The Geography Of Volcanoes

Note by , created almost 6 years ago

Leaving Certificate Geography (The geography of volcanoes) Note on The Geography Of Volcanoes, created by eimearkelly3 on 07/05/2013.

Created by eimearkelly3 almost 6 years ago
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A volcano is a structure containing a magma chamber from which molten magma erupts. The magma travels up through the vent and eventually out through a crater. Through this crater, hot ash, dust, volcanic rock bombs and molten magma erupt. Magma which has moved out of the crater is known as lava. Over time, a landform known as a volcanic cone develops.

Volcanic activity occurs : Where plates seperate (constructive plate boundaries) Where plates collide (destructive plate boundaries) At hotspots The Pacific Ring of Fire is the world's most active earthquake and volcano zone.

There are three stages in the lifecycle of a volcano: ACTIVE - erupts regularly, e.g. Mount Etna in Italy and Mount St Helens in the USA. DORMANT - has not erupted for a long time, but mat erupt again e.g. Cotopaxi, Peru EXTINCT - has not erupted in recorded history e.g. Slemish mountain Co.Antrim, Northern Ireland.

Causes of a volcanic eruption:Temperatures in the upper mantle are often hot enough for the rock of the subducted plates to melt and form into molten magma. This molten magma pushes towards the surface and builds up in a magma chamber. The magma chamber can hold a large quantity of magma. The gases contained in this chamber expand dramatically in volume and this causes the magma to rise. The gases explode and the magma is pushed upwards.As the magma nears the surface, pressure builds and the volcanic mountain bulges. The magma then forces its way up through fissures in the volcano. When the magma (and the gases within it) reaches the surface, the pressure is released and a volcanic eruption of hot ash, dust, gases, and lava occurs.

Violence of an eruption:The presence or absence of silica helps determine whether or not a volcanic eruption will be violent. Silica helps produce thick lava which traps gases within magma. The more silica present in the magma, the better its ability to trap gases and the greater the likelihood that a violent eruption will occur.

Products of volcanic eruptions: Lava Pyroclasts and pyroclastic flows. Poisonous gases Ash, pumice, and dust (tephra) Water vapour Volcanic ash and dust are blasted into the sky during an eruption. These particles then travel great distances around the world. They may also reduce temperatures by blocking out the sun.

Volcanoes are mountains formed by the build up of material which has erupted through openings in the Earth's surface.There are two types of lava: Acidic lava Basic lava

ACIDIC LAVAThis type of lava is produced at destructive plate boundaries at subduction zones. Acidic lava has a silica content of 70% or more. It is sticky and moves slowly. Because it is so sticky, gases are trapped and great pressure can build up within the magma chamber. As the magma rises to the surface huge domes are formed within the crater of the volcano. At a critical point the magma can no longer hold its gases and a massive explosion occurs. Acidic lava produces steeply convex volcanoes called dome volcanoes e.g. Le Puys in France and Mount St Helens in the USA.

BASIC LAVAThis type of lava is produced at constructive plate boundaries and at hotspots. Basic lava has a silica content of 55% or less which allows gases to escape easily forming a runny type of lava. When it erupts the lava flows like a fountain into the air forming a wide, gently sloping volcanic cone known as a shield volcano. e.g. The Hawaiian Island of Mauna Loa. The Antrim-Derry Plateau was also formed as a result of basic lava flows from fissures in the earths crust.

Pyroclasts are hot ash, lava, and rock fragments that are thrown out of the volcano. These are sometimes called volcanic bombs.Pyroclastic flows are boiling clouds of ash and rock which travel at great speeds down the sides of the volcano (up to 600km/h). These flows are also called nuée ardentes

Poisonous gases - Carbon dioxide suffocates people and animals when it bubbles from crater lakes. Chlorine and sulfur dioxide are also released. The amount of sulfur dioxide gas released increases rapidly close to the time of the eruption.

Ash, pumice, and dust (tephra)In explosive volcanoes the power of the eruption pulverises rock into a fine ash inside the vent and cone. The ash is thrown many kilometres into the sky and may be carried by winds across the world often disrupting air travel.When lava is thrown from the volcano it is whipped up and becomes full of air bubbles. If it cools quickly enough these bubbles are trapped in the rock and it is now called pumice. Some pieces contain so much air that they can float on water.

Water vapourMany volcanoes along subduction zones release huge amounts of water vapour (from the sea water carried into the manlte by the sinking oceanic plates). As this vapour rises into the air it cools rapidly creating torrential rain during the eruption. The intense rainfall can trigger lahars.

LaharsLahar is an Indonesian word that dsecribes a mixture of water and rock fragments flowing down the slopes of a volcano.Volcanic eruptions may trigger one or more lahars by quickly melting snow and ice on a volcano. Usually 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.

LOCATION OF VOLCANIC ACTIVITYWhere plates seperate (constructive plate boundaries)At these plates of seperation, the convection currents fuel the movement apart of the plates. This seperation of the plates creates a rift valley. Magma rises to fill the space left by the rift valley. When the magma reaches the surface, it begins to cool and then solidifies to form volcanic mountains. These are mid-ocean ridges. Sometimes islands form, such as the volcanic island of Iceland, which formed as part of the Mid-Atlantic Ridge.Where plates collide (at destructive plate boundaries)Where two oceanic plates collide, the heavier plate sinks below the other plate in a process known as subduction. The plate that sinks melts into the mantle. The molten magma then rises up through the cracks and makes its way to the surface through the vent. Eventually it reaches the surface where volcanic islands are formed. e.g. JapanHotspotsSometimes magma is able to reach the surface away from the plate boundary (in the middle of a plate). This is called a hotspot. There are approximately 120 hotspots across the world e.g. the Canary Islands of Hawaii. A magma plume rises through the mantle to the crust. This magma may penetrate through the crust if there is a weakness. If so it will erupt onto the surface to form a volcano. This may build up to form a volcanic island. The magma plume will remain in the same place and the overlying plate will move across it carrying away the volcanic island where it would eventually become extinct. The process will repeat and a string of volcanoes may form. This is known as a mid-plate volcanic island arc.

VOLCANIC LANDFORMSExtrusiveVolcanic coneThey form from magma rising through the crust through the central vent. Once it reaches the surface, cooling magma will harden around the central vent building up to form a volcanic cone. The type of cone will depend on the type of lava flowing from it.a) Shield volcano: these form when the basic lave (55% silica) erupts from a central vent. This lava is very fluid and it is capable of flowing great distances before it solidifies. It will therefore build into a wide cone with gentle slopes. From the air it resembles a warrior shield e.g. The Hawaiian islands.

b) Dome volcanoes: form when acidic lava (high in silica) erupts from a central vent. This usually occurs at a destructive plate boundary. It is steeply sloping beacuse the lava is thick and viscous and barely able to flow. This lava is very explosive e.g. Puy de Dome in France

c) Composite cones: They are large and symmetrical. They are built from alternative layers of lava and ash. When they erupt, they are explosive and pose a threat to nearby life and property. They are the most common of all cones e.g. Mt Vesuvius.

2. Lava plateau / basalt plateauLava plateaux are flat topped upland areas composed of basalt rock. They form as a result of fissure eruptions. This is where the lava pours out of long cracks in the crust, e.g. a constructive boundary. The lava is basic in nature (low in silica, runny - approx. 55% silica) this means it is very fluid and capable of flowing great distances before it cools. Over time with each successive lava flow it builds up to form a huge flat-topped upland, As the lava cools it contracts slightly and may form distinctive polygonal shaped columns ranging from 3-12 sides e.g. the Antrim-Derry Plateau of which the Giant's Causeway is part of. It is the largest lava plateau in Europe and formed 60 million years ago. Lava filled the river bed and cooled slowly, cracking hexagonal columns.

IntrusiveThese form when magma fails to reach earth's surface, the magma cools to form intrusive rock e.g. granite in the crust (plutonic) BatholithThese are large, intrusive/plutonic masses of rock. They may stretch up to 100 km in width. They form when a large mass of magma pushes upwards into earth's crust and cools slowly within the crust. Over many years the batholith may become exposed, at the surface as a result of denudation (weathering + erosion). They may also be exposed as a result of colliding plates which buckle the land and may expose the batholith. They are usually surrounded by metamorphic rock (rock that was once igneous or sedimentary which was changed by great heat of the mass of magma) e.g. The Leinster Batholith.

Sill -  Forms when magma enters and cools horizontally in the bedding planes of the orck (horizontal sheet of granite).Dyke - Forms when magma enters and cools in the vertical fissures of the crust.Laccolith - Forms when magma pushes overlying rock upwards. The magma cools and hardens.Lopolith - Forms when magma pushes the underlying rock downwards. It cools and hardens - bowl-shaped.

Hydrothermal areas:These are areas where volcanic activity once ocurred. Moisture collects, is then heated by magma and eventually reaches the surface.Hydrothermal areas include the following: Geysers Hot springs Black smokers Geysers are jets of steam and hot water that leap into the air at regular intervals. e.g. Old Faithful in Yellowstone National Park in the USA. Geysers are also sometimes used to generate energy e.g. The Blue Lagoon in Iceland. Hot springs are found where groundwater is heated at great depths and then rises to the surface e.g. Iceland Black smokers resemble a chimney opening. They are found at mid-ocean ridges where water hotter than 400 degrees rises out from the mantle through vents in the ocean floor.

The prediction of volcanic activity and its effects....Ground deformation:Swelling of a volcano signals that magma is rising within it. Scientists monitoring an active volcano will often measure the tilt of the slope (tiltmeter) and track changes in the rate of swelling. When the land around a volcano is deformed by magma underground, the water level in nearby wells may rise or fall. If the volcano is near the coast, the shoreline may rise or be submerged. An increased rate of swelling, especially if accompanied by an increase in sulfur dioxide emissions and earthquakes, is almost a sure sign of an impending eruption.History of volcanic eruptions:Checking historic records can identify the pattern of eruptions.Gas emissions:As magma nears the suface, gases escape more easily. Sulfur dioxide is one of the main volcanic gases, and increasing amounts of it are usually indicative of more and more magma nearing the surface.Seismic patterns:Volcanoes trigger earthquakes before they erupt. These quakes have a pattern that can help predict when the volcano will erupt. Volcanic earthquakes have three major forms: short-period earthquakes, long-period earthquakes and harmonic tremor. Short wavelength earthquakes are related to the fracturing of brittle rock as the magma forces its way upward from the magma chamber. These earthquakes signify the growth of a magma body near the surface. Long wavelength earthquakes are believed to indicate increased gas pressure in a volcano's 'plumbing system'. They are similar to the claning of a plumbing system in a building. Harmonic tremor quakes occur when there is sustained movement of magma below the surface. The volcano is continuously shaking as if it were humming.

Effects of volcanic activity POSITIVE Fertile soils --> Volcanic soils are very fertile. These rich soils are called laterite soils and are rich in minerals. These soils are sometimes called terra rossa. They are common to many countries such as Brazil, where coffee is grown, and parts of southern Italy, where fruit and vegetables are grown. Tourism --> Many dormant and active volcanoes attract hundereds of thousands of tourists each year. e.g. Mount Vesuvius in Italy and Mount Etna in Sicily. Tourists flock here to climb the sides and experience the landscape. This leads to many spin-off industries such as bus companies, restaurants, shops, souvenir shops, and hotels. This generates a lot of employment. In the case of Mount Vesuvius, nearby locations such as Pompeii, destroyed in 79 AD by the violent eruption of Vesuvius, also attract tourists. Yellowstome National Park, where the Geyser Old Faithful erupts every hour, is also very popular with tourists. Geothermal energy --> is responsible for providing approximately 80% of homes in Iceland with their electricity. Energy is harnessed in areas where water is superheated to temperatures often over 1000 degrees due to rocks being heated below them. The water does not evaporate but instead the steam and hot water are pushed to the surface where turbines harness the energy produced. This energy is then turned into electricity as is done in Iceland. Creation of new land --> Volcanoes can create new land for human habitation e.g. Iceland. Building materials --> Granite, which is used in the construction industry, is formed by volcanic activity. NEGATIVE Lahars --> Lahars are formed where ice and snow on top of a volcanic cone melt when an eruption occurs. Theses lahars create mudflows, which can travel down the slopes of a volcanic cone into the surrounding region. This can have devestating effects for towns and villages e.g. Mount Pinatubo in the Philippines. Nuée Ardentes --> These are clouds of hot ash and poisonous gases that are ejected from a volcano. They create what is sometimes called glowing avalanches. These clouds travel  down the volcanic cone sides at huge speeds of up to 200km per hour and can travel great distances. Loss of life --> The eruption of Mount Vesuvius in 79 AD caused death and destruction. More than 20,000 people died and the towns of Pompeii and Herculaneum were destroyed. Damage to property --> Lva can burn everything in its path. Effect on the environment --> Clouds of ash, dust, and gas may reduce global temperatures by several degrees. The emission of sulfur dioxide into the atmosphere may lead to acid rain. Other toxic, poisonous gases such as carbon monoxide and sulfur can also be harmful to humans. In addition pyroclastic material ejected from a volcano can damage the environment e.g. CO2 adds to the greenhouse effect. Sulfur dioxide causes environmental problems as it is converted to sulfuric acid in the stratosphere. This is a major cause of acid rain.

There are three main types of lava flow:Pahoehoe lava - ropey texture, runny or fluidAa lava - LumpyPillow lava - Rounded blobs of lava on the sea floor.



Volcanoes continued