A hot 570-1470 degrees F, dry, fast-moving 20 to more than 200 miles per hour and high-density mixture of ash, pumice, rock fragments, and gas formed during explosive eruptions or from the collapse of a lava dome. Moves away from a volcano at high speeds.
  • VEI Scale - Some scientists recently proposed the Volcanic Explosivity Index (VEI) to attempt to standardize the assignment of the relative size of an explosive eruption, using ejecta volume as well as the other criteria.

VEIDescriptionPlume HeightClassificationHow oftenExample
0non-explosive< 100 mHawaiiandailyKilauea
1gentle100-1000 mHaw/StromboliandailyStromboli
2explosive1-5 kmStrom/VulcanianweeklyGaleras, 1992
3severe3-15 kmVulcanianyearlyRuiz, 1985
4cataclysmic10-25 kmVulc/Plinian10's of yearsGalunggung, 1982
5paroxysmal>25 kmPlinian100's of yearsSt. Helens, 1980
6colossal>25 kmPlin/Ultra-Plinian100's of yearsKrakatau, 1883
7super-colossal>25 kmUltra-Plinian1000's of yearsTambora, 1815
8mega-colossal>25 kmUltra-Plinian1000's of yearsYellowstone, 2 Ma

Before a Volcanic Eruption

  • To begin preparing, you should build an emergency kit and make a family communications plan.

During a Volcanic Eruption

  • If advised to evacuate, do so immediately. Take your disaster supply kit, lock your home and choose a route away from the hazard.
  • Be aware of mudflows. The danger from a mudflow increases near stream channels and with prolonged heavy rains. Mudflows can move faster than you can walk or run. Look upstream before crossing a bridge and do not cross the bridge if a mudflow is approaching.
  • Avoid river valleys and low-lying areas.
  • If you are unable to evacuate, and in order to protect yourself from falling ash:
    • Listen to a battery-powered radio or television for the latest emergency information.
    • Use goggles and wear eyeglasses instead of contact lenses.
    • Use a dust mask or hold a damp cloth over your face to help with breathing.
    • Stay indoors until the ash has settled unless there is a danger of the roof collapsing. Close doors, windows, and all ventilation in the house (chimney vents, furnaces, air conditioners, fans and other vents.
    • Avoid running car or truck engines. Driving can stir up volcanic ash that can clog engines, damage moving parts, and stall vehicles.

After a Volcanic Eruption

  • Listen to a battery-operated radio or television for the latest emergency information.
  • Clear heavy ash from flat or low-pitched roofs and rain gutters.