Volcanic Activity:
Examples of volcanic activity:
Japan – On a destructive plate boundary as the Pacific plate subducts the Eurasian plate
nearby
Iceland – Located on the mid-Atlantic ridge where the Eurasian and N. American plates
move apart by 2-5cm per year.
Hawaii – Over the Hawaii hotspot
African Rift Valley – The African plate moves away from the Somalian plate at a constructive
plate margin.
Lava types:
Rhyolitic – 66-75% silica content, so very viscous. Gases therefore don’t dissolve in it (which
allows pressures to build). Flows slowly over shorter distances. Roughly 800C. Acidic
Andesitic – 52%-65% silica. Also, viscous (like above). Intermediate pH.
Basaltic – 45-52%. Opposite of Rhyolitic. Roughly 1200C. Basic.
Lava/eruptions at plate boundaries:
Constructive plate margins – Runny lava (basaltic), not very violent, lava and steam ejected,
frequent, longer eruptions (Kilauea has been erupting since 1983), flatter shield volcanoes –
more expansive.
Destructive – Viscous (rhyolitic/andesitic), violent, literally everything ejected, v. infrequent
(as thick magma plugs vents to build pressure must build for eruption), shorter, composite
cone.
Why?
Destructive:
1. Melting plate is made from silicon and magnesium, so magma produced has a high
silicon concentration.
2. This produces viscous lava with a lower temperature. Gases can’t dissolve in the
magma, causing pressures to build. Eventually gives way, producing explosive
eruptions.
3. Thick lava plugs air vents and holes in the volcano, preventing further eruptions,
making them infrequent.
Opposite at constructive where magma/lava is basaltic as no plate melting.
Volcanic activity at hotspots:
Lack of plate melting produces basaltic magma (e.g Hawaii, Tahiti) with low viscosity. Less
explosive eruptions are produced.
,(Very similar to constructive plate margins, as magma plume melts crust and erupts at
surface).
Volcanic hazards:
Pyroclastic flows
Max distance from volcanic vent – 40km
Average max speed – 700km/h
Max Temperature – 1000oC
Occur – With rhyolitic or andesitic lava only
Mixtures of hot rock fragments, lava particles and ash buoyed by hot gases. The flow cloud
is denser than surrounding air, so it sinks and follows the topography of the land. The high
temperature and high speeds cause them to destroy everything in their path.
They develop from either a collapsing eruption column, lava dome collapse or boil out of the
crater and slowly overflow.
Example:
Mt. St. Helens, Washington, USA, 1980:
Landslide caused by a 4.2 earthquake caused a landslide, weakening the side of the volcano
and relieving pressures as it exposed cracks and fissures, causing a lateral explosion.
Max speed – 1080 km/h
Death toll – 57
Distance – Destroyed 600km^2
Cost - $1 billion (in 1980 currency)
Lava flows
Speed – Basaltic 10km/h, Rhyolitic 1km/h
Rough max Distance – Rhyolitic 10km from source
Streams of molten rock that pour from an erupting vent.
Pose most of a threat to buildings as most are very slow.
Example
Nyiragongo Volcano, Congo, 1977:
Volcano erupted through 5 fissures, draining a lava lake that had built.
Distance – 20km^2
Damage – 400 houses.
, Ash, tephra and debris falls:
Size – Volcanic ash <2mm diameter, tephra >2mm diameter.
Ejections of very fine pieces of rock into the atmosphere, sometimes the stratosphere. The
larger pieces of which return to the surface, while small pieces can be distributed globally.
Formed when magma is fragmented by explosions or when ground water is turned
explosively into steam, and cooled as it is ejected into the atmosphere.
Ash can cause building collapse when it accumulates and destroy crops through crushing.
Example:
Mount Pinatubo, Philippines, 1991:
Ash plume – 40km
Damage – 847 deaths due to roof collapse. Cost $250 million in agricultural and forestry
loss. Destroyed every bridge within 18 miles.
Lava bombs
Size – 64mm
Very large ash particles essentially.
Example
2018 Kilauea eruption, Hawaii:
Size - Basketball sized bombs
Damage - Injured 23 people.
Mudflows (lahars)
Avg. Max speed – 80km/h
Distance – 300km
Huge amounts of ash and debris produced by eruptions mix with water from heavy rain
and/or sudden melting of snow and ice on the volcano from heat produced. Eruption events
can trigger heavy rainfall and thunderstorms also, making them more likely.
Example
Nevado del Ruiz, Colombia, 1985
Pyroclastic flows melted the mountain’s glaciers, creating 4 lahars. These reached the village
of Amero (hence called Amero Tragedy).
Speed – 30km/h
Size – 8m thick
