Volcanic hazards (intro) Flashcards
What is the definition of a volcano?
A vent in the earth’s surface where lava, rock, vapour, and gases erupt. A mountain formed by volcanic material.
What is magma?
Molten rock in the earth’s core
What is lava?
Magma that erupts onto the earth’s surface
What are pyroclasts?
Solid fragments released from a volcano
What is tephra?
All pyroclasts that fall onto the ground from eruptions
What are the four eruption types?
Monogenetic
Polygenetic
Gentle
Voilent/Explosive
Why do volcanoes erupt?
Magma is buoyant, less dense than the surrounding rocks and rises through the crust to erupt on the surface
Eruption with lots of entrapped gas
Very viscous and brittle lava doesn’t flow as quickly, explosive eruption
What does a volcanic eruption depend on?
Level of viscosity and gas content
Eruption with little entrapped gas
Not very viscous and slow moving lava, effuisve eruption
______ is a function of magma viscosity
Explosiveness
What is viscosity determined by?
Silica content (composition)
Temperature
Gas content
What causes plumes to rise to the earth’s surface?
Primordial heat- outer mantle convecting, bringing about temperature variations causing plumes to rise from the core-mantle up to the earth’s surface
What is the lower mantle?
Solid rock composed of silicon, magnesium oxides and sulphides
What is the upper mantle?
Not liquid but a ‘plastic’ so it can deform and flow, lots of heat by convention
What is the crust?
Formed by melting of the mantle. Higher silicone and less magnesium and sulphides
Fundamental relationship between volcanic activity and ____________ ___________
Plate tectonics
Where does the majority of volcanic activity occur?
On or along constructive or destructive plate boundaries
What does plate melting occur due to?
Heating e.g. hot spots
Decompression e.g. mid ocean ridge
Hydration
What is the Wilson cycle?
The process of ocean formation at spreading ridges and subduction back into the mantle at convergent margins
What % of active volcanism is spreading ridge (mid-ocean ridge)?
75
What % of active volcanism is subduction zone?
15
What % of active volcanism is intraplate?
10
How do hotspot volcanoes form?
Very hot plumes of material ascend from deep in the mantle and encroach on the base of the lithosphere causing melting
What do hotspot volcanoes look like?
Shallow, smooth, shield volcanoes
How do hotspot volcanoes erupt?
Non-explosive basaltic lava flow- if related to oceanic crust
What is basalt?
Most common rock type in earth’s crust
2/3 of surface
Shield volcanoes- lava viscosity, magma type and shape
Low viscosity lava flows (aa and pahoehoe)
Low silica magma- basalt
Gentle sloping flanks
Volcanism with oceanic and continental plates
Subduction of oceanic crust below continental crust, forming a deep trench
Volcanism with oceanic and oceanic plates
Subduction of oceanic plate below another oceanic plate, forming deep trench
Volcanism with continental and continental plates
Collision of two continents, forming a mountain range
Decreasing ability to flow = _______ viscosity
Increased
How does gas affect viscosity?
Decreasing chance of bubble to escape is what leads to explosive eruption, build up and up until it is able to escape
What is the VEI?
Volcanic explosivity index
Intensity of eruption
1 number= 10x increase
What is a strombolian eruption?
Low viscosity lava
Explosive basaltic eruption (loud blast)
Cinder cone forms
What is a vulcanian eruption?
High viscosity lava
High gas pressure
5-20 km high andesitic and dacitic eruption
What is a Plinian eruption?
High viscosity lava
>20km high dacitic and rhyolitic eruption
Large pyroclastic flows
What are the three rock types from subduction zone related volcanism?
Andesite
Dacite
Rhyolite
|- Increasing silicone content, increasing viscosity
