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Chapter 1

Study Guide For EESA05, Chapter 1

by OC2

Department
Environmental Science
Course Code
EESB18H3
Professor
Ingrid L.Stefanovic
Chapter
1

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Chapter 1: Introduction to Natural Hazards
" Tsunami in Indian Ocean on Dec 26, 2004 claimed more than 200 000 lives in 11 countries
" Subduction zone is a long, relatively narrow strip of Earth’s lithosphere where one of the large tectonic
plates that constitute Earth’s thin outer shell subducts or moves beneath another. The zone of collision
between the two plates is a huge fault that extends for hundreds of km along the ocean floor, parallel to
the edge of a continent or an island, such as Sumatra
" Giant earthquakes occur at these sites, releasing energy that has slowly built up along the fault over
centuries
" Warning systems require a communications infrastructure that permits info to rapidly reach emergency
officials in areas at risk; those officials should have plans to rapidly evacuate people from low-lying
coastal areas
1.1: Why studying Natural Hazards is Important
Processes: Internal and External
" process: ways in which events, such as volcanic eruptions, earthquakes, landslides and floods, affect
Earth’s surface
" volcanic eruptions and earthquakes are a result of internal forces explained by theory of plate tectonics
(large blocks of the Earth’s crust)
o occur at boundaries between tectonic plates
" external forces operate at or near the Earth’s surface
" wind and ocean circulation, and water evaporation determine Earth’s climate and drive the hydrologic
cyclethese forces in turn are directly related to hazardous processes such as violent storms, floods and
coastal erosion
" mass wasting is driven by both internal and external forces
o landslides result from gravity acting on hillslopes that have been formed by tectonic processes and
erosion. Gravity is the force that attracts one body to anotherin this case the attraction of surface
materials toward the center of the earth. Because of gravitational attraction, rocks and soils on
mountainsides and the water that falls as precipitation all move down slope
" processes we consider are hazards are natural and derive from internal heating of earth and external
energy from the sun
" events become hazardous only when they threaten human beings
Hazard, Risk, Disaster and Catastrophe
" hazard is any natural process that threatens human life or property; process itself is not a hazard, it
becomes a hazard only when threatening human interests
" risk: expressed as the probable severity that a destructive event will occur multiplied by the event’s likely
impact on people and property; risk integrates hazard and social vulnerability
" disaster and catastrophe refer to events that cause serious injury, loss of life and property damage over a
limited time and within a specific geographic area
" disasters may be regional or even national in scope, whereas catastrophe commonly have consequences
far beyond the area that is directly affected and require huge expenditures of time and money for recovery
" mitigation is used by scientists, planners and policy makers when describing efforts to prepare for
disasters and to minimize their harmful effects
Death and Damage Caused by Natural Hazards
" in NA, natural hazards that cause the greatest loss of life are not the same as those that cause most
property damage
" tornadoes and windstorms cause the largest number of deaths each year, lightening, floods and hurricanes
also take a heavy poll
" in most developing countries more lives are lost
" north American disaster take a large toll on the economy (loss of economic activity and employment)
" natural hazards differ greatly in their potential to create a catastrophe
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o floods, hurricanes, earthquakes and volcanic eruptions and large wildfires are most likely to create
catastrophe
o landslides, snow avalanches and tornado generally affect small area and thus are rarely catastrophic
o coastal erosion, lightening, and expansive soils do not create catastrophes but can still cause much
damage
" risks associated with natural hazards change with time because of the changes in land-use patterns
o removal of natural vegetation by agriculture, forestry, urbanization and mining can increase erosion and
sedimentation
" damage from most hazardous natural processes in Canada is increasing, but the number of deaths from
many hazards is decreasing because of better planning, forecasting, warning and engineering
1.2: Role of Time in Understanding Hazards
" hazard forecasts and warning are more accurate if we combine info about the past behavior of the process
with an understanding of present conditions
1.3 Geologic Cycle
" geology, topography and climate govern the type, location and intensity of natural processes
" geological cycle: 4 associated sequences of earth processes: tectonic cycle, rock cycle, hydrologic cycle
and biogeochemical cycles
The Tectonic Cycle
" tectonic refers to the large-scale geologic processes that deform the earth’s crust and produce such
features as ocean basins, continents and mountains
" tectonic process are driven by forces deep within earth
" tectonic plates are large blocks that form the outer shell of earth
" tectonic cycle involves the creation, movement and destruction of tectonic plates, and one cycle can last
more than 200 MY
Earth’s Lithosphere and Crust
" outermost or surface layer is called the lithosphere and is stronger and more rigid than deeper material
o average thickness is about 100 km; it ranges from a few km thick beneath the crests of mid-ocean ridges
to 400 km beneath continents
" asthenosphere: hot layer of relatively low strength rock that extends to an avg depth of about 250 km
" the upper part of the lithosphere is the crust; crustal rocks are dense than the rocks below; there are 2
kinds of crust:
o oceanic crust is denser than continental crust; it is also thinnerthe ocean floor has an avg crustal
thickness of about 7 km, whereas continental crust is about 30 km thick on avg and up to 70 km thick
beneath mountainous regions
Types of plate boundaries:
" asthenosphere is thought to be more or less continuous; the lithosphere is broken into large fragments
called lithospheric or tectonic plates that move relative to one another
" processes associated with the origin, movement and destruction of these plates are collectively termed
plate tectonics
" tectonic plates are formed and destroyed at their margins or boundaries
" plate boundaries may be divergent, convergent or transform these boundaries are not narrow cracks, but
rather broad zones of intense deformation 10-100 of km wide that extend through the crust it is at these
boundaries that most EQ and active volcanoes occur
" divergent boundaries: occur where 2 plates more away from each other and new lithosphere is produced
o places where this occur are large, underwater mountain ridges called mid-ocean ridges
o seafloor spreading lithosphere breaks or rifts apart along a series of cracks more or less parallel to the
ridge crest
o many cracks in the underwater rift zone are injected with molten rock, or magma, from below
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