Geography 1700
Vocabulary – Through Quiz #4
July 21
100-year flood – land area that is expected to be covered with damaging flood water at least once, on
average, every 100 years. A “100-year storm” may occur several times or not at all during any given
100-year period. The estimated likelihood is one percent chance of flooding during any given year, for a
cumulative likelihood of 100 percent in any century.
See Flood Plain also.
In the United States, many locations do not have more than about 150 years of direct experience with
flooding, so we really don’t know what nature can do over a long period of time, so a 100-year flood
designation is not very reliable, especially when we alter the land to make flood hazard either less or
more than what it was historically.
Aftershock –
Alluvial fan – where stream Load is deposited at the end of the stream or river. See also river delta.
Angle of Repose – “repose” is like “being at rest” or temporarily stable. The ‘Angle of Repose’ is
generally the steepest angle that earth, rock, soil or other material can hold without falling, slipping,
sliding or creeping steadily. We should not trust earth material at is normal angle of repose, because
almost anything can upset that balance and cause instability. For instance, adding water will almost
always make a slope less stable. Wildfire can expose a slope to erosion. An earthquake can greatly
reduce the stability of an earth or rock slope. For construction purposes, engineering study and grading
practices may need to be extensive, along with careful and expensive foundation and drainage features
in order for a heavy building to be stable on any angle of repose.
Asthenosphere –
Attenuation or “distance decay” –
Avalanche – a sudden fall or slide of rocks, dirt or snow. Most often, a snow slide is referred to as an
avalanche.
Basalt – relatively heavy, low-silicate lava rock that is extruded (flows) across land surfaces. Basalt is
dark rock that is not associated with explosive volcanic “activity”. Contains less viscous or “sticky”
silicon and less gas, such as carbon dioxide and/or water vapor. The result is more “flow” than blow
Block fault or “dip slip” –
Buy-Out Plan – government effort to move people out of hazardous areas by purchasing their property
and/or compensating them for relocation cost. These programs are expensive and often do not produce
worthwhile long-term result. People will try to stay or move back into attractive places in spite of
dangers.
Caldera – very large crater formed from violent explosions that destroy the volcano, rather than build it
further. After eruption, the walls cave in and a lake many form. Crater Lake in Oregon, and Yellowstone
Park are examples.
Catastrophe – a very large disaster, often requiring years for recovery and usually affecting thousands of
people, sometimes across entire regions.
Channelization – dredging or scooping out earth from the bottom of a stream or river to make it deeper
and flow more easily, increasing its capacity to carry water. Channelizing can include straightening a
stream to remove meanders and paving or lining stream walls or beds with cement or hard material to
prevent seepage and erosion.
At worst, channelization is putting streams or rivers into concrete ditches so that water flows quickly out
of the area. Sometimes channelization makes floods worse by inducing faster flow and by poor
maintenance that allows obstructions in the channel to create bottlenecks. Channelization is also
unattractive and does not support natural plants and wildlife.
Chute – a pathway that is used over and over by an avalanche. A slide area where avalanches occur
frequently may be called a ‘chute’.
See also Restoration.
Cinder cone – small volcano made of Tephra. See Strato and Shield volcano types.
Composite volcano – or “strato” (layered). Tend to be steep-sided and explosive. Compare to Dome
and Shield volcanoes. Contains more silicate rocks, providing viscosity or “stickiness” and gas,
combining to producing more “blow” than “flow”.
Convergent – either by subduction or by uplift.
Critical Facilities – hospitals, police and fire stations, city halls and other important public buildings.
Airports, freeways, power lines and power plants can also be considered ‘critical’ or vital to community
welfare. The most ‘critical’ or vital facilities would be things that are most needed during the acute
phase of a natural disaster. A fire station might be at the top of the list of services that are most vital to
preventing injury, death and loss of property caused by a natural process.
Cut Bank – See also Meander. The outside edge of a curve or turn in a stream or river. Water flows
faster on the outside of a turn, causing more erosion that eventually results in stream beds meandering
more sharply. Land is lost by erosion during floods along the cut bank side of a stream or river. In
contrast, the point bar, or inside of a river ‘elbow’ experiences slower flow, so erosion from an upstream
cut bank is often deposited along a point bar. Deposition and erosion over time cause rivers to meander
more sharply.
Dam – a wall that crosses and cuts off a stream or river creates a reservoir or lake upstream. Dams can
be produced by humans or animals (like beaver) or by natural processes such as landslides and
earthquakes. Man-made dams are built for any of six values:
1.
2.
3.
4.
5.
6.
Flood control, the single most important reason why dams are built
Water for people to use in cities and town – drinking, bathing, lawn watering
Agriculture or farming
Generation of electricity
Recreation – fishing, boating, scenic views, etc.
Silt control. Man-made reservoirs and natural lakes eventually fill up with silt that is carried
by the ability of water to erode land forms.
A levee is a dam or blocking structure that runs along a river rather than across it.
Disaster – a hazardous event that occurs over a limited time span in a define area, resulting in 10 or
more people killed, and/or 100 people affected, involving a declaration of emergency and/or a request
for outside assistance.
Displacement – how much the earth moved. How many feet or inches a building moved, or how wide a
crack opened.
Divergent or rift- where major or minor plates of earth crust pull apart. Sometimes earth in between
sinks into the void left when plates move away from each other. Death Valley, California is below sea
level because adjacent earth crust moved away from the area, allowing the space in between to sink.
Dome – explosive volcano type, made of more silicate (viscous, sticky) magma and gas. Compare to
Shield and Composite. More “blow” than “flow”.
Drainage basin – watershed, catchment, basin – the area drained by a single collection of streams. See
Headwaters.
Drought – substantially lower than normal precipitation during a season, an entire year or even longer.
Enough difference from normal to cause problems with crop production and/or require that people alter
normal habits in water consumption.
Epicenter and hypo-center or focus –
Flash Flood – a sudden rise in water level that inundates dry land and damages property. Flash flooding
occurs more often in upstream areas where precipitation may be heavy and where terrain is steep.
Flash floods are more common in dry country, or even deserts, where terrain is steep, smooth and
impermeable. “Slick rock” country in Utah is a good example of hard surfaces from which water flows
away very quickly. Narrow canyons with steep sides can become sudden death traps when high water is
concentrated and is not absorbed or slowed by surfaces. Dry country is also prone to more sharp
rainfall, which adds further to the tendency for flood waters to rise sharply and suddenly.
Flood Plain or Flood Way – A 100-year flood zone is the land area that is expected to flood at an
average Recurrence Interval of about 100 years.
See also 100-year Flood at the top of this glossary.
Flood-Proofing – raising a building floor elevation above flood elevation is sometimes allowed in flood
ways that are not expected to flood often or deeply. Installing walls or a levee can prevent entry of
flood water.
Flood stage – the elevation at which property is vulnerable to water damage. In a sense, a flood occurs
whenever “your stuff gets wet”.
Forecast – forecasts are much safer than predictions because we are more likely to be right. By the
same token, forecasts are not as precise and focused as a prediction. Forecasts are not as useful as
prediction - -if we could make a prediction, and we seldom can. So, forecasts are the best we can and
they help us a lot.
Foreshock – a smaller quake that occurs with the fault area that will host the main shock soon
afterward. Foreshocks happen often, but are not reliable indicators of a main shock. In fact, much of
the time we don’t know if the foreshock is the main shock until a larger quake occurs in the same area
soon afterward. Compare to Aftershock.
Gases – volcanic gases can include water vapor as steam, carbon dioxide, sulfur dioxide and gases that
form when hot pyroclastic material causes surface materials to heat, melt, steam or vaporize. An
example is chlorine gas that forms when hot lava spills into the ocean where it causes salt water (sodium
chloride) to boil or steam.
Geothermal – where water seeps into volcanic formations, absorbs heat from nearby magma and then
rises to the surface. Sometimes water becomes hot enough to become steam, and spouts as geysers, or
flows as “hot springs”.
Headwaters – where streams begin, usually in steeper terrain where water flows more quickly,
sometimes called ‘water shed’ or catchment area, where precipitation falls to earth and is collected into
streams. Areas that collect water are also called ‘drainage basins’.
Hindsight – it is often easy to see a pattern of events after they occur. We should try to learn from the
past, so that we don’t repeat the same mistakes.
Hotspot – a puncture or ‘mantle plume’ that pushes magma to the Earth’s surface in a small, concentric
area. Hawaii and Yellowstone Park are two examples in the United States. A hot spot may produce
magma that is explosive or not. It is important to note that mantle plumes generally do not move along
under the Earth’s crust. Instead, the Earth’s crust slides over the mantle or ‘hot spot’ causing a long line
of volcanoes or magma deposits on the surface. Compare to a plate boundary.
Hydrologic cycle – evaporation of water, movement of water vapor by wind, condensation to form
clouds, and precipitation.
Intensity or ground motion (shaking) – compare to magnitude. Intensity of rainfall means how heavy or
fast rain is falling, not its total volume. A brief, intense rainstorm may drop as much water as a longerduration rainfall over a longer period of time. Intensity of rainfall has a lot to do with the severity of
flooding: in many cases, a slower, longer rain period can be handled by downstream drainage systems,
while a brief, sharp rainstorm may overwhelm a drainage system, even though it delivers the same
amount of rain, or even less.
Lahar – a mud flow caused by hot pyroclastic material that melts, dissolves or loosens snow and dirt.
Levee – a wall or barrier running parallel to a river or stream to prevent flooding. Compare to a dam
that cuts off a river, causing a reservoir to fill upstream or behind the dam.
Levee structures are widely used, but are often not reliable because flood water can sometimes go over,
under or around a levee. Levee structures sometimes make things worse, but ‘squeezing’ or
constraining a river, creating a bottleneck that forces flood water to back up and go around.
Liquefaction – earth material, especially small sand, silt and clay may be stable when dry, but very weak
and unstable when wet. In fact, when these small particles are wet  and then shaken by an
earthquake, the cohesion or ‘stickiness’ that holds them together is often broken, so these particles
begin to move easily. If a heavy building is on top of wet soils when shaking occurs, then the lack of
cohesion or bonding strength will allow the building to sink, tip or fracture.
Load – material carried by streams: bed load, suspended load, dissolved load.
Magma Magnitude – the size of an event or process. Usually ranked or measured in terms of energy released.
Compare to intensity.
Meander – the turns back and forth that a stream takes as it goes downhill. See also Cut Bank and Point
Bar.
Modified Mercalli –
Moment Magnitude Mitigation – reduce the effect of something. Can include prevention before an event, or reducing the
cost of recovery after an event.
Natural hazard – a natural process and event that is a potential threat to human life and property.
Point Bar – See Meander.
P Wave (also S wave, and surface wave and “super shear”
Precursor – any phenomenon, clue, or signal that occurs before a main event, and is related to that
event, can be called a precursor. The precursor happens because of the same forces that will produce
the main event. Shaking often occurs before a volcanic eruption, hot, dry weather may precede a major
fire, and volcanic gases may fill the air before an eruption. An earthquake ‘foreshock’ is a type of
precursor because it is part of the main event. Some people claim that animals are able to detect
changes in air pressure, taste, odor or other signal that are precursors to natural disasters. People with
arthritis often detect a change in air pressure because a body joint will swell as air pressure drops.
Pyroclastic – “fire fragments” – the rocks, dust and other debris produced and distributed by volcanoes.
See Tephra. Sometimes pyroclastic material flows downhill, often bringing melted snow, mud and other
debris in floods.
Rebar – steel reinforcing rod – about ¼ inch to one inch in diameter – any length, inserted into wet
concrete to add tensile strength.
Recurrence Interval – an average amount of time between flood events at any given location.
Restoration. Restoring a stream to its natural state is a means of overcoming problems caused by
channelizing a stream in the past. A properly maintained natural stream is more attractive, supports
wildlife and can help reduce flooding by slowing flood water and helping it be absorbed into the earth
rather than cause damage downstream.
See Channelization.
Retrofit – an existing building can be re-built or re-fitted with new features to make it resist natural
hazards. Retrofits tend to be expensive, and sometimes it is better to demolish old buildings rather than
bring them up to standard. Adding steel to an older building is probably the most common practice to
make older buildings withstand earthquake shaking. Most buildings will never be brought up to current
code, and codes are always changing.
Rift – a region where earth crust is divergent, moving or pulling apart, sometimes because new crust is
being formed. The mid-Atlantic ridge is an example.
River delta – where a river flows into a lake or ocean, sediment is deposited. The ‘dump’ area for
sediment usually spreads out in a fan or delta.
Rock cycle – the process of creating rocks, wearing them down to dust and remaking dust into new
rocks. Energy for the rock cycle comes from a combination of solar radiation and tectonic movement.
New rocks are formed by any of many processes: erosion, dissolution, fracturing, sedimentation,
cementation, heat, pressure, and so on.
Rock types - igneous, metamorphic, sedimentary.
Scour – the erosion that occurs when fast-moving water carries away rock and soil. Scouring occurs
most often during rainy season or after a rain storm when water flow is higher and faster. Three
products are carried by streams, especially during floods:
-
Rocks and other heavy objects that move only during fast-water events – bed load or
‘traction’
Stirred-up sediment that is suspended when water is fast-moving and then settles out when
water flow is slower.
Dissolved mineral that is chemically locked in the water and will never settle out. The only
to separate water from dissolved material is to evaporate the water, leaving the solid
material behind.
Shield volcano - a more shallow or flatter volcano, not explosive, more “flow” than “blow”
Slip-strike (transform) – horizontal quake movement, with little or no vertical uplift or falling. When
plates or fault zones slide past each other along the surface of the earth we can call them ‘strike-slip’ or
‘slip-strike’. Some transform faults are very large and dangerous and a few may cause “super shear”
quaking because a horizontal slip movement along the surface can ‘rip’ for long distances almost
instantaneously. By comparison, a deep underground fault ‘dip slip’ or block fault movement will
transfer energy rock by rock in all directions. Theoretically, a super-shear transform quake can cover
distance much faster than even a ‘P’ wave traveling at more than 13,000 mph through hard rock.
Strato volcano – see “Composite”.
Subduction – where tectonic plates collide, the lighter plate will tend to rise above the heavier plate.
The sinking, heavier plate will slowly melt as it sinks back toward the asthenosphere. That molten
material may flow back along the asthenosphere to appear somewhere else millions of years later.
Dry continental crust is generally lighter and much older than crust beneath the ocean because it has
floating above heavier crust that is exposed to melting.
Supershear –see Slip-Strike above
Tectonic plates – large surface blocks of solid earth; tectonics refers to large-scale geologic processes
that deform Earth’s crust and produce landforms such as ocean basins, continents, and mountains. The
tectonic cycle involves the creation, movement and destruction of tectonic plates. Energy for tectonic
cycles comes from radioactive decay deep within the Earth. The geologic cycle includes tectonics, rock
cycle, hydrologic cycle and biologic cycle.
Tephra – small pieces of black or red lava, often lightweight, with many holes from hot gases that passed
through before cooling. A Pyroclastic rock.
Terrace – a level or flat spot on a slope. Terraces may help reduce overall angle-of-repose even though
the average slope remains the same.
Tensile – resistance to stretching or pulling apart, as compared to compression or pushing together
Tsunami – is not a wave of oscillation, but translation, because instead of just circular motion as energy
passes through, wave is actually moving somewhere.
Uplift – “convergent uplift” – when two tectonic plates collide and both are made of lighter-weight
continental crust, then both may rise together, rather than having one subduct by sinking and remelting.
Volcano – magma (lava) rising from vents in the earth to produce mountains, cones and/or hills. See
Dome, Shield, and Strato.
Weathering – the gradual process by which earth processes, including climate, storms and changes in
temperature tend to prepare rocks for collapse, falling or failure. Plants, animals and human activities
can be considered part of weathering.