Download Science A-43

EARTH STRUCTURE
Layered Earth
o Crust
ƒ Oceanic – Maximum depth generally less than 10km
ƒ Continental – Maximum depth generally less than 70km, less dense than
oceanic crust
o Mantle
ƒ Lithosphere – Rigid (solid rock)
• 10km – 100km depth
• ~90km thick
ƒ Asthenosphere – Fluid-like (plastic rock)
• 100km – 350km depth
• Approaches surface at mid-ocean ridges
• ~250km thick
o Core
ƒ Core is entirely metallic
ƒ Outer Core – Liquid metal (iron)
• 2,890km – 5,150km depth
ƒ Inner Core – Solid metal (iron)
• 5,150km starting depth
o Different density (resulting from different chemical and mineral compositions
o Different strengths
o Gets more and more dense as you go deeper
Isostasy – Less dense materials float on top of more dense material
Continental crust is less dense than the mantle or the core and so can “float” on it
o Mountains for example don’t just sit on the earth’s surface but actually embed
themselves into the lithosphere and asthenosphere
o The liquid-like asthenosphere is able to accommodate this extra continental “root”
and so a large portion of the mountain actually exists below sea-level
Behavior of Materials
o Elastic – Deformation is recovered, object returns to original shape (earthquakes)
o Ductile – Deformation is permanent, stress applied over long time or at high
temperatures
o Brittle – Deformation is permanent, stress applied very quickly to shatter or break
object (mudslide)
o Difference between brittle and ductile is the time scale over which the material
deforms
Internal Sources of Energy
o The mantle is heated by various sources
o Impact energy – Tremendous number of smaller bodies hit the Earth early after
its formation, converting energy to motion to heat
o Gravitational energy – As Earth pulled to smaller and denser mass, gravitational
energy was released as heat
o Radioactive elements – Unstable radioactive atoms decay and release heat at an
exponentially decreasing rate; rate is characterized by half-life
o Heat from these sources is still flowing to the surface today
Heat Transfer
o The mantle is solid but behaves ductily over long period of time, so it loses heat
by convection
o The asthenosphere loses heat by convection
o The crust is solid and brittle, so it cannot convect, it loses heat by conduction
o The Earth loses heat to space by radiation
Gravity is attraction between objects
Gm1m2
o Fg =
r2
ƒ G =6.673e-11 Nm2/kg2
PLATE TECTONICS
The lithosphere of the Earth is broken into pieces called plates
The gigantic pieces of lithosphere (plates) are constantly being pulled, pushed, created,
and destroyed in a process called the tectonic cycle
Plate-edge interactions are directly responsible for most of the earthquakes, volcanic
eruptions, and mountains on Earth
Tectonic Cycle
o Magma rises from the asthenosphere to the surface at mid-oceanic volcanic ridges
o New magma solidifies and adds to the plate edges
o New lithosphere cools and slowly moves laterally away from the zones of oceanic
crust formation, called spreading centers
o Subduction - When the leading edge of a moving slab of oceanic lithosphere
collides with another slab, the older, colder, denser slab turns downward and is
pulled by gravity back into the asthenosphere, where it is destroyed
o The less dense, more buoyant slab stays “afloat” while the the denser slab
subducts beneath it
The continents and seafloor are simply passengers on the moving lithosphere above
which they rest
Evidence for Plate Tectonics
o Magnetization patterns on the seafloors
ƒ Running parallel to the mid-Atlantic spreading ridge are striped parallel
bands of magnetized rock showing alternate polarities
ƒ The Earth reverses its magnetic poles every few thousand years
ƒ Magma is injected into the ocean ridges and is imprinted with the Earth’s
magnetic orientation as it cools to form new rock
ƒ The fact that the seafloor represents alternating magnetic patters suggests
that seafloor is continually being created (and presumably destroyed)
o Ages of the ocean basin
ƒ The ocean floor is only about 200M years old, significantly younger than
any other feature on the Earth’s surface
ƒ
o
o
o
o
o
o
Oceans are young features that are continually being transformed and
created along oceanic ridges and destroyed at subduction zones
Fit of the continents
Rock types and fossils on the different continents match up along boundaries
Earthquake epicenters outline plate boundaries
Oceanic mountain ranges and deep ocean trenches
Deep earthquakes
Movement of tectonic plates using GPS
EARTHQUAKES
Most earthquakes are explainable based on plate tectonics theory
The lithosphere is broken into rigid plates that move away from, past, and into other
rigid plates
As the tectonic plates move, points along the fault line are held in place due to the
friction that exists between the two surfaces of the contacting plates
The shear stress due to friction builds up at the fault line until the energy buildup can
overcome the friction, causing an earthquake
These global-scale processes are seen on the ground as individual faults where the Earth
ruptures and the two sides move past each other in earthquake-generating events
Hypocenter – Actual location of the Earthquake deep underground
Epicenter – The location on the surface directly above the hypocenter
Types of Plate boundaries
o Divergent Boundary – Plate boundaries where two plates are moving apart from
each other and so new crust must be created to fill in the gap
ƒ Most common in oceanic spreading centers located in the middle of
oceans, where new sea floor is created,
ƒ Generally higher than the average depth of the sea floor
ƒ Typically result in normal fault
ƒ Can also occur in continental rock (East African Rift)
ƒ ex. Mid-Atlantic ridge, Nazca Plate/Pacific Plate
o Convergent Boundary – Plate boundaries where two plates are moving toward
each other and so some material must get overlapped
ƒ Thrust faulting
• Southern California/Pacific Plate, Indian Plate/Eurasian Plate
ƒ Subduction zones - A convergent plate boundary where material gets
overlapped
• When a subduction zone is between oceanic crust and continental
crust, the oceanic crust subducts because it is more dense
• In continent-continent convergence zones (Himalayas) continental
crust (Indian plate) can subduct beneath other continental crust
(Eurasian plate)
• ex. Nazca Plate/South American Plate, Pacific Plate/Eurasian Plate
o Transform/Shearing – Plate boundaries where two plates are moving past each
other, parallel to the boundary
ƒ Crust is not being consumed are created
ƒ Typically result in strike-slip faults
ƒ
ex. San Andreas Fault, North Anatolian Fault (1999 Ismit earthquake),
North American Plate/Pacific Plate
Types of Faults
o Normal
o Reverse
o Strike-slip
Seismic Cycle
o Deformation during an earthquake is very quick and is greatest near the fault and
tapers off the farther away you go from the fault line
o On the long term, deformation is elastic and all points on the two plates move
away from each other equally, regardless of the distance from the fault line itself
o Between earthquakes, the deformation is relatively elastic and the plates continues
to move apart but points along the fault line are held still due to friction
Sliding and Friction
o τ = μσ N
o (Shear Force) = (Coefficient of Friction)(Normal Force)
o Byerlee’s law states that the coefficient of friction for most rocks is
0.6 ≤ μ ≥ 0.85
o Newton’s 2nd Law: F = ma
o Hooke’s Law: F = kx
o Equate the two expressions for force and we have a mathematical relationship
between elastic deformation and accelerations: ma = kx
Waves
o Amplitude – Displacement (Half the distance from peak to trough)
o Wavelength – Distance from peak to peak
o Period – Time between waves
o Frequency – Number of waves in one second (1/period)
Seismic Waves
o Body Waves – Move through the interior of the Earth
ƒ P (Primary) Waves
• Fastest of all waves
• First to reach recording station
• Move as push-pull – alternating pulse of compression and
extension
• Velocity depends on density and compressibility of substance they
are traveling through
• V = 4.8 km/s (approx.) through granite
• Can travel through air and so may be audible near epicenter
ƒ S (Secondary) Waves
• Slower
• Transverse motion – Shearing or shaking particles at right angles
to the wave’s path, actual waves
• V = 3.0km/s (approx.)
• Up-and-down and side-to-side shaking, do a lot of damage
• Can only travel through solid
o Surface Waves - Faster than body waves, short period, high frequency, most
energetic near epicenter
ƒ Travel near the Earth’s surface, created by body waves disturbing the
surface
ƒ Longer period than body waves (carry energy farther)
ƒ Do the most damage
ƒ Love Wave
• Travel similar to S- waves but side-to-side in horizontal plane
• Travel faster than Rayleigh waves
• Do not move through water
ƒ Rayleigh Wave
• Backward-rotating, elliptical motion
o Wave Speeds
ƒ Wave propagation speed depends on the material through which it is
propagating (the density and the sheer modulus)
ƒ P-Waves (Primary wave, arrives first) – Faster
k + ( 4 )G
3
• VP =
ρ
ƒ
• K-Modulus of incompressibility (function of material)
• P – Density
• G - Shear modulus
S-Wave (Secondary wave, arrives second) – Slower
G
• VS =
ρ
Wave Speed and Relative Damage
o Waves can be amplified due to slow wave speeds in week soils
o Energy of seismic wave is given by
ƒ E = cλρA2
ƒ C - Constant of proportionality
ƒ λ - Wavelength
ƒ ρ - Density
ƒ A - Amplitude
o Wavelength of seismic wave is given by
ƒ λ = vT
ƒ λ - Wavelength
ƒ T – Period
ƒ v - Velocity
o Energy is conserved as a wave passes from one type of rock to another
ƒ E1 = E2
A2
v1 ρ1
ƒ
=
A1
v2 ρ 2
o Soft Rock (less dense) – Waves move much slower, spend more time in that area,
amplitude increases, amplifies ground shaking
Creep - The amount of gradual slide that occurs between plates between earthquakes
o Causes the gradual offset of sidewalks and buildings
o Does not cause earthquakes
o A lot of build up energy can be dissipated through creep
Locating earthquake position
o Differences in wave speeds can be used to approximate earthquake positions
o Compare the arrival times of the P and S waves and calculate a general radius
using the wave speed equation above
o Using triangulation, compare seismographic readings at different stations
(minimum three) to accurately pinpoint epicenter
Seismic Moment – A measure of energy release during an earthquake
o M 0 = GLDd
o M0 – Magnitude (seismic moment)
o (G) Sheer modulus ~ 3 x 1010 Pa for most rocks
o (d) Slip of the earthquake, how far the two pieces slid past each other, movement
along the rupture
o (L) Length
o (D) Depth
o Area = LD
o Measured in Newton meters
Moment Magnitude Scale
⎛2⎞
o M W = ⎜ ⎟(log(M 0 ) − 9.1)
⎝3⎠
o MW – Moment magnitude (a number between 0 and 10)
o M0 – Seismic moment = GAd
Focal Mechanisms
o “Beach-ball” diagrams
o Shaded – Compression
o White – Tension (pulling apart)
o Strike/Slip Earthquake – Circle is divided into 4 quarters
o Dip Earthquake – Circle is shaded in more elaborate patterns
Coulomb Failure Stresses – The theory that an earthquake, by adding strain and stress
due to the displacement it causes, may be able to induce an earthquake on another plane
o At the same time, if an earthquake pushes two zones together, it may be able to
clamp it shut and retard its ability it have an earthquake
o We can use GPS to measure the deformation associated with an earthquake.
These displacements can then be used to calculate the stresses (forces exerted by
the earthquake)
o σ = Gε
ƒ Stress = (Shear Modulus) x (Strain)
ΔL
o ε=
L
o Stress is a measure of force exerted (on another fault plane that might rupture)
Change in Coulomb Failure Stresses
o Δσ = Δτ + μ ' (σ )
f
η
s
ƒ Δσ - Change in Coulomb Failure Stress
f
ƒ Δτ - Change in shear stress component
s
ƒ μ ' (σ ) - (Coefficient of friction) x (Normal stress)
η
Paleosiesmology – The investigation of past earthquakes by examination of their
geological records
o An earthquake deforms the Earth’s surface and that deformation can be preserved
beneath layers of sediment
o Allows scientists to study earthquake history and study it more closely
Assessing Earthquake Hazard
o Degrees of Seismic Risk – What sort of fault is nearby? Is it locked/creeping?
What is the seismic history of the location
o Building Type – Factors influencing vulnerability include: flexible materials,
“soft” first stories (such as garages), use of sheared walls, braced frames
ƒ Good – Soft building materials that move with the motion of the
earthquake; ex. wood
ƒ Bad – Stone masonry; soft first stories like garages simply collapse and
the entire building up above it simply falls and pancakes the first story
o Ground Type – The amplitude of seismic waves strongly depends on the type of
ground it passes through because seismic energy is conserved as a seismic wave
passes from one rock type to another, keeping E (energy) constant
ƒ E = cλρA2
• c-constant of proportionality, λ-wavelength, ρ-density,
A-wave amplitude
ƒ λ = vT
• v-velocity, T-period
ƒ Soft Rock (less dense) – Waves move much slower, spend more time in
that area, amplitude increases, amplifies ground shaking
• ex. Marina District-SF, Back Bay-Boston, Mexico City
Great Sumatra Earthquake/Tsunami (2004)
o Magnitude 9.1
o Rupture length: 1300km
o Slip 10 meters
o Dip-Slip (Reverse faulting)
o Setting: Subduction zones
o Occurred underwater
o Deadly because it caused a tsunami
o Tsunami was so deadly because it occurred far enough off shore that it was able
to grow and pick up speed
ƒ Also, because of the location, the tsunami bounced back and forth
North Ridge Earthquake (1994)
o Magnitude 6.7
o Rupture length: 30km
o Slip: 4 meters
o Continental thrust faulting
ƒ While the relative motion of the Pacific Plate and the North American
plate is generally strike-slip, some areas are prone to compression energy
and material buildup, which rupture as continental thrusting
o Rupture did not reach surface but occurred underground
Loma Prieta – World Series Earthquake (1989)
o Magnitude 6.9
o Slip: 5 meters
o Rupture length: 70km
o Continental strike-slip faulting
o Associated with San Andreas fault
o California, 10km north of Santa Cruise, 60km south of San Francisco
TSUNAMIS
Tsunamis can be triggered by earthquakes
Four Requirements for an earthquake to cause a tsunami
o Must occur under or near a large body of water
o Must be a normal or reverse fault (must cause vertical movement of the seafloor)
o Must have a magnitude greater than 7.5
o Must have a shallow focus (less than 70km)
In deep water, tsunamis travel very fast (>800km/hr) and have a very low wave height
In shallow waters, the waves slow down and the wave height increases dramatically
Wave Speed in Relation to Ocean Depth
o v = gD
o v – velocity of tsunami (m/s)
o g – acceleration due to gravity (m/s2)
o D – depth of water through which the wave is travelling (m)
VOLCANOLOGY
Volcanism is closely linked with plate tectonics, particularly subduction zones
o As a plate subducts, a few hundred kilometers away from the subduction zone, the
subducted plate heats up and begins to melt
o As a result, the molten material becomes more buoyant and begins to migrate
upwards
o The molten material breaks through the continental crust above, creating a
volcano
o Because of the linear nature of a subduction zone, often times chains of volcanoes
are created
90% of volcanism is associated with plate boundaries
o 80% at spreading centers
o 10% at subduction zones
Remaining 10% of volcanism occurs above hot spots
Variations in magma’s chemical composition, ability to flow (viscosity), gas content,
and volume determines whether eruptions are peaceful or explosive
Of 92 naturally occurring elements:
o Eight make up more than 98% of Earth’s crust
o Twelve make up 99.23% of Earth’s crust
o Oxygen and silicon are by far the most abundant
Magma – Molten or liquid rock with some crystallized solids (minerals
o Magma hardens to volcanic rock when cooled at the surface, or plutonic rock
when cooled within the Earth
Lava – Magma that flows on the Earth’s surface
Three V’s of Volcanology
o Viscosity may be low or high
ƒ Controls whether magma flows easily or piles up
ƒ High viscosity gives rise to explosive volcanism because it is more
difficult to cause that initial eruption
ƒ Factors affecting viscosity
• Temperature
• Silica (SiO2) – Higher Silica content, more viscous (∴ explosive)
• Amount of crystallized materials (mineral solid)
o Volatile abundance may be low, medium, or high
ƒ Volatiles refer to gasses, oxygen water, and other light elements that are
able to dissolve into the rock matrix
ƒ If they are trapped in a high viscosity matrix, they may cause explosive
eruptions
ƒ May ooze out harmlessly or explode
ƒ Factors affecting volatility
• Concentration of dissolved gasses (especially water)
• At high pressure and low temperature, gasses stay dissolved
• As magma rises, pressure decreases, dissolved water turns to
steam, and, in the presence of thick magma, becomes explosive
o Volume may be small, medium, or large
ƒ Greater volume Æ More intense eruptions
Subduction Zone Volcanism
o As an oceanic plate subducts, it carries water-saturated sediments down with it
o The water lowers the melting point of the subducted slab and rocks around it,
causing partial melting of these rocks and magma formation
o The magma then rises, and as it contact the rock of the overlying plate, those
rocks also experience melting
o Subduction zone volcanoes exhibit higher SiO2 content, higher water content, and
lower-temperature magmas than magmas at spreading centers
o These factors cause subduction zone volcanoes to experience more explosive
eruption
o Typical subduction zone volcano type: stratovolcano (and sometimes scoria cone)
Three Factors Will Cause Rock to Rise
o Lowering Pressure
ƒ As soon as the subterranean magma chamber is exposed to lower pressure,
it begins to rises, causing a runaway process in which magma continues to
rise further and further as it is exposed to less and less pressure
o Raising Temperature
o Increasing Water Content
ƒ Under high pressure, water mixes with magma and forms a solution
ƒ As the magma rises and the pressure decreases, water escapes as gas air
bubbles
ƒ The rapid escape of gas from the magma often causes explosive reactions
Hotspot Volcanism
o Hotspots – Areas of earthquakes and volcanoes not necessarily located on plate
boundaries
o Occur when unusually hot portions of the mantle send plumes of magma upwards
into the crust
o Locations of hotspots are fixed and therefore can provide an example of the
motion of tectonic plates
ƒ Hot spots, often times found in the middle of the ocean, create islands
ƒ As tectonic plates move above the hot spot over time, the hot spot creates
island chains (ex. Hawaii-Emperor islands)
ƒ In a chain of islands, the oldest islands have generally fallen below the
ocean surface due to continued erosion and other natural forces
ƒ Use the shape of hotspot trail to determine movement of tectonic plate
with respect to the hotspot
Rock Types
o Igneous – Formed when magma cools
ƒ Intrusive – Cools slowly underground
ƒ Extrusive – Cools quickly at surface
o Non-explosive volcanic rock types
ƒ Pahoehoe
ƒ Aa
ƒ Gabbro (formed underground)
ƒ Deposited slowly
o Explosive volcanic rock types
ƒ Pyroclastic debris – Broken up magma and rock from violent gaseous
explosions, classified by size
ƒ Deposited very quickly
ƒ May be deposited as
• Air-fall layers (settled from ash cloud)
• High-speed, gas-charged flow that surges over surface (pyroclastic
flow)
o Other volcanic material
ƒ Obsidian – Volcanic glass forms when magma cools very fast
ƒ Pumice – Porous rock from cooled froth of magma and bubbles, indicates
that the eruption had a lot of gasses associated with it
Eruption Styles
o Non-Explosive: Icelandic
o Non-Explosive: Hawaiian
o Somewhat Explosive: Strombolian
o Explosive: Vulcanian – Alternating between thick flows and pyroclastic eruptions
ƒ ex. Aeolian Islands, Sicily
o Explosive: Plinian – Gas powered vertical eruptions, lots of pumice, etc.
ƒ Often the final phase of a volcano after vulcanian phase
Signs of Increasing Volcano Activity
o Bulging of the volcano face due to increased subterranean magma movements
o Increased seismic activity also due to increased subterranean magma movements
o Increased presence of gasses (CO2) in area near volcano
InSAR and Volcanoes
o InSAR is a satellite tool that can be used to detect deformation of the Earth’s
surface from space
o These deformations, which can be caused by various factors including movement
of magma under volcanoes and displacement across faults, can be used to assess
volcano activity
Magma Type
Magma Type
Viscosity Volatility
Tectonic Setting
Rock Types
Basalt
Low
Low
45% Basilica content
Andesite
Medium
Rhyolite
High
Medium
55% Basilica content
High
70% Basilica content
Mid-ocean ridges
Shield volcanoes
Flood basalts (India)
Subduction zones
Aa
Pahoehoe
Gabbro
Diorite (intrusive)
Mt. Rainer
Cone shaped volcano
Dome shaped volcano
Granite (intrusive)
Obsidian (extrusive)
Pumice (extrusive)
Volcano Morphology and Eruption Style
Volcano Type
Shape
Magma
Eruption Style
Tectonic Setting
Shield Volcano
Width is much larger
than height, very
gentle slopes,
convex upward
Basaltic (fluid)
Hawaiian, Not very
explosive
Hot spots, Hawaii
Scoria Cones
(Cinder Cone)
Large, sloped,
straight sides with
steep slopes, large
summit crater
Andesite/Basaltic
Strombolian,
Explosive
Subduction zones,
Andes
Stratovolcano
(Composite
Volcano)
Gentle lower slopes
but steep upper
slopes, concave
upward, small
summit crater
Rhyolite, Andesite
Vulcanian, Plinian
Subduction zone,
pacific northwest
MASS WASTING EVENT
Triggers of Mass Movements
o Heavy rains
o Earthquakes
o Thawing of frozen ground
o Construction projects
ƒ Add weight to top of slope
ƒ Remove vegetation
ƒ Cut into the base of slope
Types of Mass Movements
o Slide – Material moves as semisolid mass along basal surface (planar or curved)
ƒ Creates “terraced” landscape or hillside
o Fall – Material moves as a block or coherent mass with a dominantly vertical
downward motion; free fall
o Flow – Material flows over landscape as a very viscous fluid-like mass
ƒ Turbulence within moving mass
o Subsides – Material collapses into a void as a block or coherent mass with a
dominantly vertical downward motion
Scarp – The remaining edge left after a mass wasting event removes a chunk of earth
Energy
o Kinetic Energy – Energy of motion
⎛1⎞
ƒ K = ⎜ ⎟mv 2
⎝2⎠
ƒ K – kinetic energy
ƒ m – mass
ƒ v - velocity
o Potential Energy – Stored energy
ƒ U = mgh
ƒ g – acceleration due to gravity
ƒ h - height
Slide Types
o Rotational slide
ƒ Move downward and outward above curved slip surface with movement
rotational about an axis parallel to slope
ƒ Head moves downward and rotates backward
ƒ Toe moves upward on top of landscape
ƒ Move short distance
ƒ Associated with rather steep slopes
ƒ ex. Ensenada, Baja California 1976
o Creep
ƒ Particularly common in areas that experience freeze-thaw cycles
ƒ Slowest, more widespread form of slop failure
ƒ Soil expands perpendicular to ground surface shrinks straight downward
in response to gravity
ƒ Requires water content
o Translational
ƒ Slip surface has a constant orientation
ƒ Move on planer slip surface such as a fault, joint, clay-rich layer
ƒ Associated with a shallower slope
Coulomb-Terzaghi Failure
o τ = σ + (P − Pw ) tan φ
o τ – Shear resistance
o σ – Cohesion
o P – Pressure of soil load
o PW – Pressure of water
o tan φ – Failure angle
Water reduces the contact between mineral grains, making it difficult for the surface
ground to transfer the normal load at depth
o See page 243 for chart of speed versus wetness of mass wasting events
Strength of hillside comes from cohesion (how well it sticks together) plus the weight of
all materials under gravity
Roots and vegetation hold ground together and prevent it from sliding
Strength is offset by pore-water pressure and angle of slide surface
Failure angle is low (near horizontal) for weak materials (clay) and high (near vertical)
for stronger materials
ATMOSPHERE
Adiabatic (Isocaloric) – A process in which no heat is transferred to or from working
fluid
Boyle’s Law: P1V1 = P2V2
o If pressure increases, volume decreases
o If pressure decreases, volume increases
Temperature decreases as volume increases due to fewer molecular interactions
Summary: ↑P , ↓V, ↑T AND
↓P, ↑V, ↓T
Atmospheric pressure, and with it atmospheric temperature, decreases with increasing
altitude
Parcels of air match the surrounding pressure by:
o Rising Ælower pressure Æ higher volume Æ lower temperature
o Sinking Æ higher pressure Æ smaller volume Æ higher temperature
Adiabatic Lapse Rate – The rate at which a parcel’s temperature changes with altitude
o An air parcel will rise if it is warmer than the surrounding atmosphere and sink if
it is colder than the surrounding atmosphere
o A parcel of air will continue to rise/fall until it reaches an altitude where the
surrounding atmosphere has the same temperature
o Generally constant at -9.8°C/km
Atmospheric Lapse Rate – The rate at which the atmosphere’s temperature changes with
altitude
Stable Atmosphere – The atmospheric lapse rate is less (negative) than the adiabatic
lapse rate, and rising parcels will tend to be pushed back downward
Unstable Atmosphere – The atmospheric lapse rate is more (negative) than the adiabatic
lapse rate, and rising particles will continue to rise creating rapid vertical motions in the
atmosphere and possibly leading to thunderstorms and tornados
TORNADO
Tornado - A violently rotating column of air pendant from a cumulonimbus cloud and
nearly always observable as a funnel cloud or tube
Tornados are a product of atmospheric instability
Last anywhere between 10 seconds to 30 minutes
Basic Requirements for Tornado Formation
o Warm, humid, low-level air (often flowing in from the Gulf of Mexico)
o Cold, dry, mid-level air (often flowing from Canada or the Rockies)
o High-altitude jet stream moving eastward at rapid speeds
o The different directions these air masses are travelling induces rotation
o The warm air near the surface and the cool air at higher elevations results in an
unstable atmosphere
It is very difficult to precisely calculate tornado wind speeds because they are transient
phenomena that are relatively unpredictable and also very violent, destroying most
scientific instrumentation
Tornado wind speeds are generally estimated by the damage they cause
Tornado Formation
o Warm, low altitude, humid air from Gulf collides with
cold, mid-altitude, dry air from Canada or Rockies
o High altitude jet stream moving east
o Different directions of travelling air masses induce
rotation (not necessarily a result of Coriolis effect)
Hurricane Formation
o Sea surface (upper 60m) temperature > 27°C in the
upper 60 meters of ocean
o Air must be unstable, warm, and humid
o Upper-level winds should be weak and blowing in the
same direction the storm is moving
o Low pressure zone develops, causing winds from
higher pressure zones to flow towards it
o Interaction between pressure gradient and Coriolis
force induces rotation
o Rotates around a central core
o Core draws warm, moist air up to the stratosphere,
where it cools and loses its water vapor by
condensation, releasing large amounts of latent heat
o Heat warms surrounding air, creating stronger
updrafts and fueling upward flow of wind through
central core
Require large change in wind speed and direction with
height
Require large horizontal thermal gradient (needed for
thunderstorm formation, which can form tornados)
Forms over land (solar heating of ground surface
contributes to development of thunderstorms associated
with tornados)
Only small vertical change in wind speed and direction
needed
Do not require large horizontal temperature gradient
o Small, transient phenomena
o Can be hundreds of meters wide and last up to a
couple of hours
o Large, longer-lived phenomena
o Can be hundreds of kilometers in diameter and last for
several days
o Form over water
o Lose strength as they pass over land due to loss of
power source
HURRICANES
Pressure Gradient Force – Force due to the difference in pressure over a horizontal
distance (directed from high pressure to low pressure)
Δpressure
o Fpg =
ρΔx
The Coriolis Effect – An effect caused by the rotation of the Earth, which makes
moving objects that are not in contact with the Earth’s surface appear to travel in curved
paths
o Magnitude of Coriolis Effect (Acceleration)
ƒ 2Ωvsin(λ)
o Magnitude of Coriolis Force (F=ma)
ƒ 2Ωmvsin(λ)
o Deflection due to Coriolis Force
Δx 2 Ω sin(λ )
ƒ δ =
v
ƒ Deflection is clockwise in the northern hemisphere and counterclockwise
in the southern hemisphere
ƒ Deflection is greatest at the poles
o Ω = angular velocity of Earth (rad/sec)
o v = linear velocity of object traveling above the Earth (m/s)
o m = mass of object (kg)
o λ = latitude (radians)
o Δx = distance traveled over the Earth’s surface (m)
o Explanation of Coriolis Force
ƒ A cannonball sitting in a cannon on the equator, pointed northward, has a
sideways velocity component due to the rotation of the Earth
ƒ When the cannonball is fired it gets a forward velocity component but also
retains a sideways velocity component equal to that at the equator
ƒ Because points north (or south) of the equator have a lower angular
velocity (sideways velocity component), the cannonball appears to curve
clockwise
Geostrophic Balance – The Coriolis effect and the pressure gradient are equal
Δpressure
o 2Ωvsin(λ) =
ρΔ x
ƒ Pressure (Pa), Δx (m), v (m/s), λ (rad), Ω (rad/s), ρ (kg/m3)
Atmosphere Circulation around High and Low Pressure
o Air will always seek to flow from high to low pressure
o The Coriolis Force will increase along the air’s velocity, exerting a force
perpendicular to the wind’s velocity (right in the Northern Hemisphere and left in
the Southern Hemisphere)
o The pressure gradient force and the Coriolis Force will roughly balance one
another out creating a circulation around the high or low pressure system
ƒ Circulation is clockwise around high pressure systems and
counterclockwise around low
Linear and Angular Motion
o Radians = (Degrees) x (π/180)
o Angular Velocity = Ω = (2π/t)
2πr cos(λ )
o Linear Velocity = v =
t
ƒ t - length of day (86400 sec)
ƒ λ - latitude (radian, converted from degrees)
ƒ r - radius of Earth at equator (6.3e6 m)
Basic Requirements for Hurricane Formation
o Must form over water
o Minimum sea surface temperature 27°C
o Air must be unstable, warm, and humid
o Low pressure system
o Formation must occur far enough from the equator
Bermuda High - Standing area of high pressure over the central Atlantic that pushes
new developing hurricanes from the south central Atlantic (where they are created) to
the North American continent
MASS EXTINCTIONS
Mass Extinction – The loss of 40% or more of species in a relatively short time
o Months, years (indistinguishable in geologic time), up to hundreds/thousands of
years
o Global phenomenon
Background Extinction – The loss of 10-100 species per year, according to fossil record
Major losses of marine species define geologic time
o Paleozoic, Mesozoic, Cenozoic
Five Major Mass Extinctions
o Late Ordovician (440 Mya)
o Late Devonian (365 Mya)
o Late Permian (251 Mya) – Most severe mass extinction know
ƒ Between 75 and 95% of marine species went extinct
o Late Triassic (199-214 Mya)
o Late Cretaceous (65 Mya)
Causes for the K-T Extinction 65 million years ago (killed dinosaurs)
o Flood volcanism, particularly from the Deccan Traps in Inda
ƒ Short term: Cooling (ash block sunlight)
ƒ Long Term: Increase of CO2 in atmosphere raising global temperatures,
acid rain
o Extraterrestrial Impact
ƒ Short term: Cooling (dust block sunlight)
ƒ Long term: warming, acid rain, wildfires
ASSESSING COSTS OF DISASTER PREVENTION
Decision Tree
o The cost of a disaster prevention mechanism is determined by the original cost of
implementing it added to the various other costs multiplied by the probability that
they will be incurred
Disaster Cycle
o Ideal: DISASTER Æ Rescue/Relief ÆRehabilitation Æ Reconstruction Æ
Mitigation/Prevention Æ Preparedness
o Actual: DISASTER Æ Rescue/Relief Æ Rehabilitation
o After a disaster governments are very good at responding to the immediate need
for rescue/relief/rehabilitation but often times neglect the long-term measures for
disaster prevention and mitigation
o As a result, the disaster cycle simply repeats itself with equally devastating
disasters
Hurricane Katrina
o According to the main congressional report on Katrina, the levees were all built in
the correct locations but were not built to withstand hurricanes of great magnitude
o Levee failure occurred for two main reasons
ƒ Simple overtopping of water rising above the levee wall
ƒ Breeches/failures in the levee walls, which tended to occur at levee
transitions where soil and foundation were weaker
o Improvements of levee walls
ƒ Overtopping erosion protection through provisions of rip-rap, concrete
splash slabs, or paving of the ground surface at the inboard faces of the
levee floodwalls
La Conchita Landslides
o In 1995 a slow moving landslide occurred in the town of La Conchita
o No casualties but property damage
o After this slide, debris from the foot of the new slope was removed and a
restraining wall was built
o The construction of this wall destabilized the slope
o In 2005, following two weeks of heavy rain, a second landslide occurred, this
time with high velocity killing 10 people