Natural Disasters Midterm 1

Natural Disaster

• A natural event in which a
  large amount of energy is released in a
  relatively short time with catastrophic
  consequences for life and/or infrastructure

• Natural, not manmade

• Release of stored energy

What disasters does Internal Energy cause?

• Volcanoes

• Earthquakes

• Tsunamis

What disasters does gravity cause?

• Mass movement

• Snow Avalanche

What disasters does solar energy cause?

• meteorological storm

• flood

• drought

• wildfire

• magnetic storm

• tornadoes

• Hydrologic cycle effects

• hurricanes

What disasters does Impact Energy cause?

• Impact with space junk

• meteors

Effects of Internal Heating

drives plate tectonics and
causes earthquakes, volcanoes and tsunamis

Internal Energy

● Internal heat generated primarily from ongoing
decay of naturally-occurring radioactive
elements e.g., uranium (U), thorium (Th),
potassium (K)
● Some residual heat from extraterrestrial
impacts and gravitational compaction early
in Earth’s history

Gravity

an attractive force
between any two masses,
directly proportional to the
product of the masses and
inversely proportional to the
square of their separation

Gravity Formula

Gravity /is directly proportional to/ (mass 1 x mass 2)/(separation squared)

Potential Energy

stored energy proportional to it’s elevation

Kinetic Energy

Energy of motion

What type of reactor is the sun

Nuclear fusion - 2 hydrogen combine to one helium

Hazard

potential for dangerous event

Vulnerability

Likelihood a community will suffer when exposed

Risk

vulnerability x hazard

Natural disasters occur when

natural hazards intersect with vulnerable communities. Natural hazards are inevitable but disasters are preventable

After disaster

Response (short term), recovery (long term)

Before Disaster

➢ Mitigation (long-term)
➢ Preparedness/Adaptation (long-term)

Response

Immediate actions after disaster (get situation under control)

Recovery

longer term goals to rebuild community

Mitigation

Advance Activities to reduce risk (structural, flood/earthquake proofing, etc)

Preparedness

pro-active steps such as stockpiling, drills, etc

Return Period

Average time between events

Frequency

1/period, Average number of
occurrences in a given time

Magnitude

measure of energy released

Natural disaster trends

Frequency of weather related = increasing

Frequency of geologic = stable

Fatalities = increasing (slightly), but decreasing in Canada

Economic cost = increasing rapidly

Developed countries have

fewer casualties, but higher economic costs

Stress

Force per area
• Compression (pressure):
perpendicular to surface
→ contraction
• Tension: perpendicular to
surface → extension
• Shear: parallel to surface
→ distortion

Strain

Relative
deformation under stress

Rheology of Liquids

Flow under stress

Elastic Rheology

deformation is
recoverable (object returns to
original shape when stress
removed)

Ductile Rheology

perminant deformation

Brittle Rheology

rigid, shatters

Plastic Rheology

flows like high viscosity fluid

Viscosity

internal resistance to flow

Rheology depends on

time, temperature, pressure

Brittle rupture

abrupt stress, low T and/or low P

Plastic flow

Long-term stress, high T and/or high P

Moho

Crust-Mantle compositional boundary

Continental crust

(brittle)
➢ 0—1000 oC, 2.7 g/cm3, 0.4 % of Earth’s mass
➢ Ave thickness ~35 km (up to 80 km under mountains)
➢ Granitic rock, rich in oxygen (O) and silicon (Si), ~60%
silica (SiO2)

Oceanic crust

(brittle)
➢ 0—1000 oC, 3.0 g/cm3, 0.1% of Earth’s mass
➢ Ave thickness ~10 km (thinner under mid-ocean ridges)
➢ Basaltic (volcanic) rock, ~48% silica

Inner Core

(solid)
➢ Temperature T ~ 5000 oC
➢ Density 14—16 g/cm3
➢ Makes up approx 2% of Earth’s mass
➢ Solid metal—mostly iron (Fe) with some nickel (Ni)

Outer Core

(liquid)
➢ 4000 oC, 9.7—14 g/cm3, 30% of Earth’s mass
➢ Fluid Fe/Ni

Mantle

(lower: plastic; upper: brittle)
➢ 1000—3000 oC, 3.5—5.7 g/cm3, 67% of Earth’s mass
➢ Dense, dark silicate rock rich in Fe and aluminum (Al)

Lithosphere / Asthenosphere Rheology

crust and upper-most mantle: rigid
solids fused together forming the lithosphere
(lithos ≡ rocky)
• Below lithosphere: “soft” plastic upper-mantle
layer, the asthenosphere (asthenes ≡ weak),
solid with few % partial melt (not molten), flows
slowly under stress
• Below is “stiff” plastic layer, the mesosphere

Isostasy

Gravitational equilibrium whereby lithosphere
“floats” on denser, deformable asthenosphere at an
elevation dependent on its thickness and density

ex. icebergs

Isostatic Adjustment

1. Post-glacial rebound
   ➢Glacier weight causes subsidence
   ➢Crust rebounds when ice removed

2. New volcanic islands subside,
   sometimes forming atolls if
   coral growth matches
   subsidence rate

How do we know earths interior?

Density, uniformly distributed mass, seismology, magnetic field, direct observation, lab studies

Earthquake

Shaking of the Earth due to seismic
(vibrational) waves caused by an initial disturbance

Causes of Earthquakes

➢Faulting—sudden movement of two blocks of
rock relative to each other along a fracture (most
common)
➢Volcanic activity
➢Meteorite impacts
➢Landslides
➢Explosions
➢Oil & Gas production
➢Mining
➢Caldera collapses
➢Glaciers

Fault

A fracture in Earth across which the two sides
move relative to each other (e.g., in response to long-
term tectonic forces)

Elastic-rebound theory

➢ Fault is locked by friction (no
slip)
Fault Rupture
➢ Relative motion across fault
accommodated as elastic
strain (deformation), storing
energy
➢ When stress exceeds friction,
fault ruptures in abrupt slip,
stored energy released as heat
and seismic waves
Abbott & Samson, 2009

Hypocenter

place in earth where it originates

Epicenter

place on earths surfave above hypocenter

Strike slip fault

horizontal (slide past motion)

Dip slip fault

➢Vertical motion
➢Includes normal and
reverse (thrust) faults

Oblique Faults

a mix of strike slip/dip slip faults

Strike slip types

Right lateral/left lateral

dip slip types

Normal (pull apart motion, foot wall under hanging wall), Reverse/thrust (hanging wall above foot)

Mid ocean ridges

(MOR)—great central
mountain range running through all ocean basins
(65,000 km long)

Trenches

near continental margins, as long as
5900 km (Peru-Chile Trench), as deep as 11 km
(Marianas Trench)

Volcanoes are concentrated along

Mid ocean ridges

Paleomagnetism

study of the geologic record
of Earth’s magnetic field (MF) through time

Magnetic Field

Like a bar magnet, poles at end of bar. Poles can reverse

How is the magnetic field generated

otational
flow of electrically-
conductive fluid induces
MF with magnetic poles
close to rotation poles

Seafloor paleomagnetism

magnetic anomalies in stripes

POsitive anomaly

TRM in same direction of magnetic field

Negative anomaly

TRM in opposite direction of magnetic field

Seafloor spreading

Asthenosphere upwells at MOR axis (melts due to
pressure decrease) then cools, spreading laterally
to form new seafloor
➢New material at axis obtains TRM in direction of
MF at the time it cools:
• If MF in normal polarity, positive anomaly occurs
• If MF in reverse polarity, negative anomaly

Mid ocean ridge processes

1. hot mantle rock rises

2. melt forms under lithosphere

3. magma rises into magma chamber in crust and is injected into dykes forming new crust

4. lava erupts onto ocean floor

5. plates move apart, cools and thickens

Fracture Zones

What mid ocean ridges are offset by

Hot spots

Linear chains of
volcanic islands and
seamounts observed
in ocean basins

What do hot spot tracks show

lithospheric motion due to spreading

Tenets of tectonic plates

• interlocking plates

• internally rigid

• move relative to each other

Tectonic plates explain

➢Fit of continents and geologic/fossil continuity
➢Mid-ocean ridges and trenches
➢Age of seafloor
➢Earthquake and volcano distribution
➢Magnetic seafloor stripes
➢Transform faults and Hot spot tracks

Plate Motion History: 220ma

Supercontinent of Pangaea in one
hemisphere, Panthalassa ocean in other,
Tethys Sea partially separates continent N-S

Plate Motion History: 180ma

spreading separates Pangea into Laurasia
(north) and Gondwana (south); India separates

Plate Motion History: 135ma

Laurentia begins to separate into North
America and Eurasia; India moves north

Plate Motion History: 65ma

Mid-Atlantic ridge ruptures continents N-S
opening Atlantic Ocean; Tethys Sea is enclosed
(remnant is the Mediterranean)

Plate Motion History: Present

India collides with Eurasia (pushing up
Himalayas); Australia separated from southern
continents; Panthalassa closing (remnant is Pacific)

Plate Driving Forces

➢Ridge push
➢Slab pull
➢Mantle
convection

Ridge Push

➢Outward push of upwelling hot mantle material
➢Gravitational sliding of lithosphere down-ridge

Slab Pull

➢Lithosphere spreads, cools and contracts,
eventually becoming denser than underlying
asthenosphere
➢Cold, dense lithosphere sinks at subduction
zones, pulling on plate

Mantle convection

➢Thermally-driven convective cells in
mantle, pulls like conveyor belt

3 main plate boundaries

• Shear

• Tension

• Compression

Slow spreading

1-5cm/year

Intermediate spreading

5-9cm/year

Fast spreading

9-18cm/year

Full spreading rates

rate at which plate A moves away from plate B

Half spreading rate

Rate at which plate A or
plate B moves away from the MOR axis

Rifting

Initiation of spreading margins on land

Rifting Process: Centering

Continent centers
over hot region of mantle

Rifting Process: Doming

Heat causes
expansion, uplift, stretching,
fracturing

Rifting Process: Rifting

Gravity causes
fractures to fail
forming faults,
sagging centre,
volcanism

Rifting Process: Spreading

Deep rift floods,
continued divergence
forms new oceanic
crust

African Rift Valley

Part of a triple junction, 6000 km long
60 km wide
1000 m deep
active volcanism
Deep, elongated lakes

Convergent Margins

➢Ocean-Continent (subduction zone)
➢Ocean-Ocean (subduction zone)
➢Continent-Continent (collision zone)

Subduction Zone (Oceanic-continental)

➢ Denser oceanic lithosphere subducts
➢ Trench from down-bending lithosphere (not always present)
➢ Continental volcanic arc due to melt of subducted water-
saturated sediments descending into hot mantle
➢ Largest of all EQs, up to M 9+

Subduction (oceanic-oceanic)

➢Older plate: colder & denser, subducts below younger plate
➢Volcanic island arc
➢Very large EQs, up to M ~8.5