What are the main hazards generated by volcanic activity? Flashcards
What features are associated with Strato-volcanoes?
Also known as composite cone volcanoes, made up of layers of ash and acid lava
Contain complex internal networks of lava flows with form minor igneous features e.g. sills and dykes
The vents of stratovolcanoes are often filled with a mass of solidified magma as acid lava doesn’t flow easily
Caldera
Often at convergent plate boundaries
What are calderas?
Volcanic craters, usually more than 2km in diameter.
Form when an explosive eruption destroys much of the cone and the underlying magma chamber is largely emptied.
What features are associated with Shield volcanoes?
Effusive eruption
Often at diverging plate boundaries
Basic lava can create flood basalts e.g. the Deccan Plateau in central India
When first formed these plateaux have a uniform slope of around 1 degree
Give an example of a hot spot
As the Pacific plate has slowly moved northwest over the Hawaiian hot spot (at an average rate of 10cm/year) vast amounts of basalt have accumulated on the ocean floor to produce the Hawaiian islands.
The active volcano peaks of Mauna Loa and Mauna Kea reach over 4000m above sea level
How do hot spots change over time?
Plates move away from the hot spot and volcanoes lose their source of magma and become extinct.
For example on the Hawaiian island Kauai, volcanism is no longer active
Give an example of hot spots on a continental plate
East Africa rift valley is a 4000km long rift valley containing several active volcanoes. Over the past 30 million years, the crust has been stretch, causing tension. The result if rifting with magma forcing its way to the surface and creating a line of active volcanoes.
Mount Kilimanjaro was formed this way
Give an example of a super-volcano
The Yellowstone super volcano in Wyoming has a caldera measuring 75km
A super volcano erupts 1000km^3 of material
What limitations are there of using the VEI?
The VEI is not that useful for assessing effusive eruptions
How serious are lava flows as a hazard produced by volcanic activity?
The impacts are dependent on the type of lava. basic (basaltic) lava is free-flowing and can run for considerable distances. On Hawaii in July 2015, a lava flow extended for 20km before stopping and in August a flow was reported to have covered 800m in one day.
Acidic lava flows such as rhyolite are thick and pasty so do no flow easily. Everything in the path of the lava will be either burned, bulldozed or buried. Although they destroy infrastructure, property and crops, lava flows rarely cause injuries or fatalities.
What are pyroclastic flows?
A combination of very hot gases (500C+), ash and rock fragments travelling at high speeds (100km/h). They follow the contours of the ground and destroy everything in their path. The inhalation of such hot and poisonous gas and ashes causes almost instant death.
E.g. Pompeii experienced a pyroclastic flow from Mount Vesuvius
What is tephra?
This describes any material ejected from a volcano into the air. It ranges in size from very fine ash to large volcanic bombs (>6cm across). It also includes lighter debris such as pumice. Tephra is potentially very hazardous, burying farmland in layers of ash and destroying crops.
Transport can be disrupted in the air and on ground.
. The loading of tephra on leaves can lead to the burial of plants or the stripping of branches from trees, resulting in a significant impact on agriculture
Give an example of tephra
The eruption of Iceland’s Eyjafjallajökull volcano in April 2010 led to the cancellation of 100,000 flights.
Buildings can collapse due to the weight of accumulated ash and people with respiratory diseases may have trouble breathing
What toxic gases do volcanoes emit?
carbon dioxide
hydrogen chloride
hydrogen fluoride
hydrogen sulphide
sulphur dioxide
How serious are gases erupted from volcanoes?
People can be exposed to harmful volcanic gases by breathing them in or through contact with the skin and eyes. The health effects range from mild to serious, with occasional deadly exposures. After exposure, people may report difficulty breathing and itchy skin.
However when SO2 combines with atmospheric water, acid rain is produced. This enhances weathering and can damage crops and pollute water and soils
What are lahars?
Type of mudflow with the consistency of wet concrete. 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. Lahars can travel at speeds up to 50km/hour.
