Volcanoes Flashcards
Why are eruptions of some volcanoes more explosive?
Acidic (Silica-rich) lava has more gases
- gases dissolved in magma are trapped under great pressure inside the earth
- as magma rises to the surface, pressure decreases, allowing bubbles to expand
- the more viscous the lava, the more difficult it is for bubbles to expand. So, more pressure builds up within the bubbles.
- when the bubbles arrive at the surface, an explosion occurs.
- thus, the viscosity of magma and the amount of trapped gases in magma are the two factors that are most influential in determining a volcano’s eruptive style and explosiveness.
What are the two types of volcanoes?
- Shield volcanoes (divergent)
- Stratovolcanoes (convergent)
What are the features of stratovolcanoes?
- Magma chamber
- Pipe + secondary pipe
- Alternating layers of ash and lava
- Cone-shaped
What determines the form/shape of a shield volcano?
- Basic lava has low viscosity 🡪 Flows along the surface faster 🡪 Able to travel for greater distances
- Basic lava also has a higher temperature 🡪 and takes a longer time to cool and solidify 🡪 producing shield volcanoes of great areal extent, with gentle slopes
What determines the eruptive behavior of a shield volcano?
Low basaltic lava (low silica) 🡪 low in viscosity 🡪 dissolved gases phase out readily 🡪 bubbles burst with limited force 🡪 gentler eruptions
What determines the form of a stratovolcano?
- Basic lava has high viscosity 🡪 Flows along the surface slower 🡪 Able to travel for shorter distances
- Basic lava also has a lower temperature 🡪 and takes a short time to cool and solidify 🡪 producing shield volcanoes with steep slopes nearer the summit
What determines the eruptive behaviour of a stratovolcano?
Acidic lava (silica-rich) 🡪 high viscosity 🡪 dissolved gases phase out slowly 🡪 bubbles burst with explosive force 🡪 violent eruptions
How are calderas formed?
- during an explosive eruption, the summit of a volcano may be blown off
- given that the removal of magma in the magma chamber is faster than it can be replenished, support for the volcano is decreased
- the sides of the crater will collapse inwards to form a large depression known as a caldera
- later, water may fill the caldera to form a crater lake
Subsequent minor eruptions that occur after fresh magma enters the magma chamber may produce a small volcanic island within the lake.
What are the primary volcanic hazards?
- Lava flows
- Pyroclastic Flow
- Tephra and ash falls
- Volcanic gases
What are the secondary volcanic hazards?
- Lahars(mudflows)
- Volcanic landslides
- Atmospheric effects
- Volcanic tremors and earthquakes
- Tsunami
What is pyroclastic flow?
- High-density mixtures of hot, dry rock fragments and hot gases that move away from the vent/crater that erupted them at high speeds
- Rock fragments range from ash to boulders
- Pyroclastic flows travel across the ground at speeds typically >80km/h
- The extreme temperatures of rocks and gas inside pyroclastic flows (generally between 200°C and 700°C) can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses.
What is tephra or ash flow?
- Tephra: Collective term for all airborne or ground-flowing pyroclasts
- Comprises a wide range of rock particles (size, shape, density, and chemical composition)
- e.g. Combinations of pumice, glass shards, crystals from different types of minerals, and shattered rocks of all types - The areal extent of the impact depends on
- the size of materials ejected
- the amount of tephra ejected
- the direction and strength of the prevailing wind - Volcanic ash blasted into the atmosphere.
- Dust may remain for weeks to months before they settle.
Particles block sunlight. - Cause cooling over large areas
What happened during the Mount Pinatubo 1991 eruption?
- Estimated 20 million tons of sulfur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere
- Gases and solids injected into the stratosphere circled the globe for three weeks
What is volcanic gas?
- The most abundant gas typically released into the atmosphere from volcanic systems is water vapor (H20), followed by carbon dioxide (C02) and sulfur dioxide (S02).
What is a lahar?
- Lahar, or volcanic mud-flow, is a hot or cold mixture of water and rock fragments (ranging in size from clay to boulders >10m in diameter) flowing down the slopes of a volcano and/or river valleys
- Formed when rock debris from the volcano crater, tephra or pyroclastic flows incorporate water from
- A crater lake,
- Torrential rainfall or
- Melting snow and ice from the volcano peaks - By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10 times their initial size
- Small lahars are less than a few meters wide and several centimeters deep. They may flow a few meters per second
- Large lahars are hundreds of meters wide and tens of meters deep. They can flow several tens of meters per second
