Volcanoes

BIG Questions1 – What plate settings do volcanoes occur at?

2 - Volcanoes aren’t equally dangerous....how do their hazards differ, and why?

VOLCANOES

Volcano

•an opening in a planet's

crust, which allows hot molten rock,

ash, and gases to escape from

below the surface.

What comes out of a volcano? Ash

What comes out of a volcano? GasMost common:

H2OCO2

SO2

HCl

What comes out of a volcano?Lava

So...Why are there different types of

Volcanoes•Viscosity of the magma controls the type of volcano.

•Viscosity – a liquid’s resistance to flow

•Low viscosity – flows easily

•High viscosity – flows slowly

•Viscosity is controlled by the composition and temperature of the magma.

•Silica (SiO2) content controls viscosity.

What are the different types

of Magma?

•Basaltic

•Andesitic

•Rhyolitic

Composition

Magma Source

Viscosity Gas % Silica %Explosive

nessLocation

BasalticUpper Mantle

Low 1-2 % ~ 50 %Least / Mild

Divergent Boundaries and Hot Spots

AndesiticOcean Crust & Sediments

Medium - High

3-4 % ~ 60 %Intermedi

ate

Convergent

Subduction Zones

RhyoliticContinental Crust

High - Extreme

4-6 % ~ 70 % Greatest

Convergent

Subduction ZonesHigh silica = high viscosity = explosive

eruptionLow silica = low viscosity = quiet

eruption

Viscosity and Silica content

Shape

•Shapes of volcanoes are due to the viscosity of the magma.

•Runny lava forms relatively flat shield volcanoes with quite eruptions.

•Thick lava forms steep cones with explosive eruptions.

Explosiveness

Explosiveness of the volcano

is controlled by

- the silica content of the magma

- the viscosity of the magma

- the release of gases

Types of Eruptions

Explosive “clogged”Plate 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 Boundaries and Hot Spots

Type of Volcano: Shield volcanoes

Type of Magma: Basaltic composition

Quiet Eruptions•Magma flows easily

•Gasses bubble out gently

Plate Setting: Divergent BoundaryDivergent boundary volcanoes have very low viscous magma

and non-explosive eruptions

Plate Setting: Hot SpotHot spot volcanoes form when mantle plumes rise

through the crust like a blow torch Characterized by low viscous magma and non-explosive

eruptions

Type of Volcano: Shield

•Usually start under water

•resembles a Roman shield lying on the ground

•characterized by relatively quiet eruptions with lava flows that harden on top of each other

Examples: - Hawaiian Islands - Iceland

Type of Magma: Basaltic

• runny, low viscosity lava

• relatively little explosive activity

Kilauea, Hawaii: A typical shield volcano

Basaltic eruptions produce

2 types of lavaPahoehoe – hot, fast moving lava. Surface looks like

wrinkles and rope-like coils

Pahoehoe Lava

Pahoehoe Lava

Basaltic eruptions produce

2 types of lavaAa– cooler, slow moving lava. Hardens to form rough,

jagged lava chunks

Anatomy of a Shield Volcano

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 the volcano explodes

•More dangerous and have more hazards such as pyroclastic flows

Pyroclastic Flows•Pyroclastic flows - fast-moving hot gas and rock fragments which travel away from the volcano

•speeds generally as great as 450 miles/hr

•Can reach temperatures of 1,830 °F

Plate Setting: Convergent BoundaryExplosive volcanoes form at

convergent subduction zones and have highly viscous magma

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

Type Of Magma: Andesitic

- high silica content - highly viscous magma - explosive eruptions

Examples - Paracutin, in Mexico

Anatomy of a Cinder Cone

Type 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 largest percentage of the Earth's volcanoes (about 60%)

Type Of Magma: Rhyolitic - high silica content - highly viscous - explosive eruptions

Examples: Mt. Vesuvius,

Mt. St. Helens and Mt. Rainier in the

Cascade Range

Mt. St. Helens: Before the 1980 eruption

Mt. St. Helens after its 1980 eruption

Anatomy of a Composite Volcano

Pyroclastic flow

Status of a Volcano

•Active – currently erupting or showing signs of an imminent eruption. Risk is high

•Dormant – does not show signs of an erupting in the near future or has not erupted in the recent past

•Extinct – unlikely to ever erupt again

Signs a volcano is about to erupt

•When a volcano begins to show new or unusual signs of activity, it is possible it is about to erupt

•Increased Earthquake Activity

•Ground Deformation

•Change in water composition

•Gas emissions

•Monitoring from space

Volcanic Hazards

Volcanic Hazards• Lava flows

• Ash fall

• Pyroclastic flows

• Mudflows

• Volcanic Gases

• Tsunami

Volcanic Hazards

Volcanic Hazards

Volcanic Hazards

•Volcano Hazards program

•http://volcanoes.usgs.gov/activity/alertsystem/icons.php