Lab Objectives
1. Students will learn various terms that apply to volcanology.
2. Students will explore the concept of “viscosity” by comparing the behavior of
various liquids (water, molasses, and vegetable oil).
3. Students will associate the principle of viscosity and SiO2 (silica) content to the
explosive and non-explosive characteristics of composite, shield, and cinder
cone type volcanoes.
4. Students will learn how to distinguish and classify common volcano types.
5. Students will associate the type of plate tectonic setting with formation of
common volcanoes.
Introduction
The word volcano is derived from the Roman god of fire, Vulcan. A volcano
occupies an area on the Earth's surface as well as other planets and moons, where
molten rock, gases, and pyroclastic debris erupt through the crust. As a result, magma
that exits the volcano is called lava and upon solidification produces extrusive volcanic
igneous rocks. Common volcanic rocks with aphanitic textures are rhyolite, andesite,
basalt, and vesicular textured rocks such as scoria and pumice. Rapidly cooling lava
commonly produces the glassy textured extrusive igneous rock obsidian. Additionally,
common pyroclastic material ejected from a volcano can occur as ash (powered rock
resembling smoke), volcanic bombs, and cinder (fragmented lava). The largest volcano
on earth is found on the Pacific plate in the Hawaiian Islands and is known as Mauna
Loa. Mauna Loa rises approximately 10 km (6 mi) from the sea floor to its summit and
represents approximately 42,500 cubic km (10,200 cubic mi) of earth. However, the
most active volcano within the continental USA is Mt. St. Helens, located in western
Washington state. Within our solar system, the planet Mars possesses the largest
volcano known as Olympus Mons. This volcano rises 27 km (17 mi) and spreads across
the Martian land surface over 520 km (320 mi).
Scientists that study and investigate various types of volcanoes are known as
volcanologists. Volcanologists classify volcanoes by observing the shape, type of
material that builds the volcano, and the volcano’s eruptive style. Volcanoes are
described by using terms such as: flanks, conduit, magma chamber, and crater.
Below is a diagram that shows the various terms associated with volcano morphology.
Volcano Morphology
Crater
a basin-like, rimmed structure at
top or on the flanks of a volcanic
cone
Crater
Flank
Conduit
Magma Chamber
Flank
represents the sides of a volcano
(steep or low-angled flanks)
Conduit:
represents a passage or volcanic
neck that allows the flow of magma
from the magma chamber
Magma Chamber
a reservoir of magma in the
shallow part of the lithosphere
or below a volcano
Common volcanoes that will be studied in this lab are the shield, composite, and
cinder cone type volcanoes. The shape and eruptive style of each volcano type is
influenced by the concentration of silica, which directly affects the volcano’s viscosity
characteristics. The viscosity of various substances measures its resistance to flow. For
example, the more viscous a substance, the less fluid or flow resistant the substance
becomes. The viscosity of lava and the volcano’s gas content govern the explosive or
non-explosive nature of a given volcano. Therefore, a volcano high in silica content will
typically produce high viscous lava and, thus, take on an explosive nature. A volcano
low in silica typically will produce low viscous lava (very fluid) and is considered nonexplosive. Dissolved gasses or volatiles contained within volcanoes influence the
migration or movement of magma prior to and during an eruption. For example, volatiles
stored within a volcano are under pressure and will typically expand a hundred times
their volume during an eruptive period. In fact, just vigorously shake a can of
carbonated soda and pop the top to see a great analogy. High viscous as well as high
silica and volatile content will typically produce an explosive volcanic eruption. In
contrast, low viscous along with low silica and volatile content may produce a very
fluid,“runny” volcanic eruption. Below are descriptions that depict each volcanic type
studied in this lab, which are based primarily on silica content and the nature of viscosity.
Composite Volcanoes
Composite volcanoes, also known as strato-composite volcanoes, are formed by
the buildup of alternating layers of lava, ash, and rock fragments forming steep, angled
flanks. Note the term composite used to describe the “grouping” of layered material.
Typically, composite volcanoes are large, impressive continental volcanoes that often
exceed 2500 m in height, spread over a 1000 km2 of surface area, and can occupy as
much as 400 km3 in volume. Composite volcanoes are often snow-capped through all
the seasonal changes. Strato composite volcanoes sometimes appear dormant or seem
extinct between non-eruptive periods; however, tectonic forces may be active below the
volcano, and to witness an eruption requires luck (if you are around after the eruption)
and careful surveillance. Composite volcanoes are known for their high explosive
release of pyroclastic debris and “pasty” lavas. Examples of famous composite
volcanoes include Mount Hood (Oregon State, Cascade Range), Mount Rainier
(Washington State, Cascade Range), Mount Shasta (Northern California, Cascade
Range), Mount Fuji (Japan), Mount Vesuvius (Italy), and Kilimanjaro (Africa).
Shield Volcanoes
Shield volcanoes are huge volcanoes that possess large, broad, low-angle flanks
that gently slope. These volcanoes typically result from the buildup of many layers of
fluid type lava flows. Types of lava flows associated with volcanic shield activity are aa
and pahoehoe flows. AA flows are considered very slow-moving lavas that exhibit a
rough, jagged, blocky, sharp angular textural appearance. Pahoehoe flows move rapidly
and take on a “smooth skin,” ropey textural appearance. Both lava flows are associated
with chemical compositions high in iron and magnesium; however, volatile content
distinguishes the characteristics of each flow. Shield volcanoes are known for low
pyroclastic eruptions (less than 1% pyroclastics) and primarily form at hot spots and on
the deep ocean floor. Examples of famous shield type volcanoes include the Hawaiian
Islands (Mauna Loa, Kilauea), Galapagos Islands, and Iceland.
Cinder Cones
Cinder cones are known to exist all over the earth’s surface. In fact, the
development of cinder cones typically occurs in groups. For example, the area around
Flagstaff, Arizona is known for its volcanic field containing over 600 cinder cones.
Cinder cones are formed from the accumulation of lava fragments or cinders producing
scoria and loose pyroclastic debris that result in high, steep-angled flanks (30-40
degrees). Typically, cinder cone formation occurs as a single eruptive style lasting a
short time. Cinder cones form and grow rapidly, reaching their maximum size, rarely
rising to heights that exceed 250m and 500m in diameter. Located in Paricutin, Mexico,
a cinder cone formed on February 20, 1943 in a corn field and grew to 300 feet in 5 days
before becoming silent. However, there were many ears that listened (ha, ha, ha!).
Part A – Volcano Terminology
Below are various volcanic terms that you must master in order to perform successfully
on lab tests as well as lecture tests:
volcano
magma
lava
pyroclastic
volcanologist
crater
flank
volcanic conduit
magma chamber
composite volcano
shield volcano
aa lava flow
pahoehoe lava flow
cinder cone
Part B - Viscosity and Volcanoes
The viscosity of a substance is its resistance to flow. Thus, the more viscous
a lava, the less fluidity the lava has, therefore resisting flow. The viscosity of lava and its
gas content determine the explosiveness of a volcano. In this experiment, you will
explore various viscosities by comparing the behavior of water, vegetable oil, and
molasses.
Procedure- Viscosity
1. Observe the liquids provided for this experiment (water, vegetable oil, and
molasses).
2. Based on your observation, which liquid flows the fastest? Which liquid flows the
slowest?
3. Apply a small amount of each liquid to one paper plate. While tipping the plate
and holding it at a 45-degree angle for at least two minutes, identify which liquid
moves the slowest, the most intermediate, and the fastest down the plate.
4. How do you think that the ability of a liquid to flow is affected by its temperature?
Provide an example of a liquid you heated which changed the viscosity.
5. Compare the viscosity of each liquid, and identify or rank which liquid is more,
intermediate, or less viscous.
Volcanoes and Viscosity
1. How is viscosity related to the silica content of lava?
2. Using the table below, which type of lava would produce the least explosive
eruption? Why?
Basalt
Andesite
Rhyolite
Silica content
About 50%
About 60%
About 70%
Viscosity
Least
Intermediate
Most
3. Using the table and your answer from question 2, identify the type of lavas (lava
composition) associated with:
Shield volcanoes
Composite volcanoes
4. Construct a table that compares the silica content, viscosity, volcanic
environment, volcanic rock type, and volcanic morphology associated with shield,
composite, and cinder cone volcanoes.
Part C- Volcanism and Plate Tectonics
1. Draw a diagram that represents the ocean-continent convergent boundary. In
your diagram, include the subducted plate, and identify the type of volcanoes that
form at this plate boundary. Provide at least 3 geographical locations that
represent these types of volcanoes.
2. Draw a diagram that represents the ocean-ocean convergent boundary. In your
diagram, include the subducted plate, and identify the type of volcanoes that are
formed at this plate boundary. Provide at least 3 geographical locations that
represent these types of volcanoes.
3. Draw a diagram that represents the divergent boundary. What types of
volcanoes are formed at this type of plate boundary? Provide at least 3
geographical locations that represent these types of volcanoes.
Part D – Volcano Critical Thinking Questions
1. Explain why the composition of composite volcanoes is typically much higher in SiO2 (silica)
than shield type volcanoes. (Hint: Think about different types of convergent boundaries.)
2. Explain why high lava fountains may erupt from basaltic type volcanoes.
3. Where do you think the high amounts of SiO2 (silica) originate to create rhyolitic type magmas
that result in the formation of composite type volcanoes? (Hint: Think about your answer in
question 1.)
4. Is glass a liquid or a solid? Explain the viscosity characteristics of glass.
5. What role does viscosity and SiO2 (silica) content play in the morphology characteristics of
shield, composite, and cinder cone type volcanoes?