Results from build-up of molten material emitted onto surface through fissure in
crust.
Active volcanoes: erupted in last 80 years
Dormant volcanoes: inactive but may be active again
Extinct volcanoes: will not erupt again
Primary hazards: Gases
water vapour is 80% of gases emitted and after
conversion to rain its responsible for mudflows and
lahars
CO2 (200m tonnes a year) sulphur dioxide (corrode aircraft) hydrogen sulphide
(toxic)
long period of inactivity and highly vicious magma causes solid plug to grow in
volcano vent that highly gas charged lava explodes sideways out of weakness in
flank of volcano in a nuée ardent.
very hot incandescent cloud, composed of gas and tiny fragments of solid material,
moves rapidly down the slope, keeping contact with it.
temps can reach 1000 degrees Celsius.
only 2 out of 30 000 survived in the Caribbean in 1902.
Primary hazards: Liquids
lava is liquid material that flows out of volcano crater and rock it forms when
solidified.
originates from magma, molten rock beneath the surface, which rises from a
magma chamber beneath the volcano through a vent to reach the surface.
most dangerous flows are runny basalts, occur at constructive plate margins and
oceanic hot spots
move at 50km an hour, anything in their path is rapidly covered.
fastest basalt lavas are of the aa type, formed when lots of lava erupts fast.
aa lava is thick, unto 10m deep, with a surface that breaks into rough clinkers as it
moves.
steep sloping front moves forward as a unit, with sudden, dangerous surges of
speed, destroying anything it touches
slower flowing pahoehoe lava is less than 2m thick as forms when low volumes are
ejected slower.
solidifies while moving, smooth surface has curved flow lines, giving a rope like
appearance.
slows in individual lobes, moving around obstacles, setting flammable objects on
fire.
less vicious than aa so cools slower and flows further
Primary hazards: Solids
pyroclastic material is solid particles that reach the ground in pyroclastic falls
(airborne pyroclastic material is known as tephra)
some of it is the shattered remains of the plug that solidified in the vent after the
last eruption.
other pieces form when molten lava solidifies while in the air.
solid materials are categorised by their size and shape: ashes, the smallest, are
less than 4mm in diameter while cinders are about 4-5mm in diameter and lapilli
are pebble sized.
volcanic blocks are large angular fragments resulting from the shattering of solid
lack during an eruption, whereas volcanic bombs are rounded as they form as
molten lava cools while spinning through the air.
all ejections of pyroclastic material can be dangerous as it’s hot
heavy and ash can cause roofs to collapse and damage crops, machinery,
electronics and lungs
heavier particles fall to the ground within 3km of the vent, the lighter particles can
rise high into the atmosphere where they can damage aircraft engines.
the enormous ask cloud emitted by Eyjafjallajökull in 2010 drifted over Europe from
Iceland and disrupted flights for over a week.
more than 100 000 flights were cancelled, costing airlines $US 2.5 billion
when lava domes collapse hot dry rock fragments and gases move rapidly away
from the vent down slopes and valleys by gravity
these pyroclastic flows kill almost half the people who die as a result of volcanic
eruptions
highly destructive nature results from their high density, fast speed of travel, long
distances covered and intense heat
density of materials is lowest at the top of the cloud and greatest in base flow,
which means the densest part containing boulder — sized fragments moves in
contact with the ground and destroys everything it makes contact with
above the base flow hot gases keep the ash fragments buoyant
heat sets buildings forests and crops on fire and people/animals on edges of flows
die from breathing in the hot gases
wide areas are buried by hot pyroclastic debris up to 200m thick that often welds
together
if loose, can provide material for equally hazardous lahars to occur if water is
added to the deposit
nuées ardentes are categorised as a special type of pyroclastic flow being
characterised in their incandescent ashes
that glow in the dark
Types of volcano
Cinder cones: form when gas charged lava
blobs are thrown in the air and break into
fragments. e.g. Lava Butte, Oregon. during
growth it blocked and diverted the
Deschutes River.
Shield volcanos: formed of basic lava,
containing less than 50% silica.
consequently, it is fluid, flows long
distances and solidifies slowly. if issues from fissures, forms extensive plateaus.
e.g. Deccan in India. result when there’s a central vent. big, wide bases and gentle
slopes. form along constructive plate margins. Mauna Loa is largest active volcano,
formed at hot spot. rises 9000m from ocean floor to summit.
Stratovolcanoes: form at convergent (destructive) boundaries where magma gains
added silica as it rises through continental rocks. lava is intermediate between
acidic and basic, more viscous than shield, so steeper sides with a concave shape.
base is narrower than shield. dangerous as have long dormant periods so people
don’t always follow warnings of evacuation. in dormancy a thick plug of solidified
magma builds up in the vent and as high viscosity, pressure builds to unblock vent,
usually culminating in an explosion to shatter the plug into pyroclastic fragments
then lava outpourings. parasitic cones form on volcano sides when vent is blocked,
and magma goes out another way
Lava domes: small with steep sides and rounded tops. form on slopes or craters of
stratovolcanoes. composed of silicic lava, too viscous to flow far. solidifies quickly
forming thick crust. domes swell as grow from within. very explosive eruptions
when they collapse. e.g. Mt pelée before 1902 eruption.
Types of eruption
Icelandic: fluid basalts issue quietly from fissures at mid ocean ridges
Hawaiian: fluid basalts issue from vents. Gases escape easily and quietly with
occasional spurts of gases from lava lakes causing lava fountains
Strombolian: less fluid lava, gases escape with moderate explosions in which lava
bombs are ejected
Vulcanian: more violent as more viscous, solidifies faster and traps gases. Ashes
and cinders emitted when pressure released. Dark ash laden clouds rise to form a
cauliflower shape.
Vesuvian: viscous magma gains high gas content during long inactivity periods and
deep plug forms which blown off after pressure built up. Violent eruption sends a
wide dark ash cloud and then falls.
Plinean: very explosive, gas ash and pumice results in narrower cloud that extends
into stratosphere
Pelean: highly viscous magma and long period of inactivity cause an explosion out
of weakness in side with nuee ardente falling down volcano
Volcanic explosivity index: scale from 0-8. Log arithmetic. 4 and above send
materials into stratosphere and capable of global cooling
Secondary volcanic hazards
Lahars: mudflows. Form when water mix with loose pyroclastic material. Size
varies. Forms flows (like wet concrete) down slopes and valleys. Water source can
be intense rainfall with eruption, glacier/snow melt or dam failure. Dense and
viscous but flow fast. More volcanic material, faster and more destructive. Common
on steep slopes of stratovolcanoes where strong gravity pull but also on shield
volcanoes. Increase in size as gain more water and erode material, die out far from
volcano.
Volcanic landslides: rock masses and soil moving downslope under gravity. Can be
dry/wet but drier than lahars. With water can transform into lahar if more than 3%
fine clay particles. Originate as large rock slabs but disintegrate into smaller
particles as move down. Size and speed so can rise over ridges. Result when
magma forces to surface and pushes outwards, causing rock to break/over-steepen
and collapse by gravity down sides of stratovolcano. Or triggered by large EQs
beneath volcano. Trigger explosions by removing lid on gases and rising magma.
Bury valleys and cause lahars as water drains down valley from them. when
landslides block tributary valleys, they impound lakes which can break through
dam and cause further lahars.
Nevado del Ruiz, Columbia
Stratovolcano in Andes. Nazca plate subducted by south American plate.
Prediction and monitoring: September 1985 hazard map shows danger locations
from ash rocks falls lahars. Amero town people weren’t made aware it was
included. October risk of lahars and local authorities advised to prepare
evacuation. 13th November seismograph recorded strong EQ, but people
monitoring didn’t see warnings.
Eruption hazards: ash eruptions 3pm 13th November, stopped for 4hrs so people
told stay inside. 9pm 30km ash cloud. Civil defence workers tried to warn Amero
but couldn’t contact. Eruption melted summit snow and glaciers, thick lahars down
narrow river valleys at 60km/hr, volume x4 larger.
Effects on lives and people: 1st lahar boosted by river passed through. Hit Amero
3hr after eruption, others followed up to 30m deep. People suffocated by bus and
crushed by collapsing buildings. 23,000 deaths, 20,000 in Amero, 2nd highest
deaths and 1st highest from lahars.
Risk perception: dormant for 69 years as no eruption in 140 years so reluctant to
take expensive preventative measures.
Mount St Helens, USA
Stratovolcano, subduction of Juan de Fuca plate under north American plate. 97%
more ash than Nevado del Ruiz. Biggest volcanic landslide in history.
Prediction, monitoring and perception of risk: us geological society monitor closely.
Draw hazard maps and watch gas emissions. Lasers watch ground deformation in
1980 but now GPS. Seismographs record magnitudes and send to lab. Fairly
accurate predictions. March 1980, shallow EQ north different from usual. Harmonic
tremor of continuous rhythmic EQ often come before eruption so extra
seismographs and evacuation plans by emergency services from hazard maps. Ash
eruption for few weeks and north flank bulged. Not allowed within 13km. residents
moved away, and plans made for further down valley. End of April, bulge grown
into a sizeable dome.
Eruption hazards and effects on lives: 18th may north flank broke open. Summit and
flank fell as landslide. Horseshoe shape crater left. Landslides down valleys and
over ridges. Rock deposited in north fork Toutle river valley. Landslide stopped
moving, water content melted snow and ice rose to surface and mixed with loose
rock to form lahars, destroyed 8 bridges and evacuation routes. Ships couldn’t sail
of Columbia river because of debris. After landslide, cloud of gas rock ash and ice
up and to side. Pyroclastic flows down sides and covered 550km. helicopters
rescued more than 100, 57 died. Ash had to be cleared from roads. PTSD for years
after.
Managing volcanic hazards
Prediction and monitoring: remote sensing by satellite sensors detects deformation
of surface/temp change caused by ground heating before eruption. Gravity metres
on ground detect gravitational changes due to magma movement. Geochemical
changes indicate eruption. Thermal waters have more CO2 H2 and He dissolves.
Chemical sensors measure increased sulphur.
Action taken: cold water hosed onto lava flows to cool them and reduce flow
length. Lava flows bombed and diverted away from settlements by channels. Large
concrete blocks dropped into the lava tube from which lava was flowing.
Reasons for living near tectonic hazards: hazard not predicted, people unaware of
danger, don’t want to leave birthplace family/friends, perception risk is that it
won’t happen to them, fatalistic attitude that it can’t be controlled so have to
accept. Economic reasons, hazard attracts tourism and population growth,
geothermal energy so cheap heating, people believe benefits are greater than
costs.