Volcanic hazards Flashcards
volcanoes
Where do volcanoes occur?
1.Most volcanic activity is associated with plate tectonic processes and are mainly located along plate margins/ 95% of all volcanic eruptions are associated with plate margins.
2.Found along Ocean ridges where plates are moving apart and magma is forcing its way through to the surface, cooling and forming new crust.
- On or near subduction zones (composite). Oceanic plate subducts under the continental plates, generating heat in the Benioff zone. Magma rises and forms a volcano.
- They can occur over hotspots.
- Associated with rift valleys/ (constructive plate margins) in areas such as East Africa. Magma forces its way to the surface through thinning crust to form volcanoes.
What is a pyroclastic flow? (primary hazard)
Formed from a mixture of hot gas and tephra. After ejection from the volcano, they can flow down the sides of a mountain at speeds of over 700 km per hour. Pyroclastic flows are more common in eruptions with andesitic or rhyolitic lava which tend to occur at subduction zones.
What is are nuees ardentes ? (primary hazard)
A dense, rapidly moving cloud of hot gases, ashes and lava fragments from a volcanic eruption; a type of pyroclastic flow. They contain more dense material than pyroclastic flows and therefore do not travel as far.
What are lava flows?
Lava flows- lava can flow quickly or slowly depending on its viscosity. Silica makes lava viscous and slow, which is common in explosive eruptions.
What is tephra? ( primary hazard)
The solid matter ejected by a volcano into the air. Ranges from volcanic bombs to ash. The larger fragments tend to fall closer to the volcano , while smaller particles are carried by the wind and travel greater distances both laterally and vertically.
What are Lahars? (secondary hazard)
A destructive mudflow which occurs as a result of a volcanic eruption.
What are the affects of ash fallout?
Ash can be very disruptive, it can cover a wide area, often thousands of kilometres. Predicting its range can be difficult as the strength and direction of the prevailing wind can vary. Ash is highly abrasive and can cause breathing problems, disrupt machinery and clog filters. It can obstruct sunlight, reducing visibility and temperatures. Almost, 20 million tonnes of ash erupted from Mount Pinatubo in the Philippines in 1991 causing global cooling of 0.5 degrees Celsius.
What are the other secondary hazards?
- tsunamis- a large wave triggered by seismic activity
-acid rain- volcanoes erupt sulphurous gases which can result in acidic rainwater.
What is the magnitude of volcanoes?
Vulcanicity is measured using the Volcanic Explosivity Index(VEI) the more powerful, the more explosive. It is a logarithmic scale and ranges from 0 to 8. Multiple features are considered when calculating the VEI, including how much tephra is erupted, how long it lasts, how high the tephra is ejected etc.
What is the frequency of volcanic events?
Frequency of eruptions varies per volcano. Volcanoes are classed as either active, dormant or extinct. To determine the frequency of the eruption of any volcano, its previous history of activity can be interpreted by volcanologists, using the deposits associated with the volcano itself. Usually, a higher frequency eruption means the eruptions are effusive whereas low frequency means the eruptions are explosive.
Frequency of volcanic eruptions depends largely upon their type. Kilauea, on the island of Hawaii is an active shield volcano which has erupted basaltic lava continuously since 1983. In contrast, many volcanos that erupt rhyolitic lava erupt very infrequently. The Yellowstone caldera has erupted three times in the last 2.1 million years. Generally, basaltic lava is erupted from volcanos at constructive plate boundaries and hotspots, while volcanos at destructive plate boundaries tend to erupt more vicious, andesitic or rhyolitic lava.
Frequency of eruptions varies per volcano. Volcanoes are classed as either
active, dormant or extinct. Usually, a higher frequency eruption means the eruptions are
effusive ,whereas low frequency means the eruptions are explosive.
What is regularity?
Volcanic eruptions are regular in that the eruptions on each type of boundary are similar (e.g. eruptions on destructive boundaries will regularly be explosive)
Predictability of volcanic event-
Regularity of eruptions can help estimate when eruptions will take place
signs that a volcanic eruption will happen soon:
1. animal behaviour
2.increased seismic activity
3. gases are produced/ can smell sulphur /changes in concentration of CO2 and sulphur dioxide
4.local water sources heat up
5. change in shape/ ground deformation this is measured by tiltmeters as magma starts to move and bulge beneath the volcano
-To begin to accurately predict an eruption, volcanologists need to know a volcano’s eruption history and to be currently monitoring it. They then need to interpret the data. Even with this information, it is impossible to predict the exact size, nature and timing of an eruption.
-Volcanologists can measure local seismic activity and ground deformation using tiltmeters as the magma starts to move and bulge beneath the volcano. Changes in the concentration of carbon dioxide and sulphur dioxide can also signal an imminent eruption
What are the types of magma?
-Basaltic- low silica content , low viscosity ( runny), gases escape easily , non- explosive eruptions. Generally basaltic lava is erupted from volcanoes at constructive boundaries and hotspots.
-Andesitic- medium level of silica content, intermediate viscosity and explosiveness
-Rhyoltic- high silica content, high viscosity, gases cant explosive easily and very explosive eruptions. Volcanoes at destructive plate boundaries tend to erupt rhyoltic or andestic lava.
What are the primary impacts of volcanic hazards?
Primary impacts are those that occur immediately following a eruption.
1.Lava and pyroclastic flows can destroy roads and cause buildings to collapse. Crops can also be damaged and water supplies contaminated by ash fall.
- Tephra -people can be injured or injured and can sometimes cause suffocation.
- volcanic gases
Environmental: Ecosystems damaged through various volcanic hazards. Wildlife is killed.
Economic: Businesses and industries destroyed or disrupted
Social: People killed and homes damaged by lava/ pyroclastic flows
Political: Government buildings and other important areas are destroyed or disrupted
What are the secondary impacts of volcanic hazards?
Secondary impacts are those that occur in the days and weeks that follow the eruption.
- Lahars- often occur when volcanic material mixes with water from rainfall or snowmelt. These fast flowing rivers of mud can kill and injure people. They can cause further destruction to roads and buildings, making it difficult for emergency services to reach people in need and leads to businesses being destroyed, causing high levels of unemployment.
Environmental: water acidified by acid rain. Volcanic gases contribute to the greenhouse effect and therefore global warming. Fires can also be caused by pyroclastic flows and lava flows which can lead them to spread out of control.
Economic: profit from tourism industry. However, businesses can be destroyed leading to high levels of unemployment.
Social: Fires can start which puts lives at risk. Homelessness also can occur if homes are damaged. People can suffer from psychological problems if they lose their homes or lose relatives and friends in the eruption. There also may be a shortage of food, especially if an area is dependent on agriculture.
Political: Conflicts concerning government response, food shortages, insurance etc.
Nature of Vulcanicity / association with plate tectonics:
Volcanic hazards are largely dependant upon the the position of the plate boundaries, 95 % of all volcanic eruptions are associated with plate margins , although 5% are intraplate.
The nature of volcanic hazards are dependant upon the type of eruption:
-Hawaiian eruptions are effusive and produce basalt lavas / basalt lavas have a low silica content, which means lava is fluid and can flow long distances. They have relatively high temperatures.
-In contrast, Plinian eruptions are much more explosive and produce andesitic or rhyolitic lavas. These lavas have lower temperatures and a higher silica content than basaltic lavas. This means lava is more viscous, flows more slowly and builds up in the vent of the volcano until it is suddenly released in an unpredictable and explosive eruption.
-Lava flows tend to be more of a threat to property than human life. However, when large quantities of lava are released at once, they can be incredibly dangerous.
What are fissure eruptions?
-Change landscapes
-They represent the largest contributors to global climate change and large- scale landscaping
What are shield volcanoes?
-Shallow sided and broad
-runny lava
-basaltic landscapes
-small ash clouds
-typical of CONSTRUCTIVE plate margins
-tourist attractions
-shorter periods between eruptions
-no layers as the volcano just consists of lava
What are composite volcanoes?
-steep sided as the lava does not flow far before it solidified
-alternate layers of ash and lava (aka as strato volcanoes )/ builds up the volcano in layers, layers produce weaknesses that can be exploited by the magma
-violent eruptions
-longer periods between eruptions
-produce lots of tephra
What are Calderas?
A large volcanic crater, often formed following a highly explosive, violent eruption where the summit of the volcano is removed
-creates new land
- sever impacts (Yellowstone)
What are the short term responses of volcanic events?
The responses to volcanic eruptions depend upon the magnitude of the eruption as well as the vulnerability of the local population. Short term responses are likely to involve the evacuation of people at risk from further eruptions and the deploying of emergency services to provide first-response treatment. Sometimes the short-term response is domestic. However, if the country is particularly vulnerable, then often international help is offered.
Preparedness strategies to reduce impacts of volcanic hazards
Preparedness is the state of readiness for a volcanic eruption.
-Monitoring increases the notice of volcanic eruptions, meaning warnings can be given out.
-Education of volcanoes in areas of risk so people know what to do in the case of a volcanic eruption.
-Evacuation procedures planned
-Training Response teams
What are the Long-term responses?
Long-term responses involve risk management to reduce the impacts of the volcanic hazards through preparedness, mitigation, prevention and adaption. If the volcano is monitored effectively, then it is likely that vulcanologists will be able to better predict future eruptions. Hazard maps can be created and evacuation strategies put into place if necessary. Alert systems, shelters and evacuation camps can be constructed for the local population and emergency food/water supplies can be stored.
Mitigation strategies to reduce the impacts of volcanic hazards
Mitigation is the action of reducing the severity or seriousness of a volcanic eruption
-Direct intervention to the volcano e.g.- concrete blocks to veer lava away from areas at risk.
-Strengthening buildings at risk of mudflows or of ash pileup
-Evacuation and exclusion zones
-Mitigating effects on health by having emergency aid and rescue
