AS Geology GL3 : Volcanos

In order to understand volcanos, the composition of magma must be understood. There are 3 main types of magma:

• Basaltic: this type of magma has a low viscosity, low silica content and a temperature range between 1000 to 1200 degrees centigrade. The combination of these factors means that eruptions are not violent and result in the formation of gentle-sloped volcanos. This type of lava is characteristic of the volcanos found at Hawaii.
• Andesitic: has a medium viscosity, medium silica content and can be both acidic or basic. Teperatures range form 900 to 1100 degrees centigrade. This variety of magma can produce fairly explosive eruptions and pyroclastic flows forming a volcano with a steeper gradient.
• Rhyolitic: has a high viscocity, acidity, and silica content. Temperatures can range from 600 to 1000 degrees centigrade. This type of magma results in sudden and violent eruptions, where as in the case of Mount Saint Helen's the whole side of the mountain was blown apart (which was incidentally the largest landslide in recorded history).

Different lavas have differing properties depending on their geographical location.

• Basaltic lavas are formed from melting oceanic crust
• Rhylolitic lavas are formed from melting continental crust
• Andestitic lava forms from a mixture of the above two. Rising basaltic magma melts continential, silica rich, crust before finally reaching the surface.

Volcanoes are located in areas of intense tectonic activity, on plate margins or hot-spots. The Pacific ring of fire is an area with a high frequency of volcanic activity, located around the pacific oceanic plate. Hawaii is an example of a hot-spot, where thinning in the crust as allowed magma to extrude onto the surface forming a volcanic island chain.

A volcano is essentially a vent in the earth's crust through which magma and debris from the lithosphere and asthenosphere can be extruded onto the surface. 'Volcano' comes from the latin root word 'Vulcan' the roman god of fire and destruction. The most iconic vocanos are the cone shaped strato volcanos however any vent in the earth's crust through which magma can reach the surface is considered a volcano.

Shield Volcano: Characteristic of Hawaii, has shallow angled slopes with layers composed of several lava flows. Lava types include ropey and pillow lava as a result of a lava flow coming into direct contact with sea water and rapidly cooling.

Strato Volcano: Composed of layers of lava and pyroclastic flow deposits, strato volcanos have a steeper gradient and are associated with more violent eruptions.

Fissure Vocano: A crack in the earth's crust through which magma is erupted, can be potentially many miles in length and predominantly occur at divergent plate boundries and mid-ocean ridges.

The last words of david Johnston who was a volcanologist monitoring Mount Sainy Helen's and was the first to report the eruption before being swept away by the lateral blast, reaching him at the observation post 6 miles from the volcano.

Not all volcanos are as explosive or destructive as Mount St Helen's, and the explosivity is entirly dependant on the type of lava erupted. The lower the viscosity, the more fluid the lava and so the 'gentler' the eruption.

During a volcanic eruption heated rock rises upwards from the asthenosphere to levels with lower overlying pressure. As it melts , forming magma, the surrounding rock reacts to the sudden increase in heat and pressure. As the magma rises it becomes even more fluid, dissolved gasas rise out of the magma as bubbles, helping propel the magma to the surface where it is erupted as lava.

• There are aproximatly 500 active volcanos on earth.
• Producing around 50 eruptions per year.
• Up to 80% of these volcanos are located on a subduction zone.

Humans and Volcanos

• 5% of eruptions lead to deaths (over 650 year period of study)
• The above is a deceptive figure with over half the deaths in the 20th century happening in a single event: Mount Pelee, Martinique, West Indies of which 29000 residents of Saint Pierre there were only 2 survivors.
• Hazard impacts of a vocano are relient on the population density of the surrounding area.
• The flanks of a volcano attract people to farm the fertile soil.
• Level of economic development in the country in which the volcano is located is also important in appreciating the reasons behind devistation and fatality statistics.

Primary Hazards
Lava Flows although spectacular pose more of a threat to property than human life. The most dangerous lava flows are basaltic, which have the potential to travel at high speeds and spead far from their original source. Most lava flows are slow moving and rarly travel more than 8 killometres from the source. Kilauea in Hawaii has been erupting since 1983 and has no recorded deaths to date.

Pyroclastic Flows can be extremely dangerous moving very quickly over a large area. Pyroclastic surges can develop as a flow goes downhill. he most dangerous variety being when pressure has been able to build up, resulting in a more explosive eruption. They pose an equally great threat if, as in the case of Mount Saint Helen's, the flow was released laterally rather than into the atmosphere, reaching speeds of 150mph.

Ash and Tephra Fall can also have a wide ranging effect but rarely kills directly but the risk of building collapse can present threaten to the local population. As in the case in a 1902 eruption in Guatemala where ash fall caused roofs to collaspe and the deaths of 2000 people. Ash fall, while benefical in long-term, sterilises farmland, destorying crops.

Volcanic Gases are extruded during an eruption, but are rarely the direct cause of a disaster, however instances involving carbon dioxide and carbon monoxide pose a threat. This occured in the eruption of Lake Nyos, Cameroon in 1986 resulting in the deaths of 1746 people from suffocation.

Secondary Hazards
Lahars and landslides are volcanic mudflows which are occur in wet tropics, or where volcanic materical can mix with water forming a mudslide. Lahars pose a considerable threat to people and property, and can travel downhill at up to 22 metres per second. Landslides are gravity driven and can occur before and after and eruption as a result of ground deformation. Lahars proved devistating during the eruption of Chances Peak, Montserrat, where the town of Plymouth was completly buried by lahar deposits.

Tsunamis are giant waves that can have a devistating effect on coastal areas. They are generally caused by significant mispacement of ocean floor and gather speed and intensity in shallower waters. A notable volcanic tsunamis was caused by the eruption of Krakatoa, Indonesia where a tsunamis 30 metres high resulted in the deaths of 36000 people.

Volcanic eruptions may be catagorised in an index, measuring the voilence of an eruption. The VEI was devised to provide a relative measure from which volcanic eruptions could be compared.

Volcanic Explosivity Index

0 non-explosive
1 Hawaiian
2 Strombolian
3 " " " "
4 Vulcanian
5 " " " "
6 Plinian
7 " " " "
8 Ultra Plinian

The eruption of the volcano under the ice sheet of Eyjafjallajokull was the cause of much of the air travel problems in the spring of 2010. The volcano itself was not the root cause of the problem rather the glacier that capped the top. When the volcano erupted from the main vent, the ice sheet acted as a pressure cooker, so that when the eruption broke through, it did so in a more explosive manner than would otherwise be expected. Furthermore, the thick layer of ice then caused any lava that was erupted to rapidly cool, forming ash. While larger heavier ash particles fell close to the volcano, finner grained material reached high into the atmosphere where it was caught up in the jet stream, keeping it suspended at a level where commercial aircraft also fly.

Although rules of air traffic and volcanic ash have now been amended, the reason for such caution was the Jakarta incident, in which a commercial jet plane with just under 300 passengers on board flew through a cloud of volcanic ash over Indonesia. The ash caused all four engines to fail, and sandblasted the windscreen rendering it opaque. The engines were able to be restarted only after the plane had fallen below the level of ash, and on inspection afterwards the engines were found to be coated in a fine layer of glass, as the ash which had got into the engine combustion chamber melted than re-solidified.

Although Mount Etna is the largest volcano in Europe, Vesuvius has the far more deadly legacy. When it erupted in 79AD it caused widespread destruction and death, buried the town under 8km3 of volcanic ash and pyroclastic flow debris, and was the largest natural disaster in the whole of the history of the Roman Empire.

Vesuvius is a strato volcano, fed from the collision of the African and Eurasion tectonic plates, the same movement that is causing the Medeterainian to shrink.

The eruption of Vesuvius was recorded by Roman writer and historian Pliny the younger, and so information on the eruption is relatively accurate.

Furthermore archaeological evidence of preserved bodies of those caught up in the eruption show the manner in which they were killed and how they tried to escape.

Vesuvius now stands above the Italian town of Naples, but the eruption of AD79 was by now means the last, and efforts are no being made to prevent urban sprawl onto the flanks of the volcano in an attempt to reduce the hazard to the people living below.

Benefits are associated with some natural hazards, these may include:

· Geothermal Power (Iceland, New Zealand, Italy)

· Fertile Soils in the long run although volcanic ash destroys crops and initially damages the soil productivity.

· Tourism, many volcanoes attract a high number of tourists, this can be a useful side-effect in aiding the re-establishment of communities and an economy. (Etna, Fujiyama, Bromo)

Vocanic activity can be monitored in a variety of different ways in an attemt to help predict an eruption and where the areas in the greates danger are:

• Volcano Cameras
• Hazard Mapping- Predicting the areas most at risk
• Hyrological Surveys - Monitoring water sources to determine whether magma is rising and were lahars may flow.
• Ground Deformation
• Emissions - fluctuation in emissions, such as increased temperature could indicate an imminent eruption.
• Chemical Changes in volcanic gases
• Seismic activity - magma cannot reach the surface without displacing some rock, causing multiple low level earthquakes.
• Gravitational anomolies - a change in the gravitational pull may indicate rising magma which has a lower density than the surrounding rock.