Natural Disasters (Exam 2) 03/04/2013
Lecture #14 Principles of Volcanoes I
Vulcan = Roman god of forge (fire). Volcanoes erupt hot, glowing, molten rock (sometimes explosively).
What is a volcano? Volcano mountain formed by erupted lava or pyroclastic debris
(pieces of solid rock) or both. Rock = extrusive (volcanic) igneous rock. Molten rock solidifies by
rapid cooling, therefore, fine grain size or amorphous glass (no crystals) is produced (mineral crystals need
time to grow in size). Composition of volcanic rock can vary
Composition Volcanic Rock Name
Silicic (rich in SiO2) Rhyolite (link #2)
Intermediate Andesite (link #2)
Mafic (rich in Mg + Fe, low in SiO2) Basalt (link #2) Where do volcanoes occur? Most (80%) of world's active volcanoes are located around margin of Pacific
Ocean ("Ring of Fire", map), especially New Zealand, Japan, Alaska, Mexico, Indonesia, Central America,
Philippines, + Chile. In USA, Alaska, Hawaii, northwest USA (Washington, Oregon, + California).
Volcanoes, like EQ's, are concentrated at tectonic plate boundaries, but mainly divergent +
convergent (not transform).
Nature of volcanic eruptions Volcanoes that erupt explosively can be extremely hazardous, but lava flows
are much less hazardous. Qualitative scale to describe explosiveness of eruption = volcanic explosivity
index (link #2), values range from 0 (gentle effusion, i.e., small flow) to 8 (cataclysmic). Explosivity is
characterized based on volume of ejected rock, height of eruption column, + duration of eruption. Mount St.
Helens eruption of 1980 was 4.
What causes explosive vs. nonexplosive eruptions? To get an explosive eruption, you need highly
viscous (i.e., stiff or thick) magma (one that resists flow) with lots of gas (mainly water vapor).
Presence of gas is driving force for explosion. Gas expands (boils away) as magma reaches surface,
causing explosion.
Silicarich magma – high viscosity, explosive eruption, slower flow
Maficrich magma – low viscosity, nonexplosive, faster flow
Volcanoes and Plate Tectonics
Eruptive Style Erupted Product
Nonexplosive Basalt lava flows (+ lava fountains, if gas pressure builds up)
Explosive Andesite + rhyolite ash fal l large plume of finegrained volcanic glass, which settles from air like dust or snow Eruptive Style Tectonic Setting
Nonexplosive Divergent boundary (oceanic ridge) and intraplate (hot spot) localized zones of upwelling mantle
Explosive Convergent boundary subduction zones (oceanic lithosphere sinking into mantle) Lecture #15 Case Studies of Volcanoes I
Submarine Volcanism Most lava eruptions occur under ocean (where tectonic
plates move apart), where there is very fast cooling + formation of spherical
shapes called pillow basalt
black smokers = very hot, undersea geysers, produced when seawater descends through cracks in
oceanic crust, gets superheated + then rises to produce jetstream of extremely hot water; black color from
iron sulfide minerals.
I) Shield volcanoes (nonexplosive eruption of basalt lava flow) Most famous examples = Hawaiian
volcanoes (link #2). Intraplate ("hot spot") type of volcano. Chain of islands that extends to Emperor islands,
all formed by basalt lava flows. Hawaii = largest mountain in world.
Characteristics of Shield volcanoes:
• Broad, shieldlike profile
• Formed by eruption of lowviscosity lava
• Lava low in silica
• Nonexplosive erupting
• Hawaii islands Kilauea
Kilauea = one of most thoroughly studied volcanoes because extremely active, many US volcanologists, +
almost always quiet outpourings of lava that can be destructive (e.g., Kalapana Gardens), but not lethal.
Photo sequence of lava covering Kalapana Gardens in 1990 April, May, June. Photo sequence of lava
covering Walter's Kalapana Store and Drive Inn in 1990 April, June 6, June 13.
Other Features of shield volcanoes =
lava fountain (geyser due to buildup of gas pressure)
fissure eruption (long crack that forms away from central crater
lava tube (underground river of lava, allows lava to travel much greater distances before cooling;
eventually they can empty + be underground cavity)
pahoehoe lava flow (smooth, ropey surface due to low viscosity from high temperature + high gas
content);
aa lava flow (very rough, jagged surface due to higher viscosity from lower temperature + lower gas
content; moves very slowly)
can get both pahoehoe + aa lavas in single flow. How? It is common to get both pahoehoe and `a`a flows from the same eruption with no difference in chemical
composition, showing that what causes pahoehoe or `a`a to form is largely related to the physical
conditions the lava undergoes once erupted.
Often what starts out as a pahoehoe flow may go through the transition to `a`a when there is a change in
conditions, such as a sudden steepening in slope, or just by the continuous loss of heat and gas as
distance from the vent increases.
II) Composite (strato) volcano (explosive eruptions of volcanic ash, ashfall eruptions) MORE
COMMON THAN SHIELD VOLCANOES
Characteristics of Stratovolcanoes:
• tall conical mountains composed of lava flows and other ejecta in alternate
layers
• high viscosity lava, high in silica, cools and hardens before travelling far
• Krakatoa, Mt. Vesuvius, Mt. St. Helens
A) Mt. Vesuvius active volcano next to Naples, Italy (densely populated city of ~2 million).
Subduction zone volcano. In 79 AD famous eruption buried ancient Roman city of Pompeii.
Important for historians because instantaneous burial + preservation of typical Roman city. Ashfall also
killed many people. During late 1700's archeologists discovered Pompeii, began to excavate it, + found
holes in volcanic ash. They filled holes with plaster + found shapes of bodies of people. Vesuvius = possible
major disaster to Naples area in future. Phlegrean Fields another volcanic hazard just west of Naples!
Lecture #16 Case Studies of Volcanoes II
II) Composite (strato) volcano (continued)
B) Mt. St. Helens (MSH) one of many volcanoes in Cascade Mountains (northern CA through southern
British Columbia). MSH is in SW Washington. Subduction zone volcano (plate tectonic setting).
Major eruption on May 18, 1980. Before 1980, it was symmetrical, after eruption large part of mountain
exploded away.
Sequence of Events
1) Pre 5/18/80 (~50 days) Gascharged silicic magma filled area below volcano causing ground to swell
~300 ft. Geologists warned of possible explosive eruption. 2) 5/18/80 Two strong earthquakes (from magma moving) caused largest landslide in recorded history.
Landslide quickly released pressure on magma + caused upwardly directed explosion and laterally directed
explosion of volcanic ash. Enormous force knocked over trees for large distances. Ash blown 10 miles
upward (photo) and darkened skies as far away as western Montana (ash carried by jet stream to eastern
USA). Ashfall was mainly nuisance.
3) 5/18/80 Hot ash melted snow to produce enormous lahar (mudflow), which created flooding. 63 people
were killed, including one geologist (David Johnston). Many more deaths without warnings from geologists.
III) Other explosively erupting volcanoes (generally subduction zone volcanoes)
Cinder cone consists entirely of pyroclastic debris of all shapes and sizes (no lava flows). Small,
symmetrical cones + usually with ~short lifespans.
Characteristics of cinder cone volcanoes:
found on the flanks of shield, stratovolcanoes, and calderas
steep conical hill of tephra (volcanic debris)
Lava dome bulbous mass of extremely viscous lava that accumulates around vent, which can get
plugged; potential for extremely explosive eruption
Characteristics of lava dome:
resulting from the slow extrusion of viscous lava
high viscosity: high levels of silica in the magma, or by degassing of fluid
magma
forms when high viscosity lava doesn’t flow far from the vent from which it
extrudes, creating a domelike shape of sticky lava that then cools slowly in situ
if it collapses, creates pyroclastic flows (lethal)
Caldera rare, but extremely violent eruption that produces huge crater (10's of km wide); largest volcano
explosivity indices (VEI > 6). Calderas commonly begin as composite cone, then enormous eruption blows
away top part of volcano. Magma chamber empties, leaving large underground cavity and ground over it
collapses, e.g., Crater Lake in Oregon and Long Valley, CA
Characteristics of Calderas:
formed by the collapse of land following a volcanic eruption
triggered by the emptying of the magma chamber beneath the volcano
if magma is high in silica, gas pressure builds causing explosion of ash and a’a
lava flow (Krakatoa) if magma is high in basalt, less viscous, magma chamber drains by large lava
flows (Kilauea, shield volcanoes – Hawaii)
if Yellowstone has a calderaforming eruption, super volcano will be catastrophic
Examples of calderaforming eruptions include:
Krakatau in SW Pacific (in 1883 it created crater that extended 300 m below sea level and produced giant
tsunami that killed >30,000 people)
Mount Mazama in SW Oregon (~6,800 years ago it created Crater Lake)
Santorini in Greece (1600 BC) which destroyed Late Minoan civilization (lost city of Atlantis?)
Yellowstone 3 large calderaforming eruptions over past 2 million years (1.9, 1.3, + 0.6 million years ago)
with 600,000 year recurrence, eruptions were ~1000 times larger than Mount St. Helens, continues to show
signs of activity with hot springs and geysers, but difficult to predict future catastrophic eruption; ongoing
threat of catastrophic eruption of Yellowstone was theme of 2005 movie Supervolcano a fictional
"docudrama" that depicts the worst case scenario of another calderaforming eruption at Yellowstone.
Lecture #17 –Volcanoes IV Benefits and Hazards
Benefits of Volcanoes and Volcanic Eruptions
A) Erupted rock
pumice (highly porous volcanic rock) used in various products (skin care, Lava brand soap, dentist's
polish, + pencil eraser)
obsidian (glassy volcanic rock) used as decorative stone, cinders used in construction, + volcanic ash
can provide fertile soil.
Creation of new land underwater lava flows just east of Hawaii (youngest volcano in Hawaiian
chain, Loihi seamount) + Heimaey, Iceland.
B) Geothermal energy heat from shallow magma used to generate electricity.
Need underground water to circulate + transfer heat to surface as hot water + steam, which drive
turbines.
World's largest geothermal plant = The Geysers, central California.
C) Tourism National parks + national monuments
D) Metallic ore deposits many kinds of metallic ore deposits are in roots of old volcanoes.
Hazards of Volcanoes + Volcanic Eruptions A) Lava flows usually nonlethal but can cause considerable damage. Lava flow will burn or bury
everything in its path until it stops.
B) Explosion + ashfall explosive eruption of plume of volcanic ash (small, abrasive pieces of rock)
can cause crop damage, livestock deaths, structural failure,
airplane engine failure (planes flying through ash plume
breathing problems
Where are volcanic hazards in USA?
C) Explosion + ashflow explosive eruption of magma too viscous to be sent vertically, creates
turbulent mixture of superheated gas + pyroclastic debris (nuee ardente = "glowing cloud") that flows down
mountainside with great speed (up to 150 km/hr). Ashflows can cause enormous destruction, common
during calderaforming eruptions.
Example = Mt. Pelée eruption in 1902 on Caribbean island of Martinique destroyed city of St. Pierre,
killing ~28,000 people in ~30 seconds
Same area has been resettled. Mt. Saint Helens 1980 eruption had small ashflow.
(D) Lahar (mudflow) mixture of water + volcanic debris that flows downslope following river valleys.
Water is from snow + ice on mountain slope (large composite cone at high elevations or in polar regions,
Nevado del Ruiz, Columbia), or from eruption itself, or from rainfall. Lahars can move quickly or slowly,
depending on amount of water. Fast lahars can be lethal (lahar from Nevada del Ruiz volcanic eruption in
Columbia, South America in 1985 killed 25,000, + slow moving lahars can be very destructive (moving mud
has great weight + force). Mt. Saint Helens 1980 eruption had major lahar
(E) Tsunami volcanic eruptions rarely create tsunamis, most formed in SW Pacific (around Indonesia).
Krakatoa eruption (between Java + Sumatra) created very large (35 m high) tsunami, which killed 36,000.
(F) Effect on climate large, explosive ashfall eruptions can cause global cooling of up to several
degrees for 1 2 years after eruption. Climate change can cause crop failure + famine, e.g., Tambora, 1815
caused "Year without a Summer". Cooling is due to SO2 gascoated airborne volcanic ash, which reflects
sunlight. (G) Gases water is major gas released in volcanic eruptions, also can get other more harmful gases
(e.g., CO2, CO, SO2, H2S, H2SO4, HCl, HF) released. Sulfurbearing gases can oxidize to sulfuric acid +
be highly corrosive. In 1986 in Cameroon (central Africa), 1,700 died overnight due to volcanoderived CO2
gas that was released quickly from Lake Nyos (crater lake on top of dormant volcano). People died of
asphyxiation (O2 deprivation) when cloud of dense CO2 gas came rolling down hillside. CO2 gas from
magma below seeped upward + dissolved in lake waters. Gas was kept in solution by pressure of overlying
water column. Something happened to overturn water (natural overturn, submarine landslide, underwater
eruption, or EQ) + huge amount of CO2 gas (80 million m3) was released, creating large waves when it
broke water surface. Relatively heavy gas moved downslope + displaced O2 in low areas; very rare.
Predicting volcanic eruptions
involves monitoring precursors (ground swelling, underground temperature, EQ activity,
+ composition of volcanic gases); some success (e.g., Mount Pinatubo in 1991), but much more
progress is necessary.
Mitigation
for explosive eruptions there is only one alternative EVACUATE
for nonexplosive eruptions = diversion using quickly built piles of earth + rubble (e.g., Mount Etna +
Heimaey, Iceland), chilling with water (e.g., Heimaey, Iceland), + bombing (e.g., Mount Etna).
Includes all the steps we discussed early in course, e.g. mapping and zoning and disaster preparation
Lecture #18 Mass Wasting/Landslides Types + Processes
Mass wasting (= slope failure = landslide)
downslope movement of rock + sediment that occurs at Earth's surface in response to gravity; variable
speeds
extremely fast to extremely slow
shapes our landscape (with river erosion). Why hasn't gravity completely flattened Earth's landscape since
it formed?
• Plate tectonics rejuvenate landscape (collisions create mountains, etc.)
• Slow movement – property damage
• Rapid movement – damage and deaths ~ $1 billion PROBLEM = slow movement induces property damage + rapid movement induces damage + deaths
(annual damage from landslides in USA = >$1 billion; 25 50 deaths per year in USA and thousands of
deaths per year worldwide from landslides).
Where is landslide problem worst in USA? Why in these areas?
• Eastern USA – Appalachians
• Western USA – Rocky Mountains
• West Coast – small mountains and coastal erosion by Pacific Ocean
Where is landslide problem worst in Illinois?
• Western, IL – Mississippi River Valley
• NorthernCentral – Illinois River Valley
Landslides can be greatly influenced by human activities (often making problem worse).
I) Landslide Classification numerous systems + none accepted by all. Most landslide classification
systems are based on
type of downslope movement
speed of movement (fast slow)
earth material involved (rock vs. soil)
amount of water.
(A) Type of downslope movement
(1) Flow turbulent movement of sediment, i.e., individual grains are mixed around during movement.
(2) Slide/slip earth material moves as coherent block in contact with slope.
(3) Fall earth material moves as free fall (out of contact with slope). In contrast, movement occurs along
free face in flows + slides.
II) Landslide Types (some of most important + common ones) (A) Flows Speed ranges from imperceptibly slow (creep) to extremely fast (avalanche). Creep is very
slow (few mm per year), continuous downslope movement of rock or sediment. Process involves outward
expansion of earth + downward contraction of earth.
Expands outwards, contracts downwards
Why would earth/soil expand outward?
Swelling clay, expands wet/frozen, contracts dry/thawed
Too slow to see actual movement, but can see effects of movement
Avalanche = fastmoving (>15 km/hr), dense mixture of rock (rock avalanche), sand, mud, + water
(debris avalanche). In 1970 (M = 7.7) EQtriggered debris avalanche (thousands of tons of rock) in Andes
Mountains (Nevados Huascaran) moved at >300 km/hr + killed 20,000 people in Yungay, Peru. Similar
event occurred in 1962, killing 4,000 people
Area is currently repopulated + hazardous conditions continue.
Mudflows = intermediate speed landslide consisting mainly of mud. Example = Lahar from Nevado del
Ruiz volcano killed 22,000 in Armero, Columbia
(B) Complex –
Slumps/rotational slide/earthflow have combinations of different characteristics. Upslope portion moves
as coherent mass (i.e., slide) + produces cliff. Downslope portion flows + has rolling surface. Failure
surface is curved in crosssection + spoonshaped in map view. Slumps are most common type
of landslide + can be small (few meters wide) or huge (hundreds of meters to kilometers).
Landslide hazard maps show landslide deposits, risk of future landslides, + recommended land use
Lecture #19 Landslides Slope Stability
To understand why landslides occur, need to consider balance between forces involved:
driving forces = downslope weight of slope material (from gravity) that pulls it downward
resisting forces shear strength (amount of sideward directed force needed to cause failure) that
holds slope in place. Evaulate slope stability by calculating safety factor (SF):
SF = Resisting Forces/Driving Forces (R/D)
If SF > 1 then R > D + slope is stable. Most building codes require SF > 1.5. Driving force (weight directed downslope) is easily
calculated using simple trigonometry. Shear strength can be measured in lab. Examples of balance of
forces for 2 slides (one with potential failure surface that is planar + one with potential failure surface that is
curved).
Planar failure surface = translational slide (common for bedrock).
Curved failure surface = rotational slide (common for soil) Rotational slide is self
stabilizing; resisting mass will quickly >, stopping landslide.
Shear strength (resisting force) depends on two physical properties: cohesion (ability of particles to stick
together due to electrostatic forces; clay vs. dry sand?) + friction (resistance to motion between grains,
related to grain size + shape, sand vs. clay).
What factors lead to slope stability or instability?
Controls on slope stability/instability
(A) Slope weight instability can result from either natural processes or humaninduced processes that
add "driving mass" to slope (dirt, house, water, vegetation) or remove "resisting mass" or support (erosion
of sea cliff by waves, new housing development
(B) Slope angle > slope angle = less stable slope. Slope angle can change from human intervention
(e.g., development regrades slopes for buildings or roads) or from natural processes (fault cliffs).
(C) Geologic material earth materials with low shear strength are susceptible to failure. Sediment is
generally weaker than rock + clay is typically very weak (e.g., swelling clay). Weak rocks include shale or
other rocks with prominent plane of weakness, e.g., foliated metamorphic rocks, sedimentary layering,
jointing (fractures) in any rocks. Orientation of weakness plane is critical.
If weakness plane is parallel to slope direction, slope is susceptible to failure. Example of quick clay = nonswelling clay minerals that were deposited in ocean environment that caused
flat clay plates to accumulate into "house of cards" structure (up to 90% water/10% solid). While saltwater is
present, clays have some strength, but if fresh water enters system, strength < weight (driving force) by filling pore space (up to
30% of soil volume). Water could be from natural rainstorm or human intervention (lawn watering, septic
tanks). Heavy rains in California commonly produce landslides. Water in pore space also affects shear
strength by producing pore water pressure. Waterfilled pore space creates high pore water pressure (water
supports overlying weight), which decreases grain to grain friction + shear strength. Partly filled pore space
can actually strengthen soil; creates negative pore water pressure.
(E) Vegetation plant roots > soil cohesion (+ therefore > shear strength), making slope more stable.
Plants also act as umbrella, shielding soil from erosive impact of rain (> slope stability), but they also add
weight ( resisting force (strengthening rock/soil) or both.
(A) Water drainage don't allow water to build up within slope ( shear strength); install
horizontal plastic pipes that drain water from within slope (photo), surface drains, or impermeable seals on
top of slope (prevent infiltration).
(B) Proper grading of slope remove material from top of landslide ( resisting force). Can also create benches (small steps in slope) to prevent large
landslides.
(C) Retaining walls steelreinforced concrete wall at vertical face to strengthen slope (> resisting
force); need holes in wall for drainage; wire mesh fencing (photo) or sprayed concrete (shotcrete). For steep
bedrock slopes and tunnels in bedrock, bolts can be installed to strengthen rock.
(D) Regrowing vegetation roots bind soil and canopy protects soil from impact by raindrops.
Case Histories
Portuguese Bend, CA hillside community (~200 houses) that overlooks Pacific Ocean, ~ 40 km
south of Los Angeles. From 1956 1986, area (and most houses) experienced steady downslope
movement (~200 m of total movement, averaging 0.3 2.5 cm per day!). Many houses had to be
abandoned, others are still occupied, but only with constant maintenance (hydraulic jacks to keep houses
and garages level).
Combination of hazardous local geology and negative impact of development. Local bedrock = weak rock
(shale and siltstone) with bentonite (swelling clay). Also, layering is ~parallel to slope and waves undercut
slope at sea cliff ( slope weight and < driving force). Result was continuous downslope movement of large block
(sliding movement).
Homeowners successfully sued LA County for adding fill dirt. Movement was < 100 meters several kilometers) results in compaction of sediment due
to removal of buoyant support provided by fluid (
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