Characteristics and Formation of Extrusive Igneous Landforms
The type of volcanic cone formed during a volcanic eruption, is dependent upon the type of lava (i.e.
basaltic, andesitic (or even rhyolitic - the most explosive)) and the nature of the eruptions. This is
greatly dependent on the type of plate margin on which these volcanoes form.
Constructive plate margins - tend to give rise to eruptions associated with basaltic magma - forming features
such as shield volcanoes and fissure eruptions which often form lava plateaux.
Destructive plate margins - tend to give rise to eruptions associated with more explosive andesitic or rhyolitic
magma - giving rise to features such as acid domes, cinder cones, composite cones and calderas.
REMEMBER - there is no volcanic activity associated with either collision or conservative plate margins this is because there is no magma source as (i) they are not above the rising limb of a convection current so there
is no rising magma (ii) there is no existing crust being destroyed by subduction to create a magma source;
Characteristics / Formation of Extrusive
Landform Feature:
Diagram and Named Examples of Features:
Acid / Lava Dome Volcano
Characteristics:
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dome shapped accumulations of volcanic rock
these features have steep, convex sides;
narrow base and high cones
may have secondary cones;
Formation of:
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Examples:
steep, convex sides due to acidic, andesitic
lava, which is too viscous to flow far from the
Lassen Peak Volcanic Dome (NW USA)
source and it soon cools and solidifies
domes, often plug vents as lava solidifes,
these may lead to the formation of a
secondary vent on the side of the volcano due
to the build up of pressure
due to the plugging of vents, gasses are
trapped and eruptions are often explosive in
nature.
Common at destructive margins
Shield Volcano
Characteristics:
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gentle sloping cones
wide base and not particularly high
consists of layers of basaltic lava
Formation of:
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Examples:
Medicine Lake Shield Volcano (Cascades)
built up due to succesive flows of basaltic lava
due to the low viscosity and high
Hawaiian Islands - e.g. Mauna Loa
temperatures, the lava flows over wide
distances away from the vent;
the lava gradually cools as thin, gently dipping
sheets of volcanic rock
Common at constructive margins and hot spots
(e.g. Hawiian Islands)
Cinder Cone
Characteristics:
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Slightly concave sides
consist of successive layers of material (ash
and cinders)
Formation of:
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largest materials fall closest to the summit
forming steep but stable sides close to the
vent
finer materials are carried further away and
form gentle slopes at the base of the cone.
Examples:
Lava Butte, Oregon (Cascades)
Composite Cone / Stratovolcano
Characteristics:
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often very large structures with steep and
fairly symmetrical sides;
consist of alternating layers of acidic lava and
ash;
often has parasitic or secondary cone (several
are common due to earlier eruptions);
Formation of:
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These cones are associated with andesitic
magma, during violent eruptions, when the
Examples:
vent has been blocked, ash and other
pyroclasts are produced during the eruption;
Mount St Helens, Washington State (Cascades)
In subsequent less explosive eruptions, lava
may be released - giving rise to the
alternating layers of lava and ash;
these cones hava conduit system through
which magma rises to the surface, solidfied
lava in the conduit system helps to strengthen
the cone;
Where the passage of magma becomes
blocked and pressure builds, magma may flow
from fissures on the volcanoes flank, forming
parasitic or secondary cones.
Common at destructive boundaries (subduction
zones)
Caldera
Characteristics:
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large. steep walled, basin shaped depressions;
ofen have a lake (or lagoon, if below sealevel) inside the crater
may have new volcanic cones growing inside; Examples:
Formation of:
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Formed due to highly explosive volcanic
activity - fresh andesitic magma entering and
filling the magma chamber, may trigger a
highly explosive volcanic (eruption) (diagram
2)
Crater Lake Caldera, Oregon
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As the eruption progresses, the magma
chamber gradually empties and there is no
magma left to support the roof of the
chamber. The mountain summit therefore
collapses into the chamber creating a caldera
(earth movements may cause further
subsidence of the sides); (diagram 3)
The bowl-shaped depression often fills with
water, forming a lake (or lagoon, if below sealevel);
If new magma continues to rise, small, new
volcanic cones may form within the caldera
(e.g. Crater Lake)
After 1000s of years, fresh magma may enter the
magma chamber causing it to re-inflate and the
caldera floor to dome up as a Resurgent Caldera when these erupt, they can be the most destructive
natural phenomenum on earth. They form as large
amounts of highly viscous and explosive rhyoltic
magma rises to the surface.
Lava Plateaux
Characteristics:
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Extensive, flat / very gentle sloping,
Layered structure (layers of lava from series
of eruptions over time)
flat, featureless (usually limited soil /
vegetation cover)
Formation of:
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Lava Plateaux are formed during fissure
eruptions, where basalt lavas (known as flood
basalts) pour out of cracks in the surface
(fissures) rather than from a central vent;
as the lava is runny (low viscosity) and takes Examples:
a long time to cool, it travels long distances
Columbia Plateau - NW USA (Cascades area)
from the fissure;
these features are layered structures caused
by the accumulation of lava over a series of
Deccan Plateau - NW India
lava flows, forming as broad plateaux rather
than piling up as a volcanic mountain;
slow cooling can lead to the formation of
columnar jointing;
Rivers often cut across the lava plateau,
forming gorges - for example the Columbia
river which has cut across the Columbia
Plateau
The Deccan Plateau is a huge expanse of lava covering
an area of 700,00km2 and consisting of 29 lava flows
which have occured within the last 2 million years. A
number of river valleys (including the Wardha river)
are eroded into the plateau which lies at 700-900m
above sea level.