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rockfall debris blocking a park road
Rockfall blocking Scenic Drive in Zion National Park, Utah.

A geologic hazard is a naturally occurring, dynamic geologic process capable of causing damage, loss of property, and/or injury and loss of life. Geologic hazard processes can happen slowly over days or years, or have a sudden onset occurring in seconds or minutes. Engineering solutions, such as stabilizing hillslopes or building flood walls, can mitigate some serious hazard-related safety issues, but many naturally occurring hazards are more difficult to mitigate, such as a rockfall or sinkhole formation. These hazards may pose a threat to the safety of visitors and employees, the structural integrity of facilities, and the preservation of irreplaceable cultural resources.

Listed below are categories of geohazards and brief descriptions of the types of dangers associated with each hazard.

 

Natural Process Geohazards

Dust storms can move in quickly and reduce visibility, cause damage infrastructure and machinery, and be hazardous to people. Wind blown dust—AKA Loess (fine sand, silt, and clay particles)—can cause eye injuries, damage to breathing passages, and greatly increasing the chances of becoming disoriented and lost.

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A flow of snow, ice, rock, and other material that occurs as a result of thawing or agitation (can be started by the weight of skiers), are called avalanches. These events occur mostly late in the snow season when the snowpack is deep and the thaw is just beginning. Especially common are avalanches from steep slopes with a deep snow cover. Fast-moving, deep snow-flows and rapid burial are the biggest danger to people and facilities.
A karst landscape is characterized by sinkholes, caves, sinking streams, and springs. These features typically develop in limestone terrain, but can also be found in areas with abundant evaporate deposits. Dissolution of these subsurface layers can lead to surface failures. The major dangers in caves are rockfalls, falling, or getting lost. In karst topography, the biggest danger, though unpredictable, is the sudden formation of sinkholes.

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Numerous hazards face parks located on ocean coasts and lake shores. The regions of our country with these types of parks are the Atlantic coast, Gulf coast, Great Lakes parks, and Pacific Rim locations. Major hazards that make up this broad category include: tropical storms, hurricanes, coastal flooding, tsunamis, beach erosion, ship groundings, and submarine landslides (within park boundaries). Storms and floods can cause widespread facility damage as well as expose harmful or toxic components from debris. Tsunamis are extremely dangerous events, sending a large wave or series of large waves toward the shore as a reaction to a coastal landslide or submarine earthquake. Massive damages can result from a tsunami and only some of them give advanced warning. Those waves that propagate from long distances can be prepared for, but in the case of small, nearly-closed basins, little can be done to reduce risks.

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Flooding is an overflow of water onto land. Floods at parks can occur on the floodplains of any body of water or as a result of major storm events, spring snowmelt, or rain-on-snow events. Coastal flooding is also an issue at parks, but has been categorized as a coastal/shoreline hazard for this study.

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At volcanically active parks and those with heat at depth in their geologic strata, geothermal hazards become a concern for visitor and employee safety. Boiling mudpots, geysers, steam-vents and hot springs are all geothermal hazards present at certain parks. Most of these hazards are well mapped and serious injury can generally be avoided if warning signs are followed. Simply staying on the trail or boardwalk and using handrails could have prevented many injuries and fatalities. The biggest dangers posed by geothermal hazards are severe skin burns which can lead to hospitalization and often fatal injuries.

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Glacial hazards are common in the high-elevation, mountainous regions of the country. The major hazards to people are accidental slips into crevasses and glacial outburst flood events (or Jökulhlaups). A crevasse is a deep crack in an ice sheet or glacier. Although a range of sizes and shapes exist, the deepest crevasses can be up to 45 meters deep. Falling into a crevasse while on a backcountry hike or climb could prove fatal to both visitors and rescuers alike. A glacial outburst flood is any large and abrupt release of water from a backcountry or subglacial lake. These flooding events can have disastrous results for homes and facilities downstream.

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Landslides, mudflows, and debris flows are all forms of mass wasting that can cause severe facility damage and endanger lives. Often lubricated by rainfall or agitated by seismic activity, these events occur very rapidly and move as a flow. The runout of a mass wasting event depends on the volume of material, water content, and slope steepness. Buildings, trails, campgrounds, and cultural resources can be deeply covered with debris, damaging them beyond repair. Lahars, avalanches, and rockfalls (all forms of mass movement) are categorized as different hazard types for this study because they occur under very unique conditions.

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Although Rockfall are technically a form of mass wasting, they are unique because either their volume or viscosity prevents them from behaving as a flow. Fragments of rock that fall off vertical or sub-vertical cliffs and travel down slope, by bouncing or sliding, can pose a serious threat in our parks. Those parks in mountainous terrain or with any kind of steep, sheer cliffs have rockfalls dangers. Roads and facilities built along cliffs or below cliffs are most sensitive.

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Seismic waves are generated from slips and sudden releases of energy at depth in the Earth’s crust. The surficial expression of this energy is called an earthquake and can range widely in severity. Strong earthquakes can destroy buildings and roads, start landslides or rockfalls, and can even cause wide-spread flooding. Earthquake events are tightly linked to geographic location, focused mostly around the Pacific Basin and on major fault lines.

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Violent eruptions are only one of the hazards associated with volcanic hazards. Volcanic mudflows, called lahars, carry a mixture of hot or cold water and volcanically-derived debris down the flanks of a volcanic feature. They vary in size and speed which, in turn, varies their destructive power. A pyroclastic flow is a high-density mixture of rock fragments and hot gases that move away from the volcanic vent at high speeds. Eruptions can cause lava flows, clouds which rain down hot rock (tephra deposits), seismic activity, and poisonous ash and gas. All these hazards can prove very dangerous to park visitors and staff as well as the facilities and cultural resources that may be located near a volcanic field.

No eruption is necessary for cracks in the ground to allow gases to reach the surface. More than 90% of all gas emitted by volcanoes is water vapor (steam), most of which is heated groundwater. Other common volcanic gases are carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, and fluorine. Sulfur dioxide gas can react with water droplets in the atmosphere to create acid rain, which causes corrosion and harms vegetation. Carbon dioxide is heavier than air and can be trapped in low areas in concentrations that are deadly to people and animals. Fluorine, which in high concentrations is toxic, can be adsorbed onto volcanic ash particles that later fall to the ground, poisoning livestock and domestic water supplies.

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Additional Geohazards

Human activities can influence the the rate and magnitude of geologic processes, and introduce or concentrate hazardous materials. One example is hazards related to mineral extraction, especially abandoned mines which are no longer maintained for entry.

Reference
Schaller, E. M., V. L. Santucci, S. B. Newman, T. B. Connors, and E. L. Bilderback. 2014. Summary and categorization of documented geologic hazards of the National Park System. Natural Resource Report NPS/NRSS/GRD/NRR—2014/813. National Park Service, Fort Collins, Colorado.

Last updated: August 31, 2018

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