Flashcards in Volcanic Hazards Deck (13)
What is the nature of vulcanicity and relation to plate tectonics?
-uneven distribution of volcanoes around the world
- spread along plate margins, such as Pacific and North American margin
- found at constructive and destructive margins
How are volcanoes formed at destructive margins?
The denser plate (oceanic) sinks beneath the continental plate at the subduction zone. The plate melts to form magma and eventually rises up through cracks in the crust. As pressure builds, a eruption could occur.
How are volcanoes formed at constructive margins?
The pressure where the plates move apart is low so the magma rises up from the mantle and erupts to the surface. Once the magma cools it forms new crust which builds up into a volcano.
What is a shield volcano?
They are low with gently sloping sides and formed from layers of lava. They are found at constructive boundaries. E.g. Mauna Loa.
- non violent eruptions
- lava flows of high speed over long distance, non viscous lava
- frequent eruptions of basaltic lava
What is a composite volcano?
They are steep sided, formed from layers of ash and acidic lava flows. Found at destructive margins. E.g. Mt st Helens
- explosive eruptions of lava and ash
- viscous lava
- when main vent gets blocked, secondary cones form
What are the volcanic gases?
The gases released are CO2, sulfur dioxide, hydrochloric acid and carbon monoxide etc. They can cause acid rain and suffocation.
What is acid rain?
Rainfall made so acidic by atmospheric pollution that it causes environmental harm. The pollution is caused by gases that are released from volcanic rocks and magma, which condense and fall as acid rain. It leads to release of aluminium that does lots of damage.
What is a mudflow?
A fluid or hardened stream or avalanche of mud. Contains suspended particles and silt. A high water content can result in high velocities so can be dangerous.
What is a lava flow?
It’s a mass of flowing or solidified lava. Most flows are basaltic in composition. Two types of basaltic lava flow:
Aa - higher viscosity, slightly cooler and forms large channels
Pahoehoe- lower viscosity, slightly hotter and forms lava tubes
What are pyroclastic flows?
They are fluidised masses of rock fragments and gases that move quickly in response to gravity. Gases within can explode and cause ash to fall on near areas. They can form when: eruption column collapses, result of gravitational collapse or explosion of lava dome/flow.
What is a ash fallout?
They are small jagged pieces of rock, minerals, and volcanic glass the size of sand and silt. They are formed when gases in magma escape into air and when water is heated by magma and flashes into steam. The force of the expanding and escaping gas blasts magma and rock into the air. Can form a eruption cloud.
What is tephra?
This is bits of rock and other particles ejected from a volcano. They can release blocks and bombs, which are pieces of tephra with diameter greater than 64mm, can be put to 30 tons. They can create lightning due to electrically charged material in air and can disrupt communications.