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  adapted to html from notes by Cliff Ollier and Stephen A. Nelson

see also... Volcanic / Astronomy Terms (Glossary)



Australian Volcanos

Basic Lava Volcanoes
Lava shields  
Lava dome    
Lava cones    
Lava mounds  
Lava disc

Acid Lava Volcanoes
Cumulo-domes  
Plug domes   
Spines

Pyroclastic Volcanoes
Scoria cone  
Scoria mound   
Nested scoria cones    
Littoral cones   
Maars (tuff rings)

Mixed Eruption Volcanoes
Strato-volcanoes

Miscellaneous Volcanoes
Composite volcanoes
Parasitic cones (also called adventive cones and secondary cones)
Multiple 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.

Basic Lava Volcanoes

Basic lavas are characteristically very fluid, so spread easily and give rise typically to volcanoes of low gradient.

Basaltic Cones

Figure 4: Basaltic Cone types

Lava shields

Lava ShieldA succession of wide sheets of volcanic rock, built by repeated outpourings of basaltic lava, can pile up to produce huge 'shields' with gentle slopes (less than 7º) and convex outlines. The Hawaiian islands are the classic example. Mauna Loa, the largest island, rises 19,000 m above its sea-floor base, which is 109 km wide, and projects over 4000 m above the sea. Shields of this size are made up of a number of separate volcanoes; for instance Kilauea and Hualalai are mere flank volcanoes on Mauna Loa. Volcanic masses such as the island of Hawaii, formed by the overlapping of a group of shield volcanoes, may be termed volcanic shield-clusters.

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.

Lava dome

Lava DomeA smaller-scale volcano that erupts liquid lava may produce a convex dome rather than a shield. Any of the individual peaks on Hawaii, such as Mauna Kea, may be regarded as lava domes. The distinction between size used here is not standard practice and many authors use shield and dome interchangeably.

Lava cones

Lava ConeCentral eruption on a still smaller scale may give rise to simple straight-sided cones built of successive lava flows, such as Mt Hamilton, Victoria. These usually have flanks of low angle (7º or less), but some examples are much steeper.

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.

Lava mounds

Lava MoundSome basaltic volcanoes have no sign of a crater, but are gently sloping mounds, such as Mt Cotterill, Victoria. These extinct volcanoes may owe their shape partly to erosion, although they probably never had very pronounced craters but had lava welling right to the brim before solidification. Such volcanoes, distinguished from cones by their lack of crater, may be termed lava mounds by analogy with scoria mounds.

Lava disc

Lava DiscIn Victoria there are a few anomalous volcanoes which have been described as lava discs. They are made of basalt, and display jointing perpendicular to the lava skin on both the upper surface and the sides. The smallest one, Lawaluk, has the form of a steep-edged, flat-topped disc of basalt. Mondilibi is probably of the same type, and the main lava sheet within the ring barrier of Mt Porndon appears to be a similar feature though larger (3 km in diameter). These hills appear to be made by eruption of single flows that develop a tough skin and spread out from the centre without breaking the skin, in the manner of a water-filled balloon collapsing into a disc.

Acid Lava Volcanoes

Acid igneous rocks are generally very viscous, and if they do not explode their lack of flow gives rise to a number of distinctive landforms

Acid cones

Figure 5: Acid Lava forms

Cumulo-domes

Cumulo-domeWhen viscous lava is extruded, it sags and spreads into convex dome-like bodies called cumulo-domes. These may be almost independent, or may be associated with and partly intrusive into previously deposited pyroclasts.

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.

Plug domes

Plug DomeIn its most viscous form, the magma extruded from a vent may be so rigid that it moves up like a piston, producing a roughly cylindrical body known as a plug dome. [In American usage plug dome may refer to what are here called cumulo-domes.] In New Zealand, Mt Edgecumbe is an andesite volcano that was apparently extruded through a jagged orifice, for it has grooves on the side and top that are not due to erosion, but are giant scratches.

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.

Spines

Whereas plug domes are large bodies of nearly mountain size, smaller-scale extrusion of very rigid lava, through chinks in the cracked skin of plug domes or cumulo-domes, gives rise to spines. The spine of Mont Pelée, Martinique, which was produced after the catastrophic eruption of 1902, reached a height of over 300 m, but was rapidly eroded. At one stage it grew 13 m in a day. Spines are frequently irregular in shape, and are not extruded uniformly as cylindrical pillars. A spine on Santa Maria, Guatemala, that grew between 1922 and 1925, reached a maximum size of 500m high and 1300m across the base.

Pyroclastic Volcanoes

When explosively produced fragments of lava fall around a volcanic vent they build up a heap of debris, the slope of which depends on the angle of rest of the fragments concerned (Figure 8). Fine particles have lower slopes than coarse ones, and as the coarser fragments tend to accumulate near the vent, beautiful concave slopes are formed, like those of Fujiyama and Mt Egmont.

Scorai cones

8: Scoria volcanic types

Scoria cone

The ideal scoria cone is single, steep, with straight or gently concave sides, and with a crater at the top. Mt Elephant, Victoria, 240 m high, is a good example. The even height of the crater rim often causes scoria cones to appear flat4opped when viewed from a distance.

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.

Scoria mound

Scoria MoundSome scoria volcanoes have no apparent crater and may be termed scoria mounds to distinguish them from normal scoria cones. The Anakies, Victoria, are examples.

Nested scoria cones

Nested Scoria ConeScoria cones are frequently produced as the last phase of an eruption on the site of larger volcanoes of other type. When they are in the centre of a large crater or caldera they are called nested cones. The V-sectioned trough between the inner cone and the crater wall is called a fosse.

Littoral cones

When aa lava reaches the sea it explodes and the ejecta pile up to form a cone up to 100 m high and 1 km in diameter. There is often a double hill, one hill built on each side of the lava stream (Wentworth and Macdonald, 1953).

Maars (tuff rings)

MaarMaars are landforms caused by volcanic explosion and consist of a crater which extends below general ground level and is considerably wider than deep, and a surrounding rim constructed of material ejected from the crater. The rim consists of pyroclastic material, either igneous or comminuted bedrock, and is often markedly asymmetrical, with greater deposition on the downwind side of the crater. The rim deposit is also asymmetrical in cross section, with a steep side towards the crater, and a gentle slope (commonly 40º or less) away from the crater, parallel to the bedding of the pyroclastics. The craters have a diameter often about 1 km and the rims are commonly less than 50 m high, although they may reach 100 m.

Maars are usually associated with basaltic igneous activity, but andesitic maars are known in Chile.

Mixed Eruption Volcanoes

In many volcanoes there is a mixture of lava and fragmental deposits.

Strato-volcanoes

Many of the world's great volcanoes, such as Vesuvius, Fujiyama, Egmont, and many others, are strato-volcanoes, with both lava flows and pyroclastic deposits. Many of these have erupted over a long period, and indeed the strato-volcano is the commonest form developed by long-lived central volcanoes. The cones become gullied by erosion and lava flows commonly follow such gullies. New gullies then form on the edges of flows, and so on. Scoria cones are built around the top of the volcano and pyroclastic flow and fall deposits may have a wide distribution on the flanks. Ngauruhoe, New Zealand, is a typical strato-volcano, almost perfectly conical and about 1000 m high. The slopes are about 30 degrees steep and the crater is about 400 m across. The highest point on the rim is to the east, possibly because of the prevailing westerly wind. Young aa lava flows have reached the base on all sides except the east.

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.

Miscellaneous Volcanoes

Composite volcanoes

This term in a strict sense refers to those volcanoes which have a mixture of lava and scoria forms, but not in a simple layered sequence. Mt Rouse, Victoria, is an example, with an elongate crater in a scoria hill and with a smaller and more distinct crater with a basalt rim on the south. The composite Mt Porndon, Victoria, has a large basalt disc with a diameter of about 3 km, in the centre of which are a number of scoria cones and mounds. Staughtons Hill, Victoria, consists of a maar, a scoria cone, and a basalt-rimmed separate crater. Most composite hills appear to consist of individual hills which are genetically related, but it is possible that some, such as Staughtons Hill, are due to accidental superimposition of unrelated eruption points of different ages.

Parasitic cones (also called adventive cones and secondary cones)

When a volcano becomes very high, very great pressure is required for the rising lava to reach the summit crater. It is sometimes possible for the lava to find an easier route to the surface, and erupt on the flanks of the main volcano. Once such an eruption has taken place, the solidified lava in the conduit plugs that outlet, and in succeeding eruption another opening must be made. In this manner large volcano comes to have many small parasitic cones on its flanks. Mt Etna, Sicily, with over two hundred parasitic cones and over eight hundred small mounds of lava known as boccas, is the finest example. Here each new series of eruptions occurs along a rift, and succeeding new cones appear higher and higher up the fissure until it is sealed.

Multiple cones

In some areas, as, for instance, on the volcanic plains of Victoria, a number of scoria cones are built very close together. The general mechanism appears to be the same as for parasitic cones, that is the first cone blocks the vent, and the second one occurs on a new vent close by. The difference here is that no cones grow to any great size, and all the separate cones tend to be of about the same size; that is there is no main volcano with parasites, but a series of equal volcanoes. These may be called multiple cones.


Volcanic Hazards/Case Studies

adapted to html based on notes of  Stephen A. Nelson

Primary Effects of Volcanism
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


Primary Effects of Volcanism

Violent Eruptions and Pyroclastic Activity Poisonous Gas Emissions



Secondary and Tertiary Effects of Volcanism sthelensmap.gifOn November 13, 1985 a mudflow generated by a small eruption on Nevado del Ruiz volcano in Columbia flowed down slope and devastated the town of Armero, 50 km east of the volcano and built on prior mudflow deposits.  The town had several hours of warning from villages higher up slope, but these warnings were ignored, and 23,000 people died in the mudflow that engulfed the town.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Debris Avalanches and Debris Flows                       

Flooding Tsunamis Volcanic Earthquakes and Tremors Atmospheric Effects Famine and Disease Beneficial Aspects of Volcanism

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.

Predicting Volcanic Eruptions

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.

Extinct Volcano - An extinct volcano is a volcano that has not shown any historic activity, is usually deeply eroded, and shows no signs of recent activity.  How old must a volcano be to be considered extinct depends to a large degree on past activity. Dormant Volcano - A dormant volcano (sleeping volcano) is somewhere between active and extinct.  A dormant volcano is one that has not shown eruptive activity within recorded history, but shows geologic evidence of activity within the geologic recent past.


Long - Term Forecasting and Volcanic Hazards Studies

Short - Term Prediction based on Volcanic Monitoring

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.

volcseismonit.gifSeismic 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 Resistivity

 - 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.

Ground Deformation

 - 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.

rruptprecurs.gifAfter 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 the southeast of Naples, Italy along the Bay of Naples.  Prior to the catastrophic eruption of 79 A.D. Vesuvius was considered an extinct volcano by the inhabitants of the area, who were flourishing at the height of the Roman Empire.  The area northwest of Naples, known as the Phelegrean Fields, was known to be an active volcanic area, however.  Nevertheless, when earthquakes  became frequent in the Vesuvius area in 62 A.D., they caused no alarm because earthquakes were quite common throughout Italy.
 

vesuvius.jpgWhat 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 for several hours of sleep.  During his sleep tephra began piling up in the courtyard of the house and earthquakes and loud explosions occurred in the area of Vesuvius.  Pliny awakened by the trembling and loud noises and conferred with Pomponianus.  They decided that there best escape was to go to the shore and try to leave by ship.  They tied pillows to their heads 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, but found that the wind was not blowing in a direction favorable to sail away from the area.
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 the Elder died, his wife and nephew were still in Misenum.  Conditions there were also growing worse.   Although Misenum was upwind from the volcano, there were still frequent earthquakes and the sky filled with ash and became dark.  Pliny the Younger and his mother decided to leave their house and move farther away from the volcano.  They set off in chariots, but the shaking of the ground and the accumulation of ash soon made it impossible to continue.  They observed sharp retreats in the sea, followed by waves that crashed high on the shore, probably due to tsunamis as pyroclastic flows from Vesuvius entered the sea.  They attempted to move farther inland, but soon the ash from the eruption 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.

Mt. Pelée, Martinique, 1902

mtpele.jpgMount 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.

Because an island wide election is to take place on May 11, political leaders encourage the populace to remain in the towns, villages, and cities to vote.  Still, many people leave St. Pierre fleeing to Fort de France on the southern part of the island.  Many more, however, flee the surrounding countryside and enter St. Pierre, boosting the population to about 29,000.

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!"

A powerful nuée ardent rolls down the valley of the Riviere Blanche.  The densest part of the cloud remains in the valley, but part of the pyroclastic flow overtops the valley walls and engulfs St. Pierre.  All but 2 people out about 29,000 are killed in the next 2 to 3 minutes.  Ships in the harbor catch fire and sink.  Walls up to 1 meter thick are knocked down, roofs are blown off, and many burn, iron bars are bent and twisted, but some areas of the city are affected more than others.  Still, nearly all humans and animals die, even in parts of the city not knocked down by the blast.

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.

This eruption is an extremely important event because it was the first time that the scientific community became aware of the phenomena of pyroclastic flows.

Mt. St. Helens, 1980

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.

sthelenseruptstages.gif8:32 AM on MAay 18, 1980 a magnitude 5.0 earthquake occurred beneath Mt. St. Helens.  This led to a violent eruption that took place over about the next minute.  The earthquake triggered a large landslide that began to slide out to the north, initially as three large blocks.   As the first block, containing the old Goat Rocks lava dome, began to slide downward, the magma chamber beneath the volcano became exposed to atmospheric pressure.   The gas inside the magma expanded rapidly, producing 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

sthelensmap.gifLogs 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, several volcanic domes have been emplaced in the crater and some have been blasted out.  In the future, it is expected that new domes will continue to form, eventually building the volcano back to a form that will look more like it did prior to the 1980 eruption.

Volcano Types

Cinder
Spatter
Composite
Strombolian
Hawaiian
Submarine
Pelean
Surtseyan
Shield
Vulcanian

see...Australian Volcanos for more links
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