Volcanic Hazards Flashcards
How are volcanoes built
They are built by the accumulation of their own eruptive products: lava, bombs (crushed over ash deposits) and tephra (airborne ash and dust).
What are ‘bombs’
Crusted over ash deposits
What’s tephra
Airborne ash and dust
What is a volcano most commonly like
A conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the earth. There are approximately 500 active volcanoes around the world. Only a few of them are erupting at any one time.
What is a volcanic eruption
An eruption is when a volcano gives off quantities of lava, ash or volcanic gas. A few volcanoes erupt more or less continuously, but others lie dormant between eruptions, when they give out very little gas and lava.
The type of volcano and volcanic activity depends upon the…
The nature of the lava. In turn this depends upon the location of the volcano with regard to tectonic plate margins.
If the lava is a thin fluid (not viscous)….
The gases may escape easily
If the lava is thick and dense (highly viscous) the..
Gases will not move freely but will build up tremendous pressure, and ultimately example with explosive eruptions.
Define vulcanicity
The progress through which gases and molten rock are either extruded on the earths surface or intruder into the earths crust: it is clearly linked to the existence of plate margins.
Tell me about the volcanic activity at a destructive margin
The magma source is a mix of old oceanic plate, ocean sediments and continental fragments, often weathered by water
Rock name: andesite/rhyolite
Magma chemistry: medium to high acidity, greater than 63% silica content.
Magmas physical character: viscous (solidifies quickly), flows over short distances, solidifies even on steep slopes, allows gases to build up pressure - can explode violently.
Tell me about the volcanic activity at a hotspot
Magma source: deep in the asthenosphere (mantle)
Rock name: basalt/gabbro
Magma chemistry: quite basic (alkaline), sometimes relatively rich in sodium and potassium, low silica content (around 50%)
Magmas physical character: quite non-viscous (fairly runny), flows over low angled slopes or can erupt as an ash.
Tell me about the volcanic activity at a constructive margin
Magma source: deep in the asthenosphere (mantle)
Rock name: basalt/gabbro
Magma chemistry: very basic (alkaline), low silica content, typically high iron and magnesium content.
Magmas physical character: very non viscous (runny), flows over Long distances over very low angled slopes or can create a black ash (tephra) when exploding with water vapour (steam)
Tell me about the volcanic activity of basaltic lava
It has a 45-50% silica content level, the lowest of the three
Eruption temperature: 1000 degrees C, the highest of the 3
Viscosity: lowest of the three types of lava, very runny, low gas
Volcanic products: very hot, runny lava (shield volcanoes, low land or plateaux)
Eruption interval is the shortest, can be almost continuous, as on Hawaii
Tectonic setting: organic hotspots and constructive margins
Processes: dry partial melting of the upper mantle/lower lithosphere, basaltic magma is generally uncontaminated by water.
Not really hazardous
Tell me about the volcanic activity of Andesitic lava
Silica content: 55-60%
Eruption temp: 800 degrees c
Viscosity and gas content: sticky, intermediate gas
Volcanic products: sticky lava flows, tephra, ash, gas (composite volcanoes)
Eruption interval: decades or centuries
Tectonic setting: destructive plate margins (ocean/continental and ocean/ocean)
Processes: wet partial melting of subducting oceanic crust contaminated by water and other material as magma rises.
Very hazardous
Tell me about the volcanic activity of rhyolitic lava
Silica content: 65% (the highest of the 3)
Eruption temperature: 700 degrees c
Viscosity and gas content: very sticky, high gas
Volcanic products: pyroclastic flows, gas and volcanic ash (domes)
Eruption interval: millennia
Tectonic setting: continental hotspots and continental/continental margins
Processes: in situ melting of lower continental crust, most rhyolitic (granitic) magmas cool before they reach the surface.
Very hazardous but rare
The impact of a volcanic event is only deemed hazardous when it has an effect on…
People
A volcanic event can produce a number of hazardous effects, the impact of which can range from the area immediately around the volcano to the entire planet.
List me hazardous effects of a volcanic event
Tephra
Pyroclastic flows
Nuée ardente
Lava flows
Volcanic gases
Lahars
Flooding
Tsunamis
Climatic change
What is tephra
Solid material of varying grain size (from fine ash up to volcanic bombs) ejected into the atmosphere. Buildings often collapse under the sheer weight of ash falling onto their roofs. Air, thick with ash, is very difficult to breathe, and can cause serious respiratory problems. Fine tephra can also contribute to acid rain.
What are pyroclastic flows
Very hot (800 degrees c), gas charged, high velocity flows (over 200km/h) of a mixture of gases and tephra. These flows devastate everything in their path.
What’s nuée ardente
A glowing cloud of hot gas, steam and dust, volcanic ash and larger pyroclasts produced during a violent eruption, which can descend the slopes of a volcano at high velocity.
What are lava flows
At the speed at which most lava flows, it does not usually pose a threat to life. Lava flows do, however, represent a threat to farmland, property and infrastructure.
What are volcanic acids
These include carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur dioxide and chlorine.
What are lahars
Volcanic mudflows
How can a volcanic event lead to flooding
Caused by the melting of ice caps and glaciers, such as glacial bursts
