adapted to html from notes by
Cliff Ollier and Stephen A. Nelson
Basic Lava Volcanoes
Acid Lava Volcanoes
Nested scoria cones
Maars (tuff rings)
Mixed Eruption Volcanoes
Parasitic cones (also called adventive cones and secondary cones)
see... Volcanic Hazards and case studies for more links
see... Volcano types for more links
In popular imagination volcanoes are always symmetrical cones like Fujiyama, but in reality volcanoes vary widely in shape, size, and composition.
As a first classification we shall consider the common forms of volcano associated with basic lava, acid lava, scoria eruption and mixed eruption.
Figure 4: Basaltic Cone types
Parasitic cones, flank eruptions, and fissure eruptions are commonly associated with shield volcanoes. 'Shield volcano' is a less exact term that usually refers to a lava shield, but may be used for a large strato-volcano or volcanic complex.
Beerenberg on the Arctic island of Jan Mayen, for example, consists of a broad basalt lava dome some 15-24 km in diameter at sea level, on the crest of which is a lava cone with steep 450 slopes, some 5 km in diameter and 750 m high (Fitch, 1964). In the Victorian volcanic province a final stage of scoria eruption often obliterates lava cones formed earlier. In Iceland a rim of scoria round the crater is charactenstic.
The Kolotta Dyngja, a typical Icelandic volcano, rises to a height of 460 m with an average slope of 70, approaching 80 towards the summit. The volcano has a diameter of 5 km and the crater a diameter of 550 m. A ring of scoria about 15 m high surrounds the crater, and has slopes of up to 30 degrees.
Figure 5: Acid Lava forms
The main part of Lassen Peak, California, is a large-scale example, rising 800 m above pyroclastics and having a diameter of 2.5 km.
In the Mullaley district of New South Wales there are two simple cumulo-volcanoes with the form of domes and three similar structures that are breached by trachyte (Wilshire and Standard, 1963) Mt Macedon, Hanging Rock, and Brocks Monument, Victoria, are examples of fairly small cumulo-domes.
In the Central Volcanic Region of the North Island of New Zealand there are many hyolite domes, akin to cumulo-domes, but with more mobile flows or coulees on their flanks. These appear to erupt in the late stage of a volcanic cycle after vast ignimbrite flows and caldera formation.
Sometimes several cumulo-domes may coalesce. Tauhara in New Zealand is a multiple volcano of late Pleistocene age consisting of five coalescing dacite cumulo-domes. Internal flow structures suggest that each dome was formed from lava continuously extruded, but each had a separate vent.
The Tarawera Rift explosion of 1886 exposed excellent sections through a number of domes, enabling a better interpretation of internal structure than is usually possible. There is well-developed circular jointing at the centre of domes, becoming vertical towards the edge and in the coulees. The tops of both domes and coulees are very irregular due to fissuring.
The puys or volcanic hills of the Puy-de-Dome landscape of Auvergne are typically scoria cones with craters, but some, such as the craterless Grand Sarcoui, are trachytic cumulo-domes, and the term puy is occasionally and unfortunately used to mean cumulo-dome.
The Pitons of Carbet, Martinique, are thought to be plug domes, and Merapi, Indonesia, is an active volcano which builds successive plug domes that are explosively destroyed.
Plug domes can grow rapidly, but during growth they are shattered by explosions and broken by uneven growth, and the accumulation of broken spines and extrusion ridges causes many plug domes to be covered by a jumble of debris, which make a scree-like deposit around the flanks with rocks piled up at their angle of rest.
8: Scoria volcanic types
Scoria cones may be built very rapidly. Monte Nuovo near Naples, Italy, was built to a height of 130 m in a single eruption lasting a few days in 1538. Barcena, on the island of San Benedicto, Mexico, built a cone of 300m in twelve days in 1952.
In the last stages of eruption basaltic magma tends to build up scoria cones. Thus in Victoria there are far more scoria cones than other types of volcano, though the province as a whole is dominated by flows of basic lava.
Maars are usually associated with basaltic igneous activity, but andesitic maars are known in Chile.
Some strato-volcanoes are isolated, but many occur in groups. In McMurdo Sound, Antarctica, for instance, the large young strato-volcanoes, Mts Erebus (altitude 3700 m), Bird, Terra Nova, and Terror coalesce to form Ross Island.
Hekla, Iceland, is intermediate in many respects between a typical Icelandic shield volcano and a strato-volcano such as Vesuvius. It has been built by repeated eruptions from a fissure, often with several craters active at the same time.
adapted to html based on notes
of Stephen A. Nelson
Violent Eruptions and Pyroclastic Activity
Poisonous Gas Emissions
Secondary and Tertiary Effects of Volcanism
Beneficial Aspects of Volcanism
Predicting Volcanic Eruptions
Vesuvius, 79 A.D.
Mt. Pelée, Martinique, 1902
Mt. St. Helens, 1980
see... Volcano types for more links
see...Australian Volcanos for more links
Lava flows are common in Hawaiian and
Strombolian type of eruptions, the least explosive.
Although lava flows have been known to travel as fast as 64 km/hr, most are slower and give people time to move out of the way.
Thus, in general, lava flows are most damaging to property, as they can destroy anything in their path.
Control of lava flows has been attempted with limited success by bombing flow fronts to attempt to divert the flow, and by spraying with water to cool the flow. The latter is credited with saving the fishing harbor during a 1973 eruption of Heimaey in Iceland.
Pyroclastic activity is one of the
most dangerous aspects of volcanism.
Hot pyroclastic flows cause death by suffocation and burning. They can travel so rapidly that few humans can escape.
Lateral blasts knock down anything in their path, can drive flying debris through trees.
Tephra falls can cause the collapse of roofs and can affect areas far from the eruption. Although tephra falls blanket an area like snow, they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow.
Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation.
Tephra falls can cause loss of agricultural activity for years after an eruption, a secondary or tertiary effect.
emit gases that are often poisonous to living
these poisonous gases are: Hydrogen Chloride (HCl), Hydrogen Sulfide
Hydrogen Flouride (HF), and Carbon Dioxide (CO2). In 1984, CO2
gas escaping from the bottom of Lake Monoun, a crater lake in the
country of Cameroon, killed 37 people. In 1986 an even larger CO2
gas emission from Lake Nyos in Cameroon killed more than 1700 people
Secondary and Tertiary Effects of Volcanism
Volcanoes can emit voluminous
quantities of loose, unconsolidated tephra which become deposited on
landscape. Such loose deposits are subject to rapid removal
are exposed to a source of water.
The source of water can be derived by melting of snow or ice during the eruption, emptying of crater lakes during an eruption, or rainfall that takes place any time with no eruption.
Thus, mudflows can both accompany an eruption and occur many years after an eruption.
Mudflows are a mixture of water and sediment, they move rapidly down slope along existing stream valleys, although they may easily top banks and flood out into surrounding areas.
They have properties that vary between thick water and wet concrete, and can remove anything in their paths like bridges, highways, houses, etc.
During the Mt. St. Helens
May 18, 1980, mudflows were generated as a result of snow melt on the
itself, and deposition of tephra in streams surrounding the mountain.
Debris Avalanches and Debris Flows
Volcanic mountains tend to become oversteepened as a result of the addition of new material over time as well due to inflation of the mountain as magma intrudes.
Oversteepened slopes may
unstable, leading to a sudden slope failure that results in landslides,
debris slides or debris avalanches. We will cover these types
hazards in more detail later in the course and in the next lecture.
During the May 18, 1980 eruption of Mt. St. Helens, Washington, a debris avalanche was triggered by a magnitude 5.0 earthquake. The avalanche removed the upper 500 m of the mountain, and flowed into the Spirit Lake, raising its level about 40 m. It then moved to the west filling the upper reaches of the North Fork of the Toutle River valley (see map above).
Debris avalanches, landslides, and debris flows do not necessarily occur accompanied by a volcanic eruption. There are documented cases of such occurrences where no new magma has been erupted.
Drainage systems can become blocked
by deposition of pyroclastic flows and lava flows. Such
may create a temporary dam that could eventually fill with water and
resulting in floods downstream from the natural dam. Volcanoes in cold
climates can melt snow
and glacial ice, rapidly releasing water into the drainage system and
causing floods. Jokaulhlaups occur when heating of a glacier results in
rapid outburst of water from the meleting glacier.
Debris avalanche events, landslides,
caldera collapse events, and pyroclastic flows entering a body of water
may generate tsunamis (more detail on tsunamis will found later in the
During the 1883 eruption of Krakatua volcano, in the straits of Sunda between Java and Sumatra, several tsunamis were generated by pyroclastic flows entering the sea and by collapse accompanying caldera formation. The tsunamis killed about 36,400 people, some as far away from the volcano as 200 km.
Earthquakes usually precede and accompany
volcanic eruptions, as magma intrudes and moves within the volcano.
Although most volcanic earthquakes are
small, some are large enough to cause damage in the area immediately
the volcano, and some are large enough to trigger landslides and debris
avalanches, such as in the case of Mount St. Helens.
Volcanic Tremor (also called harmonic tremor) is a type of continuous rhythmic shaking of the ground that is generated by magma moving underground.
Since large quantities of tephra and
volcanic gases can be injected into the atmosphere, volcanism can have
a short-term effect on climate.
Volcanic ash can cause reflection of solar radiation, and thus can cause the temperatures to be cooler for several years after a large eruption.
The 1860 eruption of Tambora volcano in Indonesia, was the largest in recorded history. The year following the Tambora eruption (1861) was called the "year without summer". Snow fell in New England in July.
Volcanic gases like SO2 also reflect solar radiation. Eruptions in 1981 at El Chichón Volcano, Mexico, and 1991 at Pinatubo, Philippines, ejected large quantities of SO2 into the atmosphere. The effects of the El Chichón eruption were masked by a strong El Niño in the year following the eruption, but Pinatubo caused a lowering of average temperature by about 1oC for two years following the eruption.Volcanic gases like CO2 are greenhouse gases which help keep heat in the atmosphere. During the mid-Cretaceous (about 90 to 120 million years ago) the CO2 content of the atmosphere was about 15 times higher than present. This is thought to have been caused by voluminous eruptions of basaltic magma on the sea floor. Average temperatures were likewise about 10 to 12oC warmer than present.
As noted above, tephra falls can cause
extensive crop damage and kill livestock. This can lead to
Displacement of human
of sewerage and water systems, cut off of other normal services can
to disease for years after an eruption, especially if the
is not in place to provide for rapid relief and recovery.
Since this course concentrates on the damaging effects of volcanism, we won't spend too much time on the topic of the beneficial aspects of volcanism. We note here, that volcanism throughout Earth history is responsible for outgasing of the Earth to help produce both the atmosphere and hydrosphere. Volcanism helps renew the soil, and soils around active volcanoes are some the richest on Earth. Hydrothermal processes associated with volcanism produce rich ore deposits, and the heat rising around magma bodies can sometimes be tapped to produce geothermal energy.
Active Volcano - An active volcano to volcanologists is a volcano that has shown eruptive activity within recorded history. Thus an active volcano need not be be in eruption to be considered active.
Currently there are about 600 volcanoes
on Earth considered to be active volcanoes.
Each year 50 to 60 of volcanoes actually erupt.
For example, Yellowstone Caldera is
about 600,000 years old and is deeply eroded. But fumorolic
hot springs, and geysers all point to the fact that magma still exists
beneath the surface. Thus, Yellowstone Caldera is not
extinct. Other volcanoes that are deeply eroded,
smaller, and much younger than Yellowstone, that show no hydrothermal
may be considered extinct.
Because the lifetime of a volcano may
be on the order of a million years, dormant volcanoes can become active
volcanoes all of sudden. These are perhaps the most dangerous
because people living in the vicinity of a dormant volcano may not
the concept of geologic time, and there is no written record of
These people are sometimes difficult to convince when a dormant volcano
shows signs of renewed activity.
Yellowstone Caldera would be considered a dormant volcano.
Mount St. Helens was a considered a dormant volcano, having not erupted for 123 years, before its reawakening in 1980.Mount Pinatubo in the Philippines had been dormant for over 400 years before its eruption in 1991.
Mount Vesuvius, near Naples,
considered an extinct volcano prior to its devastating eruption of 79
Long - Term Forecasting and Volcanic Hazards Studies
Studies of the geologic history of
a volcano are generally necessary to make an assessment of the types of
hazards posed by the volcano and the frequency at which these types of
hazards have occurred in the past.
Once this information is available, geologists can then make forecasts concerning what areas surrounding a volcano would be subject to the various kinds of activity should they occur in a future eruption, and also make forecasts about the long - term likelihood or probability of a volcanic eruption in the area.
During such studies, geologists examine sequences of layered deposits and lava flows. Armed with knowledge about the characteristics of deposits left by various types of eruptions, the past behavior of a volcano can be determined.
Using radiometric age dating of the deposits the past frequency of events can be determined.This information is then combined with knowledge about the present surface aspects of the volcano to make volcanic hazards maps which can aid other scientists, public officials, and the public at large to plan for evacuations, rescue and recovery in the event that short-term prediction suggests another eruption.
Such hazards maps delineate zones of danger expected from the hazards discussed above: lava flows, pyroclastic flows, tephra falls, mudflows, floods, etc.
Short - term prediction of volcanic eruptions involves monitoring the volcano to determine when magma is approaching the surface and monitoring for precursor events that often signal a forthcoming eruption.
Exploration and Monitoring
- Since seismic waves are generated by both earthquakes and explosions, and since S-waves cannot pass through liquids, arrays of seismographs can be placed around a volcano and small explosions can be set off to generate seismic waves. If a magma body exists beneath the volcano, then there will be zone were no S-waves arrive (an S-wave shadow zone) that can be detected. Monitoring the movement of the S-wave shadow zone can delineate the position and movement of the magma body.
As noted above, as magma moves and deforms rocks it may be responsible for the generation of earthquakes. Thus, there is usually an increase in seismic activity prior to a volcanic eruption. Focal depths of these precursor earthquakes may change with time, and if so, the movement of magma can sometimes be tracked. In addition, volcanic tremor, as noted above, can also be indication that magma is moving below the surface.
Changes in Magnetic Field
- Rocks contain minerals such as magnetite that are magnetic. Such magnetic minerals generate a magnetic field. However, above a temperature called the Curie Temperature, these magnetic minerals show no magnetism. Thus, if a magma body enters a volcano, the body itself will show no magnetism, and if it heats the surrounding rocks to temperatures greater than the Curie Temperature (about 500oC for magnetite) the magnetic field over the volcano will be reduced. Thus, by measuring changes in the magnetic field, the movement of magma can sometimes be tracked.
Changes in Electrical
- Rocks have resistance to the flow of electrical current which is highly dependent on temperature and water content. As magma moves into a volcano this electrical resistivity will decrease. Making measurements of the electrical resistivity by placing electrodes into the ground, may allow tracking of the movement of magma.
- As magma moves into a volcano, the structure may inflate. This will cause deformation of the ground which can be monitored. Instruments like tilt meters measure changes in the angle of the Earth's surface which are measured in microradians 0.00018o. Other instruments track changes in distance between several points on the ground to monitor deformation.
Changes in Groundwater System
- As magma enters a volcano it may cause changes in the groundwater system, causing the water table to rise or fall and causing the temperature of the water to increase. By monitoring the depth to the water table in wells and the temperature of well water, spring water, or fumaroles, changes can be detected that may signify a change in the behavior of the volcanic system.Changes in Heat Flow
- Heat is everywhere flowing out of the surface of the Earth. As magma approaches the surface or as the temperature of groundwater increases, the amount of surface heat flow will increase. Although these changes may be small they be measured using infrared remote sensing.
Changes in Gas Compositions
- The composition of gases emitted from volcanic vents and fumaroles often changes just prior to an eruption. In general, increases in the proportions of hydrogen chloride (HCl) and sulfur dioxide (SO2) are seen to increase relative to the proportion of water vapor.
In general, no single event can be used to predict a volcanic eruption, and thus many events are usually monitored so that taken in total, an eruption can often be predicted. Still, each volcano behaves somewhat differently, and until patterns are recognized for an individual volcano, predictions vary in their reliability. Furthermore, sometimes a volcano can erupt with no precursor events at all.
After the catastrophic eruption of Mount St. Helens on May 18, 1980, a volcanic dome began to grow in the crater. Growth of this dome occurred sporadically, and sometimes small eruptions occurred from the dome. After several years of dedicated monitoring, scientists are now able to predict with increasing accuracy eruptions from this dome. An example is shown in the graphs, modified from figure 4.3 in your text. In the weeks prior to an eruption on March 19, 1982, the amount of seismic energy released increased, the amount dome expansion increased, tilt increased, and SO2 emissions increased prior to the event. Beginning on March 12, a prediction was made that an eruption would be likely within the next 10 days. On March 15, the prediction was narrowed to likely within 4 days, and on March 18 scientists predicted that an eruption would occur within the next two days. On March 19 the eruption did occur.
Note that eruption predictions such as in this example are only possible if constant monitoring of a volcano takes place. Monitoring is an expensive endeavor, and not all active or potentially active volcanoes are monitored. Still, if people living around volcanoes are aware of some of the precursor phenomena that occur, they may be able to communicate their findings of anomalous events to scientists who can begin monitoring on a regular basis and help prevent a pending disaster.Vesuvius, 79 A.D.
Mount Vesuvius is located to
of Naples, Italy along the Bay of Naples. Prior to the
eruption of 79 A.D. Vesuvius was considered an extinct volcano by the
of the area, who were flourishing at the height of the Roman
The area northwest of Naples, known as the Phelegrean Fields, was known
to be an active volcanic area, however. Nevertheless, when
became frequent in the Vesuvius area in 62 A.D., they caused no alarm
earthquakes were quite common throughout Italy.
What we know of the eruption that took place in 79 A.D. comes from two different sources: (1) Two letters written by the Pliny the Younger, the nephew of Caius Plinius (Pliny the Elder), to the historian Tacitus concerning the death of his uncle. Pliny the Elder was an admiral in the Roman navy and well known in his time. (2) Information from geological and archeological studies that have examined the area around Vesuvius.
We start with the account given by Pliny the Younger in his letters to Tacitus. About 1 P.M. on August 24, 79 A.D. the Plinys were in Misenum, about 30 km across the Bay of Naples from Vesuvius. A large cloud appeared above the volcano, growing rapidly, rising many thousands of meters and spreading out laterally in the atmosphere. Pliny the Younger states that the cloud had the shape of a Mediterranean pine tree, with a narrow trunk at the base, spreading out like branches above.Pliny the Elder was curious about the cloud and began to plan a trip across the bay to investigate. Before he could get his ship ready, he received word from people living closer to Vesuvius that people needed help, and his investigative mission became instead a rescue mission. He organized his fleet and set sail with the intention of rescuing people living near the base of the volcano at Torre del Greco. As he crossed the bay his ship was showered by hot ashes and pumice which accumulated on the deck. He soon found it was impossible to make the rescue attempt and he turned in the direction of the wind and instead landed at Stabiae, to the south of Vesuvius.
At Stabiae, conditions were still mild and he encountered his friend Pomponianus who was trying to escape to the sea, but the winds were not favorable. In order to reassure Pomponianus that there was no danger, Pliny went to the local baths and sat down for a meal.
As night came, Pliny retired
hours of sleep. During his sleep tephra began piling up in
of the house and earthquakes and loud explosions occurred in the area
Vesuvius. Pliny awakened by the trembling and loud noises and
with Pomponianus. They decided that there best escape was to
the shore and try to leave by ship. They tied pillows to
for safety from the falling pumice, but darkness enveloped the area as
a result of the falling tephra, and it remained dark even after the sun
should have come up. They eventually made it to the shore,
that the wind was not blowing in a direction favorable to sail away
Suddenly Pliny the Elder began to feel poorly. He laid down on a cloth near the shore and asked for water twice. Then sulfurous fumes descended on the area making staying put an unpleasant experience. Pliny rose to move away from the area, but fell dead. His friends thought that he had been poisoned by the fumes, but no one else was affected. His friends all later escaped. Three days later Pliny's body was found, unmarked by any damage. It is highly unlikely that Pliny died of poisoning from the fumes because Stabiae is so far from Vesuvius; it is more likely that he died from a heart attack resulting from his physical exertion in attempting to flee the falling ash.
On the same morning that Pliny
died, his wife and nephew were still in Misenum. Conditions
were also growing worse. Although Misenum was
volcano, there were still frequent earthquakes and the sky filled with
ash and became dark. Pliny the Younger and his mother decided
leave their house and move farther away from the volcano.
off in chariots, but the shaking of the ground and the accumulation of
ash soon made it impossible to continue. They observed sharp
in the sea, followed by waves that crashed high on the shore, probably
due to tsunamis as pyroclastic flows from Vesuvius entered the
They attempted to move farther inland, but soon the ash from the
became so thick that they were engulfed in total darkness.
Pliny the Younger compared the darkness to that of a sealed room with no source of light. This lasted for many hours, but eventually the air cleared and they were able to observe Vesuvius in the distance.
What had once been a smooth gentle mountain was now seen to be reduced in size by about one half, and what once had been lush vineyards in the countryside surrounding the mountain was now a plain of gray colored ash mantling the countryside like snow.The account given by Pliny the Younger says nothing about what happened to the towns of Pompeii and Herculaneum, close the volcano. But his account of the "pine tree" like cloud above the volcano was the first description of an eruption column, and thus volcanic eruptions that produce eruption columns and fall deposits have taken on his and his uncle's name, and are referred to as Plinian eruptions.
The fate of Pompeii and Herculaneum had to await further exploration by geologists and archeologists. Note that Pompeii was located about nine kilometers down wind from Vesuvius, and was buried by close to 3 meters of tephra that had fallen from the eruption cloud. Since the eruption killed most of the inhabitants of Pompeii it remained abandoned for hundreds of years.
In 1595 some of the ruins of Pompeii were discovered when excavation for a new aqueduct began. Coins were found, along with marble tablets with inscriptions referring to Pompeii. People then began the unsystematic excavation of parts of the former city in search of buried works of art and other treasures. In the 1800s true archeological excavations began to uncover the buried city in a more systematic way. The pumice that had buried the city was dug away to reveal a well preserved instant in time. It appears that disaster came rapidly, as food laid out in some houses showed that life must have continued much as normal until the end. Many human fossils were found. The bodies had been buried in ash and rain falling on the ash had cemented it together to form molds around the bodies before they decayed. After the bodies decayed an open cavity was preserved and archeologists made plaster of paris casts of these cavities to reveal remarkable detail about the last moments of Pompeii .
Many of the bodies showed that the victims had likely died of asphyxiation from breathing the hot gas and dust. Their hands were clenched around their mouths probably in an attempt to filter out the dust prior to their deaths. Many corpses were found clutching bags of gold that they had gathered in an attempt to flee. Although some people likely died as their roofs collapsed due to the weight of the accumulating tephra, geological evidence indicates that their were periodic blasts of ash coming from the volcano that produced pyroclastic flows. These were likely the major cause of death.
The destruction of Herculaneum was somewhat different. Herculaneum was located on the shore of the Bay of Naples at the base of Mount Vesuvius. Herculaneum has not been excavated to the extent of Pompeii because the modern city of Resina is built on top of it. Herculaneum was buried by different kinds of deposits than was Pompeii. Instead of loose tephra fall deposits, Herculaneum was buried by a highly unsorted mixture of rock fragments and fine-grained tephra that became much more consolidated into a hard rock after deposition. Such consolidation makes excavation much more difficult at Herculaneum.Early geologic investigations suggested that these deposits were mudflows or lahars, but more recent investigations show that they are instead deposits of pyroclastic flows. Within the 8 square blocks of Herculaneum that have been excavated, it first appeared that most of the residents had escaped death before the advance of the pyroclastic flows.
But recent excavations near the shoreline have found buildings supported by arched chambers open to the beach. Within these chambers hundreds of skeletons were found. Thus it is likely that the residents ran to these buildings for shelter from the volcano, but were eventually engulfed in the pyroclastic flows and died.
Although Vesuvius has been intermittently active throughout the years since the 79 A.D. disaster, the population has returned to the Vesuvius area and Naples and its suburbs is one of the largest modern cities built in the danger zone of an active volcano.
Mount Pelée is a volcano on the northern part of the French Caribbean Island of Martinique. The city of St. Pierre lies about 6km to the southwest of the volcanic peak on the shores of the Caribbean Sea. Prior to 1902, the last eruption from the volcano had occurred in 1851. Below is a chronology of events leading up to and including the devastating eruption that took place on May 8, 1902.
1889 - Fumaroles appear in the summit
January, 1902 - Fumaroles in summit crater increase in number and strength.
March 2, 1902 - Climbers enter the crater and find that a dry lake bed in the crater is now emitting sulfur fumes from several points.
April 23, 9:00 PM - Minor explosive
activity takes place in the dry lake bed in the crater. Ash
consists only of pre-existing rock fragments.
April 27 - An investigating party climbs into the crater and discovers that the dry lake now contains water and a small tephra cone.
April 27 through May 4 - Explosions continue from the summit crater, with occasional ash falls and sulfurous fumes reaching St. Pierre. Still, all of the ash is pre-existing fragments of the volcano.
May 5 - The water that has accumulated in the crater suddenly bursts through the walls of the crater and flows into the valley of the Riviere Blanche. As the water moves down the valley it picks up loose debris to become a mudflow that travels at 90 km/hr. It reaches a rum distillery near the coast, burying all but its smokestack, and killing 24 workers. The mudflow enters the Caribbean Sea and generates a tsunami with 3 to 4 m waves that flood the low lying parts of St. Pierre.
A scientific commission is appointed to investigate the volcanic phenomena. One of the commissioners reports to the local newspaper that "Mt. Pelée presents no more danger to St. Pierre than Vesuvius does to Naples!"
May 6 - Ash eruptions begin to contain
new magma and a small lava dome forms in the crater. May 7 -
Several small nuée
ardents flow down the Riviere Blanche valley as a result of small
events from the lava dome in the crater.
May 8, 7:50 AM - A loud blast is heard from the summit of Mt. Pelée.
May 8, 7:52 AM - All clocks in St. Pierre stop.
Many of the victims are badly burned, yet their clothing is not singed. Most appear to have died from asphyxiation due to breathing the hot mixture of gas and dust.
One survivor was locked in a small prison cell with only a small barred opening on facing the north. His body was badly burned and he reported that the heat only lasted a few seconds. This survivor later joined the circus and traveled around the world telling his story of survival and showing his badly burned back.
May 20 - A second nuée ardent
sweeps through St. Pierre. There are no deaths (They all died
May 8!) May 30 - A third nuée ardent takes
a somewhat different path, engulfing not only St. Pierre, but also the
town of Morne Rouge where it kills an additional 2,000 people.
The video on Mount St. Helens is spectacular. The following are the major points of the events leading to the eruption, the eruption itself, and following the eruption, shown in the video. There will likely not be enough time to discuss these points in class. The last portion of the video discusses the prediction of dome eruptions that have taken place since the May 18 eruption and was discussed in the previous lecture.
In a 1975 volcanic hazards study of Mount St. Helens, U.S. Geological Survey geologists D.R. Crandell, D. Mullineaux and M. Rubin noted that Mount St. Helens had been built within the last 2,500 years with the last eruption in 1857. Major eruptions had occurred every hundred years or so and St. Helens was one of the most active volcanoes in the Cascade Range. In their paper they stated that Mount St. Helens "will erupt again in the next 100 years, perhaps before the end of the century."Mount St. Helens did reawaken on March 21, 1980 when a magnitude 4.2 earthquake occurred beneath the volcano. Earthquakes continued until March 27 when a phreatic eruption occurred at the summit. Similar small eruptions took place through mid April and the summit of the mountain developed a new crater due to the explosions. Note that only pre-existing rock fragments were ejected during these initial eruptions.
By mid-April volcanic tremor had begun beneath the mountain indicating the movement of magma. By the end of April surveys showed that the north face of the mountain above an older volcanic dome, known as the Goat Rocks Dome, had begun to bulge upwards and outwards at rates up to 1 m per day. By May 12, the bulge had displaced parts of the northern part of the volcano a distance of about 150 m. Geologists now recognized that this bulge could soon develop into a landslide, but were uncertain when or if it would occur and how big the slide would be. Most of the area in the danger zone (called the red zone) had been evacuated, but intense pressure from people with cabins and homes within the red zone forced the Governor of Washington to open the area to allow land owners to return under police supervision to retrieve their belongings. The first of these escorted entries into the red zone took place on May 17, and a second caravan was scheduled for the morning of May 18. Fortunately the latter expedition was not scheduled to leave until 9:00 A.M.
AM on MAay 18, 1980 a
magnitude 5.0 earthquake occurred beneath Mt.
St. Helens. This led to a violent eruption that took place
the next minute. The earthquake triggered a large landslide
began to slide out to the north, initially as three large
As the first block, containing the old Goat Rocks lava dome, began to
downward, the magma chamber beneath the volcano became exposed to
pressure. The gas inside the magma expanded
a lateral blast that moved outward toward the north.
As the second slide block began to move downwards a vertical eruption column began to form above the volcano. The lateral blast rapidly overtook the slide block and roared through an area to the north of the mountain, knocking down all trees in its path and suffocating all living things, including humans. Within the next 10 seconds the third slide block moved out toward the north. All slide blocks banked up against the mountain north of Spirit Lake and were then diverted down the North Fork of the Toutle River valley toward the west, breaking up into smaller blocks as they moved down slope. The landslide thus became a debris avalanche and left a deposit extending about 20 km down the valley (see map below). The southern shores of Spirit Lake were displaced about 1 km northward and the level of the lake was raised about 40 m. Harry Truman's Lodge was buried by the debris avalanche.
Within about the first minute of the eruption the lateral blast had ceased, the debris avalanche had been deposited, and the summit of Mount St. Helens had been reduced by about 500 m. The magma however continued to erupt in a Plinian eruption column that reached up to 26 km into the atmosphere. The eruption column collapsed several times to produce pyroclastic flows that moved into Spirit Lake and the upper reaches of the Toutle River Valley. This Plinian phase lasted about 9 hours and spread tephra in a plume to the east, darkening the area at midday to make it appear like night.During this first minute of the eruption everything within the lateral blast zone had been knocked down and covered with a thin layer of ash. Trees along about a 100 m wide zone surrounding the blast zone had been stripped of their vegetation or severely singed to a brownish color. In all, 62 people lost their lives, either by being buried by the debris avalanche deposit, or suffocating by breathing the hot gases and dust of the blast. Some also died in the cars or trucks when pieces of wood were driven through the cars by the blast.
Over the next several days melted snow combined with the new ash to produce mudflows that roared down the North and South Forks of the Toutle River and drainages to the south of the volcano
Logs were carried into the Columbia River and so much sediment accumulated in the shipping channel of the Columbia River that it cost several hundred million dollars to dredge the channel and remove the logs so that shipping could continue. Eastern Washington was covered in volcanic ash for months after the eruption and relief efforts were slow. Although the tephra fall deposits that accumulated in eastern Washington were anticipated by the prior volcanic hazards study, most planning for rescue and recovery had concentrated on the area immediately around Mount St. Helens.
In general, the eruption had been much larger than most anticipated, but the fact that a hazards study had been carried out, that public officials were quick to act and evacuate the danger zone, and that the volcano was under constant monitoring, resulted in the minimization of loss of life to only 62 instead of a much larger number that could have been killed had not these efforts been in place.
Since the 1980 eruption,
domes have been emplaced in the crater and some have been blasted
In the future, it is expected that new domes will continue to form,
building the volcano back to a form that will look more like it did
to the 1980 eruption.
see...Australian Volcanos for more links
see... Volcanic Hazards and case studies for more links