2.b. Volcanic eruptions generate distinctive hazards

Question 1

As with all tectonic activity, volcanoes become hazardous when they interact with what?

(Hazards produced by volcanic activity)

Answer 1

When they interact with human communities and activities.

Question 2

Outline the nature of volcanic eruption hazards.

(Hazards produced by volcanic activity)

Answer 2

Most of the hazards they pose are relatively short term.

However, eruptions (VEI - 8 and above) emit very large quantities of ash into the upper atmosphere and can have implications for longer-term climatic change, e.g. Mount Tambora.

Question 3

Outline the hazards generated from the super-volcano Toba in Indonesia.

(Hazards produced by volcanic activity)

Answer 3

Led to reductions in global temperatures as ash blocked sunlight from reaching the Earth’s surface; linked with a dramatic fall in the global human population at that time.

The release of SO₂ also added to cooling; as SO₂ mixed with water, forms sulphuric acid, which reflects insolation.

Question 4

Outline lava flows as a volcanic hazard.

(Lava flows)

Answer 4

The impacts are dependent on type of lava.

Basaltic lava is free-flowing and can run for considerable distances.

Acidic lavas such as rhyolite are thick and pasty so do not flow easily.

Everything in the path of lava will be either burned, bulldozed or buried, e.g. infrastructure, property and crops.

They rarely cause injuries or fatalities.

Question 5

What is the named example for lava flows? Explain.

(Lava flows)

Answer 5

Hawaii, 2015.

A lava flow extended for 20 km before stopping.

In August the same year a flow was reported to have covered about 800 m in a day.

Question 6

Outline pyroclastic flows as a volcanic hazard.

(Pyroclastic flows)

Answer 6

These are a combination of very hot gases (500°C+), ash and rock fragments travelling at high speed (100 km/h).

They follow the contours of the ground and destroy everything in their path.

The inhalation of such hot and poisonous gas and ash causes almost instant death.

Question 7

What is the named example for pyroclastic flows? Explain.

(Pyroclastic flows)

Answer 7

Pompeii, AD 79.

The Roman town was overwhelmed by a pyroclastic flow from Mount Vesuvius.

Question 8

What is tephra?

(Tephra)

Answer 8

Any material ejected from a volcano into the air.

Ranges in size from very fine ash to large volcanic bombs (>6 cm across), including lighter debris such as pumice.

Question 9

What are the potential environmental impacts of tephra?

(Tephra)

Answer 9

Potentially very hazardous, burying farmland in layers of ash and destroying crops.

Eruption columns can carry material into the stratosphere because the temperature of the ejected material is always much hotter than the surrounding air, so it rises very fast, in addition to the propulsion of the explosion itself.

Eruptions emit a wide range of toxic gases, including CO, CO₂, and SO₂; when SO₂ combines with atmospheric water it produces acid rain, damaging crops and polluting surface water and soils.

Question 10

What are the potential human impacts of tephra?

(Tephra)

Answer 10

Transport can be disrupted both on the ground and in the air.

Buildings can collapse due to the weight of accumulated ash.

People with respiratory diseases may have difficulty breathing.

Eruptions emit a wide range of toxic gases, including CO, CO₂, and SO₂; which can pose a deadly threat to human populations.

Question 11

What is the named example for tephra? Explain.

(Tephra)

Answer 11

Iceland, 2010.

The eruption of Eyjafjallajökull volcano led to the cancellation of 100,000 flights.

Question 12

What are lahars? How are they formed?

Answer 12

A type of mud flow with the consistency of wet concrete, with the ability to travel at speeds up to about 50 km/h.

Snow and ice on a volcano summit melt during an eruption and flow rapidly down the cone.

Rock fragments large and small, as well as ash and soil, are mixed together.

Everything in their path is either destroyed or buried under thick layers of debris.

Question 13

What is the named example for lahars? Explain.

(Lahars)

Answer 13

Nevado del Ruiz, 1984.

Following the eruption, the Colombian town of Armero was overwhelmed by lahars; killing 23,000 people.

Question 14

Where are lahars often seen? Why?

(Lahars)

Answer 14

Southeast Asia.

This is because ash-covered slopes of volcanoes continue to generate lahar hazards after periods of heavy rain.

Question 15

Outline floods as a volcanic hazard.

(Floods)

Answer 15

Volcanic eruptions beneath an ice field or glacier cause rapid melting.

During an eruption, vast quantities of water accumulate until they find an exit from under the ice.

Question 16

What is the named example for floods? Explain.

(Floods)

Answer 16

Iceland.

Several active volcanoes lie under the Vatnajökull ice field.

Question 17

What are tsunamis? How do they form?

Answer 17

The violent eruption of some island volcanoes can cause massive displacement of ocean water and tsunami waves capable of travelling at speeds of up to 600 km/h.

In deep water they have a height that is usually less than 1 metre and a very long wavelength of up to 200 km.

Approaching the shore, tsunami waves increase rapidly in height and when they break, transfer vast amounts of energy and water along the shore and inland.

Question 18

What is the named example for tsunamis? Explain.

(Tsunamis)

Answer 18

Indonesia, 1883.

The tsunamis created by the eruption of Krakatoa in 1883 are believed to have drowned 36,000 people.

Question 19

What is the named example for toxic gases? Outline it.

(Toxic gases)

Answer 19

Lake Nyos, Cameroon.

One of a number of deep lakes that occupy volcanic craters in Cameroon in West Africa.

It is 2 km wide and 200 m deep.

Question 20

What happened to Lake Nyos in August 1986? Why?

(Toxic gases)

Answer 20

1700 people and all animal life in the area around the volcano were asphyxiated.

The cause was a leak of CO₂, from a volcanic crater lake.

The gas had built up at the bottom of the lake after being emitted from the underlying magma chamber.

CO₂ is a dense gas and when it escaped it flowed down the volcano slopes as a 50 m thick ground-hugging layer travelling at about 70 km/h.

It is not known for certain what caused the CO₂ to escape the lake.

Possible explanations are a deep movement of magma, an earthquake, a change in water temperature in the lake, or strong winds stirring up the lake waters.