|
Overview/Introduction:
“In
Roman mythology, Vulcan, the god of fire, was said to have made tools
and weapons for the other gods in his workshop at Olympus. Throughout
history, volcanoes have frequently been identified with Vulcan and
other mythological figures. Scientists now know that the "smoke"
from volcanoes, once attributed by poets to be from Vulcan's forge,
is actually volcanic gas naturally released from both active and many
inactive volcanoes.”
Excerpt from: McGee and Gerlach, 1995, Volcano Hazards Fact Sheet,
USGS Open-File Report 95-85.
In one version
of the Vulcan myth, Vulcan’s father Jupiter threw him from heaven
when he sided with his mother Juno in an argument. He became a smithy,
making weapons for the gods and thunderbolts for his father inside
the crater of Volcano, one of the Aeolian Islands off the coast of
Sicily. Vulcan is the origin of the name we now give to those areas
on Earth where magma comes forth as lava or pyroclastic flows, creating
what we call “volcanoes.”
The theory of Plate Tectonics--the existence of pieces of Earth’s
crust that move about on the asthenosphere--describes the mechanism
that causes volcanic eruptions. As plates subduct under others and
melt, or move apart from each other and open a fissure in the crust,
mantle material (magma) moves up and out onto the Earth’s surface.
Purpose or Objective:
In this lesson,
students will learn:
• the various types of volcanoes
• the emissions that come from them
• why some erupt by “quietly” pouring lava, while others
blast out chunks of rock and
clouds of ash and gases
• where shield and composite volcanoes usually occur and why
Time Required:
Five 50 minute
periods
Materials, Tools, and Resources Needed:
(KWL chart-LS*)
Literacy
Strategies: (These
strategies are noted by LS* where they appear in the lesson.)
KWL Chart—Introductory
Activity
Read-aloud of Primary Source documents-- Introductory Activity
Quick-write—Activity 2
Venn Diagram—Activity 3
Matching of pictures with written description of volcano types—Activity
3
Volcano Type Comparison Chart—Culminating Activity
•
Letters of Pliny the Younger—Account of Mt. Vesuvius eruption
• Chart paper, markers
Prerequisite Student Knowledge:
Students should
know:
• that magma is molten material in the mantle of the Earth.
• that gases take up more space than solids or liquids.
• the theory of Plate Tectonics and the various movements of
the plates (convergent,
divergent, and transform).
• the states of matter, and that gases take up more space than
solids or liquids.
Procedure:
Have students work
individually by writing on paper what they think they already know
about volcanoes. Then have them share in groups of four what they
wrote. Each group should come up with a single list of things that
they think they know about volcanoes based on their individual lists.
Have each group
share one statement at a time with the whole class and write their
statements on chart paper. Save for a follow-up to the lesson in which
you ask the class if the statement was found to be true, false or
not addressed in the lesson.
Next, ask students
what they still Want to learn about volcanoes. Write on a separate
chart paper. [Note: It is fun for the students if you include the
initials of the student who asks the question.] When the lesson is
completed, come back to the chart and ask the student if he or she
has learned the answer to the question.
After the class
KWL chart is completed, read the first-hand account of the eruption
of Mount Vesuvius to the class from the letters of Pliny the Younger
(LS•). Several websites have translations of his letters. One
site is: http://faculty.cua.edu/pennington/pompeii/PlinyLetters.htm
Activity 1 --Viscosity
and Gas Flow Test:
For demonstration:
Two clear plastic dishes (such as a petri dishes)
Two droppers
For each
team of 4 students:
•
2 small paper cups: one with a small amount of water, the other with
a small amount of
molasses (about 1/4 cup)
• 1 straw for each student
Procedure:
Explain
that magma—molten material in the Earth’s mantle—can
sometimes contain gases such as water vapor and carbon dioxide. There
is a distinct difference in the kind emission from a volcano if the
trapped gases can escape freely and easily (as in very fluid magma
which is low in silica), or remain trapped and cannot escape easily
(as in very thick magma which has a high silica content).
Demonstration:
Illustrate
the meaning of the word “viscosity” – the measure of
a fluid’s resistance to flow – by putting both clear plastic
dishes on the overhead. Drop a couple of drops of water in one dish
and a couple drops of the molasses in the other. Tilt the dishes and
show how each substance moves around the dish. Explain that the slow
movement of the molasses is because it is a viscous material –
something that does not flow easily.
In this simulation,
the water represents the thin, runny, magma that is low in silica.
The molasses represents the thick, high-silica content magma.
1. Have each student
take the straw and blow gently first into the water, watching the
carbon dioxide bubble freely out of the water.
2. Next, s/he places the straw into the molasses and blows bubbles.
3. Pass the cups on to the next student in the group.
[Be sure to have the students use the straw in the water first, so
as not to mix the molasses into the water.]
Ask:
1.
Which “magma” held the bubbles longer?
2. Into which “magma” was it harder to blow bubbles?
3. Which “magma” would take up more space—the one which
holds the gas longer, or the one which lets the gas out faster?
Explain:
1.
Thick, viscous magma traps dissolved gases and holds them inside.
2. The thin, runny magma lets the gases out quickly.
3. The more gases that are trapped in the magma, the more the pressure
builds up.
Ask:
1.
What do you think will happen if lots of gas pressure builds up in
magma under the Earth’s surface? (An explosion will result.)
2. What is holding the gas in the magma? (The more viscous the magma,
the less easily the gas can escape.)
3. Why is the gas trying to get out of the magma? (Gas occupies more
space than liquid, so it is trying to get free of the liquid in order
to expand.)
Activity
2 – Gas Under Pressure:
For
each student:
1 35 mm film canister (preferably the white type)
1/4 of an Alka-Seltzer tablet (or generic store brand)
Water
Safety goggles (4 pair can be shared)
Procedure:
Refer
to the previous activity about gas dissolved in the magma. Tell students
that they are going to learn what happens when magma that contains
a lot of dissolved gases is confined in a closed in space--that is,
put under pressure.
Place the 1/4
tablet of Alka-Seltzer in each student’s film can. Take students
outside away from buildings or other students. Have four students
put on the goggles and place their cans on the ground in a line about
1 foot apart. Pour a small amount of water in each can and have students
cap them up, then move away quickly (about 5 feet or more).
Students watch
as the cans “blow their tops” in succession. Continue until
all students get a chance to do the activity. Have them do a quick-write
paragraph on what they think happened and how this activity is similar
to an explosive volcano (LS*).
Ask:
1.
Why did the caps blow off? (The pressure of the escaping gas became
too great for the cap to keep inside the can.)
2. What caused the lids to “blow off” at different times?
(Some pieces of the tablet were larger than others, producing more
gas; some caps had a stronger grip on the cans, increasing the amount
of pressure needed to pop the top.)
3. How are the film cans like volcanoes? (Thick silica-rich magmas
can plug up the vent, thereby increasing the pressure of the dissolved
gases. When enough pressure builds up to overcome the plug, an explosive
eruption occurs.)
4. How does this activity relate to the predictability of eruptions?
(Eruptions cannot be accurately predicted due to the variation of
the composition of the magmas and the overlying pressure of rock layers.)
Activity
3 – What Are the Different Types
of Volcanoes?
For
each student:
• Written description of the three volcano types (unlabeled)
included in lesson
• Pictures of the three volcano types and cross-sections of volcanoes
For each group of 3-4:
• Venn Diagram Sheet (LS*) – 3 overlapping circles
One for the class:
A large version of the Venn Diagram on Chart paper
Have students
read each volcano description and try to match what they read with
the pictures (LS*)
Have students
look at the photos and diagrams of the three types of volcanoes (Composite,
Cinder Cone and Shield). Ask them to discuss in groups of 3-4 how
the three types are similar, and how they are different. They should
then list these similarities and differences in the Venn Diagram.
Put up the large
Venn diagram on the wall. After each group completes their own diagram,
have the groups read their diagrams to the whole class. Write their
statements on the large version of the diagram.
Here
are some possible answers:
All
are volcanoes; all emit some type of material; all are formed from
magma beneath the Earth; all are hills or mountains; all have vents
or openings at the top.
Composite only: Formed from alternating layers of lava and pyroclastic
debris; broad at the base and narrower at the top.
Composite/Cinder Cone: Both are made from pyroclastic debris; both
are somewhat pointed.
Cinder Cone only: Made from only pyroclastic debris; smallest in size
of the 3.
Shield only: Made from fluid lava flows only; shield shaped; can occur
in the middle of tectonic plates; largest mountains on Earth.
Shield/Cinder Cone: Both have vent in the top; can occur together.
Activity
4 – Locating Non-explosive and Explosive
Volcanoes
Demonstration:
1 birthday candle (OR permanent red marker)
Sheet of paper
For each
team of 2 students:
•
Map of Tectonic Plates of the world (Most 6th grade Earth Science
textbooks have one.
There is an excellent one in Ranger Rick’s Naturescope: Geology:
The Active Earth—
Copycat Page: Plates on the Go)
• World Plotting Map from GEMS Guide: Plate Tectonics—the
Way the Earth Works
• Lists of Shield and Strato (Composite) Volcanoes and their
latitude and longitude from
The GEMS Guide (see above)
Procedure:
Discuss
and review the theory of Plate Tectonics with the class. Review the
various plate motions: divergent—moving apart; convergent—moving
together; transform—sliding past each other. Remind them that
in divergent motion as plates pull apart, magma comes up to fill the
gap created. As plates converge and subduct, crystal melting takes
place as the plate dives down into the mantle. Both of these kinds
of plate movements cause volcanoes to be created, as magma moves up
to the Earth’s surface.
Introduce “hot
spots” to the students: thin areas in the crust where magma comes
out in a volcano. These hot spots remain fixed under the plates as
they move above. Hawaii is the clearest example of hot spot volcanoes.
Demonstration:
Light
a birthday candle and hold it under a paper, only long enough to singe
the paper. (Be sure to have a bucket of water near to douse any flames
if they arise.) Move it gradually and singe another spot. Do this
several times to simulate the plate movement over a hot spot.
If preferred,
use a permanent red marker instead of the candle and very absorbent
paper.
Student
Activity:
1.
Ask students what type of plate movement they think produces composite
and shield
volcanoes.
2. Tell students that they are going to find out by mapping the locations
of volcanoes and
drawing conclusions.
3. Pass to each team of two both maps and the lists of shield and
composite volcanoes.
4. Have them plot the locations of the volcanoes on the lists.
5. Ask students to analyze their plotting data to determine where
each type of volcano
most often occurs.
6. Have the teams report out what they discovered.
(Most shield volcanoes occur over hot spots in the interior of plates,
or at divergent boundaries. There, magma is less gaseous and therefore
not under much pressure.
Eruptions are
“quieter” and emit fluid lava. Composite volcanoes occur
usually at convergent plate boundaries. In subduction zones, where
magma contains gas under pressure from the trapping of water as the
plate dives down, explosive eruptions are created.)
Student
Handouts/ Record Sheets:
Assessment:
Culminating
Activity/Assessment:
• Create a chart (LS*) of the three types of volcanoes, comparing:
Where they occur
• Type of eruption
• Material extruded
• Shape
• Drawing of each type
Rubric:
Volcano Types Comparison Chart
|
