1. Intro to Geology 4. Earth’s Materials and minerals 3. Rock Cycle and Rock types 4. Structure of Earth

Earth’s Materials & structure

Part II 10. Hydrologic Cycle 8. Rock Weathering & Soils 6.

Mass wasting 10. Streams Running water 11. Groundwater 12 13. Glaciers Deserts & winds 14. Work of Ocean Shorelines External processes

External factors, Water resources

Part III 8. Geologic time 17. Rock deformation

Internal Processes

Part IV

2 . Plate Tectonics

16. Earthquakes, Tsunamis 17.

Volcanoes Hazards

Hawaiian Islands

18. Hawaii Geology 19 Geothermal resources

Geology, Volcanic

20. Maui, Molokai, Lanai, Kahoolawe

Processes, Hazards,

21. Kauai 22. Oahu

Geothermal Resources

Volcanic eruptions are one type of phenomena that have shaped the Earth over the past four billion years. The products of the volcanic eruptions are creation of more land and islands in the deep open sea. Hawaiian Islands are one example of active volcanic processes. However, there is still much to learn about how Hawaiian volcanoes erupt and how island develops from the ocean floor. The history of the volcanic islands of Hawaii and the land formed by the volcanoes is very contrasting and\mysterious.

KILAUEA'S FUMES OVER HAWAII A CONCERN

LONG-TERM EFFECTS ON HEALTH, CROPS, BUSINESS CONCERN RESIDENTS

Vog Volcanic fog safety tips •Stay indoors and use an air conditioner, if available.

•Do not smoke and avoid second-hand smoke.

•Limit physical exertion.

•Drink plenty of fluids to loosen mucus. Warm beverages seem to work best.

•If you take medications, make sure you have an adequate supply and keep them readily available in a convenient place.

•Contact your physician as soon as any respiratory problem develops.

While these recommendations are intended primarily for persons having respiratory or chronic lung disease, they are also useful for healthy persons during vog episodes. To obtain additional information on respiratory health, contact your personal physician or the American Lung Association of Hawai`i at (808) 537-5966.

So-called vog, or volcanic fog, forms when sulfur dioxide gas reacts with sunlight, oxygen, dust particles and water in the air. Tiny droplets known as sulfate aerosols are created, along with sulfuric acid and other substances.

Elevated levels of sulfur dioxide gas could cause breathing problems, especially among those with respiratory conditions such as asthma, emphysema and bronch

Halau makes offering to Pele at Kilauea Volcano lookout :Dancers with Halau Ka Liko Pua O Kalaniakea, under the direction of Kumu Kapua Dalire-Moe danced and chanted to pay their respects to Pele on April 2, 2008. Volcanic fumes spewed from Halemaumau in the background. After the halau danced and chanted, they threw all their leis into the crater, returning them back to the earth

Air Pollution

Dispersion and Air Quality Forecasting

Located in the Central Pacific, just south of the Tropic of Cancer and east of the International Date Line, the islands of Hawai‘i enjoy a tropical climate. The general climate of the islands features moderate temperature from the high 70s to the mid 80s (degrees Fahrenheit) throughout the year. The islands are also subjected to winds generally from the northeast and localized rainfall storms.

Chemicals and Ingredients of the Volcanic Eruptions determine the Health Risks

Dispersion and Air Quality Forecasting

WHAT IS AIR POLLUTION

Definition Chemical, physical or biological agents that alter the natural characteristics of the Earth’s atmosphere Examples Chemical: Polycyclicaromatic hydrocarbons (PAH) Persistent Organic Pollutants (POPs) Physical: Particulate matter (PM), nuclear radiation Biological: Pollen, mold, germ warfare

SOURCES OF AIR POLLUTION

Human activities Manufacturing, trade & warfare Fuel & power production Agricultural production & burning Transportation Household activities & waste disposal Construction Natural?

Volcanic eruptions, wildlife emissions, dust, wildfires, radiation

Air Pollution – Natural Pollution, Volcanic Air Pollution Types of Contamination Gases, Vapors, Aerosols, Particulates, Volcanic Ash Public Health How does air pollution affect me?

Air Quality Index More Information All around the earth there is a thick blanket of air called the atmosphere. Air, like other gases, does not have a fixed shape. It spreads out to fill any available space so nothing is really empty. But air cannot escape from the atmosphere as the force of gravity keeps it from floating away from the earth.

Volcano Smoke Particles

Volcanic degassing vog The volcanic smog--or vog, as it is called- contains sulfur dioxide, which turns rainwater acidic and causes respiratory health problems.

Long-lasting Eruption of Kilauea Volcano, Hawai`i Leads to Volcanic-Air Pollution of volcanic smoke particles

Vog may pose health hazard

Volcanic Air Pollution—A Hazard in Hawai`i When Lava Meets the Sea – Lava Haze or Laze Air Pollution Pahoehoe lava enters sea.

Extreme heat from lava entering the sea rapidly boils and vaporizes seawater, leading to a series of chemical reactions. The boiling and reactions produce a large white plume, locally known as lava haze or laze, which contains a mixture of hydrochloric acid (HCl) and concentrated seawater.

Avoid standing beneath a laze plume.

Dense laze plumes contain as much as 10-15 parts per million of hydrochloric acid. These values drop off sharply as the plume moves away from the lava entry areas. During along-shore or on-shore winds, this plume produces acid rain that may fall on people and land along the coast. This rain (pH 1.5 to 2), often more acidic that lime juice or stomach acid, is very corrosive to the skin and clothing. Visitors to the lava entry areas should avoid standing directly in, under, or downwind of the laze plume. \

Much is still unknown about vog's composition and its effects on health.

On the Island of Hawai`i, the trade winds (blue arrows) blow the vog from its main source on the volcano (white plume) to the southwest, where wind patterns send it up the island's Kona coast. Here, it becomes trapped by daytime (onshore) and nighttime (offshore) sea breezes (double-headed arrows). In contrast, when light "kona" winds (red arrows) blow, much of the vog is concentrated on the eastern side of the island, but some can even reach Oahu, more than 200 miles to the northwest.

GAS ADVISORY SYSTEM HELPS ALERT PEOPLE OF HAWAI‘I due to Volcano Smoke Particles Current Conditions Last Updated: 04/07/2007 • 10:45 AM HST Sulfur Dioxide

Good Moderate Unhealthy

SO2 gas plume crosses road near Halemaumau and low on Chain of Craters Road. Sensitive individuals should limit exposure in these areas .

The hydrochloric acid (HCl) comes from the breakdown of seawater-derived chlorides during sudden boiling. Because the lava is largely degassed by the time it reaches the sea, any HCL coming from it is insignificant by comparison. Analyzed samples of the plume show that is is a brine with a salinity of about 2.3 times that of seawater and a pH of 1.5-2.0.

Key seawater chloride breakdown reactions that produce HCl gas

•MgCl 2 (sea salt) + H 2 O (steam) = MgO (periclase) + 2HCl (HCl gas) •2 NaCl (sea salt) + H 2 O (steam) = Na 2 O (sodium oxide) + 2 HCL (HCl gas) •CaCl 2 gas) (sea salt) + H 2 O (steam) = CaO (lime) + 2 HCL (HCl

Volcanic Air Pollution—A Hazard in Hawai`i

Health officials have warned people with asthma, bronchitis, emphysema, and allergies to stay indoors where there is air-conditioning or filtered air, to avoid strenuous exercise, and to drink lots of water. The vog can also affect children and the elderly.

http://pubs.usgs.gov/fs/fs169-97/ Noxious sulfur dioxide gas and other pollutants emitted from Kilauea Volcano on the Island of Hawai`i react with oxygen and atmospheric moisture to produce volcanic smog (vog) and acid rain. Vog poses a health hazard by aggravating preexisting respiratory ailments, and acid rain damages crops and can leach lead into household water supplies. Much is still unknown about vog's composition and its effects on health.

The U.S. Geological Survey's Hawaiian Volcano Observatory is closely monitoring gas emissions from Kilauea and working with health professionals and local officials to better understand volcanic air pollution and to enhance public awareness of this hazard.

Air pollution

Pollution usually refers to the presence of substances that are either present in the environment where it doesn't belong or at levels greater than it should be.

Air pollution is caused by any undesirable substance, which enters the atmosphere. Air pollution is a major problem in modern society. Even though air pollution is usually a greater problem in cities, pollutants contaminate air everywhere. These substances include various gases and tiny particles, or particulates that can harm human health and damage the environment. They may be gases, liquids, or solids. Many pollutants are given off into the air as a result of human behavior. Pollution occurs on different levels: personal, national, and global.

Some pollutants come from natural sources.

Forest fires emit particulates, gases, and VOCs (Volatile organic compounds and substances that vaporize into the atmosphere) Ultra-fine dust particles created by soil erosion when water and weather loosen layers of soil, increase airborne particulate levels. Volcanoes spew out sulfur dioxide and large amounts of pulverized lava rock known as volcanic ash and volcanic smoke particles.

Passive Eruptive

Which emission source is larger? Passive or Eruptive?

Nyiragongo (DR Congo )

• No eruption of magma • Long-lived (weeks-centuries)

Reventador (Ecuador)

• Magma erupted (ash) • Short-lived (hours-days) • Low altitude - tropospheric (<5 km) • High altitude - stratospheric (<40 • Environmental/health hazard • Poorly quantified km) • Climate impacts poorly understood • Aviation hazard (ash) • Global climate effects possible

Steam rises under a forbidding sky, as lava from the Piton de la Fournaise volcano drains into the Indian Ocean. The 2,632 meters (8,635 feet) mount, on Reunion Island, has erupted three times this year. The latest phase began on 2 April. The island, a French territory, neighbors Mauritius.

Natural Background Aerosol

 Stratospheric and tropospheric aerosol  Stratospheric aerosol • 11-50 km • Formation of sulfuric acid droplets by gas-to-particle conversion of SO 2 injected into the stratosphere by major volcanic eruptions  Tropospheric aerosol • <11 km • Direct emissions from natural sources: desert, ocean, and vegetations • Gas-to-particle formation Does tropospheric aerosol concentration vary with altitude? Why?

Eruption of Mt. Pinatubo (in the Philippines) in 1991

Do You Know…

During the eruption of Mt. Pinatubo, 14-20 Tg (1 Tg= 10 6 tons) of SO 2 was injected into the stratosphere, causing the aerosol concentration to increase from 2-5 µg/m 3 µg/m 3 to 20-100

Net solar radiation at Mauna Loa Observatory, relative to 1958, showing the effects of major volcanic eruptions. Annual variations are due to transport of Asian dust and air pollution to Hawaii

www.research.noaa.gov/climate/t_greenhouse.html

Volcanic Gas Emissions Volcanic Smoke Particles

Vog

Contour map of volcanic air pollution in the Kau District, Big Island of Hawai'i, during September 2003. Eruption at Kilauea Volcano produces a visible plume from the eruption vent and degassing from the summit.

Hawaiian residents who live downwind from the long-active Kilauea volcano may have elevated risks of adverse health conditions because of high levels of sulfur dioxide and aerosol particulates that drift downwind, Eye irritation Headache Upper respiratory Irritation Nausea, dizziness Fatigue, lethargy Sore/dry throat Odor Memory impairment

Vog and acid rain affect water quality Many residents on the island of Hawaii depend on rainwater collected by rooftop catchment systems for drinking water. The continuous release of volcanic gases, especially sulfur dioxide, causes rainwater to become acidic downwind of Kilauea's erupting vents. When it falls on roofs, this acid rain leaches lead from roofing nails and paint. The lead contaminated rainwater then fills the water catchment tanks, creating a health hazard.

Volcano

Tectonic Style Temperature Water Vapor H 2 0 Carbon Dioxide C0 2 Sulfur Dioxide S0 2 Hydrogen H 2 Carbon Monooxide CO Hydrogen Sulfide H 2 S Hydrochloric Acid HCl Hydrofluoric Acid HF

Kilauea Summit

Hot Spot 1170 °C 37.1 48.9 11.8 0.49 1.51 0.04 0.08 ---

Erta` Ale

Divergent Plate 1130 °C

Momotombo

Convergent Plate 820 °C 77.2 11.3 8.34 1.39 0.44 0.68 0.42 -- 97.1 1.44 0.50 0.70 0.01 0.23 2.89 0.26 Examples of volcanic gas compositions, in volume percent concentrations (from Symonds et. al., 1994 )

A volcanic eruption may send ash and sulfate gas high into the atmosphere. The sulfate may combine with water to produce tiny droplets (aerosols) of sulfuric acid, which reflect sunlight back into space. Large eruptions reach the middle stratosphere (19 miles or 30 kilometers high). At this altitude, the aerosols can spread around the world.

The Earth's atmosphere is made up mostly of nitrogen (78%) and oxygen (21%), with a small amount of "trace gases" (1%) mixed in. But, that tiny percentage of trace gases - such as carbon dioxide, ozone, methane, and carbon monoxide - contribute in a big way to changes in the Earth's climate. Such trace gases, also called greenhouse gases, allow energy from the sun (known as shortwave radiation) to reach the earth's surface, but absorb energy emitted from the earth (known as longwave radiation); this affects the surface energy balance of the planet by warming the atmosphere directly above it resulting in long-term changes to global climate. Although a greenhouse also works by trapping energy from the sun, the physics is different. The roof of a greenhouse is a slab of glass that traps radiation emitted from the ground which prevents convection (i.e. rising hot air) from allowing heat to escape. The atmospheric greenhouse is based on certain molecules (e.g. carbon dioxide) absorbing radiation at particular wavelengths (such as that emitted from the ground) and reemitting a portion back to the ground. Although an excess of greenhouse gas results in global warming, naturally occurring greenhouse gases are beneficial in keeping our planet at a comfortable temperature.

Aerosols

Small particles in the atmosphere - from smoke, dust, manufacturing, and other sources - can affect how the Earth system behaves. For example, aerosols can absorb and scatter radiation , which can cause either warming or cooling of the atmosphere. They also are important to the formation and behavior of clouds, and can influence the water cycle and the Earth's radiative balance.

Sierra Negra (Galapagos Is) eruption October, 2005

22 Oct 2005 27 Oct 2005 • Effusive eruption • Oct 22 - Nov 1, 2005 23 Oct 2005

Topography • At night, cold, dense surface air drains downhill into low-lying areas, making them susceptible to strong temperature inversions • Hills and mountains can block flow (like Mauna Loa) • Sea breezes can trap pollution in its vertical circulation (East Maui)

A volcanic eruption may send ash and sulfuric acid (SO2) into the atmosphere, which increases planetary reflectivity causing atmospheric cooling. Over time precipitation will remove these aerosols from the atmosphere. Volcanic eruptions can have a worldwide impact.

A massive volcanic eruption can cool the Earth for one or two years. The 1982 El Chichon eruption and the 1991 Pinatubo eruption caused the globally averaged surface temperature to cool less than 1°F

Acid Rain • Note that “clean” rain is naturally acidic (pH of 5.6) due to carbon dioxide in the atmosphere dissolving in the water to create carbonic acid H 2 O + CO 2 —> H 2 CO 3 • Other acids are created when pollution aerosols dissolve in water • Important acids created due to man’s activities are sulfuric acid and nitric acid

Sulfuric Acid

• Sulfur dioxide dissolves in water to create a weak acid, sulfurous acid SO 2 + H 2 O —> H 2 SO 3 • Sulfurous acid then can react with oxygen to create sulfuric acid O 2 H 2 SO 3 + —> H 2 SO 4 • Once again, sulfur dioxide is emitted primarily by coal-burning power plants

Nitric Acid

• Nitric acid is created when nitric oxide (NO) oxidizes, yielding nitrogen dioxide (NO 2 ). The nitrogen dioxide then dissolves in water creating nitric acid (HNO 3 ) and nitrous acid (HNO 2 ) 2NO 2 + H 2 O —> HNO 3 + HNO 3 • High temperature combustion, such as automobiles, produced the nitric oxide that began this process

Acid Rain Consequences • Crop and forest damage • Increase in acidity for lakes, rivers • Death of fish and wildlife • Weathering of monuments and buildings • Health impacts for those prone to respiratory ailments

Particulate Pollutants

Sources of Atmospheric Aerosol TABLE 1 Sources and Estimates of Global Emissions

of Atmospheric Aerosols (Data from: W.C. Hinds, Aerosol Technology, 2nd Edition, Wiley Interscience) Source Amount, Tg/yr [10 6 tons/yr] metric Range Best Estimate

Natural

Soil dust Sea salt 1000 - 3000 1000 10000 26 - 80 1500 1300 Botanical debris Volcanic dust Forest fires Gas-to-particle conversion Photochemical

Total for natural sources

4 - 10000 3 - 150 100 - 260 40 - 200 2200 24000 50 30 20 180 60 3100

Anthropogenic

Direct emissions Gas-to-particle conversion Photochemical

Total for anthropogenic sources

50 - 160 260 - 460 5 - 25 320 - 640 120 330 10 460 Which emission source is larger? Natural or anthropogenic?

The Air Quality Index is a tool used by EPA and other agencies to provide the public with timely and easy-to-understand information on local air quality and whether air pollution levels pose a health concern. The AQI tells the public how clean the air is and whether or not they should be concerned for their health. The AQI is focused on health effects that can happen within a few hours or days after breathing polluted air .

Toxins in the Air • As a result, EPA is now using an exposure assessment methodology that measures all the exposures to toxics regardless of media (e.g., air, water, land).

What Is Exposure Assessment?

• Scientists and government officials use a four-step process called risk assessment to estimate people's increased risk of health problems as a result of exposure to a toxic air pollutant.

THE 4-STEP EXPOSURE ASSESSMENT

Hawaii Volcanoes National Park is perhaps the only place on earth where visitors arrive in continuous carloads to peer at volcanic landscapes, erupting ashy craters, calderas, cones, plumes of gas, and skeletons of trees and to clamber over sharp rock, desperate to see lava. Despite many posted warnings, people in open-toed sandals and shorts eagerly trot along newly hardened, still hot lava to peek through a sudden opening and catch a glimpse of the red flow. The desire to be as close as possible to this force of nature has cost five people their lives in the past decade because they ignored warnings either about lava hazards or about medical conditions that can be aggravated by sulfuric fumes. At the same time, however, Hawaiian volcanoes are, as volcanoes go, gentle giants ones--so if you are going lava hunting, this is the place to do it. Hawaiian lava here is more fluid than most and contains less gas, so it is less explosive and gives rise to what are called shield volcanoes because of their sloping profiles. landscape, a battlefield of burned trees and buried houses--although a few structures stand stranded in small patches of rain forest, spared by the flow. Pu`u `O`o destroyed the Royal Gardens community here between 1983 and 1986 and covered part of the park's Chain of Craters Road. From the air it is easy to appreciate the power and reach of the volcano, the primal force shaping this island, creating new land (about 600 acres' worth so far), incinerating everything in its path

Gas composition: monitor the composition of gases that are continually vented from the volcano, and note some unique changes in the gas composition have correlated with eruptions that followed

Air mass moves in

An air mass moves toward a mountain.

This air mass holds water vapor that has evaporated from an ocean

Air mass forced upward

:The slope of the mountain forces the air mass upward. .As the air moves higher, it becomes less dense. The reason is that, at higher elevations, there is less air above to push down on the air mass. This decrease in air pressure causes the air to expand. As it expands, it cools. This process, called adiabatic cooling, typically results in a cooling of about 1*C per 100 meter ( 5.5°F for every 1,000 feet).

Clouds form

:As the air is forced higher, it cools even more. When it reaches its dew point -- the temperature at which water vapor in the air becomes saturated—water molecules within the air start to condense, forming water droplets. These droplets are visible as clouds.

Rain, sleet, and snow

: The air continues to cool as it rises, but not as fast as before because condensation heats the air. As it cools, more water drops form. When they get large enough, they fall to the Earth in the form of rain, sleet, or snow. This is why the windward side of a mountain is usually wetter than its leeward side.

Over the summit:

After the air mass passes over the mountain's summit, it begins to descend. Just as the air cooled as it became less dense, it now begins to warm as it becomes denser at lower elevations—again, at a rate of about 5.5°F for every 1,000 feet. This process is called adiabatic warming.In addition to becoming denser and warmer, the air's relative humidity decreases as it descends. Relative humidity is the total amount of water vapor in the air, measured as a percentage of water vapor the air can hold at a given temperature .

Rain shadow

: The warmed air mass, which lost most of its water content on the other side of the mountain, warms quickly as it descends. Because heat was added to the air mass when, on the other side of the mountain, its water vapor condensed, it can be much warmer when it reaches the base of the mountain—as much as 50°F warmer than it was before it started its ascent. The lack of moisture, and rain, on the leeward side of a mountain is known as its "rain shadow."

For those who live on or near mountains, a change of climate can be just a short walk away. That's because mountains create their own microclimates—areas in which the climate differs from the prevailing climate. Take Mauna Loa, Mauna Kea or Haleakala. These volcanoes in Hawaii have microclimates that range from scorching at the base to frigid at the summit. (For a look at the mountain's six ecological zones, see Figures.) The flow of air masses over these volcanoes and other mountains also influences microclimates, often causing wet (or snowy) weather on one side and a dry, clear climate on the other.

The volcanic smog--or vog, as it is called--contains sulfur dioxide, which turns rainwater acidic and causes respiratory health problems.

Flank eruption

Vog, volcanic smog, is a concern to the residents of the Big Island of Hawai'i because of the possible harm it is doing to their health, agriculture, and the tourist industry. (Monastersky, 1995) Sore throats, headaches, allergies, and bronchitis have been blamed as some of the health hazards of vog

VOG

:

Volcanic

Smoke+

Fog

) lava slowly comes out, but so does smoke containing sulfur, mercury, and arsenic. 80% of the time the NE Tradewinds blow and keep this VOG (volcanic fog) off Maui. When the trades don't blow, the VOG drifts over Hawaii and Maui, and it gets hazy

During prevailing trade wind conditions, the nearly constant stream of volcanic smog (vog) produced by Kilauea Volcano on the Island of Hawai`i is blown to the southwest and west (satellite image shows increasing amounts of vog aerosol particles in yellow, orange, and red, respectively); traces have been detected as far away as Johnston Island, 1,000 miles to the southwest. On the Island of Hawai`i, the trade winds (blue arrows) blow the vog from its main source on the volcano (white plume) to the southwest, where wind patterns send it up the island's Kona coast. Here, it becomes trapped by daytime (onshore) and nighttime (offshore) sea breezes (double-headed arrows). In contrast, when light "kona" winds (red arrows) blow, much of the vog is concentrated on the eastern side of the island, but some can even reach Oahu, more than 200 miles to the northwest. (The names of the five volcanoes that make up the Island of Hawai`i are shown in yellow. National Oceanic and Atmospheric Administration (NOAA) satellite image processed by John Porter and collected by Pierre Flament, University of Hawai`i.)

VOG

:

Volcanic

Smoke+

Fog

HVO shows how scientists take temperature readings in the volcano's crater to help them predict how far below the surface the magma lies. Molten lava from Kilauea Volcano frequently flows through underground lava tubes to reach the Pacific Ocean, where it vigorously reacts with cold seawater to create large steam plumes laden with hydrochloric acid (HCl). These plumes, known as "laze", are another form of volcanic air pollution and pose a local environmental hazard along the Island of Hawaii's southeast coast and southeastern Maui, especially to people who visit these ocean-entry sites. Kilauea is the single largest source of volcanic sulfur dioxide in the world, emitting as much as 1,800 tons of the gas a day, and 40,000 truckloads of volcanic materials on the land.

When a volcanic erupts, sulfur dioxide within the molten rock is converted to sulfuric acid. The resulting plume is known as vog (volcanic fog). Gases are also produced at the ocean. When the lava enters the ocean, hydochloric acid is produced - called laze (lava haze). Both of these gases can contain particulate matter, such as volcanic glass or trace metals.

Vog and laze are carried by winds across the county. The impacts include: obscured views; lower agricultural yields for certain crops; adverse health effects for people with respiratory or heart conditions; and acidified rainwater catchment tanks (which, in turn, produces a secondary hazard of leached lead in local water supplies).

The goals of the Lung Assessment during Volcanic Activity study, LAVA for short, include: Teaching residents research skills enabling them to participate in their environmental health and safety. Assessing the community's exposure to vog over the last 10 years, based on historical records of weather patterns and volcanic emissions. Measuring current acidity and amount of particles small enough to breathe. Studying the respiratory systems and lung functions of children across the island who have lived their entire lives with vog. Studying the lung growth and health of these children five areas on the Big Island: the Kona Coast (west Hawaii), Hilo (east Hawaii), Ka’u, (on Hawaii’s south coast), Waimea and the Kohala Coast (on the northern tip of the island). LAVA project : Elizabeth Tam, a pulmonologist at the University of Hawaii's John A. Burns School of Medicine on Oahu

Mt Ruapehu Crater Lake Lahar Threat Response - Image of Whangaehu Valley, March 18, 2007.

What is a lahar?

Lahar is an Indonesian word that refers to a rapidly flowing mixture of rock debris and water (other than normal water flows) from a volcano.

Large lahars can present a significant natural hazard. When they overflow their channels they can destroy, erode or bury obstacles in their path. There are various kinds of lahar. A debris flow lahar contains large amounts of sediment (more than 60% of volume) of varying size (from small particles to boulders) and flows like a slurry. A hyperconcentrated flow lahar contains less sediment (mainly sand-sized particles or smaller) and flows more like water. Management precautions for the predicted Crater Lake lahar are based on a debris flow lahar.

A train passes over a bridge over the Whangaehu River at the scene of the historic Tangiwai Rail disaster after a mud flow from the crater lake of Mount Ruapehu, in the central North Island, New Zealand, Sunday, March 18, 2007. A potentially lethal mix of mud, acidic water and rocks tore down the slope of New Zealand\'s Mount Ruapehu on Sunday, emergency officials said, but there was no immediate threat to life. (AP Photo/NZPA, Stephen Barker)

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