1
Hazardous Environments notes
Advanced Physical Geography Options
Cambridge International AS and A Level Geography
for Syllabus 9696
Hazards resulting from tectonic processes
Global distribution of tectonic hazards
Distribution of earthquakes
Tec. Hazards = seismic activity like earthquakes, volcanoes + tsunamis. Most World's
earthquakes occur in defined linear patterns. These chains follow plate boundaries. E.g. Centre
of Atlantic Ocean. Distinct lines of earthquakes around Pacific. Some cases broad like Nazca
subduction S. American. Broad belts associated subduction zones. Narrower plates =
constructive margins. Collision boundaries (Himalayas) associated broad belts earthquakes.
Conservative plates (San Andreas Fault) narrow belt earthquakes. Isolated earthquakes caused
by humans + isolated plumes rising magma (hotspot).
Distribution of Volcanoes
Most volcs. found plate boundaries. Some exceptions such as Hawaii (over hotspot). 550
historically active volcs. ¾ lie on “Pacific Ring Fire”. Includes new volcs like Mt. Pinatubo
(Philippines) + St. Helens (US). Most studied volcs above land but some submarine.
Volcs found on boundaries earth's major plates. Deeper levels earth not molten as pressure too
high. Plate boundaries = molten rock (aka magma) supplies volcs. In Ring Fire caused by
subduction oceanic/continental crust. Sub. in ocean creates chains volc islands (aka island arcs)
like “Aleutian Islands” (Pacific under N. American). Where sub. of oceanic crust under cont. =
formation young fold mountains. “Andes” (Nazca under S. American)
,2
Not all volcs at plate boundaries. In Hawaii found middle ocean. These islands = line increasingly
older volc islands, stretch N-W across Pacific. These volcs related movement of plates above hot
part fluid mantle.
- Mantle Plume (hotspot) = Jet of hot material rising from deep within mantle.
Responsible for volcs. Can be found beneath continents like E. African Rift Valley as well.
Produce isolated volcs. Play part in break-up of continents + formation of new.
At subduction zones, volcs produce viscous lava (erupt explosively + produce much ash). Volcs
found mid-ocean ridges (hotspots) produce fluid basaltic lava (Iceland + Hawaii).
At mid-ocean ridges, hot fluid rock from deep mantle rise cause of convection currents. Upper
mantle begin melt + basaltic lava erupts, creating new oceanic crust.
, 3
At subduction zone cold slab (ocean floor) slides down subduction zone slowly warming.
Volatile compounds (water +CO2) leave slab + move upwards into mantle so melts. Hot magma
able to rise.
Explosion occur where water meets hot rock. Water vaporises, increases pressure until rock
explodes. Gases within molten rock also pressure build up. Explosion depends viscosity of
magma. Gases dissolve easily in molten rock deep underground as pressure high. As it rises to
surface pressure drops + some gas becomes insoluble creating bubbles. In fluid magma bubbles
rise surface. In viscous traps gas building enough pressure create volc eruption. Style eruption
influenced by processes at different plate boundaries. Partial melting (process of different
melting point of minerals).
Partial melting Earth's mantle creates basalt. At sub zones, older + deeper slabs = greater partial
melting producing silica-rich magma.
Tsunamis
90% world's tsunamis in Pacific Ocean as associated with sub zones.
Earthquakes and resultant hazards
Equake = series vibrations/seismic waves (shock) that originate from focus (point where plate
suddenly releases tension). Epicentre = point on surface of Earth above focus of equake. Large
equake preceded by smaller tremors (aka foreshocks) + followed by aftershocks (devastating
as damage buildings already damaged by first main shock. Seismic waves travel along surface +
through Earth.
Following quake 2 types body waves (within Earth's interior) occur. First = P-waves
(primary/pressure waves) Second = transverse S-waves (Oscillations at right-angle to direction
movement).
P-waves travel by compression + expansion, able pass through rocks, gases + liquids.
S-waves travel with side to side motion, able pass through solids but not liquids + gases as no
rigidity to support sideways motion.
1909 Yugoslavian geophysicist detected 4 kinds seismic waves:
- 2 pressure waves (slow travelling near epicentre + faster moving waves further away)
- 2 shear waves (slow travelling near epicentre + faster moving waves further away)
Hazardous Environments notes
Advanced Physical Geography Options
Cambridge International AS and A Level Geography
for Syllabus 9696
Hazards resulting from tectonic processes
Global distribution of tectonic hazards
Distribution of earthquakes
Tec. Hazards = seismic activity like earthquakes, volcanoes + tsunamis. Most World's
earthquakes occur in defined linear patterns. These chains follow plate boundaries. E.g. Centre
of Atlantic Ocean. Distinct lines of earthquakes around Pacific. Some cases broad like Nazca
subduction S. American. Broad belts associated subduction zones. Narrower plates =
constructive margins. Collision boundaries (Himalayas) associated broad belts earthquakes.
Conservative plates (San Andreas Fault) narrow belt earthquakes. Isolated earthquakes caused
by humans + isolated plumes rising magma (hotspot).
Distribution of Volcanoes
Most volcs. found plate boundaries. Some exceptions such as Hawaii (over hotspot). 550
historically active volcs. ¾ lie on “Pacific Ring Fire”. Includes new volcs like Mt. Pinatubo
(Philippines) + St. Helens (US). Most studied volcs above land but some submarine.
Volcs found on boundaries earth's major plates. Deeper levels earth not molten as pressure too
high. Plate boundaries = molten rock (aka magma) supplies volcs. In Ring Fire caused by
subduction oceanic/continental crust. Sub. in ocean creates chains volc islands (aka island arcs)
like “Aleutian Islands” (Pacific under N. American). Where sub. of oceanic crust under cont. =
formation young fold mountains. “Andes” (Nazca under S. American)
,2
Not all volcs at plate boundaries. In Hawaii found middle ocean. These islands = line increasingly
older volc islands, stretch N-W across Pacific. These volcs related movement of plates above hot
part fluid mantle.
- Mantle Plume (hotspot) = Jet of hot material rising from deep within mantle.
Responsible for volcs. Can be found beneath continents like E. African Rift Valley as well.
Produce isolated volcs. Play part in break-up of continents + formation of new.
At subduction zones, volcs produce viscous lava (erupt explosively + produce much ash). Volcs
found mid-ocean ridges (hotspots) produce fluid basaltic lava (Iceland + Hawaii).
At mid-ocean ridges, hot fluid rock from deep mantle rise cause of convection currents. Upper
mantle begin melt + basaltic lava erupts, creating new oceanic crust.
, 3
At subduction zone cold slab (ocean floor) slides down subduction zone slowly warming.
Volatile compounds (water +CO2) leave slab + move upwards into mantle so melts. Hot magma
able to rise.
Explosion occur where water meets hot rock. Water vaporises, increases pressure until rock
explodes. Gases within molten rock also pressure build up. Explosion depends viscosity of
magma. Gases dissolve easily in molten rock deep underground as pressure high. As it rises to
surface pressure drops + some gas becomes insoluble creating bubbles. In fluid magma bubbles
rise surface. In viscous traps gas building enough pressure create volc eruption. Style eruption
influenced by processes at different plate boundaries. Partial melting (process of different
melting point of minerals).
Partial melting Earth's mantle creates basalt. At sub zones, older + deeper slabs = greater partial
melting producing silica-rich magma.
Tsunamis
90% world's tsunamis in Pacific Ocean as associated with sub zones.
Earthquakes and resultant hazards
Equake = series vibrations/seismic waves (shock) that originate from focus (point where plate
suddenly releases tension). Epicentre = point on surface of Earth above focus of equake. Large
equake preceded by smaller tremors (aka foreshocks) + followed by aftershocks (devastating
as damage buildings already damaged by first main shock. Seismic waves travel along surface +
through Earth.
Following quake 2 types body waves (within Earth's interior) occur. First = P-waves
(primary/pressure waves) Second = transverse S-waves (Oscillations at right-angle to direction
movement).
P-waves travel by compression + expansion, able pass through rocks, gases + liquids.
S-waves travel with side to side motion, able pass through solids but not liquids + gases as no
rigidity to support sideways motion.
1909 Yugoslavian geophysicist detected 4 kinds seismic waves:
- 2 pressure waves (slow travelling near epicentre + faster moving waves further away)
- 2 shear waves (slow travelling near epicentre + faster moving waves further away)
