Introduction to volcano
characteristics and activity
Lesson 1
What determines volcanic activity?
Volcanic activity is an eruption of magmatic material onto the Earths surface. The types of
volcanic activity is often determined by it’s geographical and tectonic plate setting. Most
volcanic eruptions occur on a divergent or convergent plate boundary (many around the Pacific
Ring of Fire)..
Pacific Ring
of Fire
African Rift
valley
Chilean
Volcanoes
Siebert L, Simkin T (2002-). Volcanoes of the World: an Illustrated Catalog of Holocene Volcanoes and their Eruptions. Smithsonian
Institution, Global Volcanism Program Digital Information Series, GVP-3, (http://www.volcano.si.edu/world/).
Volcanoes at divergent plate
1) Convecting magma from the Earth’s
asthenosphere undergoes
boundaries
depressurisation as it nears the crust,
causing it to become more molten.
2) This rising magmatic material
applies pressure causing deformation
of the Earth’s crust, there by allowing
magma to rise to the surface through
cracks.
3) Due to the relatively thin crust found
at oceanic divergent plate boundaries
magma cools only slightly so that when
it reaches the surface it erupts
between 1200-1000 degrees Celsius,
resulting in a low viscosity.
4) Due to lack of gas bubble formation
in the magma the eruptions are often
low in explosivity, leading to fire
fountain eruptions or low viscosity
basaltic lava flows from the rift valley
source.
5) Over time eruptions may build up to
Source: www.indiana.edu
Volcanoes at convergent plate
1) Volcanism occurs at two types of
boundaries
convergent plate boundary:
i) oceanic and oceanic
ii) oceanic and continental
2) These boundaries are often
associated with more explosive
volcanism and a wider range of
volcanic products.
3) The subducting oceanic plate
undergoes an increase in heat and
pressure thereby causing its upper
layer to melt.
4) This created magmatic material
starts meets the overlying crust (either
oceanic or continental) and applies
pressure. It forces its way through
cracks in the crust.
5) On route to the surface it start to
cool, becoming more viscous and also
melts other surrounding rock, adding
water vapour. These create gas which
6) Magma may become stored on route to the
surface
in magma chambers as it reaches similar density to
the surrounding rock.
7) If magma takes a long time to reach the surface it
may
become pressured by trapped gas therefore
leading to
explosive eruptions.
Types of volcanic cone
Source; http://www.earthlyissues.com/volcano.htm
The type of cone is often a reflection of the material erupted.
Fissure and shield which have low slope angularity are associated with divergent plate boundaries
where less viscous lava is more common. There are often volcanic products associated with
explosions such as ash.
Ash-cinder, and composite (stratovolcano) are often composed of tephra which has been erupted
in explosive events therefore are more common at convergent plate boundaries. These may be
associated with ash, lapilli, bombs, pyroclastic flows and viscous lava.
Images of volcanic cones
This is a volcano rift (fissure) at Laki in Iceland which famously erupted in 1783. This eruption
was associated with a divergent plate boundary. It covered the surrounding area in tephra and
large viscous lava flows. None of the volcanic slopes are steep in gradient and few of the
volcanic cones reach great heights (altitude).
Source: Wiki commons
Images of volcanic cones
Mt St Helens before and after the 1980
eruption
Source: Wiki commons (both images)
This volcano found near a convergent plate
boundary produced extensive ashfall and
pyroclastic blasts, destroying vast areas of
forest around the volcano.
The blast was so strong the volcanic
eruption removed the top of the mountain in
the blast. Tephra mixing with ash caused
extensive lahar damage.
Volcanic products
Fluid lava flows (pahoehoe)
often associated with low
viscosity eruptions
Source: Wiki commons
AS pahoehoe lava flow
cools it often becomes
slower moving and more
blocky
Source: Wiki commons
Volcanic products
Volcanic material erupted in
explosive activity is often
referred to as tephra or
pyroclastic material. This is
classified by size
Volcanic ash is any particle
below 2mm, lapilli between
2-64mm and volcanic
bombs above 64mm.
Volcanic products
This image shows a pyroclastic flow
from the Mt Mayon volcano in the
Phillipines. These are superheated
(and fast travelling) clouds of
pyroclastic material, formed by
collapse of the volcanic column or
collapse of accumulated viscous
lava. They are deadly and few
have survived a direct hit by one.
Volcanic mudflows, also more
commonly known as lahars, their
Indonesian name where they are
common. Lined to steep sided
cones, they form where ash mixes
with rainfall or melts snow to create
fast-moving mudflows. This image
is a lahar from the Indonesian
volcano Galunggung on the island
of Java.
Volcanic eruption styles
The style of volcanic eruption is
linked to the level of explosivity.
This can range from the least
explosive Icelandic to the most
explosive Plinian.
Source: http://media.web.britannica.com
Eruption characteristic
Description
Icelandic
Low viscosity and highly effusive. May have water therefore phreatic.
Hawaiian
Effusive, minor explosivity, with fire fountain and low viscosity lava flow.
Strombolian
Eruption with gas bubbles so eject tephra (ash & lapilli).
Vulcanian
Higher gas build up than Strombolian, highly viscous lava. A series of
short-lived explosions, with tephra including bombs.
Pelean
Explosive eruptions with range of tephra with nuee ardent (glowing
cloud) pyroclastic flows
Plinian
Highly explosive, large eruption column (up to 45km), range of tephra
and pyroclastic fallout, including flows.
Classifying volcanic explosivity
Volcanic explosivity is measured
using the VEI scale (Volcano
Explosivity Index) which is a
measurement of the volume of
erupted material.
Each volcano does not have a
characteristic mark on the scale.
Instead each individual eruption is
given its own point on the scale.
Hawaiian and Icelandic eruptions
are commonly found near the
bottom of the VEI scale, while
supervolcanic eruptions are up to
VEI 8, e.g. the Yellowstone
eruptions.