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Hawaiian Islands 2. Yellowstone
examples of intraplate volcanoes (hotspot volcanoes)
Hot Spot Volcanoes
form when an area of the mantle melts in the middle of a plate and the magma rises (mantle plume) and collect in the crust
location of a mantle plume
fixed in place but plate moves over it
What plate is the Hawaiian Islands on ?
Oceanic plate (1)
What kind of magma does the Hawaiian Islands have?
basaltic magma = low viscosity (runny)
What kind of eruptions do the Hawaiian Islands have?
non-explosive eruptions - gentle eruptions of lava flow
Pahoe-oe
type of Hawaiian island lava flow, warmer and smoother lava
A-A
type of Hawaiian island lava flow, colder, sharper lava ( painful surface to walk on)
shield volcanoes
gently-sloping volcanoes that form when the runny lava flows out and spreads laterally
Hawaiian Volcanoes
chain of volcanoes with NW trend; hot spot fixed in place, but Pacific Plate moves to the NW
What happens as the Hawaiian volcanoes move to the NW?
All volcanoes form above same hotspot - they become older and inactive as you move NW
What plate is Yellowstone on?
North American Plate = Continental
What kind of magma does Yellowstone have?
Rhyolitic Magma - very high viscosity (explosive)
Is Yellowstone made up of Volcanoes?
No, but large calderas
What is a caldera?
very large craters formed by collapse of ground surface
The 3 Calderas of Yellowstone
3 eruptions in last 2 million years
How long ago was the last eruption of a Yellowstone Caldera?
600,000 years ago
Is there a Yellowstone eruption to be expected?
Super volcano = new mega eruption to be expected
huge amount of ash will be all over US
Long Valley Caldera by Mammoth
Super Volcano, blew up last time was 700,000 years ago
What are volcanic predictions based on?
Seismicity - many small earthquakes indicate magma is pushing higher
Gases - Composition changes or increases (sulfur dioxide, CO2
Ground Deformation - same areas start swelling up when magma is near the surface
Mass Wasting
downslope motion of Earth material due to gravity
What is slope stability?
forces acting up a slope
Driving force or shear stress
drives rocks in to motion = reduces stability
resisting force or shear strength
works against stress = increases stability
Shear Stress
depends on slope steepness and weight
Oversteepening
process that increases slope steepness (may be caused by natural or manmade processes)
Natural Processes of Oversteepening
wave or river erosion
Man-made Activities of Oversteepening
road cuts and house building
by removing lower portion of slope, you remove support for the upper slope
Road cut
slope is cut to create a flat surface (oversteepening)
Road fill
material is added to the slope to create a flat surface (added weight)
Examples of added weight
road fills, houses, large amounts of water
Water as an added weight
can come from rainfall or irrigation
increases the shear stress because it adds weight (water is heavy) which increases gravity
it acts as a lubricant by floating the sediment grains apart - thus reducing friction and favoring motion
Vegetation as an example of Shear Strength
protects slope from impact of rain
roots bind sediment grain together
Small amounts of water as an example of Shear Strength
increases cohesive forces - help keeping sediment grain together
What are the factors that contribute to mass wasting in Southern California?
dry climate
wildfires
sudden, heavy rainfall
Factor of Safety = FS
Shear Strenghth/ shear stress
FS could be
1 or >1 or <1
if FS = 1
stress = not good because things might change in the future; stress might increase
if FS <1
slope is unstable = stress is greater than strength
if FS>1
slope is table = strength is greater than stress
what should the FS be for building purposes?
>1.5
what are the three types of mass wasting ?
falls, slides, flows
Falls
rocks separate from steep slope and free fall
what are the two types of falls?
rockfall and rock avalanche
Slides
layers slide down slope
what are the two types of slides?
rock slide and slump
Flows
earth materials are flowing downhill due to presence of water
what are the three types of flows?
mudflow/ debris flow
earth flow
creep
where are rock falls common?
common along mountains where it's called = frost (ice) wedging
what is water's role in rock fall?
when water freezes, it expands thus breaking the rock apart = gravity pull rocks down
what is an indication that rocks will fall in the future ?
slopes that are formed by the accumulation of rocks that fell from top of cliff
rock avalanche
large chunks of rocks breaks off at high elevation and falls for hundred of feet
what happens to the rocks during a rock avalanche?
rocks pulverize, displace air which turns into abrasive dust +air blast
what are the examples of rock avalanches?
Yosemite, Andes, Alps
rock slide
sliding occurs on a flat surface between layers
where does a rock slide occur?
occurs in a slope that is layered
Example of rock slide
Vajont dam
rock slide falls into reservoir, creating a huge wave of water that goes over damn, washing town away
slump
sliding down a curved surface - rotation breaks slope into blocks that are tilted backwards
why are slumps common?
they are common because of oversteepening
What are examples of slumps?
Point Ferman (sunken city), San Pedro = wave erosion
Ocean trails Golf Course ( now trump ), PV = caused by irrigation
example of slumps
La Conchita (1995) & La Conchita (2005)
La Conchita 1995
area is unstable because of steep/weak slope, road cut (oversteepening), orchard on top of cliff (irrigation adds water). occurred after rainfall
La Conchita 2005
debris flow occurs after storms, but water has time to soak into slope leaving top layer dry
what kind of casualties happened during La Conchita 2005?
many houses were buried and several people re killed
debris flows/mudflows
usually mixed with a lot of water which moves fast
examples of debris flows/ mudflows
La Conchita (2005) and Wrightwood, CA,
Wrightwood, CA
by San Andreas Fault (1940); debris flow was caused by early snowmelt, melted water mixed with loose rocks from frost wedging
destroyed many houses
earth flows
less water than debris flows, moves slowly
What does an earth low start out as?
starts out as a slump; shaped like a tongue or lobe
Example of earthflow
Portuguese Bend
Portuguese Bend
very unstable because layers point down towards ocean, oversteepened by wave erosion
Why would the slop of Portuguese Bend move after rain fall?
lots of clay = expands and becomes slippery; naturally slope would move after rain because of clay or after earthquakes
Portuguese Bend 1950
more houses and roads are built after area development
Portuguese Bend 1950 Crenshaw extension
road fill added weight triggers. slump on upper portion and continues downhill as an earthflow
Besides wave erosion and new area develop what else adds to the earthflow and slumps of the Portuguese Bend?
irrigation of the newly built houses added water which makes earthflow move all the time
Creep
Affects all slopes due to gravity; slow motion
Where are creeps the biggest problem?
A problem in cold climate where ground is frozen and then thaw
Subsidence
sinking; slow and progressive sinking of ground
natural subsidence
due to compaction of sediment grains in flood plains or river deltas
example natural subsidence
Mississippi --> New Orleans is below sea level due to subsidence of Mississppi Delta
Man-made subsidence
result of human activities; pumping of water or oil
examples of man-made subsidence
Venice, Italy = ground water pumping
San Joaquin Valley of CA = of 45 feet due to ground water pumping
Long Beach + caused by oil pumping
sink holes
sudden collapse of ground surface
Note
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