Volcanic ash is formed during explosive volcanic eruptions when gases dissolved in magma (molten rock) expand and escape violently into the air, or when water is heated by magma and abruptly flashes into steam. The force of escaping gas shatters solid rocks. Expanding gas also shreds magma, blasting it into the air, where it solidifies into fragments of volcanic glass.
"Tephra" is the general term for volcanic rock fragments exploded or carried into the air during an eruption. Such fragments can range in size from less than 2 mm (0.08 inches) to more than 1 m (3.2 feet) in diameter. The smallest tephra is called volcanic ash, pieces of pulverized rock and volcanic glass the size of sand or silt. Tiny ash particles can be less than 0.001 mm (0.00004 inches) in diameter.
Volcanic ash can be carried great distances by wind, unlike larger tephra, which typically falls back to the ground at or near the vent. Thus, ash can cover a much larger area and disrupt the lives of far more people than other tephra.
Volcanic ash is not the product of combustion, like the soft, fluffy material created by burning wood, leaves, or paper. Volcanic ash is hard and does not dissolve in water. It is also extremely abrasive and mildly corrosive.
The volcanic ash produced by small explosions at Halema‘uma‘u Crater's Overlook vent is a mixture of lithic fragments (pulverized rock) and tiny spheres and shards of volcanic glass, including Pele's hair and Pele's tears.
Kīlauea Volcano is known for its relatively benign eruptions of fluid lava flows. Therefore, many people were surprised by the small explosions that occurred in Halema‘uma‘u Crater in March and April 2008. However, ash emissions from Halema‘uma‘u Crater are not unknown in the volcano's history.
Kīlauea 's summit has erupted explosively throughout the history of the volcano, producing ash deposits that date back at least 30,000 years and probably older. In 1790, at least 80 people were killed in the Ka‘ū Desert by searing hot gas and ash produced by a devastating explosion. More recently, in 1924, a series of steam explosions in Halema‘uma‘u Crater blasted columns of volcanic ash and dust as high as 3 km (2 mi) into the air. These plumes were blown downwind beyond the community of Pahala, 32 km (20 miles) away, where ash fallout turned day into night. Muddy ash also fell in lower Puna, making railroad tracks so slippery that trains couldn't travel in MAKUU. One person was killed near Halema‘uma‘u when he ventured too close to the vent and was hit by falling rocks.
Most mornings, rain or shine, USGS HVO scientists go to the rim of Halema‘uma‘u Crater downwind of the gas vent to collect ash that has fallen during the past 24 hours. The ash is taken back to HVO, where it is weighed to calculate average accumulation rate. It is also inspected under a microscope to determine composition - lithic fragments (pulverized old rocks), volcanic glass (juvenile material), or a combination of both. The glass component of the ash is then sent for chemical and petrographic analyses.
The highest rate of ash deposition during the current summit eruption of Kīlauea occurred on March 19 when the new vent opened on the floor of Halema?uma?u Crater. Presently, rates of deposition range from about 500 kg to about 3,000 kg per day, with about 1,000 kg per day (i.e., a ton) as a typical value.
Like airborne particles from dust storms, forest fires, and air pollution, volcanic ash particles are often small enough (less than 10 microns) to be inhaled deeply into the lungs. Thus, volcanic ash poses a health risk, especially to children, elderly people, and individuals with cardiac or respiratory ailments, such as asthma, chronic bronchitis, and emphysema.
Heavy ash fall can collapse roofs, cause short circuits and damage in electronic components, interrupt telephone and radio communications, cause power outages, clog air filters in vehicles and machinery, create poor visibility, make roads slippery or impassable, damage crops, and cause harm to grazing livestock.
Airborne volcanic ash poses a serious hazard to aviation because it can diminish visibility, damage flight control systems, and cause jet engines to fail. The greatest danger is to aircraft near the source of the ash cloud, but winds can blow volcanic ash great distances, so it can also pose a hazard to aircraft far downwind of an erupting volcano.
Due to ongoing concerns about explosions from Halema‘uma‘u, the Federal Aviation Administration has closed the airspace at or below 5,000 feet above ground level and three nautical miles from Halema‘uma‘u Crater to minimize risks to aircraft flying near the summit of Kīlauea.
As with any natural disaster, communities and homeowners should develop an emergency-response plan that can be activated if Kīlauea erupts large amounts of volcanic ash. Having a plan in place before an eruption occurs can greatly reduce the harmful and disruptive effects of ash.
Heavy ash fall is not expected from Kīlauea's current summit eruption, but even small amounts of ash can be problematic to humans, animals, and plants. Web sites listed below provide helpful suggestions on how to prepare for, and contend with, ash fallout.