global distribution of volcanoes
- located along plate boundaries or in the middle of the plate away from plate boundaries
- shield volcanoes usually found at constructive (divergent) plate boundaries
- composite volcanoes usually found at destructive (convergent) plate boundaries
how are volcanoes formed
- magma forces its way between two tectonic plate boundaries
- lava cools and solidifies and turns into rock. years later, magma forces its way up again
- process repeats and cooled lava forms layers of rock
- in between, volcano spills out ash and steam. ash settles onto volcano and is cemented into rock
- over millions of years, layers build up to form a volcano
process of eruption of a volcano
- preparation phase
- in the volcano, magma rich in dissolved gas fills the magma chamber. tremors are often felt at the surface. the volcano expands - explosion
- the explosion of pent up gases pushes magma up the conduit. fused particles fall as ash. the pressure enlarges the conduit - eruption ends
- the magma chamber empties. the walls of the conduit are weakened and collapse, creating a crater/caldera
characteristics of shield volcano
-formed by frequent eruptions of thin runny lava
volcano
- broad-based
- cone-shaped
- gentle slopes
lava
- fluid
- basic
- flows fast(er than acidic lava)
- cools and solidifies slowly
eruptions
- frequent but gentle
- less powerful due to easier release of gases
- liquid lava emitted from a (large) central vent (which sometimes has a collapse caldera)
characteristics of composite volcano / stratovolcano
- more viscous lavas with large pyroclastic debris; large, emitted from central vent
volcano - cone-shaped
- concave
- alternate layers of acid lava and ash+cinder
eruptions - violent
- lava may escape through secondary cones
lava - viscous
- acidic
- flows slowly
- presence of ash and cinder
shield vs composite volcano
frequent gentle eruptions vs rare violent eruptions
broad-based vs concave
fluid, less viscous (basic) lava vs viscous (acidic) lava
basic lava vs acid lava
low viscosity, hot (1200ºC) and runny vs viscous, less hot (800ºC) and slows slowly
lower silica content vs higher silica content
takes longer time to cool and solidifies (hence flows longer distances) vs soon cools and solidifies (hence flowing short distances)
eruptions frequent but relatively gentle vs produce steep-sided, more localised features
- lava and steam ejected vs ash, rocks, gases, steam, lava ejected
primary effects of volcanic eruptions (volcanic hazards)
pyroclastic flow (composite), volcanic gases (both), volcanic bombs, tephra (composite), lava (both)
what is pyroclastic flow
- dense cloud of fast flowing volcanic ash and gas
- reach temperatures between 200-800ºC
- reach speeds of 100km/h
what are volcanic gases
- poisonous gases released during eruption
- include carbon dioxide, carbon monoxide, hydrogen sulphide, sulphuric dioxide and chlorine
- can cause climate change and breathing problems
- in composite volcanoes, impacts are worse as gases travel higher and further and impurities in lava create more gas
what are volcanic bombs
large volcanic rocks
what is tephra
- solid material of varying grain size from ash to volcanic bombs ejected into the atmosphere
- can bury crops, contaminate water sources, cause transport and breathing problems
- contribute to climate change
what is lava
- liquid rock that reaches very high temperatures
- travel slow enough to escape from but tremendous temperatures burn and destroy fixed items (impact more serious for shield volcanoes)
secondary effects of volcanic eruptions
lahar (composite), landslides, acid rain, flooding, tsunamis, climate change
what is lahar
- ash mixing with water (snow melt and lakes) to create a fast flowing river of volcanic mud down the steep sides of volcanoes
- destroys / buries everything in its path
what are landslides
- collapsing mountain material
what is acid rain
rain mixed with volcanic gases that can damage crops
what is flooding
- melting of glaciers and ice caps
what are tsunamis
- giant sea waves generated after violent caldera-forming events
what is climate change
- ejection of vast amounts of volcanic debris can reduce global temperatures
example of damage caused by pyroclastic flow
During the 1902 eruption of Mont Pelee in Martinique, pyroclastic flow demolished the coastal city of St. Pierre, killing nearly 30,000 inhabitants.
examples of damage caused by lahar
On November 13, 1985, the eruption of Nevado Del Ruis produced an enormous lahar that buried and destroyed the town of Armero in Tolima, causing an estimated 25,000 deaths.
examples of damage caused by tephra
During the eruption of Rabaul in Papua New Guinea in 1937, 375 people were suffocated or buried by tephra fall.
examples of damage caused by volcanic gas
The 1991 eruption of Mt. Pinatubo is thought to have injected more than 250 megatons of gas into the upper atmosphere on a single day. The eruption of Santa Maria volcano in October 1902 killed 350 people due to inhalation of deadly fumes.
Eruption of Mount St. Helens in 1980 led to 57 people deaths, most of them probably from asphyxiation after inhaling hot ash.
