How are lava type, volcano shape and eruption style linked?

Role of silica in determining volcano shape and eruption style and why is can dictate such characteristics. Basic explanation of magmatic differentiation and notes on volcano shape and style.

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MAGMA = molten rock below the surface
Only called LAVA once it has come out of the top of a volcano
Magma is a mixture of molten rock, volatiles and solids. It may also contain suspended
crystals and dissolved gas and sometimes gas bubbles.
We make the assumption that the mantle is homogenous (the same) BUT not all igneous
rocks are the same, i.e. cooling rates dictates crystal size, thereby appearance, and magma
differs due to the nature of tectonics.
Silica is the key ingredient in determining how a volcano erupts and its shape.
More Silica = more VISCOUS lava
more gas trapped, lava flows less common but more pyroclastic flows as eruptions are
explosive not effusive, produces steep-sided volcanoes
All magma does contain dissolved gases but as they rise to the surface to erupt, some
gases are released quietly and others explosively. Thin fluid lava (described as not viscous)
permit gases to escape with ease whereas thick lava (highly viscous) prevents release of
gases so pressure builds up, until an explosive eruption allows the gases t o escape.
Silica content varies in volcanoes around the world.......…read more

Slide 2

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Why are there such differences in volcanoes and igneous rocks?
Magma `evolves' as it rises
3. Silica is the last mineral
and as some plate boundaries
to form and the first to
are thicker than others,
melt, therefore any
magma evolves to a differing
volcanoes above a
extent around the world
subduction zone will
- The thicker the plate the
already be enriched in silica
more evolved the magma
2. As this cools as it
travels closer to the
surface, more and more
crystals form, further
enriching the melt with
1. At about 900 degrees
Celsius, heavy non-silicates
turn into solid crystal within
the melt, becoming heavier
= FRACTIONAL CRYSTALLISATION (denser) and so sink to leave
/ MAGMATIC DIFFERENTIATION the melt enriched in silica…read more

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BASIC ­ Percentage Silica = 45% - 52%
Lava Type Basalt (Intrusive = Gabbro)
Eruption Style Icelandic/Hawaiian
Volcano Shape Shield
Plate Tectonic Setting Divergent margins and above Hotspots
Magma Source Direct from mantle = unevolved
Eruption Frequency Big eruptions every 5 years
Hazards Lava flows, gas, acid rain
MDC Example Iceland E-15 in 2010, Hawaii Kilauea in 2011
LDC Example DR Congo Nyiragongo in 2002…read more

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INTERMEDIATE­ Percentage Silica = 52% - 66%
Lava Type Andesite (Intrusive = Diorite)
Eruption Style Vulcanian/Strombolian
Volcano Shape Composite/Stratocone
Plate Tectonic Setting Convergent: Oceanic vs. Oceanic convergence
Magma Source Partially evolved and partial melting of subducting
Eruption Frequency Less frequent then basic. Big eruptions every 10-50
Hazards Fire fountains, ash, gas, tsunamis. pyroclastic flows,
MDC Example Russia, Kamchatka Peninsula = 30 active volcanoes
and over 100 dormant
LDC Example Vanuatu 2008
Average of 2.6 natural hazards per year…read more

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ACIDIC ­ Percentage Silica = Over 66%
Lava Type Rhyolite (Intrusive = Granite)
Eruption Style Plinean/Pelean
Volcano Shape Composite/Caldera
Plate Tectonic Setting Convergent: Oceanic vs. Continental subduction
Magma Source Fully evolved and full melting of subducting slab
Eruption Frequency 100-9000 years
Hazards Pyroclastic flows, lateral blasts, lahars, ash fall
MDC Example Mount St Helens 1980 (lateral blasts)
Aleutian Islands
LDC Example Columbia Nevada de Ruiz, 1985…read more

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Volcano Shapes...
Most commonly found around spreading ridges and
normally basaltic so lots found in Iceland and Hawaii i.e.
1973 eruption of Heimaey. This was also responsible for the
flood basalts of Fingals Cave and Giants Causeway.
Eruptions like this form plateaus as the lava has low
viscosity so flow rapidly and far away from its source.
Basaltic Rifts/early divergent Gentle, persistent
Basaltic rock forming gently sloping cones from layers of
less viscous lava. Form some of the largest structure on the
Earth i.e. Mauna Loa in Hawaii is largest volcano in the
Basaltic Hotspots and Oceanic vs. Gentle, persistent
Oceanic convergence
Most common type found on land. Created by layers of ash
from initial explosive phases of eruptions and subsequent
layers of lava from main phase. Lava tends to be more
acidic so more viscous, therefore does not flow as far =
steep sides. Typical examples are Vesuvius, Etna and
Andesitic Convergent margins Explosive, unpredictable…read more

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Dan Morris


Fantastic! Just what I was after, thanks.



This was perfecto!

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