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Geol 2001 Exam 2

Why are most active volcanoes associated with plate tectonic boundaries?
spreading or sinking plates interact at plate boundaries with other materials to produce magma
Viscosity of a magma is controlled by
temperature and silicon content of the magma
The shape of shield volcanoes is a function of
lava viscosity
Why do composite volcanoes consist of alternating lava and pyroclastic layers?
composite cones are created by a mixture of explosive activity and lava flows
Mid-ocean ridge volcanism produces what type of volcanic rock?
basalt
Composite cones typically are associated with what type of plate tectonic feature?
subduction zone
What is the principal difference between craters and calderas?
calderas are much larger depressions created by collapse of the upper portions of the volcano
Why were the citizens of Heimaey, Iceland, successful in stopping the lava flow that threatened their harbor?
they had a ready supply of water with which to chill the lava
What is one reason that ash fall is such a significant hazard?
the weight of the ash can cause structural damage to buildings
A lahar is produced when
ash and other pyroclastic ejecta become saturated with water
Hundreds of residents near a dormant volcano at Lake Nyos died when
a dense cloud of volcanic gas was released
The number of fatalities from the Mt. Pinatubo eruption was relatively low because
the eruption was predicted successfully, and the area was evacuated
Seismic activity under serves as a tool of volcanic forecasting because
moving magma causes earthquakes
Why is the geologic history of a volcano important in hazard prediction?
the geologic history reveals the frequency and style of eruptions
Why is topographic monitoring sometimes useful in forecasting a volcanic eruption?
a volcano sometimes swells as lava moves into the edifice
Viscosity is a primary control on the nature of volcanic activity
True
Composite volcanoes are typically associated with subduction zones.
True
Lahars are usually produced by saturation of thick volcanic ash.
True
A shield volcano is typically andesitic in composition.
False
Composite volcanoes consist almost completely of lava flows.
False
Volcanic domes form on oceanic plates above hotspots.
False
Caldera eruptions are very common features of subduction zones.
False
Lava flows are the most dangerous of the primary volcanic hazards.
False
The 1980 eruption of Mt. St. Helens produced large lava flows that filled the nearby river valleys.
False
Volcanic gas emissions are important because they tell geologists how flammable the gases in the magma are likely to be.
False
The eruption of Mt. __________ in 1991 killed several tens of people with pyroclastic flows.
Unzen
Shield volcanoes typically consist of the rock type ____________.
basalt
Volcanic domes typically consist of the rock type _____________.
rhyolite
___________ __________ are small volcanoes formed from tephra accumulation near a volcanic vent.
cinder cones
The Hawaiian islands were formed volcanism associated with a _______ ___________.
hot spot
_____________ eruptions are among the most violent and largest known.
Caldera
_____________ ___________ are dangerous, hot masses of ash and gas that travel down the flanks of a volcano.
Ash flows
_____________ ___________ is a volcanic gas associated with acid rain and vog.
Sulfur dioxide
The May 18, 1980 eruption of Mt. St. Helens began with a massive ___________.
landslide
________________ is the primary human adjustment to volcanic activity.
Evacuation
Why did extensive flooding occur upstream of St. Louis during the 1993 Mississippi River flood?
floodwalls protecting St. Louis caused a bottleneck in the river’s flow
The portion of rainfall that flows off the land and directly into a river is called
runoff
What is river base level?
the lowest level to which a river may erode
The suspended load of most rivers consists mostly of
silt and clay
If a stream channel is of constant width and depth, how must increased discharge be accommodated?
increased velocity
The Ventura Beach RV Resort was flooded because
it was constructed in an unrecognized distributary channel that became active
The total load of sediment that a river carries in a given period of time is called
capacity
Why might logging of a drainage basin change the slope of the river?
increased sediment load causes the river to steepen its gradient to transport more sediment
What type of river channel is characterized by multiple channels and gravel bars?
braided
How do point bars and cut banks differ from one another in a stream system?
cut banks are sites of erosion, while point bars are sites of deposition
Which type of flood is typically caused by intense rainfall of short duration?
upstream flood
What is the best description of the “10-year flood”?
a flood that has a 10% chance of occurring each year
The degree of urbanization consists of
percentage of impervious cover and the percentage of area served by storm sewers
In what way can structural control of floods actually increase flood damage?
by encouraging development on the floodplain
Why is flood hazard mapping considered an important step in floodplain management?
flood hazard mapping guides appropriate development in the floodplain
The floodplain is usually considered as a distinct system with respect to the river system.
False
The bed load is the dominant portion of the total load for most rivers.
False
Braided channels are typically characterized by fine-grained bed load.
False
The 100-year flood is the flood that occurred exactly 100 years ago.
False
Urbanization generally increases the lag time between the peak of rainfall and the peak of the resulting flood.
False
The base level of a stream is typically sea level.
True
For the same amount of discharge, a narrow, shallow channel will have a higher water velocity than will a wide, deep channel.
True
A faster-moving stream has greater competence than a slow-moving stream.
True
Downstream floods typically cover larger areas than do upstream floods.
True
The principal objective of channelization is to control floods.
True
The region drained by a single river or river system is the ____________ _____________.
drainage basin
The total load of a river is dominated by the ________________ load.
suspended
The continuity equation pertains to the accommodation of ____________ by a stream channel.
discharge
Streams generally maintain a _______________ _________________ between the sediment transported and the sediment delivered to the stream.
dynamic equilibrium
______________ are shallow portions of a meandering stream, in which the water is typically fast-moving at low flows.
Riffles
The _________________ __________________ is the period of time in which a flood of given magnitude is expected.
recurrence interval
__________________ changes many characteristics of floods in smaller streams.
Urbanization
Protection of banks by planting trees and other vegetation is a form of ______________ _____________.
channel restoration
The objective of __________ ____________ is to maximize beneficial use of the floodplain while minimizing flood damage and the cost of flood protection.
floodplain regulation
Following Hurricane Floyd in 1999, residents in one part of the state of _____________ ____________ were relocated from a badly impacted floodplain.
North Carolina
A comprehensive term for any type of downslope movement of Earth materials is
mass wasting
What strategy stabilized the Portuguese Bend landslide?
water drainage
In Yosemite National Park, strong bedrock influences the formation of what types of hillslopes?
free face and talus
How do slumps and translational slides differ?
slumps are rotational, while translational slides are planar
The safety factor for landslides is
the ratio of resisting to driving forces
Why are landslides more common on steep slopes?
slope angle determines the relative magnitude of driving forces
Vegetation influences landslides in part by
anchoring the slope materials
Why might the infiltration of water deep into a slide mass increase landslide hazards?
the water increases the pore water pressure, effectively lifting the slide mass from stable ground beneath
How did geology influence the Vaiont Dam disaster?
the geology in the adjacent hillslopes was dominated by weak rocks
How does road construction associated with timber harvesting contribute to landslide hazards?
the roads alter the movement of surface water and groundwater
What aspect of hillside development has contributed to the frequency of landslides in southern California?
grading of hillslopes to create pads for home construction
On what information is a landslide hazard map based?
geology and inventory of past landslides
Why is drainage control sometimes an effective method of landslide control?
it reduces the amount of water in the hillslope materials
How might the 1983 Thistle landslide have been prevented?
examination of landslide history and installation of drains
Why does withdrawal of fluids sometimes lead to subsidence?
the fluid pressure provides partial support for the overlying rock mass
Hillslopes typically are static systems.
False
Slumps and debris flows tend to occur separately in time and space.
False
A safety factor of 1 implies that driving forces are 10 times greater than resisting forces.
False
The expense of preventing landslides is usually too great for the prevention strategies to be feasible.
False
Hillslopes in weaker rocks tend to be more gently sloped.
True
Debris avalanches tend to occur on steep slopes.
True
Climate influences landslides by determining the vegetation present on a slope.
True
Erosion of the base of a slope may increase landslide hazards.
True
Grading of slopes can either increase or decrease landslide hazards.
True
Sinkholes often result from water table fluctuations.
True
All slope movements fall under the term _________ _____________.
mass wasting
_____________ is the downslope movement of a coherent block of Earth material.
Sliding
The lower portion of a slump typically evolves into a ____________.
flow
The ratio of resisting to driving forces is called the ______________ ______________.
safety factor
The noncohesive downslope movement of saturated Earth materials is called an _______________.
earthflow
The spontaneous liquefaction of clay-rich sediment is called _____________ ___________.
quick clay
The Thistle landslide, in the state of _____________, was one of the most expensive landslides in U.S. history.
Utah
One of the most effective methods of stopping an active slide is _____________ ___________.
water drainage
Snow avalanches tend to move down tracks called ___________.
chutes
______________ are caused by the collapse of caverns.
Sinkholes

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