Download Volcanic Features Information

Volcanic Features Information
Caldera
A caldera is a bowl-like volcanic feature usually formed by the collapse of land
following a volcanic eruption. They are sometimes confused with volcanic craters. The
word comes from Spanish caldera, and this from Latin CALDARIA, meaning "cooking
pot". In some texts the English term cauldron is also used.
A collapse is triggered by the emptying of the magma chamber beneath the volcano,
usually as the result of a large volcanic eruption. If enough magma is erupted, the
emptied chamber is unable to support the weight of the volcanic cone above it. This
causes a roughly circular fracture, the ring fault, to develop around the edge of the
chamber. The collapse may occur as the result of a single large eruption, or it may occur
in stages as the result of a series of eruptions. The total area that collapses may be
hundreds or thousands of square kilometers.
Lava Dome
In the study of volcanoes, a lava dome is a roughly circular mound-shaped
protrusion resulting from the slow leak of thick (high viscosity) lava from a volcano. The
geologic chemistry of lava domes can vary from basalt to rhyolite although most
preserved domes tend to have high silica content. The characteristic dome shape is
caused by high viscosity that prevents the lava from flowing very far. This high viscosity
can occur in one of two ways: by high levels of silica in the magma, or by fluid magma giving off gases that are
dissolved in it.
Pyroclastic Flow
A pyroclastic flow is a fast-moving current of hot gas and rock
(collectively known as tephra), which reaches speeds moving away from a
volcano of up to 700 km/h (450 mph). The gas can reach temperatures of
about 1,000 °C (1,830 °F). Pyroclastic flows normally hug the ground and
travel downhill, or spread laterally under gravity. Their speed depends
upon the density of the current, the volcanic output rate, and the gradient of the slope. They are a common
and devastating result of certain explosive volcanic eruptions.
Lahar
Lahar is an Indonesian term that describes a hot or cold mixture of water
and rock fragments flowing down the slopes of a volcano and (or) river
valleys. When moving, a lahar looks like a mass of wet concrete that carries
rock debris ranging in size from clay to boulders more than 10 meters (33
feet) in diameter. Eruptions may trigger one or more lahars directly by
quickly melting snow and ice on a volcano or ejecting water from a crater
lake. More often, lahars are formed by intense rainfall during or after an eruption--rainwater can easily erode
loose volcanic rock and soil on hillsides and in river valleys.
Blue Group: Mt St Helens
Volcanic Eruption 1980
Where is the volcano?
Mt St Helens (8365 ft , 22,550m) is in the Cascade Volcanic
Range in Washington State, USA. The Cascade Range has
160 volcanoes and Mt St Helens is considered the most
active, and lies 100 miles south of Seattle and 50 miles from
the coast at Portland
Why is there a volcano here?
Mt St Helens is a stratovolcano within the Cascade
Volcanic range in Washington State and had been
dormant for 100 years prior to 1980. As an Eastern
remnant of the Pacific Plate, the Juan da Fuca
plate, has subducted beneath continental North
America the Cascade volcanic chain has formed.
The Mt.St.Helens volcano has been active for
around 40,000 years but the now damaged cone we
see today has formed over the last 2,200 years
Eruption Style
The volcano is highly explosive and erupts in the Plinian style like Vesuvius. Sudden violent
explosions are accompanied by towering clouds of hot ash, dust and rocks thrown high into
atmosphere. Pyroclastic flows and volcanic mud flows called Lahars flow down the mountain at speed
and consume the surrounding landscape. Lavas from basalt to andesites have been erupted forming
layers of ash and lava. In many stratovolcanoes a central summit crater has dome of intermediate lava
type which is squeezed out of the main vent, similar to Montserrrat. In 1980 this did not occur and
instead a crypto dome formed (crypto means hidden) causing the Northern slope of the volcano to
bulge and grow. From mapping the deposits around the volcano it seems that lateral blasts and side
domes were not unprecedented.
Sequence of Events
On May 18 1980 Mt.St.Helens produced an enormous horizontal
(sideways) blast which completely destroyed everything for 20
miles north of the volcano. The elevation of mountain peak
decreased from 2930m to 2550m and a gaping crater 1.5km wide
was created. The sequence of events that occurred had not been witnessed before as until then
geologists had always seen volcanoes erupt upwards but this was different this was sideways!
In March 1980 earthquakes and a steam eruption column alerted the United State Geologic Survey
(USGS) that the volcano was certainly waking up. Throughout the spring the mountain continued to
splutter and steam the northern side of the mountain was bulging out by 1.5m/day. A 5 miles exclusion
zone was placed around the volcano and famously Harry Truman owner, of the Spirit lake lodge resort,
refused to leave.
On the morning of May 18 Geologist David Johnson was on a ridge 10km north of the volcano when a
5.1 earthquake struck. Suddenly the bulge on the north side of the volcano slid downwards in a
massive landslide at 300km/hour and crashed into Spirit lake causing an enormous wave of water the
rest of the landslide ploughed down the Tootle river valley and mixed with vast quantities of lake
water and ice . Davids last message was to his colleagues in the nearby Vancouver USGS base ( not
Canada) "Vancouver,Vancouver .. this is it!"
The landslide uncorked the magma in the crypto dome
and released the pressure. The gas in the magma
instantly expanded and shattered the rocks at
supersonic speed creating a lateral of blast ash and hot
rocks which overtook the landslide at reached speeds
of 1,000km/hour (670 miles/hour) . The blasts power
has been estimated at 500 times that of Hiroshima.
600km2 of forest was blown away and trees combed
down like hair, sap exploded in trees in the intense
heat. Trees up to 20 miles away were engulfed in the
blast 4 times the distance of the exclusion zone.
The third string to Mt.St.Helens bow was a 19km (12miles) high eruption cloud which powered
vertically out of the volcano in a matter of minutes. The cloud turned day to night and ash fall was
heavy. Lightning crackled around the cloud.
Mudflows or Lahars formed within minutes of the
eruption. lake water, melted glacier ice, hot ash and
debris all combined to send hot ( 29-33C) cement
like flows down the Toutle river. Pyroclastic flows
started to form from the crater just after the blast as
fresh magma came into contact with the air.
Material appeared to overflow the craters edge and
flow down the Toutle valley as a grey turbulent
cloud.
Harry Truman and David Johnson along with 55 other
people did not survive. Both have had ridges named
after then within the Mt.St.Helens national monument.
The area within the monument is being left to recover
naturally. Within 8km of the volcano all trees were
obliterated, from 8km to 30km flattened and outside this
dead but left standing. Outside the boundary logging
companies have replanted trees.
Mount St. Helens showed significant activity on March
8, 2005, when a 36,000-foot (11,000 m) plume of steam
and ash emerged. A new dome is growing and will eventually fill the 1980 crater.
Effects and Aftermath
Rescue helicopters looking for survivors were grounded or
could not land due to ash. Ash removal from roofs and roads
was costly and lengthy costing 2 million dollars just in one
town(Yakima)
Hazard Management
All volcanoes in the Cascades are monitored by the Cascades Volcanic Observatory CVO part of the
United States Geological Survey USGS from their base in Vancouver, not Canada but Washington
State USA. On March 27th 1980 a phreatic eruption signaled the beginning of an active phase. The
CVO moved in a team of Geologists to monitor the volcano. Small eruptions of steam and ash blasted
from the old summit dome continued and attracted attention from the press and tourists. Evacuation
plans were prepared and roads closed leading to the volcano. As the mountain swelled with magma,
and bulged outwards at a rate of 1.5m/day, it was obvious that the Spirit Lake area to the north of the
volcano needed to be evacuated. The Washington State Governor placed a 5 mile red zone around the
summit only Harry Truman an 83 year old resort owner refused to move. When the volcano erupted on
May 18th the lateral blast extended 20 miles 4 x the exclusion zone.
Red Group: Kilauea Volcano Hawaii
Where is the volcano?
Located in the south of the Big Island within the
Hawaii Volcanoes National Park. Kilauea sits on the
shoulder of Mauna Loa and the summit is only 1247m
(4091ft). There are no steep cinder cone slopes here as
this is a shield volcano and slopes are very low angled
and gentle due to successive lava flows. Kilauea has
been active for 23,000 years and has been continuously
erupting since 1983 ; 90% of the volcano is less than
1,100 years old. The summit caldera is around 11
miles in circumference and contains the Halema'uma'u
crater (active 1974), the home of Pele, and the Kilauea
Iki (little Kilauea) crater which was active in 1959. The floor of the caldera contains lava flows from
the 1970s and 1980s, the most recent being lava from 1982.
The Pu'u' oo ( "Poo-oo Oh Oh") vent on the southern
slope of the volcano ,along the south eastern rift zone,is
the site of current activity and first became active in
1983 with lava fountains 460m high.
When has Kilauea erupted?
Kilauea first emerged from the sea floor around 500 000 years ago as the next in the Hawaiian chain of
volcanoes. There have been 61 historical eruptions with the oldest dated rocks being 23,000 years old.
Since 1952 there have been 34 eruptions and Kilauea has been erupting continually since 1983. Firstly
with the eruption of Pu oo oo vent and now from the Kamoamoa Fissure which erupted in March 2011
generating 2.5million cubic meters of lava/day. In 1990 the towns of Kalapana and Kaimu were totally
destroyed by lava flows from the Kupaianaha vent plus a long stretch of Route 130,the coastal road,
which now abruptly dead-ends at the lava flow.50 ft of lava covers the towns. Homes have also been
destroyed in 2008,2010 and 2011. In March 2008 the summit crater of Halema'uma'u produced its first
explosive event since 1924, showering the summit area in debris and causing the closure of roads to
tourists. In 2011 the crater has a lava lake within a collapsed section of the cater floor.
Why is there a volcano here?
Hawaiian volcanic activity is due to a Mid-ocean Hot Spot
or mantle plume firing a jet of hot rock at the base of the
Pacific Ocean crust over the past 70million years to form
the Hawaiin-Emperor Semount chain . The jet of hot rock
punches a hole through the crust and creates a volcano
which eventually emerges from the ocean as an island. As
the Pacific plate has moved to the NW over the Hot Spot
at a rate of 5-10cm /year, islands have moved away from
the Hot Spot and new volcanic islands have been created.
The older volcanic islands erode until they become coral
atolls and eventually disappear beneath the waves to become sea mounts. The entire chain is 5000km
long the oldest sea mount being near the Aleutian trench near Alaska and the youngest volcano is
Lo'ihi, which is still submerged off the coast of Big Island( Hawaii Island).
Eruption Style
Hawaiian volcanoes erupt basaltic lava which has low viscosity and erupts at around 1000C. Most
eruptions are gentle effusive lava flows which can travel far from the vent. Spectacular lava fountains
can also be produced, as in the Kilauea Iki eruption of 1959 which reached 320m high.
Hazard Management
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 and south east
coast. Hawaii is also prone to tsunamis which
originate from around the Pacific Ring of Fire .
Tsunamis from Chile (1960), Alaska (1949/64) and
Japan. The Pacific Tsunami Warning Centre is
located in a volcanic crater on Oahu. Key to map here.
Hawaii volcano observatory
Black Group: Montserrat Volcanic Eruptions /
Soufriere Hills Volcano
Where is the volcano?
The Soufriere Hills volcano lies in the small
Caribbean Island of Montserrat in the Antilles
Islands. The Island is a British Protectorate and
lies to the SW of Antiqua. The Island at its largest
is no more that 12km by 8km and before the
eruption had a population of 11000.
When was this eruption?
The current eruptive phase began on 18th July 1995 when large
billowing clouds of ash and steam erupted from Soufriere Hills
volcano in the south of the island. The eruption has left the southern
two thirds of the island uninhabitable and it remains so to this day.
Pyroclastic Flows still pour down the slopes of he volcano. Check out
the latest eruptions at the MVO- Montserrat Volcanic Observatory
Why is there a volcano here?
The Soufriere Hills volcano is situated above a destructive plate margin, or subduction zone which
forms the Peurto Rico Trench. Oceanic crust from the North American plate is sinking under the
Caribbean plate, forming the Antilles volcanic island arc. As the slab of crust descends, sediments,
water and the heat of the mantle cause the wedge of mantle above the slab to melt. The molten rock is
less dense than the surrounding crust and rises to the surface. The magma formed at a depth of around
6km, with a temperature of 820–885°C, and then partially crystallized before a new injection of deep
magma boosted it towards the surface.
At the surface, a thick viscous dome of lava appeared in English’s
Crater in the Soufriere Hills. Most of the gas in the lava had
already escaped but some remained in pockets, which caused
explosive eruptions.
The volcano is a strato volcano, composed mainly of consolidated
ash layers from previous eruptions. Initially pyroclastic flows
flowed eastwards from the open crater down the Tar River valley,
but as the dome grew, eventually flows were able to come down
any side of the volcano. The flows particularly followed four
main river valleys: Tar River, White River, Mosquito Ghaut and
Fort Ghaut. Where the flows reached the sea they created new
deltas of land e.g. Tar River valley delta.
Eruption Style
The volcano is a strato-volcano . Pyroclastic Flows and eruption
columns characterize the Soufriere Hills volcano . Such flows occur
when the dome collapses or explodes. Tonnes of hot rock, lava and
ash explode from the crater in a fast-moving cloud. The cloud can
race straight down the mountainside, or if there is a large vertical
explosion, which creates an ash column, come crashing down as the
ash column collapses. Such clouds have two parts: on the ground,
an avalanche of large boulders and rocks, and above this a cloud of
ash and gas with temperatures of 400°C+. The cloud moves at
speeds of 100mph, obliterating and incinerating everything in its
path.
Sequence of Events











1992/1994 Earthquake swarms first detected
1995 July Phreatic eruption of ash and steam
August 5,000 evacuated as a precaution
December Plymouth evacuated as new dome
grows.
1995 Nov New dome grows
1996 January English’s Crater and rapidly
buries old dome. Residents allowed to return
during quiet phase. March–Sept First
pyroclastic flows down the Tar River valley, creating a new delta in the sea. April South of island
evacuated. Plymouth abandoned.
1997 June 25/6 Most devastating flow yet down NE slopes of volcano. 19 people killed who had
returned to their homes in the danger zone.9000m high eruption cloud.
1997 August Pyroclastic flows destroy Plymouth, Montserrat’s capital 3 miles from crater. Permanent
relocation of inhabitants away from island is considered by British government. Exclusion zone
expanded, residents advised to seek shelter, wear helmets and masks outdoors. Warning of
cataclysmic eruption.
1997 September Montserrat’s Volcanic Observatory advises all residents to move as far north as
possible immediately. 5,000 now live in shelters. Flows destroy Bramble airport 3.5 miles from crater.
People may be moved forcibly from the exclusion zone.
1997 October Eruptions approximately every 9 hours. Ash and tephra land in safe zone. Dome is 250ft
above old crater and growing at 280ft3/second.
1997 November Dome collapse remains a threat, causing large eruptions.
1997 December Boxing Day eruption. Pyroclastic flows down the White River valley. Eruption clouds to
36,000ft.
1999 July 20 Large eruption from dome collapse. Eruption cloud to 35,000 ft. Threat of dome collapse
is still real. People largely out of danger unless in the exclusion zone. Restructuring of the north
continues.
Effects and Aftermath
Montserrat’s population have had their lives completely changed. Their economic, administrative and
cultural capital lies in ruins like a modern Pompeii. Jobs, schools and community activities have been
disrupted. At one point 5,000 people were living in temporary shelters in the north, in some cases for
over two years. The north of the island had few settlements of any size and found it difficult to cope
with the influx from the south. Salem is now being repopulated and is being developed together with St
Johns as new population centers. The following are some of the problems:









Plymouth contained all the main services – hospitals, banks, schools, government and many private
businesses.
collapse of the tourist and rice processing industries
unemployment has risen from 7% to 50%.
agriculture is at a standstill. • respiratory problems caused by ash.
psychological problems and discontent with relocation package
traffic congestion
shortage of available building land
70% rise in rent for accommodation
skills shortage as people left.
Hazard Management
Once the volcano was deemed dangerous to
life. Evacuations plans and Hazard maps
were put into place. The maps contained
areas of increasing risk including the highest
risk area- the exclusion zone. As the eruption
progressed the zones were modified until the
southern two thirds of the island was
declared an exclusion zone.
Monitoring






An extensive seismograph network was
established around the volcano to
measure earthquake strength and
depth.
Earth deformation meters and tiltmeters
were also put in place to show any signs
of the ground swelling or deflating as
magma rose within the volcano.
A satellite location GPS (global
positioning system) was also used to check ground movements.
An instrument called COSPEC was used to measure gas emissions from the volcano, particularly
sulphur dioxide which is a good indicator of magma type and movement.
The pH of rainwater was monitored as it gives an indication of the magmatic gas content.
Geologists regularly flew into the most dangerous areas to sample new flow deposits, measure cracks
caused by the volcano swelling and check the growth of the dome.