AZGS Geologic Hazard Glossary

Sources: Dictionary of Geological Terms, 3rd edtn.  American Geological Institute,
                      R.L. Bates & J.A. Jackson, eds, 1983. 
	   Wikipedia: http://en.wikipedia.org
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Creep - A form of mass wasting involving the slow (mm to cm per year) downslope movement of mineral, soil and rock material.  Creep occurs on slopes as low as 1 degree. 

Debris flows – A rapidly moving mass of rock fragments, soil, and mud, with more than ½ the particles being larger than sand size. Velocity varies greatly, but 10s of kilometers per hour is not uncommon.

Earthquake – A sudden motion or trembling in the Earth caused by the sudden release of slowly accumulated strain.  Seismic waves – secondary and primary waves -- propagate outward in all directions at velocities of 3 to 6 km/second (approximately 6600 to 13,400 miles per hour!).

Earth Fissure – A tension fracture in the Earth’s surface that results from substantial overdrafting (pumping) of ground water and subsequent subsidence of the ground surface.  Fractures may extend for 100s of feet to several miles, from inches to 15 ft wide, and may be 100s of feet deep.  

Fault – A fracture zone along which there has been displacement of earth materials/rock on either side of the zone.  The action of fault rupture releases seismic energy, thereby producing earthquakes. 

Flash Flood -- A flash flood is a rapid flooding of geomorphic low-lying areas - washes, rivers and streams - caused by the intense rain fall associated with a thunderstorm, or multiple thunderstorms.

Flood – High stream flow that overtops the stream banks in any part of its course, covering land that is not normally underwater. 

Geomagnetic storm -- A temporary disturbance of the Earth’s magnetosphere caused by a disturbance in space weather.  Geomagnetic storms electrical power generation and distribution. 

Geologic Hazard – A geologic condition or phenomenon that poses a risk or is a potential danger to human life or property.  

Landslide – A general term for a wide variety of processes and landforms involving the downslope movement, under the influence of gravity, of masses of soil and rock material.

Mass wasting – A general term for the downslope movement of soil and rock material under the influence of gravity.  Rates (velocities) of mass wasting phenomenon range dramatically from centimeters (inches) per year to 160 km (100 miles) per hour.  

Radon – Rn 222 is a naturally occurring radiogenic gas with a half-life of 3.8 days.  Radon is a major cause of lung cancer in the U.S.  

Risk – The likelihood or magnitude of loss associated with a specific geologic hazard.  

Rockfall – The relatively free falling of a newly detached segment of bedrock from a cliff, steep slope, or arch.

Seiche – An oscillation of a body of water in an enclosed basin that varies in period.  It is caused chiefly by local changes in atmospheric pressure, tidal currents, and less commonly earthquakes.    

Shrinking (or swelling) soil – Shrinking soil is caused by a loss in water moisture due to evaporation, ground water lowering, or plant uptake of water.  Swelling or expansive soils are ones that swell when subjected to moisture.  

Volcanic activity – Eruption of molten rock onto the Earth’s surface.  Eruptions range from effusive or passive (e.g., lava flow extrusion) to explosive and sometimes catastrophic pyroclastic eruptions (pyro – hot, clastic – broken or fragmental).  There are a variety of volcanic hazards including lava flows, tephra fall (ashfall), pyroclastic flows and debris avalanches, to name a few.