A2 Physical Geography - Plate Tectonics and Hazards

Plate Tectonic Theory and Evidence

CONTINENTAL FIT: some continents seem to fit together if placed beside each other. This is particulary true if continental shelves are taken into account.

GEOLOGICAL EVIDENCE:  rocks of the same age and type have been found on either sides of the atlantic. Also they have the same formation in south east Brazil and South Africa. Also trends in mountains suggest that they were once connected.

CLIMATOLOGICAL EVIDENCE: places as far apart as Antartica, the UK and Svarlbard all have similar coal deposits which would have been formed in tropical conditions. As they are no longer in a tropical position, it shows that they have drifted.

BIOLOGICAL EVIDENCE: similar fossil formations are found on either side of the Atlantic. For example, the same reptile remains, called Mesosaurus, is only found in south america and south africa, suggesting they were once connected. Plant remains from the humid swamps that later formed coal deposits have been found in Antartica and India.

Palaeomagnetism

Constructive Boundary Landforms

OCEANIC RIDGES: form the longest uplifted feature on the earths surface - combined length of 60000km. Where two plates pull apart and expose weak parts which are hotter and crust splits, causing rifts to appear.

RIFT VALlEYS:

1) Convection currents cause the continential plate to dome upwards

2) The upward pressure causes the crust to crack leaving an unsupported middle

3) The unsupported section collapses forming a rift valley with steep stacks

4) If the convection currents are strong enough, a line of volcanoes will form at the base of the valley, which will eventually form an ocean form.

Oceanic and Continental Convergence

OCEANIC/CONTINENTAL: due to oceanic crust being denser than continental curst, when plates collide the oceanic crust is subducted, which could cause major earthquakes if pressure builds up over a peroid. Destructive margins are the most seismically active.

OCEANIC/OCEANIC: where two pieces of oceanic plates are moving towards each other. The more denser or the quicker plate will be subducted. These plates are the main cause of island arcs which are just a trail of volcanoes caused by oceanic/oceanic subduction.

CONTINENTAL/CONTINENTAL: as an oceanic plate is being subducted, it forces two land masses to come together. As they have similar density so similar buyoncy, none are subducted but instead they collide. Volcanoes associated with earlier subduction and scraping of the ocean floor, build up to form fold mountains on the fault. Fold mountains are just a build up and pressure of small sediments.

Hotspots

The middle of the Pacific Ocean is the volcanic Hawaiian Islands that aren’t connected with any plate boundary normally associated with plate margins. The volcanoes are caused by a localised hotspot within the Pacific Plate.

The concentrations of radioactive elements inside the mantle cause the hotspot to develop, from this a plume of magma rises to eat into the plate above. Where lava breaks through to the surface, active volcanoes occur above the hotspot. Hotspot is stationary so as the Pacific plate moves over it a line of volcanoes is created.

One above the hotspot is active and the rest form a chain of extinct volcanoes. Hotspots are evidence that the Pacific plate is moving to the North-West.

Extrusive Volcanic Landforms

BASALTIC LAVA: originate largely from the upward movement of mantle material. They are most common along spreading ridges but are also found at hot spots and within more developed rift systems.

ANDESITIC LAVA: are typical of destructive plate margins where crust is being detroyed (destructive plate boundarties.)

RHYOLITIC LAVA: are most often found at destructive and collision margins.

Pyroclastic material comprises a wide range of volcanic fragments from finer-grained ash and lapilli to larger volcanic bombs. They are characteristic of more gaseous phases of eruption, where the build-up of has beneath blocked volcanic vents creates a violent explosion, shredding the magma into final particles.

Volcanic Classification

FISSURE ERUPTIONS: occur where an elongated crack in the crust allows lava to spill out over a large area. It is basaltic rock and occurs mainly on rifts or early constructive margins. Eruptions are generally gentle and persistant.

SHIELD VOLCANOES: are made of basaltic rock and form gently sloping cones made from layers of ash and lava. It contains less viscous lava and basaltic rock. Found usally on hotspots or where oceanic meets oceanic. Eruptions are gentle and predictable.

COMPOSITE VOLCANOES: most common type of volcano found on land and are created by layers of ash from the initial explosive phases. The rock is andesitic and are found mainly on destuctive margins where they're explosive and unpredictable.

ACID OR DOME: steep sided volcanoes formed from very viscous lava. It has rhyolitic rock and is found on continental rock. Also its explosive and unpredictable.

Minor Extrusive Features

GEYSERS/HOT SPRINGS: even in the areas where vulcanism does not produce active volcanoes, water heated at depth in the crust by magma chambers can periodically escape as steam and hot water. A geyser is an intermittent, turbulent discharge od superheated water ejected and accompanied by a vapour phase. Where hot water on its way upwards mixes with muds near the surface, a bubbling, boiling mud volcano may be formed. In some places, hot springs have become a tourist attraction.

FUMAROLES: are areas where superheated water transforms into steam as it condenses on the surface. These features are typical of areas such as Solfatara in Italy, where the escape of steam and water mixed with sulpher-rich gases gives rise to the collective name 'solfatara'.

Instrusive Features

Case Study - Mount Etna July 2001

EFFECTS - SHORT TERM:

• South side buldged and caused eartquakes
• Lava bubbled to the surface and could be observed from the main cone
• Huge plumes of ash and volcanic bombs made there way towards Nicolosi
• The lava carried on the next 24 days, but did not reach any major settlements.

EFFECTS - LONG TERM:

• After a couple of weeks, the ash fell and covered vineyards and the slopes
• Lava damaged ski facilities. This took expensive renovations before the winter ski season
• The eruption made news and people were panicking and cancelled holidays to Sicily and even main land Italy.

Volcanic Case Study - Mount Etna July 2001

OTHER POTENTIAL HAZARDS:

• Seismic activity connecting with eruption activity
• Gas plume emission, volcanic dust amd ashfalls
• Flank collapse before or follwing
• Phreatic eruptions (steam-driven explosions)

IMPACT OF THE EVENT:

• 77 confimed deaths due to people straying into dangerous areas
• In the last 20 years, all deaths hav been due to lightening strikes
• Eruption completely destroyed the tourist station at Piano Provenzana
• Lava took two week to reach other tourist areas
• July 2002, the airport was forced to close while the runways were cleared
• Winter tourist areas were affected as people stayed away

Volcanic Case Study - Mount Etna July 2001

MANAGEMENT AND RESPONSES:

• Etna has a long history of eruptions so people know how to react
• Lava tubes ad explosives were used to force the lava down the hill without hitting the major slopes. However the initial blast shattered the pipes but the lava was still effectively diverted
• During 2002 eruptions, damns of soil and volcanic rock were put up to protect the tourist base
• The Italian Army's heavy earth mvng equipment was brought to block and divert lava flows

MONITORING:

• INGV has monitored the volcano for 20 years with permanent network of remote sensors
• Data is continuously recordd by permanent stations are also integrated with discrete observations, surveys and laboratory analysisto evaluate activity level
• Geochemical monitoring programmes test gas/fluid emissions to help predict new eruptions and warn of dangerous gas emissions. 

Volcanic Case Study - Chaiten, Chile, May 2008

BACKGROUND INFO:

• Initial eruption produced an ash cloud which reached 17km high
• The town of Chaiten was covered with ash
• The ash cloud was so thick, near airports like Argentina were forced to close
• Also some schools were forced to close
• One of the largest chain of volcanoes in the world
• Composed of rhyolitic lava and pyroclastics 

TIMELINE:

• First earthquakes felt 30/04/08 local time
• Eruption began around midnight 01/05/08
• Nearly continuous ash cloud as high as 30km which was between 2-8 May
• Lava dome extrusion with sustained vapour and ash column

Volcanic Case Study - Chaiten, Chile, May 2008

IMPACT OF EVENT:

• Laharas were generated by intense rainfall mixing with the ash
• 90% of the town Chaiten was flooded and the eruption also triggered thunderstorms
• Only 1 death but many livestock were lost
• 20-30% of buildings were completely destroyed 
• Minimal damage to aircraft
• Chaiten airport was closed indefinitely
• Also other aiports in Chile closed, like Chaiten, Osorno, Puerto Montt
• Many domestic and international flights were cancelled as people were scared to visit in case of other eruption.
• Extensive damage to airport and marine facilities further hampered rescue operations as people were trying to evacuat
• The fall of ash covered and asphyxiated some animals as well as blocking many major roads around the country

Volcanic Case Study - Chaiten, Chile, May 2008

MANAGEMENT AND RESPONSES:

• Residents were told to not drink the water s the resrvoirs in the area were all covered with a layer of ash
• Chilean officials distributed fresh clean water as well as protective masks to minimize the choking on ash
• The government issued a monthly disaster stipend of the equivilent between $1200-$2200 per month per family
• The government also ordered a 50km exclusion zone around the town to decrease the amount of people put at risk
• Financial aid to small businesses which have been affected by the eruption have been issued in the form of a 90-day freeze on any exisiting loan repayments to the bank Banco Estado so the money can be spent on renovating and cleaning up 
• They updated their volcanic monitering about bought in the VEI which began on the 17th of May so all real time activity can be viewed

Seismicity

EARTHQUAKES: occur when a build up of prssue over time is released which sends shockwaves through the earths surface causing the ground to shake violently. They occur almost continuously over the surface of the Earth. Theres only been 18 earthquakes which have regsistered 7.0 or over and these are the most deadily of earthqaukes; killing thousands

P-WAVES (PRIMARY): are te fastest waves and shake the waves backwards and forwards. These travel the fastest and move through solids and liquids

S-WAVES (SECONDARY): are slower and move with a sideways motion, shaking the earth at right angles to the directin of travel. They cannot pass through liquids but do more damage than primary waves.

SURFACE WAVES: these travel much nearer to the surface and more slowly than P or S waves but are mre destructive than either. They include long waves and cause the waves to move up and down. They also include Rayleigh waves. 

Magnitude and Frequency 

MAGNITUDE: the amount of energy released by the event and is usually measured on the Richter scale. This is a logarithmic scale, with each unit representing a 10-fold increase in strength and a 30-fold increase in energy released. 

FREQUENCY:  this varies between seismicaly active regions and seismic zones within the shield areas of ancient crust. 

Effects of Earthquakes

TSUNAMIS: enromous sea waves generated by disturbances on the sea floor. They are most often triggered by earthquakes and submarine landslides The most devastating recent example occured in December 2004 in Indonesia

LIQUEFACTION: where a violet distruption of the ground causes it to become liquid-like when strongly shaken. Such extreme shaking causes increased pore water pressure which reduces the effective stress, and therefore reduces the shear strength of the soil so it fails more easily.

LANDSLIDES/AVALANCHES: where slope failure occurs as a result of ground shaking and will cause serious consequences

HUMAN IMPACT: this covers a wide range of effects and depends upon population density and distance from the epicentre. Strong shaking of the ground can cause buildings, roads and bridges to collpase, and disruption to gas, electricity and water supplies. Others occur later as a result of the primary impacts

Earthquake Prediction

SEISMIC RECORDS: studying patterns of earthquakes and using these to predict the next event and its severity. Seismic chock waves and recorded on a seismometer or seismograph.

RADON GAS EMISSIONS: radon is an inert gas that is released from rocks such as granite at a faster rate when they are fractured by deformation.

GROUND WATER: deformation of the ground can cause water levels to rise (compression) or fall (tension) independently of atmospheric conditions

REMOTE SENSING: there is some evidence that electromagnetic disturbances in the atmosphere directly above areas about to have an earthquake can be detected

LOW FREQUENCY ELECTROMAGNETIC ACTIVITY: a sudden change in the ionospheric electron density and temperature was recorded a week before a 7.1 magnitude earthquake occured in southern Japan in September 2004

Protection

As earthquakes can't be prevented, some countries that are prone to them have a range of ways of reducing potential damage. For example:

• making buildings/cities more earthquake resistant
• raising public awareness about diaster prevention via an education programme
• improving earthquake prediction

Tsunami Case Study - Indonesian December 2004

BACKGROUND INFO:

• Triggered by Andaman earthquake of 26 December 2004 which registered 9.1 magnitude, was the third biggest earthquake ever

• While large earthquakes are not uncommon, major tsunamis are rare.

NATURE OF THE SEISMIC HAZARD:

• Trench off the south-west coast of Indoneisa marks where the Australia Plate is subducted under the Burma plate.
• A 15-20m slip occured along 1,600m of faultline in two phases over a peroid of 2 to 3 minutes
• The epicentre was approximately 160km offshore
• Places all over the world experienced freak high tides, an average 1.5 metres above average, 16 hours after the eartquake occured
• The water refracted round land and caught unprepared countries like Kerala

Tsunami Case Study - Indonesian December 2004

IMPACT OF THE TSUNAMI:

• 130,736 confirmed deaths but 167,736 estimated deaths, just in Indonesia
• In total, 230,210 where estimated to be dead, including countries like Maldives and Bangledesh
• Over 1.69 million people were displaced or homeless as their houses were destroyed or damaged by the tide waters
• Contamination of drinking water supplies and farm fields by salt water was a significant problem in many coastal regions
• Tourism was hit badly as tourists chose to avoid the area for fear of repetition
• Enormous environmental impact with severe damage to ecosystems such as mangroves, coral reefs, forests and coastal wetlands
• High economic impacts as Sri Lanka is mainly a fishing country, and as the water destroyed all the boats, no many was being bought in

Tsunami Case Study - Indonesian December 2004

MANAGEMENT AND RESPONSES:

• $7 billion in said was promised for damages regions, mainly from richer countries like Australia and America
• While tsunamis warning systems were in place for the Pacific, the indian ocean has no such system, which in part reflects the economic status of the countries that border the coast. There was no waarning of the event
• The sheer scale of the event meant broken communication links and when the need to communicate across the border is this great, communication is vital
• The fear of disease from the vast number of bodies led to rapid burial or burning in some instances.
• The british public gave 330 million, a sum greater than that was donated by the british government.