Three major magma
compositions: basalt,
intermediate, and rhyolite.
SiO2 content controls the
relative viscosity of a magma.
The total amount of dissolved gasses that
are trapped within the magma. Magmas
that are more rhyolitic have a higher gas
content. This gas content and relative
viscosity leads to the more explosive and
violent eruptions that occur at composite
volcanoes.
VISCOSITY - the ability of a
liquid to resist flowing.
MAGMA’S VISCOSITY AND
VOLCANIC ERUPTION
Basaltic Lava
Magma that has a lower viscosity and
lower silica content produces the volcanic
rock called basalt and forms shield
volcanoes.
commonly associated with Hawaiian
eruptions. These eruptions are
characterized by the gentle and effusive
outpouring of lava with low viscosity.
formed by Strombolian eruptions are a
form of relatively viscous basaltic lava,
and its end product is mostly scoria.
Pahoehoe
has a smooth, billowy look to it. It is
nicknamed ‘ropy lava’ because it forms
a braided or ropy texture. Pahoehoe
lava flows in rivers down the volcano.
As the lava is flowing, the part of the
flow that is in contact with the air cools
quickly into rock.
A’a’ (pronounced 'ah-ah')
is the second type of lava that forms.
This lava is blocky and forms when the
magma cools quickly and large chunks
of cooled lava get reincorporated back
into the moving flow.
due to the higher amounts of trapped
gasses and the higher viscosity. This
leads to an increased pressure beneath
the surface as the magma attempts to
rise.
Rhyolitic/ Andesitic Lava
Magma that has a higher viscosity and
silica content produces rocks that are
called rhyolite and andesite.
Formed by Plinian eruptions result from
a sustained ejection of andesitic to
rhyolitic magma into eruption columns
that may extend up to 45 km above the
vent.
due to the higher amounts of trapped
gasses and the higher viscosity. This
leads to an increased pressure beneath the
surface as the magma attempts to rise.
Vulcanian eruptions are also associated
with high-silica magma. These eruptions
are explosive in nature, similar to Plinian
eruptions, but are typically less powerful.
Get ¼ sheet of paper answer the
following statements Write ↑ if it
is shows high viscosity, and write
↓ if it shows low viscosity.
1. Magma that is thick and flows
slowly.
2. Magma that is more likely to
lead to explosive eruptions.
3. Magma traps gases more
easily.
4. Magma that is thin and flows
quickly.
5. Magma that is more likely to
lead to gentler eruptions.
