In this lesson we discuss why some volcanic eruptions are violent while others are mild
mannered.
As we will discover, the relationship between the gas content and magma viscosity is critical
in determining just how explosive an eruption will be.
We have three learning objectives:To define the term "viscosity"; to provide examples
of common materials with different viscosities and to explain how gas content and magma viscosity
influence the style of a volcanic eruption.
The story we want to tell is why composite and shield volcanoes erupt so differently.
Composite volcanoes are known for their explosive eruptions that toss clouds of tephra far into
the atmosphere.
In contrast, shield volcanoes have runny lava that builds wide, gently sloping landforms.
First, lets consider the gases in magma.
Opening a bottle or can of soda after shaking it causes the contents to overflow.
In such cases, the carbonation in the soda represents gases dissolved in the liquid.
Under normal conditions, we can't see any bubbles in the drink.
However, if we shake the bottle, we can see some of the bubbles come out of solution.
And when the bottle is opened, pressure decreases and the escaping gas causes the bottle to
overflow.
The same kind of thing happens as magma rises toward Earth's surface.
At depth in the crust, volcanic gases such as sulfur dioxide and carbon dioxide are dissolved
in the magma.
As the magma rises, it is under less pressure and gases start to come out of solution causing
the volume to increase.
As the gases come out of solution, it forces the magma upward and under appropriate conditions
has the potential to propel the magma out of the volcano.
So gases drive volcanic eruptions, but gases don't escape from all magma in the same way.
Depending on the viscosity of the magma, gases can either get trapped in the rising magma
or escape readily from the magma.
Viscosity, or the resistance of the material to flow, depends on the temperature and composition
of the magma.
Everyday materials have a range of viscosities.
Liquids that flow easily like water and milk have a low viscosity.
Oils, things like olive oil or canola oil, typically have more resistance to flow and
will take longer to flow a similar distance.
Thicker substances, such as syrup or honey, have higher viscosities.
They will exhibit more resistance to flow and will take a much longer time to travel
the same distance.
To consider how magma might influence the violence of a volcanic eruption, lets do a
simple demonstration.
Imagine blowing air through a straw into a glass of water.
The low viscosity water will bubble readily as the air escapes without much effort.
Now, try doing the same thing again but this time replace the water with a milkshake.
First, it will be more challenging to blow air through the mixture.
Second, if you succeed, it will react more vigorously and splatter the milkshake over
anyone standing nearby.
The higher viscosity milkshake makes it difficult for air to escape, causing the pressure to
build up, and producing bigger bubbles.
Back to our consideration of the gases and viscosity in magma.
So, gases drive volcanic eruptions, but gases don't escape from all magmas in the same way.
Depending on the magma viscosity, gases can either be trapped in the rising magma or escape
easily as the magma approaches the surface.
Low viscosity magmas of mafic composition allow these gases to escape.
Eruptions of these magmas are commonly associated with shield volcanoes and are characterized
by flowing lava and occasional lava fountains.
In contrast, higher viscosity magmas of intermediate or felsic composition trap gases causing pressure
to build up internally and eventually producing a violent eruption.
Eruptions of these magmas are commonly associated with composite volcanoes that are characterized
by tephra blasted high into the air.
So viscosity is the resistance to flow and low viscosity magmas that allow gas to escape
produce mild eruptions and high viscosity magmas that trap gases produce explosive eruptions.
That basically sums up our lesson for today.
These are our three learning objectives.
How confident are you that you could complete these tasks.