Volcanic Hazards

Vulcanicity, Magma and Lava Types, Volcanic Hazards, Types of Volcanoes,

Why learn about volcanoes?

1. They kill people every year when they erupt. They also cause economic problems such as airport closures

2. Some are deadly and have the potential to wipe out most/all life on our planet (supervolcanoes, e.g. Yellowstone)

3. The more we learn about them, the more we can predict and prepare for them

4. They created our atmosphere (all 4 layers) and still release CO2 into our atmosphere

5. They release water vapour onto our planet’s surface so they created 50% of the water on our planet

6. Volcanoes bring social and economic opportunities (tourism)

What is meant by the “nature” of a volcanic hazard?

• The distribution - where they are

• The frequency - how often they erupt

• The magnitude - size, type and violence of the eruption on the VEI scale

What is the VEI scale and what does it measure up to?

Volcanic explosivity index

1-8

Where are volcanoes located? (distribution)

• 95% of all volcanoes are on/near plate boundaries

• 5% are intraplate

Where are most volcanoes found?

60-70% are found around the Pacific plate

Volcanoes at constructive and destructive plate boundaries (frequency and magnitude)

• More frequent and lower magnitude eruptions tend to be found at constructive plate boundaries. Lava will be more runny, travel further, gasses will be able to escape easily

• Less frequent and higher magnitude eruptions tend to be found at destructive boundaries. Thicker, slower moving lava, gases trapped inside are not able to escape as easily

Magma

A mixture of lava (molten rock) AND gas found in the mantle

Lava

Hot molten or semi-fluid rock erupted from a volcano

If magma reaches the planets surface, the gas escapes and leaves behind just the lava

What are the three types of lava?

1. Basaltic (thin and runny)

2. Andesitic

3. Rhyolitic (thick and pasty)

What do the silica contents of lava determine?

How thick or viscous it is

Silica levels determining the characteristics of lava

Above 55%, lava will be thicker, more viscous and slower moving. Gases trapped inside are not able to escape as easily

Below 55%, lava will be more runny, travel further and gases will be able to escape easily

Characteristics of basaltic lava

FOR REFERENCE:

• Location

• Silica levels

• Eruption temperature

• Viscosity

• Hazard risk

• Plates

• From deep in the mantle

• Low silica content (40%)

• Eruption temperature 1000°C

• Non-viscous (runny), flows over long distances and can create black ash (tephra); gentler eruption

• Low hazard risk

• Mostly found at constructive plate boundaries

Characteristics of andesitic lava

FOR REFERENCE:

• Location

• Silica levels

• Eruption temperature

• Viscosity

• Hazard risk

• Plates

• A mix of old and new plates

• High silica content (55%)

• Eruption temperature 800°C

• Viscous (solidifies quickly); flows over short distances; pressure can build into violent eruptions

• Sticky consistency

• High hazard risk

• Mostly found at destructive (O&O or O&C) plate boundaries

Characteristics of rhyolitic lava

FOR REFERENCE:

• Location

• Silica levels

• Eruption temperature

• Viscosity

• Hazard risk

• Plates

• A mix of old and new plates

• High silica content (65%)

• Eruption temperature 700°C

• Viscous (solidifies quickly); flows over short distances; pressure can build into violent eruptions

• Very sticky consistency

• High hazard risk but very rare

• Mostly found at destructive (C&C) plate boundaries or hot spots

Pyroclastic flows

Burning clouds of hot rocks, lava, ash and gases released during a volcanic eruption, typically from eruptions of composite volcanoes

What are pyroclastic flows also known as?

Nuee Ardentees

What speeds and temperatures can pyroclastic flows reach?

• Speeds of up to 700km per hour

• Temps between 350 and 1000°C

The scale of pyroclastic flows

The scale of pyroclastic flows can be vast, covering 1-10km³

Lava flows

A mass of flowing lava caused directly by the eruption of the volcano

Temperature range of lava flows

600-1250°C

Give an example of a volcano with basaltic lava flows and how fast it flows

Mt. Nyiragongo, 60mph

Give an example of a volcano andesitic/rhyolitic lava flows

• Mt. Pinatubo

• Mt. St Helens

• Mt. Vesuvius

What gases can volcanoes emit?

• Carbon dioxide - CO₂

• Carbon monoxide - CO

• Hydrogen sulphide - H₂S

• Sulphur dioxide - SO₂

• Chlorine - Cl

Ash fallout

Volcano ejects ash high into the atmosphere (>30km)

Danger to aircraft and can reduce sunlight

Give an example of a volcanic eruption which produced ash fallout

Soufriere Hills Volcano - 1997 eruption

Lahar

100°C boiling mudflow, can reach speeds of up to 60km per hour and are highly erosive

How are lahars formed?

Heat melts snow on volcanoes which then turns into water and mixes with the gases to make mud

Give an example of a volcanic eruption which produced a lahar

Nevada Del Ruiz eruption - 1985

100°C, 60km/h, 25,000 dead

Tephra

Rock fragments that are ejected into the sky during a volcanic eruption

Range of ash particles d<2mm to rock of d≤1m e.g. volcanic bombs

Nuee ardentees

Pyroclastic flows that are composed of only hot gas, and no tephra

How do volcanoes cause acid rain

When sulphur (from SO₂) released by the volcano combines with moisture in the air, it can cause acid rain

Consequences of acid rain

• Dangerous for ecosystems - can raise the acidity of water (toxic to marine life)

• Can cause weathering of buildings and corrode metals

Magma chamber

Large underground pool of molten rock

Volcanic bombs

Large lump of molten rock that gets fired out of the top of a volcano

Crater

Roughly circular opening at the top of a volcano

Vent

Tube through which magma travels to the surface

Ash

Fine material blown out of the top of a volcano

Cone

Shape a volcano looks like

Secondary cone

Small cone on side of volcano

Active volcanoes

A volcano which is erupting or has erupted recently

Dormant volcanoes

A volcano that is not currently active but could erupt in the future

Extinct volcanoes

A volcano that will never erupt again

Describe the processes of a volcanic eruption

1. A large pool of molten rock collects near the surface in the magma chamber

2. Pressure builds inside magma chamber as more molten rock forces its way upwards

3. Molten rock (magma) moves towards the surface through cracks in the Earth’s crust

4. Eventually the crust gives way above and the magma rushes to the surface through the vent and out through the crater. Lava, ash and volcanic bombs erupt from the volcano

What are all of the different types of volcano?

1. Shield

2. Fissure

3. Acid dome

4. Composite

5. Ash-cinder

6. Caldera

Shield volcanoes

• Constructive plate boundaries

• Built of fluid lava flows which cools and solidifies from the summit vent creating a gentle slope

Fissure volcanoes

• Constructive/destructive plate boundary

• Basaltic lava slowly seeps onto a flat plain; volcano is hardly visible

Acid dome volcanoes

Very viscous lava which solidifies quickly giving a steep-sided cone

Composite volcanoes

• Destructive plate boundaries

• Typically large and steep sided with alternating layers of lava flow and ash

• They have an internal pipe system which feeds the volcano from below the Earth’s crust

• They tend to have a crater as well as vents and when it erupts it may rise above 2500m in height

Ash-cinder volcanoes

• Destructive plate boundary

• Loose debris from the explosion falls around the volcano and builds a conical shape

• e.g. Eric Barone crashed his bike on Cerra Negro, an active cinder cone volcano in Nicaragua

Caldera volcanoes

• Huge explosions remove the cone summit leaving a large opening which may become flooded

• When supervolcanoes erupt, they will also leave behind a caldera

Mnemonic for describing distribution

GCSE

• General trend

• Calculation (% change, etc.)

• Specifics (time, dates, amounts)

• Exceptions (anomalies)