4. Primary and secondary volcanic hazards

Describe tephra as a primary hazard giving reference to a case study and further impacts?

• Solid material of varying sizes that is ejected by a volcano – ranges from ash (less than 2 mm in diameter) to volcanic bombs (more than 64mm).

• Lapilli sized material (6-64mm) can be carried upward within a volcanic plume and downwind within a volcanic cloud but is dropped to the ground when the eruption cloud cools. Volcanic ash (ash fallout) (<2mm) can be carried within the plume for long distances

• Eg: Eyjafjallajokull 2010: airspace closed for a week, 10 million people left stranded, Kenyan farm workers laid off because fresh flowers and veg for European markets couldn’t be transported (disruption to trade blocs). Locally, it contaminated water sources, and thick deposits of ash made farming hard

Describe volcanic gas as a primary hazard giving reference to a case study and further impacts?

• Eruptions releasing large quantities of gases such as sulphur dioxide and carbon dioxide

• Eg: Lake Nyos 1986: 1,700 killed when large amounts of CO2 were released. Flowed silently downhill and released odourless, colourless gas so people weren’t aware/didn’t know to evacuate

• LIC - Cameroon - no scientific measurements or schemes to prepare, now there is

Describe pyroclastic flows as a primary hazard giving reference to a case study and further impacts?

• Very hot (800 degrees C), high velocity (700km per hr) made up of gas and tephra. They form when the eruption column collapses and the gas and tephra rush down the sides of the volcano

• Eg: Chances Peak, Montserrat 1995: Been dormant for over 300 years and the flows covered the surrounding area and capital (Plymouth) causing many homes, hospitals etc to be destroyed. However, evacuation was successful and only 19 died.

• LIC - helped by international aid: British government provided a £41 million aid package, and British Navy evacuated 7,000 residents

Describe lava flows as a primary hazard giving reference to a case study and further impacts?

• Lava flows travel at different speeds depending on slope, temperature and viscosity

• Most move slowly so don’t kill people but can cause significant damage to property and infrastructure, can melt snow and ice causing flooding

• The fastest flows can reach speeds of up to 60 mph

• Eg: Mount Nyiragongo, Democratic Republic of Congo 2021 - lava flows travelled almost 10 km, destroying properties and killing over 30 people

Describe nuées ardentes as a primary hazard giving reference to a case study and further impacts?

• A more dense, red glowing type of pyroclastic flow only flowing 50km from the source

• Eg: Mount Pelée - worst volcanic disaster of the 20th century, 30,000 killed, Saint Pierre city destroyed within minutes, only 2 survivors

Describe mudflows/lahars as a secondary hazard giving reference to a case study and further impacts?

• These are fast-moving mixtures of water, volcanic ash, rock fragments, and debris that flow down the sides of a volcano and along river valleys. They can travel tens to hundreds of kilometers from the volcano.

• Eg: Mount Pinatubo, Philippines 1991 - Huge amounts of ash were deposited during the eruption → Seasonal monsoon rains repeatedly mixed with the ash. Lahars continued for years after the eruption, burying villages and farmland.

Describe tsunamis as a secondary hazard giving reference to a case study and further impacts?

• Eg: Krakatoa 1883 - the volcano was magnitude 6 and destroyed itself collapsing into the ocean generating a 120 feet tall tsunami (sea wave). Carried a steamship 1 mile inland, wiped out 165 villages in Java and Sumatra. 90% of people who died, died from the tsunami

Describe acid rain as a secondary hazard giving reference to a case study and further impacts?

• Forms when sulphur dioxide emitted during the eruption reacts with water vapour in the atmosphere to form sulphuric acid → acid rain

• Eg: Katmai, Alaska 1912 - harmed many ecosystems killing vegetation and animals, however, it is a rural, remote area so not many people were effected just small villages

Describe landslides as a secondary hazard giving reference to a case study and further impacts?

• Mount St Helens 1980, USA - volcano was bulging due to a blocked vent and upwelling magma so when a 5.1 magnitude earthquake struck that part of the volcano collapsed → largest landslide in history up to 249km/hr → filled Toutle River valley up to 180m deep with avalanche debris

Describe flooding as a secondary hazard giving reference to a case study and further impacts?

• Eyjafjallajokull was dormant for nearly 200 years but in 2010 when it exploded it melted parts of the volcano’s icecap → >140 floods → 800 people were evacuated so there were no deaths

Describe climatic change as a secondary hazard giving reference to a case study and further impacts?

• Ejection of huge amounts of volcanic ash into the atmosphere can block the sun reducing global temperatures and slowing down climate change

• Eg: 1815 eruption of Mount Tambora, Indonesia. The most powerful eruption in recorded history and directly caused the "Year Without a Summer" in 1816.