Volcanic Hazards
LAVA FLOW
 Product of quiet effusion of molten
rock or magma from beneath a volcano
 When it comes out from a volcanic
vent, its temperature ranges from 700 to
1,200 C.
Lava effusion
 When magma erupts onto the
surface and flows away from a vent, it
creates a lava flow. Erupted lava can be
fairly fluid, flowing downhill like a fast river of
molten rock. Some lava flows are not so
fluid and travel only a short distance before
solidifying.
Explosive eruption
 Some explosive eruptions send
molten lava into the air. A lava fountain can
accompany basaltic volcanism and results
from high initial gas content in less viscous
lava.
 The amount of gas involved is
dependent on the viscosity of magma,
which in turns depends on the composition
and temperature of the magma
 Magma with high silica content have
higher viscosity than magma with low silica
content
 Viscosity increases with decreasing
magma temperature.
Basaltic Lava
 Fastest flowing lava
 Low viscosity
 Usually associated with broad
landforms of shield volcanoes
 Lava flow styles are pahoehoe and
AA
Andesitic lava
 Usually associated with stratoVolcanoes and commonly form lava domes
 Volume of andesitic lava is small so
they do not usually go beyond the foot of
the volcano
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RHYOLITIC LAVA
lava flow have higher viscosity
Associated with violent eruptions
involving pyroclastic flow
LAVA FLOW HAZARD ZONING
1. Viscosity
 Viscous lava discharged very slowly,
these cover only a few kilometers and form
a short, stubby lava flow
2. Effusion rate
• The higher the discharge rate, the
wider/ longer the area in which the lava will
flow
the ground. Therefore, areas subject to
this hazard from poisonous gases will be
downslope.
3. Wind directions
3. Slope of the land
• Fluid basalt flow will not go very far
from a volcano vent on gentle slopes
• On steep slopes, basaltic lava flows
can travel as fast as 10km/hr
Ways to Mitigate the Effect of Lava Flow
1. Construction of barrier and lava channels
2. Cool advancing front with water
VOLCANIC GASES
Toxicology of Volcanic Gases
PYROCLASTIC FLOW
 Hot mixtures of fresh lava, gas, rock,
pumice and ash that move down the sides
of a volcanic crater at high speeds during
an eruption
Ways of Pyroclastic Flow Generation
1. Soufriere type
 The eruption column can no longer
be sustained so the column collapses
forming pyroclastic flow on the flank of a
volcano
2. Merapi type (Gravitational dome
collapse)
 a dome of viscous magma grows
in the crater. Eventually it gets so large, it
collapses under gravity producing a
pyroclastic flow.
3. Pelee type (Eruption dome collapse)
a dome of viscous magma is blocking the
conduit. Eventually, it explodes under
pressure blasting pyroclastic material down
one of the flanks of the volcano.
Effects of pyroclastic flow
1. Burn
 The extreme temperatures of rocks
and gas inside pyroclastic flows can burn
people, houses and vegetation.
2. Impact and burial
 By its speed, mass and volume a
pyroclastic flow may knock down, shatter,
burry or carry away nearly all objects and
structures in its path.
3. Inhalation of hot ash an gases
 Hot ash and gas impair breathing.
The compounds that come with pyroclastic
flows are toxic and will cause immediate
asphyxiation of people and animals
Identifying Volcanic Gas Danger Spots
1. Gas emission site locations
 Places close from emission sites
have increase hazard since airborne gases
become diluted by winds and so people
farther are seldom affected
2. Low lying areas
 These are gases (CO, CO2, SO2)
that are denser than air and thus flow near
4. Lahar and flooding
 Valleys and other low- lying areas
along the slopes of volcanoes act as
channel ways and receptors of pyroclastic
flow deposits. The thick accumulation of
loose pyroclastic flow deposits is easily
mobilized by stream flow to become part of
the lahar.
Pyroclastic flow Hazard Zoning
 Pyroclastic hazard zones largely
depends on the extent of pyroclastic flows
from previous eruptions
 Prehistoric eruptions may be few
times bigger than a volcano’s string of
historical eruptions
 Possible shifts in vent location and
slope topography should also be considered
in determining the potential extent of
pyroclastic flow
TEPHRA FALL AND BALLISTIC
PROJECTILE
 Tephra refers to volcanic rock and
lava materials that are ejected into the air by
explosions or carried upward by eruption
column’s hot gases.
Types of Tephra
1. ASH -- Very fine-grained fragments (< 2
mm), generally dominated by broken glass
shards, but with variable amounts of broken
crystal and lithic (rock) fragments.
2. LAPILLI -- Pea- to walnut-size pyroclasts
(2 to 64 mm). They often look like cinders.
In water-rich eruptions, the accretion of wet
ash may form rounded spheres.
3. BLOCKS AND BOMBS -- Fragments >64
mm. Bombs molten when ejected and
assume various shapes upon cooling.
Blocks are large broken pieces of solid vent
material or surrounding rocks.
4. PUMICE - Frothy felsic rock formed by
vigorous vesiculation (bubbling) due to rapid
gas escape.
5. Scoria -- Scoria is a vesicular mafic rock,
commonly of lapilli to bomb size. It
accumulates as scoria air-fall deposits
derived from Strombolian-type eruptions.
6. PELE'S TEARS -- Molten basaltic lava
quenches in the air to form teardrop-shaped
glass lapilli. Named after Pele, the Hawaiian
goddess of fire.
7. PELE'S HAIR -- These thin filaments of
stretched out basaltic glass form during
times of high fire-fountaining, often in the
presence of strong winds.
DANGERS FROM TEPHRA FALL AND
BALLISTIC PROJECTILE
Tephra Fall and Ballistic Projectile
Hazard Zoning
 How far the projectiles go from the
vent partly depends on the size of ejected
fragments.
 Ballistic projectiles had been found
to rarely go beyond 5 km from the vent
 As the distance from the vent
increases, the volume of the particles from
the tephra decreases
 As the distance from the vent
increases, the ash deposit thickness
decreases
LAHAR
 Is the process wherein wet cementlike mixture of volcanic material and water
flows down the slopes of a volcano. Lahar
usually carries fresh eruption material like
pyroclastic flows and tephra fall.
 Lahar are generated when rainwater
erodes loose volcanic material deposited on
the upper slopes of a volcano or along the
path of a river on its way down the slopes
Many Faces of lahar