Impacts of Volcanic Activity (AQA A-Level Geography): Revision Notes

Impacts of Volcanic Activity

Volcanic eruptions can create a wide range of effects that vary from localised impacts near the volcano to global consequences. These impacts are classified into primary effects (direct results of the eruption) and secondary effects (indirect consequences that follow).

Understanding volcanic explosivity

Volcanic eruptions show enormous variation in their intensity and scale. The nature of the eruption depends largely on the characteristics of the lava itself. Lava that is not viscous (meaning it's thin and runny) will escape from the volcano quite easily. However, when lava is highly viscous (thick and pasty), gases become trapped and cannot move freely. This causes tremendous pressure to build up inside the volcano, eventually leading to explosive eruptions.

These explosive eruptions can launch volcanic dust high into the upper atmosphere. This reduces the amount of incoming solar radiation reaching Earth's surface, which can cause short-term global climate change.

The VEI scale helps scientists and geographers classify eruptions and compare their relative sizes. For example, the 1980 eruption of Mount St Helens had a VEI of 5, whilst the massive 1815 Tambora eruption reached VEI 7. The largest eruptions on the scale (VEI 8) are rare super-volcano events that occurred thousands of years ago.

Primary effects

Primary effects are the immediate impacts caused by material directly ejected from the volcano during an eruption.

Tephra

The size of tephra particles determines how far they travel and what damage they cause:

• Volcanic bombs (as large as a bus) are extremely dangerous but affect only a small area because few people venture close enough to the vent to be struck by them
• Fine ash can be thrown high into the atmosphere where strong winds carry it around the Earth, spreading over vast distances
• Ash reduces incoming solar radiation and has a cooling effect on global temperatures

Pyroclastic flows

These flows are among the most deadly volcanic hazards:

• They typically hug the ground and flow down the sides of the volcano
• Speeds can reach up to 700 km per hour
• The extreme heat and speed make them virtually impossible to outrun

Lava flows

Lava flows rarely cause injury to people because of their relatively low velocity. People can usually move out of their path in time. However, lava flows cause significant damage in other ways:

• They destroy crops and agricultural land
• They damage and destroy buildings and infrastructure
• They block roads, cutting off access routes

Volcanic gases

Volcanoes release various gases during eruptions, including:

• Carbon dioxide
• Carbon monoxide
• Hydrogen sulphide
• Sulphur dioxide
• Chlorine

Secondary effects

Secondary effects are indirect consequences that result from the initial eruption, often causing damage after the eruption itself has finished.

Lahars

The water that triggers lahars can come from several sources:

• Heavy rainfall during or after an eruption
• Melting snow and ice on the volcano's summit
• The collapse of crater lakes

Flooding

When an eruption melts glaciers and ice caps on a volcano, serious flooding can result downstream. The rapid melting of large volumes of ice can release enormous quantities of water in a very short time.

Volcanic landslides

The high velocity and great momentum of volcanic landslides enable them to:

• Cross between valleys
• Run up slopes for several hundred metres
• Travel long distances from their source

Tsunamis

Sea waves can be generated by violent volcanic eruptions, particularly when they occur in or near the ocean. These can be triggered by underwater explosions, landslides entering the sea, or the collapse of volcanic calderas.

Acid rain

Volcanoes emit gases that include sulphur compounds. When sulphur mixes with atmospheric moisture, acid rain forms. This can damage:

• Vegetation and crops over a wide area
• Buildings and infrastructure
• Water supplies
• Ecosystems

Climatic change

When huge amounts of volcanic debris and gases are ejected into the high atmosphere, they can:

• Reduce global temperatures by blocking incoming solar radiation
• Create a "volcanic winter" effect
• Cause short-term climate disruption lasting months or years
• Affect weather patterns across the planet

Major eruptions with high VEI values have been linked to past climatic changes, with ash and gas remaining in the atmosphere for extended periods. For instance, the 1815 eruption of Mount Tambora led to the "Year Without a Summer" in 1816, causing widespread crop failures and food shortages across Europe and North America.

Responses to volcanic hazards

Preparedness and prediction

Early prediction provides the opportunity to evacuate people from danger zones before an eruption occurs, potentially saving many lives. Scientists can now predict volcanic eruptions with increasing certainty by monitoring several warning signs:

• An increase in the release of various gases, particularly sulphur dioxide and carbon dioxide
• A rise in the level of lava lakes in volcanic craters
• The bulging upwards of surrounding land due to pressure from rising magma below
• An increasing number of relatively small earthquakes caused by the rising magma

Studying the previous eruption history of any volcano is crucial. By examining ash, lahar and pyroclastic flow deposits around volcanoes, scientists can:

• Determine the frequency of eruptions
• Identify the types of hazard that occurred
• Predict what might happen in future eruptions
• Map areas at greatest risk

Risk assessments

Governments in volcanic regions, such as the Philippines, conduct comprehensive risk assessments that:

• Map hazard zones around volcanoes
• Identify populations at risk
• Establish evacuation procedures
• Create alert level systems

Mitigation measures

Mitigation involves taking actions before, during and after an eruption to reduce or eliminate long-term risks.

Physical actions during eruptions:

Several practical measures can reduce impacts:

• Diverting lava flows: On Mount Etna in Sicily, trenches have been dug, blocks dropped into lava streams, and explosives used to slow down or divert lava flows. These methods have been successful in some cases
• Protecting water supplies: In 1973, inhabitants of Heimaey in Iceland poured seawater onto the front of an advancing lava flow. This successfully solidified it before it could cut off their vital fishing port from the open sea
• Building barriers: In parts of the Hawaiian Islands, barriers have been constructed across valleys to protect settlements from lava flows and lahars

Evacuation strategies:

The most common way to reduce human impacts is to evacuate vulnerable areas when risk becomes intolerable. However, evacuations present their own challenges:

• If evacuations are carried out needlessly, future warnings may be ignored
• People may be reluctant to leave their homes and property
• Evacuations are more difficult to manage if carried out repeatedly

Sometimes permanent evacuation becomes necessary. For example, two-thirds of the Caribbean island of Montserrat has been designated an exclusion zone following the 1995 eruption of the Soufrière Hills volcano. People cannot return to live in this area.