Volcanic Hazards

Question 1

How are volcanoes built

Answer 1

They are built by the accumulation of their own eruptive products: lava, bombs (crushed over ash deposits) and tephra (airborne ash and dust).

Question 2

What are ‘bombs’

Answer 2

Crusted over ash deposits

Question 3

What’s tephra

Answer 3

Airborne ash and dust

Question 4

What is a volcano most commonly like

Answer 4

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.

Question 5

What is a volcanic eruption

Answer 5

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.

Question 6

The type of volcano and volcanic activity depends upon the…

Answer 6

The nature of the lava. In turn this depends upon the location of the volcano with regard to tectonic plate margins.

Question 7

If the lava is a thin fluid (not viscous)….

Answer 7

The gases may escape easily

Question 8

If the lava is thick and dense (highly viscous) the..

Answer 8

Gases will not move freely but will build up tremendous pressure, and ultimately example with explosive eruptions.

Question 9

Define vulcanicity

Answer 9

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.

Question 10

Tell me about the volcanic activity at a destructive margin

Answer 10

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.

Question 11

Tell me about the volcanic activity at a hotspot

Answer 11

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.

Question 12

Tell me about the volcanic activity at a constructive margin

Answer 12

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)

Question 13

Tell me about the volcanic activity of basaltic lava

Answer 13

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

Question 14

Tell me about the volcanic activity of Andesitic lava

Answer 14

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

Question 15

Tell me about the volcanic activity of rhyolitic lava

Answer 15

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

Question 16

The impact of a volcanic event is only deemed hazardous when it has an effect on…

Answer 16

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.

Question 17

List me hazardous effects of a volcanic event

Answer 17

Tephra

Pyroclastic flows

Nuée ardente

Lava flows

Volcanic gases

Lahars

Flooding

Tsunamis

Climatic change

Question 18

What is tephra

Answer 18

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.

Question 19

What are pyroclastic flows

Answer 19

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.

Question 20

What’s nuée ardente

Answer 20

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.

Question 21

What are lava flows

Answer 21

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.

Question 22

What are volcanic acids

Answer 22

These include carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur dioxide and chlorine.

Question 23

What are lahars

Answer 23

Volcanic mudflows

Question 24

How can a volcanic event lead to flooding

Answer 24

Caused by the melting of ice caps and glaciers, such as glacial bursts

Question 25

How can a volcanic event lead to tsunamis

Answer 25

Giant sea waves set off by huge explosions

Question 26

Tell me how a volcanic event can lead to climatic change

Answer 26

The ejection of vast amounts of volcanic debris into the atmosphere can reduce global temperatures and is believed to have been an agent in past and present climatic change.

Question 27

Tell me about one way of classifying a volcanoes hazardous nature

Answer 27

We can classify a volcanos hazardous nature by its explosivity, using the volcanic explosivity index (VEI).

A volcanoes impact can be judged in terms of its primary and secondary effects, and the environmental, social, economic and political consequences, both short term and long term.

Question 28

What two categories can the management of and responses to volcanic events fall into

Answer 28

Prediction and protection

Question 29

Tell me about using prediction for the management of and responses to volcanic events

Answer 29

Study the eruption history of the volcano

Measure gas emissions, land swelling, groundwater levels

Measure the shock waves generated by magma travelling upwards

Question 30

Tell me about using protection for the management of and response to volcanic events

Answer 30

Asses the hazard, ie try to determine the areas of greatest risk that should influence land use planning

Dig trenches to divert lava

Build barriers to slow down lava flows

Administer explosive activity to try to divert a lava flow

Pour water on the lava front to slow it down