58
Experiential Science 30—Freshwater Systems
Activity 9
field activity
4 lab activity
library activity
classroom activity
4 chapter project
4 research team activity
Procedure
1. Work in your research team. Clamp one end of the plastic
tubing to the bicycle pump and the other to the balloon.
Modelling a Volcanic Eruption
Purpose
To simulate a volcanic eruption and determine how the
simulation models a real volcanic eruption.
Materials and Equipment
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¾” plywood (60 cm × 60 cm)
round balloon (minimum 20 cm in diameter)
bicycle pump
1–2 m flexible tubing
clamps with rigid connector
pile of sand, gravel, and soil
SAFETY
PRECAUTION
toothpicks
Wear safety goggles.
file folder labels
Debris may be rapidly
camera
expelled if the balloon
metre stick
bursts.
protractor
2. Place the balloon in the centre of your plywood sheet.
3. Use the dry sand, soil, and gravel to build a volcano
surrounding and on top of the balloon. Photograph or
sketch each stage. (Refer to diagram A.)
4. Make some toothpick flags (minimum six flags) to represent
natural or built structures near the volcano. Cut flag shapes
from the labels and stick them to the toothpicks. Place the
toothpick flags on your volcano, being careful not to pierce
the balloon. The flags act as a crude tiltmeter to measure
the degree of inflation. Tiltmeters measure the change in
angle on a surface.
5. Measure the height of the volcano and the angle of each
flag to determine the slope. When measuring the height of
each flag, ensure you are at eye level parallel to the flag’s
base. Also, when measuring the slope of the volcano at
each flag, you may need to readjust the flag to make sure
it is perpendicular to the plywood (see diagram B). Record
your data in your notebook.
¾” plywood
Diagram A
Illustration of the activity set-up.
Metre stick
Balloon
(magma chamber)
Clamps
60 cm
Flexible
tubing
Bicycle pump
60 cm
Chapter 1 Plate Tectonics and Volcanoes59
6. Take a photo and video of the volcano from the side. In
steps 7 to 9 below, stand on a chair and take photos and
videos from above (or make sketches).
7. Slowly start pumping up the balloon so that the top of the
volcano rises slightly. Stop and measure the height and the
slope at each flag. Do this numerous times as you pump
more air into the balloon. Record your data and note any
changes you see, such as cracks forming and ground rising.
Keep track of elapsed time in your data table.
8. Observe the model with caution and make sure everyone is
wearing safety goggles. Continue to pump up the balloon
until it bursts. Record what happens to your volcano and
your flags.
9. Measure the height of your volcano again. Photograph or
sketch what it now looks like.
Reflections and Conclusions
1. If the air in the balloon represents magma and the
balloon represents the magma chamber, explain how this
experiment models what happens in a real volcano. What
does the sand, gravel, and soil represent?
2. Why do you think the volcano became higher before it
erupted?
3. Describe the formation of the crater after the eruption.
4. How did the location of the toothpick flags affect what
happened to them during and after the eruption? How does
this correlate with structures located near a volcano?
5. Based on what you have observed in this experiment, how
can an engineer help determine when a volcano might
erupt?
6. Add your answers to your chapter project folder along with
photos, sketches, or video.
Diagram B Close-up views illustrating how to measure the slope of the volcano at each flag.
Total angle = 100°
Total angle – 90° = slope
100 – 90° = 10°
90°
10°slope
60
Experiential Science 30—Freshwater Systems
A lava flow in
Hawaii.
How Volcanoes Erupt
Some volcanic eruptions are very explosive, but others are not. Whether a volcanic eruption is explosive
or non-explosive depends on the amount of gas in the
magma and the viscosity of the magma. Generally, the
greater the gas content and viscosity, the more violent
the explosion.
The effect of gases
Magma contains a mixture of dissolved gases, including
significant amounts of water vapour and carbon dioxide. As magma moves up towards the Earth’s surface
and the surrounding pressure decreases, the dissolved
gases start to turn to vapour and form bubbles. The volume of the gas bubbles expands as the external pressure
continues to decrease and more dissolved gases rapidly
escape from the magma.
When magma erupts from a vent, the escaping gas
bubbles burst and force lava into the air in a volcanic
eruption. If the gas escapes easily, as in mafic magma,
the eruption is a relatively quiet flow of lava, as is the
case with some volcanoes in Hawaii. If the gas can’t
escape easily, as in felsic magma, pressure builds up in
the gas bubbles, and when they reach the surface they
burst explosively, causing an explosive eruption such as
the one that occurred at Mount St. Helens or Krakatoa.
Whether the dissolved gases can escape easily or not
depends on the viscosity of the magma.
Viscosity
The viscosity of a fluid is its ability to resist flowing. For
example, cold molasses is thick and doesn’t flow easily,
so we say it has high viscosity. Water flows easily, so we
say it has low viscosity. The viscosity of magma depends
on two factors: temperature and silica content.
If you heat molasses, it will flow more easily. The
same thing happens with magma. Higher temperatures
decrease the viscosity of magma, and lower temperatures increase its viscosity, making it more sticky.
As well, magma that contains a high concentration
of silica has higher viscosity. Magma with a low concentration of silica has low viscosity.
If magma has low viscosity, then the gas in the
magma can expand and escape relatively easily. For
example, mafic magma has lower viscosity, so the
escaping gases cause relatively quiet eruptions and lava
flows that can spread widely and travel long distances.
Kilauea volcano in Hawaii is an example of a lowviscosity magma flow.
Felsic magma has high viscosity, and intermediate magma has medium viscosity. In both felsic and
intermediate magma, gases cannot escape easily. This
results in explosive eruptions containing large amounts
of pyroclasts, also known as tephra. The eruption of
Mount Pinatubo in the Philippines is an example of a
felsic-magma explosive eruption.
SCIENTIFIC TERMS
pyroclast: hot ash and rock fragments ejected during a volcanic
eruption.
tephra: collective term for the pyroclasts that are ejected from
a volcanic vent.
Chapter 1 Plate Tectonics and Volcanoes61
Types of Volcanoes
Volcanoes are classified according to their size and
shape. The size and shape of a volcano depend largely
on the chemical composition of the magma that forms
it. Figure 1.16 illustrates the three main
types of volcanoes: cinder cones, composite
volcanoes, and shield volcanoes.
Because mafic lava has low viscosity, the lava spreads
widely, creating a volcano that looks like a warrior’s
shield. Shield volcanoes may form over hot spots, such
as Hawaii, or where sea-floor spreading occurs.
Cinder cones
Steep, cone-shaped volcanoes are called cinder cones. They are formed when mafic or
intermediate magma with a high concentration of gas but low viscosity erupts from
a narrow vent. The loose pyroclastic material that is ejected piles up around the vent,
forming a circular hill with a central crater.
Cinder cones, such as Volcano Mountain,
YT, are not as large as shield or composite
volcanoes. They are rarely more than 500–
1,000 m high, and they erode easily.
1 km
Cinder cone
Composite volcanoes
Composite volcanoes, also called stratovolcanoes, are formed from alternating layers of
tephra and lava that are a few kilometres in
diameter. They are usually created by intermediate magma but may also erupt mafic
and felsic magmas. Composite volcanoes
are larger than cinder cones because they
have erupted many times over hundreds of
thousands of years, but they are smaller than
shield volcanoes.
Composite volcano
10 km
Shield volcanoes
Shield volcanoes, such as Mount Okmok
in Alaska, are very large, wide, gently sloping structures formed by mafic lava flows.
They may be tens of kilometres in diameter.
SCIENTIFIC TERMS
cinder cone: a volcano made mostly of cinder
and other loose rock particles that have been
blown into the air by volcanic eruption and
have fallen into layers around the vent.
composite volcano: a tall, conical volcano with
many layers (strata) of hardened lava, tephra,
and volcanic ash.
Shield volcano
20 km
Figure 1.16 Block diagrams illustrating the three main types
of volcanoes. Note the scale, where cinder cones are the
smallest, under 1 km, while shield volcanoes can be tens of
kilometres wide.