OCR a Level Sociology Lesson Element

Learner Resource 10

Examining the different types of volcanic eruptions and the various hazards they generate

Read the Following

Since the year A.D. 1600, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by pyroclastic flows and mudflows, deadly hazards which often accompany explosive eruptions of subduction-zone volcanoes.

The main hazards generated by the different types of eruption include;

lava flows, pyroclastic flows, gas emissions, tephra and ash
lahars and flooding associated with the melting of ice
tsunamis associated with explosive eruption.

The level of death and destruction depends on the nature of the volcano.

Volcanoes generally occur in three distinct areas.

1.Hot spot or mid plate volcanoes

Eruptions of Hawaiian and most other mid-plate volcanoes differ greatly from those of composite cones. Mauna Loa and Mauna Kea, on the island of Hawaii, are known as shield volcanoes, because they resemble the wide, rounded shape of an ancient warrior's shield. Shield volcanoes almost always erupt non-explosively, and they are associated with the outpouring of huge volumes of fluid lava.

Hawaiian-type eruptions are rarely life threatening because the lava advances slowly enough to allow safe evacuation of people. You will often see scientists being able to approach quite close to the eruption. However large lava flows can cause considerable economic loss through the destruction of property and farmland.

In recorded history, explosive eruptions at subduction-zone (convergent or destructive boundary) volcanoes have presented the greatest hazard to civilisations.

At divergent boundaries about three-quarters of the material erupted on Earth is deep in the ocean. Where it does reach the surface the story is mixed. In Iceland, where the Mid-Atlantic Ridge is exposed on land, many volcanoes erupt non-explosively from fissure vents, in similar fashion to typical Hawaiian eruptions, however others, like Hekla Volcano, erupt explosively.

2.Volcanoes at Divergent boundaries

Often a volcano may cause little loss of life but the effects can be dramatic as it was with Eyjafjallajökull on Iceland in 2010. The story can be followed here;

How volcano chaos unfolded;

(a)Summarise the nature of the eruption, the spread of the ash cloud and its impact

This is a useful follow up video about the likelihood of a further eruption;

3.Volcanoes at subduction-zone (convergent or destructive boundaries)

(a)Revisit this webpage to review volcano formation at convergent boundaries at

(b)Then explore the Geological Society website (below) and explain the processes operating and in particular the role of and its effect on the melting point of the mantle and the production of magma.

Two key impacts of Volcanic Eruptions – Lahars and Pyroclastic Flows

(a)Explore the websites and videos below to explain the main impacts of Lahars and Pyroclastic flows giving located examples to illustrate your answer

Lahars – the Hazard

Lahars – the impact

Pyroclastic flows – the hazard

Pyroclastic flows – the impact

Tsunamis

The NOOA website offers a detailed explanation of the formation of tsunamis and their impact. Further useful sites include;

Please complete the question below using your Tsunami research.

(a)Describe and explain the causes of Tsunamis

Review your Learning

Pyroclastic flows, also called nuéesardentes (“glowing clouds” in French), are fast-moving, avalanche-like, ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that can travel at speeds in excess of 150 km per hour.

Mudflows (debris flows or lahars) are mixtures of volcanic debris and water. The water largely comes from one of two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to volcanic material, mudflows can range from syrupy water to thick flows similar to wet cement. As mudflows race down the steep sides of the volcano, they have the strength and speed to destroy or bury everything in their paths.

A Tsunami is a set of ocean waves caused by a sudden large disturbance of the sea-surface. Usually due to an earthquake or landslide.

Hazardous Earth