5.3: Volcanic Hazards

Question 1

Where are the four places where volcanic activity is found?

Answer 1

• along ocean ridges
• hotspots
• subduction zones
• rift valleys

Question 2

Outline how volcanic activity can be found along oceanic ridges.

Answer 2

Along oceanic ridges, plates move apart and magma is forced up to the surface, cooling and forming new crust.

As the plates move further apart new crust is carried away from the ridge (sea floor spreading).

Question 3

Outline how volcanic activity can be found on or near subduction zones.

Answer 3

The deeper the oceanic plate descends, the hotter the surroundings become. This, together with the heat generated by friction, begins to melt the oceanic plate into magma (occurring in the Benioff zone).

As it’s less dense than the surrounding material, this molten magma begins to rise as plutons of magma. Eventually reaching the surface and forming volcanoes.

Question 4

What happens if volcanic eruptions from subduction zones take place offshore?

Answer 4

A line of volcanic islands (aka an island arc) can appear eg in West Indies.

Question 5

Outline how volcanic activity can be found near hotspots.

Answer 5

In certain places, a concentration of radioactive elements below the crust causes a hotspot to develop. From this, a plume of magma rises to eat into the plate above. When this lava breaks through to the surface, active volcanoes form above the spot.

Question 6

What are the two types of plate margins where volcanic activity is found?

Answer 6

Constructive and destructive

Question 7

Which type of plate margins has no volcanic activity occurring?

Answer 7

Conservative.

Question 8

Can volcanic activity occur in the middle of a plate?

Answer 8

Yes - eg the Hawaiian hotspot and along rift valleys.

Question 9

How is the magnitude of volcanic activity measured?

Answer 9

Using a logarithmic scale from 0 to 8 known as the Volcanic Explosivity Index.

Question 10

Characteristics of an Icelandic volcano?

Answer 10

Basaltic lava.

Lava flows gently from fissures.

Question 11

Characteristics of Hawaiian volcanoes?

Answer 11

Basaltic lava.

Lava flows gently from a central vent.

Question 12

Characteristics of a Strombolian volcano?

Answer 12

Thicker basaltic lava.

Frequent, explosive eruptions of tephra and steam. Occasional short lava flows.

Question 13

Characteristics of Vulcanian volcano?

Answer 13

Thicker basaltic and rhyolitic lava.

Less frequency, but more violent eruptions of gases, ash and tephra.

Question 14

Characteristics of Vesuvian eruptions?

Answer 14

Thicker basaltic and rhyolitic lava. Following long periods of inactivity, very violent gas explosions blast ash high into the sky.

Question 15

Characterics of Pelèean volcanoes?

Answer 15

Andesitic and rhyolitic lava.

Very violent eruptions of nuèes ardentes.

Question 16

Characteristics of Plinian volcanoes?

Answer 16

Rhyolitic lava.

Exceptionally violent eruptions of gases, ash and pumice. Torrential rainstorms cause devastating lahars.

Question 17

How can the frequency of volcanic eruptions be determined?

Answer 17

By volcanologists interpreting its previous history of activity using the deposits associated with the volcano itself and those within the wider region in can affect.

Question 18

What equipment can be used to predict volcanic eruptions?

Answer 18

Seismometers and seismographs

Question 19

What does seismic activity (measured using seismometers) indicate?

Answer 19

Microquakes which indicate rising magma fracturing and cracking the overlying rocks.

Question 20

Ground deformation is measured using tiltometers. But what does this indicate?

Answer 20

Bulging (inflation) of the ground is caused by riding magma.

Question 21

Upward movement is iron rich magma is measured using magnetometers. What does this indicate?

Answer 21

Changing magnetism within the volcano is a common geophysical indication of rising magma.

Question 22

How can volcanic eruptions cause acid rain?

Answer 22

Steam is produced during volcanic eruptions, which may also contain nuclei of sulphur.
Heat from the eruption encourages convection currents to rise.

High above the volcano, the steam condenses into localised cumulus clouds, resulting in bursts of acid rain.

Question 23

Outline primary effects of volcanic activity.

Answer 23

• tephra: solid material of carrying size (ranging from volcanic bombs to ash) are ejected into the atmosphere).
• pyroclastic flow: very hot (800*C), gas charged, high velocity flows made up of gas and tephra. Hug the ground and flow down sides of volcano at 700km per hour.
• lava flows
• volcanic gases: including CO2, carbon monoxide, sulphur dioxide, chlorine.

Question 24

Secondary effects of volcanic activity?

Answer 24

• lahars (volcanic mudflows): melted snow as a result of the eruption combined with volcanic ash forms mud flows that can move down the course of the river valleys at high speeds.
• flooding: when an eruption melts glaciers and ice caps
• tsunamis: sea waves generated by violent volcanic eruptions eg Krakatoa.
• acid rain: volcanoes emit gases including sulphur. When this combines with atmospheric moisture, acid rain results.
• climatic change: the ejection of huge amounts of volcanic debris into the atmosphere can reduce global temperatures.

Question 25

What did the people of Mt Etna in Sicily do once the lava had started to flow and was fairly viscous?

Answer 25

They dig trenches, dropped blocks into the lava stream and used explosives to slow down the flow and diverted it from the built environment.

Question 26

What did the people of Haeimaey (Iceland) do to divert lava flow?

Answer 26

They poured seawater on the for the so that it would quickly solidify