HAZARDS: VOLCANO

Key features of strato volcanoes?

• Composite Cone Volcanoes

• Layers of ash, acid & lava

• Concave symmetrical profile

What do most strato volcanoes contain?

• Complex internal networks of lava flow

• Form minor igneous features e.g. sills & dykes.

Strat-volcanoes: Features of acid magma?

• Does NOT flow easily

• Vents often filled with a mass of solidified magma.

• Prevent magma rising freely from depth.

• Thus, enormous pressure builds inside volcano = explosive eruption.

What are calderas?

• Volcanic craters - >2km in diameter.

• Explosive eruption destroys much of cone

• Underlying magma chamber is largely emptied.

• W/out support of underground magma - sides of volcano collapse & form caldera.

Example of a caldera?

• Krakatoa, Indonesia: 1883

• Left caldera 7km wide.

What gives rise to chains of strato-volcanoes?

Convergent plate boundaries & explosive eruptions

Example of this?

• Indonesia

• 130 active strato-volcanoes along its archipelago.

What happens at constructive plate boundaries to form volcanoes?

• Tectonic plates move away from one another.

• Earth’s crust pulled apart - creates new pathway for rising hot magma to flow onto surface.

• Volcanoes sometimes form here.

Example of where volcanoes formed in this setting?

• Iceland

• Lies on Mid Atlantic Ridge = constructive boundary, North American & Eurasian plates move away from each other.

• Plates pull apart, magma rises up & erupts as lava = new ocean crist.

Volcanic setting of Iceland?

• Over 100 volcanoes.

• Some extinct, but 30 still active.

• Majority occur along volcanic rift zones that cut through centre of the island.

At what plate boundaries are tectonic plates moving towards each other?

Destructive & convergent

What 2 settings do volcanoes form in on destructive & convergent boundaries?

1. Oceanic plate descends below another oceanic plate.

2. Or, oceanic plate descends below a continental plate.

   = SUBDUCTION

What 2 distinctive types of volcanoes are created during subduction, depending on the setting?

1. Ocean-Ocean subduction = Island-Arc volcano.

2. Ocean-Continent subduction = Andean-type volcano.

What does subduction provide?

A mechanism for introducing water-bearing sediments into the mantle.

How does subduction provide mechanism for introducing water-bearing sediments into the mantle?

• As subducted oceanic plate sinks & heat up, water is gradually released from sediments & minerals within plate ‘slab’.

• Water reduces melting temps of mantle by 60-100 C.

What does this process allow?

The generation of magma at depth - feed volcanoes that are formed on surface.

Where is basaltic lava found?

• Mostly shield volcanoes

• E.g. Mauna Lao, Hawaii

Viscosity of basaltic lava?

Low

Silica content in basaltic lava?

45-52% = low

Temp of eruption for basaltic lava?

1100-1250C = high

What type of volcanic eruption for basaltic lava?

Effusive

Where is andesitic lava found?

• Volcanoes on convergent plate boundaries.

• E.g. Krakatoa, Indonesia

Silica content in andesitic lava?

55-65% = higher

Viscosity of andesitic lava?

High

Temp of eruption for andesitic lava?

900-1100C = lower

Type of eruption for andesitic lava?

Explosive

Where is rhyolitic lava found?

• Explosive

• E.g. Composite & Caldera volcanoes

Silica content of rhyolitic lava?

70-80% = higher

Viscosity of rhyolitic lava?

High

Temp of eruption of rhyolitic lava?

800-1000C = lower

Type of eruption for rhyolitic lava?

Explosive

7 hazards from volcanic eruptions?

1. Lava flows

2. Pyroclastic flows

3. Tephra

4. Gas emissions

5. Lahars

6. Floods

7. Tsunamis

What are Lava flows dependent on?

• Impacts depend on lava type.

• Basaltic: free-flowing, can run considerable distances.

• Acidic: E.g. Rhyolite, thick/pasty, don’t flow easily.

Impacts of lava flows?

• Everything in path of lava = burned, bulldozed or buried.

• Destroy infrastructure, property & crops.

• Lava flows rarely cause injuries/fatalities.

Example of lava flows?

• July 2015: Hawaii

• Lava flow extended 20km before stopping.

• August same year: flow reported to have covered 800m in a day.

What are pyroclastic flows?

• Combination of very hot gases (>500C), ash & rock fragments.

• Travel at high speeds = 100km/h.

• Follow contours of ground & destroy everything in their path.

3 examples of types of pyroclastic flow debris?

1. Volcanic bombs

2. Ash (<2mm)

3. Lapilli (2-16mm)

Impacts of pyroclastic flows?

• Inhalation of hot & poisonous gas + ash.

• Causes almost instant death.

Example of pyroclastic flow?

• AD 79: Mount Vesuvius

• Pyroclastic flow overwhelmed town of Pompeii.

What is tephra?

• Material ejected from a volcano into air.

• Ranges from fine ash to large volcanic bombs = over 6cm across

How big are large pieces of tephra?

• Over 64mm

• Blocks & bombs

• Shot ballistically from volcano

features of Blocks & bombs?

• 8-30 tonnes.

• Fall as far as 1km from source.

How far do small blocks/bombs fall?

• Far as 20-80km

• Velocities: 75-200m/s.

Example of tephra?

• 1980: Mount St Helens

• Secondary thickening occurred.

• Ash deposits thicker in one area than others.

• When ash particles & water form clumps - produce larger particles with higher terminal velocities.

What did ash cover in Mount St Helens eruption?

• Ash covered town of Yakima

• Approx. 80 miles east from volcano

• 10mm of ash.

What are gas meissions?

• Wide range of toxic gases.

• CO2, carbon monoxide & sulphur dioxide.

• Gases pose deadly but silent threa

Impacts of lahars?

• Everything in path destroyed or buried under thick layers of debris.

• Places e.g. SE Asia - ash-covered slopes of volcanoes generate lahar hazards after heavy rain.

Example of lahars?

• 1984: Nevado del Ruiz

• (Columbia town) Armero = overwhelmed by lahars.

• 23,000 died.

Eruptions on Hawaii?

• Effusive

• Characterised by outpouring of lava

What do both Kilauea & Mauna Loa have?

• Rift systems

• Created by extensional forces = occasional sites of fissure eruptions.

Key feature of Kilauea craters?

• On eastern flank of Kilauea craters line a rift.

• Known as ‘Chain of Craters’

• Pu’u O’o crater = most active at present.

Where is all Hawaiian volcanic activity at present?

• Big Island

• Currently above mantle plume

What is the only potentially active Hawaiian volcano outside of Big Island?

• Haleakala on Maui

• Last erupted around 1790

Danger of lava on Hawaii?

• Lava flows frequently & freely

• Not considered major hazard

• May be locally destructive

When was the Kilauea eruption?

1983-2001

Impacts of Kilauea eruption?

• Destroyed 13km of highway

• Approx. 180 buildings

• Almost entire destruction of village of Kalapana.

What type of lava is on Hawaii?

Basaltic

2 types of basaltic lava on Hawaii?

1. Pahoehoe - smooth, rope-like surfaces.

2. A’a - Jagged, rough or clinkery surfaces.

Vog & Kilauea?

• Kilauea - erupting almost continuously since 1986.

• Eruption associated with release of 2,000 tonnes of sulphur dioxide per day into atmosphere.

How is vog formed?

• Gas carried up Kona coast by low-altitude winds - react with moisture in air.

• Forms sulphuric acidic droplets = vog.

How far does this vog travel?

100 miles or more - depends on wind patterns.

Impacts of vog?

• Not fully understood

• Believed to lead to respiratory problems.

• Kona coffee plants & other vegetation damaged.

• Drinking water - potentially contaminated.

Example of explosive activity on Hawaii?

Kilauea - 2 main recorded periods of explosive activity = 1790 & 1924.

Why did this explosive activity happen?

• Probably from expanding steam.

• Generated from heated water on/near ground - mixed with magma & hot rocks.

Impacts of explosive eruption from Kilauea?

• Fields of volcanic debris around Kilauea.

• 1790 eruption.

• 35 deep in some places.

Where else is there evidence of volcanic debris?

• Around summit crater of Mauna Loa.

• Some blocks bigger than 7 feet across.

• Smaller blocks projected - 2 miles.