Section 18.2 Volcanoes -- Eruptions
The type of eruption that a volcano has (explosive or non-explosive) is dependent on the composition
of the magma (what the magma is made up of).
Dissolved Gases – as the amount of dissolved gases in magma increases, the
magma’s explosiveness increases. Water Vapor is the most common
dissolved gas in magma.
Think of the gases dissolved in soda.
Viscosity – a material’s resistance to flow (is it thick and slow or is it thin and runny).
Temp.:
Think of warm
* Cooler magma has a higher viscosity (thick and slow moving).
honey & cold honey.
* Warmer magma has a lower viscosity (thin and runny).
Composition:
Think of silica like the * Magma with high silica content will have a higher viscosity (thick and slow moving).
flour that is put into
* Magma with low silica content will have a lower viscosity (thin and runny).
gravy to thicken it.
The factors that influence whether a rock will melt are:
 temperature
 pressure
 the presence of water
3 Types of Magma
Basaltic – low silica content, low viscosity (thin & runny), gases escape easily (not trapped),
non-explosive eruptions (most likely shield cone volcanoes have this type).
Andesitic – a medium level of silica content, intermediate viscosity, intermediate explosiveness.
Rhyolitic – high silica content, high viscosity (thick & slow moving), gases cannot escape easily
(they get trapped), very explosive eruptions (most likely cinder cone volcanoes have this type).
Explosive Eruptions
Tephra – the material that is blown out of a volcano and into the air during a violent eruption.
This is also known as pyroclastic material. There a 5 general categories of tephra that
are primarily seperated according to size: Volcanic Dust, Volcanic Ash, Lapilli,
Volcanic Blocks, and Volcanic Bombs.
Volcanic Dust – A powder-sized material
[<under 0.063 mm (0.0025 inches)], that appears
to be a soft, harmless powder. It is composed of
very small irregularly-shaped particles with
sharp, jagged edges (see image at right)
It has a high hardness rating, so with its irregular
particle shape it can be an abrasive material.
Because of its extremely small size, it can be
blasted out of a volcano and transported very far
distances from the volcano (sometimes all the
way around the globe). It can block out some of
the sun’s rays and affect local and global climate.
With its irregular particle shape it
can be an abrasive material.
This gives these tiny particles
the ability to damage aircraft
windows, be an eye, nose and
throat irritant, as well as damage
moving parts within electric
motors.
ASH -- Very fine-grained fragments (< 2 mm), generally dominated by broken
glass shards, but with variable amounts of broken rock fragments. Because
of its small size, it can be blasted out of a volcano and transported far
distances from the volcano (a few thousand miles from the volcano).
Shown here is the eruption of Mt. Saint
Helens. A huge amount of tephra (volcanic
dust & ash) was blown out of the volcano.
Some of this material was blown high into the
atmosphere and the upper atmospheric winds
carried it across the United States and around
the globe. The volcanic dust made it around
the world in approximately 2 weeks.
As can be seen in this diagram, the largest particles will
fall out of the volcanic cloud first. The smallest particles
can stay suspended for long periods of time and be carried
great distances from the volcano.
Lapilli -- (it means “little stones” in Latin) they range in size
from 2 mm to 64 mm in diameter and are simply small
chunks of hardened or semi-hardened rock that are
blasted from the volcano. Because of their larger size
they do not travel as far as the volcanic dust or ash
(up to a few hundred miles).
Volcanic Blocks -- Volcanic blocks are solidified, angular rock
fragments greater than 64 mm in diameter (larger than the size of a
football). Blocks commonly are ejected during explosive eruptions
and consist of older pieces of the volcano's edifice (the main structural
part), e.g. parts of the conduit, lava domes or older lava flows.
During violent eruptions, blocks of up to several meter size can be
thrown to several km distance (which is equal to a few miles).
The time that some of these blocks spend on their flight out of the
volcano can be easily calculated to be around 30-40 seconds, while
they can travel at speeds of typically 200-300 meters / sec.
Volcanic Bombs – is a mass of molten rock (tephra) larger than 65 mm (2.5 inches)
in diameter, formed when a volcano ejects viscous (thick) fragments of lava during an
eruption. They cool into solid fragments before they reach the ground (spinning and
rotating as the fly thru the air). They will have a hard outer rock shell with liquid lava
inside. Lava bombs can be thrown many kilometers (which is equal to a few miles) from an
erupting vent, and often acquire aerodynamic shapes (round or spindle shaped) during
their flight. Bombs can be extremely large.
Volcanic bombs are known to occasionally explode from internal gas pressure (the
gases that are still trapped in the lava are trying to escape) as they cool, but explosions are
rare; in most cases most of the damage they cause is from impact when they hit the
ground and crack open spilling out the still liquid lava inside.
Pyroclastic Flows
Pyroclastic flows are high-density mixtures of hot, dry rock fragments and
hot gases that move away from the vent that erupted them at high speeds
(hundreds of miles per hour)..
Pyroclastic flow animation # 1
Pyroclastic Flow animation # 2
Pyroclastic Flow
Maurice and Katia Krafft studied and
photographed erupting volcanoes all over the world,
witnessing and documenting more active volcanoes
than any other investigators. Their popular books,
lectures, and videotapes -- all done in their exuberant
style -- greatly increased public awareness of the
nature and dangers of volcanic eruptions. Ironically,
they were killed by a pyroclastic flow while
photographing an eruption in Japan in 1991.
Maurice and Katia Krafft were right here
when this pyroclastic flow hit them.
Eruption of Mount Unzen Showing Pyroclastic Flow, Japan, 1991