Post on 30-May-2018
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Volcanoes
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BIG Questions
1 What plate settings dovolcanoes occur at?2 - Volcanoes arent equallydangerous....how do their hazards differ, andwhy?
VOLCANOES
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Volcano
an opening in a planet'scrust, which allows hot molten rock,
ash, and gases to escape from
below the surface.
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What comes out of a volcano?Ash
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What comes out of a volcano?GasMost common:
H2OCO2SO2HCl
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What comes out of a volcano?
Lava
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So...Why are there differenttypes of Volcanoes
Viscosity of the magma controls the type ofvolcano.
Viscosity a liquids resistance to flowLow viscosity flows easilyHigh viscosity flows slowly
Viscosity is controlled by the compositionand temperature of the magma.
Silica (SiO2) content controls viscosity.
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BasalticAndesitic
Rhyolitic
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Composition
MagmaSource
Viscosity Gas % Silica % Explosiveness
Location
Basaltic UpperMantle
Low 1-2 % ~ 50 % Least /Mild
DivergentBoundaries and HotSpots
Andesitic OceanCrust &Sediments
Medium -High
3-4 % ~ 60 % Intermediate
Convergent
Subduction Zones
Rhyolitic Continent
al Crust
High -
Extreme
4-6 % ~ 70 % Greatest Convergen
tSubduction Zones
High silica = high viscosity = explosive eruption
Low silica = low viscosity = quiet eruption
Viscosity and Silicacontent
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Shape
Shapes of volcanoes are due to theviscosity of the magma.
Runny lava forms relatively flat shieldvolcanoes with quite eruptions.Thick lava forms steep cones with explosive
eruptions.
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Explosivenes
Explosiveness of the volcano
is controlled by
- the silica content of the magma
- the viscosity of the magma
- the release of gases
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Types of Eruptions
Explosive cloggedPlate Setting: Convergent Boundary Subduction Zones
Type of Volcano: Composite volcanoes and Cinder Cones
Type of Magma: Andesitic or Rhyolitic composition
Quiet runny
Plate Setting: Divergent Boundariesand Hot Spots
Type of Volcano: Shield volcanoes
Type of Magma: Basaltic composition
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Quiet Eruptions
Magma flows easilyGasses bubble out gently
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Plate Setting: Divergent Boundary
ivergent boundary volcanoes have very low viscous magma
and non-explosive eruptions
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Plate Setting: Hot Spot
Hot spot volcanoes form when mantle plumes rise
through the crust like a blow torchCharacterized by low viscous magma and non-explosive
eruptions
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Type of Volcano:
Shield
Usually start under water
resembles a Roman shield lying on the ground
characterized by relatively quiet eruptions with lavaflows that harden on top of each other
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Examples:- Hawaiian Islands- Iceland
Type of Magma: Basaltic
runny, low viscosity lava
relatively little explosiveactivity
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ilauea, Hawaii: A typical shield volcan
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Basaltic eruptions produce 2 types of lava
oehoe hot, fast moving lava. Surface looks lwrinkles and rope-like coils
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Pahoehoe Lava
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Pahoehoe Lava
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Basaltic eruptions produce 2 types of lava
cooler, slow moving lava. Hardens to form roujagged lava chunks
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Anatomy of a Shield Volcano
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Explosive Eruptions
Magma is thick and sticky
Magma slowly builds up in the volcano's pipe
Gasses cannot easily escape from the magma
Trapped gasses build up pressure until thevolcano explodes
More dangerous and have more hazards such aspyroclastic flows
Pyroclastic Flows
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Pyroclastic Flows
Pyroclastic flows - fast-moving hotgas and rock fragments which travelaway from the volcano
speeds generally as great as 450miles/hr
Can reach temperatures of 1,830 F
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Plate Setting: Convergent Boundary
Explosive volcanoes form atconvergentsubduction zones and have highly viscous
magma
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Type of Volcano: Cinder Cone
relatively small (less than 300 m or 1000 ft high) relatively steep slopes (30 - 40 degrees)
made of pyroclastic material
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Type Of Magma: Andesitic
- high silica content- highly viscous magma- explosive eruptions
Examples- Paracutin, in Mexico
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Anatomy of a Cinder Cone
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ype of Volcano: Composite
large (1 - 10 km across) Also called a Stratovolcano layered structure, consisting of alternating
layers of lava and pyroclastic material
These volcanoes make up the largestpercentage of the Earth's volcanoes (about
60%)
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Type Of Magma: Rhyolitic- high silica content- highly viscous
- explosive eruptions
Examples:Mt. Vesuvius,
Mt. St. Helens andMt. Rainier in the Cascade
Range
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Mt. St. Helens: Before the 1980 eruption
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Mt. St. Helens after its 1980 eruption
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Anatomy of a Composite Volcano
Pyroclasticflow
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Status of a Volcano
Active currently erupting orshowing signs of an imminent
eruption. Risk is high
Dormant does not show signs of anerupting in the near future or hasnot erupted in the recent past
Extinct unlikely to ever eruptagain
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Signs a volcano is about toerupt
When a volcano begins to show new orunusual signs of activity, it ispossible it is about to erupt
Increased Earthquake Activity
Ground Deformation
Change in water composition
Gas emissions
Monitoring from space
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Volcanic Hazards
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Volcanic Hazards
Lava flows
Ash fall
Pyroclastic flows
Mudflows
Volcanic Gases
Tsunami
l i d
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Volcanic Hazards
V l i H d
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Volcanic Hazards
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Volcanic Hazards
Volcano Hazards program
http://volcanoes.usgs.gov/activity/alertsystem/icons.php
http://volcanoes.usgs.gov/activity/alertsystem/icons.phphttp://volcanoes.usgs.gov/activity/alertsystem/icons.php