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Pyroclastic flows / surges
• They are mixtures of pulverized rock,
ash, and hot gases from the volcano,
formed when magma interacts
explosively with water.
• Pyroclastic flows can move at speeds of
hundreds of miles per hour (650mph)
and are hundreds of degrees (400°c)
• They destroy everything in their path by
crushing or burning – they cannot be
The 1980 Mount St Helens (Washington, USA) eruption killed 57 people. Those
closest to the eruption fell victim to the landslide and pyroclastic blast that was
triggered by an earthquake. The pyroclastic flow of very hot volcanic gases, ash and
pumice formed from new lava, while the pulverized old rock hugged the ground
reached speeds as high as 670 mph (and may have even broken the sound barrier). It
spread outward, devastating a fan-shaped area 23 miles across by 19 miles long; 230
square miles of forest was knocked down.
• Lava is molten rock that flows out of a
volcano or volcanic vent.
• Can have a high or low viscosity - fluid
flows are hotter and move faster than
viscous flows, which are cooler and travel
• Lava flows generally travel slowly and can
be easily avoided by a person on foot, but
cannot be stopped or diverted.
• Lava flows are extremely hot - between
1,000-2,000°C – so can cause severe
burns and burn down vegetation and
The volcano is monitored by the United States Geological Survey(USGS) who have an
observatory on the Kilauea crater rim. Lava flows are the main volcanic hazards and
VOG- volcanic gases mixing with clouds or steam. Lava flows are currently flowing from
Kilauea but it is Mauna Loa which poses the biggest risk to the largest settlement in the
South- Hilo. Currently lava flow hazard areas are mainly on the south
Volcanic ash & tephra
• Made up of a range of rock particles;
different shapes sizes and types -
combinations of pumice, glass shards,
crystals, minerals, and shattered rock.
• Ash falls cover everything, infiltrates most
openings, and is highly abrasive.
• Airborne ash can obscure sunlight, creating
• On roads can become slippery when wet,
posing risk to vehicles.
• Automobile and jet engines may stall from
ash-clogged air filters, and moving parts
can be damaged from abrasion.
• Roofs may collapse under the weight of
• Farmland covered and plants smothered
• Power systems may shut down and waste
May 18, 1980 eruption of Mount
St. Helens covered areas in ash
180 miles from the volcano. Ash
covered homes, farms, and roads
to a depth up to four-inches.
• Magma contains dissolved gases that are
released into the atmosphere during
• Gases are also released from magma that
either remains below ground rising toward
the surface, like volcanic vents, fumaroles,
and hydrothermal systems.
• Vog contributes to breathing problems and
acid rain damages crops and corrodes facing
stones on buildings and damages metal.
• C02 emissions and sulphur dioxide can also
affect the ozone layer.
Whilst most of the time lava merely oozes out of volcanic vents in Hawaii,
Sometimes clouds of steam and sulphur dioxide rise from Kilauea volcanic system.
The regular release of gas since 1983 has created volcanic smog, ‘vog’ and acid-rain
conditions on the Big Island of Hawaii.
• Areas of volcanic activity are usually in
areas of earthquakes, or are prone to
earthquakes triggered by volcanic
• This could lead to general impacts of
earthquakes, such as building collapse,
land deformation and cracks.
• Earthquakes tend to happen before a
volcanic eruption, so could be taken as
an early warning to evacuate the area.
Chances Peak in Monteserrat, a small Caribbean island, had been dormant for over 300 years.
It erupted in 1997, preceded by earthquakes – these were recognised as a sign of volcanic pre-
eruption the small population of the island (11,000 people) was evacuated in 1995 to the north
of Montserrat, neighbouring islands and the UK. However 19 people died, who had riskily
decided to stay. In many other cases, the earthquakes that come before volcanic eruptions
have acted as a warning indicator and allowed people to get to safety.
Debris, avalanches &
• Caused by intrusive magma
chamber collapse, earthquakes, or
explosive volcanic erruptions.
• Landslides commonly originate as
massive rockslides or avalanches
which disintegrate during
movement If the moving rock debris
is large enough and contains a large
content of water it could lead to the
formation of mudslides and lahars.
• The obvious hazards of landslides
are the enormous power and speed
at which the material will travel –
covering or destroying everything in
its path. Landslides into the sea may
also cause tsunamis.
The landslide at Mount St. Helens on May 18,
1980, had a volume of 2.5 km3, reached
speeds of 50-80 m/s (180-288 km/hr), and
surged up and over a 400 m tall ridge located
about 5 km from the volcano.
• Sudden, violent discharge of glacial
meltwater. Jökulhlaups occur when
glacier ice dam bursts creating a large
but quick flood, usually triggered by
• Risks of Jokulhlaups are roughly similar
to severe flash flooding risks – flooding
settlements, carrying debris, and
damaging roads etc.
On the 14th of April in Iceland, Eyafjallajökull’s explosive eruption caused
melting through the 200 m thick ice cap within hours. The rapid melting of the
ice cap produced volcanogenic jökulhlaups that cascaded on the northern and
southern flanks of the volcano. The flood reached damaged Iceland’s ring road,
and the town Markarfljót was evacuated.
• A lahar is a mudflow of debris
containing pyroclastic material, rocky
debris, and water flowing down from a
volcano. It can carry sediment as large
as boulders, and tears through
vegetation and the land and can grow
to 10 times their original size.
• Lahars are triggered by volcanic
eruptions quickly melting snow / ice on
a volcano, ejecting water from a crater
lake, but more often are due to intense
rainfall that loosens material on
An explosive eruption from Nevado del Ruiz, Colombia, in 1985 sent a series of pyroclastic
flows and surges across the volcano's broad summit. The pyroclastic flows and surges quickly
eroded and mixed with Ruiz's snow and ice. Flowing mixtures of water, ice, pumice and other
rock debris then poured from the summit and sides of the volcano into rivers draining the
volcano. lahars had travelled 100 km and left behind a wake of destruction: more than
23,000 people killed, about 5,000 injured, and more than 5,000 homes destroyed.