Download Chapter 4 - Heritage Collegiate

Chapter 4
Volcanoes and Other Igneous Activity
Notes
(3 classes notes + 1 class Xword & review + 1 class test)
Objectives:
1. Define the term volcano. p.96
2. Name and describe the three (3) main types of volcanoes. pp. 97102
3. Compare rift valley, hotspot, and subduction boundary eruptions
and the types of volcanoes produced. pp.116-122
4. Examine the global effects of volcanic activity. pp. 122-124
Def n :
A volcano is a mountain formed of lava and/or pyroclastics.
Pyroclastics are pieces of solid material that are ejected from a
volcano.
Types of Volcanoes
There are three (3) main types of volcanoes (see Figure 4.12 p. 98
text).
1. Cinder Cone - a small volcano with steep sides
 30  40 slope  built primarily of pyroclastics ejected from
a single vent. Examples are Paricutin, Mexico and Mount
Edziza in B.C (see Figure 4.14, p. 100). Typically scoria and
basalt are the rocks associated with these volcanoes.
2. Shield Volcano - a broad, gently sloping   5 slope 
volcano built from fluid mafic (basaltic) lavas. They have
little pyroclastic material. The islands that make up Hawaii,
created by a hot spot, is an example (see Figure 4.10 p. 97 and
Figure 4.11A p. 98). Another example is Iceland which was
formed when lava from the Mid-Atlantic Ridge rose above
the surface of the ocean.
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3. Composite Cone (stratovolcano) - a volcano composed of
alternating layers of lava flows and pyroclastic material. They
are usually the most explosive type of volcano. The sides are
usually steep   40 slope  and the base is wider than that of
a cinder cone but narrower than that of a shield volcano.
Stratovolcanoes typically occur in the Ring of Fire that
encircles the Pacific Ocean (see Figure 4.33 p. 117). Recall
that tectonic plates are converging there (subduction zone).
Typically rhyolite, andesite, and basalt are the rocks
associated with these volcanoes. An example is Mount St.
Helens (see Figure 4.9 p. 96 text). See animation here.
Sample Exam Question
1. What type of volcano is produced from an eruption of fluid
basaltic lava?
(A) cinder cones
(B) composite cones
(C) shield volcano
(D) stratovolcano
Activity: Complete the following table.
Type of
Type of
Size of
Slope Material
Volcano
Base
Ejected
Gentle
  5
Shield
Basalt
Mostly
Pyroclastic
Cinder
Composite
Associat
ed Rock Example
Types
Mount St.
Helens,
USA
Intermediate
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The Nature of Volcanic Eruptions - How Violent is the
Eruption?
How violent an eruption is depends mainly on viscosity
(stickiness) of the magma in the volcano. Volcanoes with highly
viscous (really sticky) magma tends to erupt violently and eject
pyroclastic material (composite volcano, Mount St. Helens),
whereas those with less viscous (runny) magma tend to erupt
quietly and eject quiet, free flowing lava (shield volcano,
Hawaii).
The magma viscosity depends on three (3) factors:
i. Temperature – if the magma is hot, it will be runny (less
viscous) and the eruption quieter.
ii. Amount of dissolved gas – if the magma contains a lot of
dissolved gas, it will be runnier (less viscous) and the
eruption quieter.
iii. Amount of silica – if the magma contains little silica, the
magma will be runnier (less viscous) and the eruption
quieter.
Describe the type of magma that would produce the most violent
volcanic eruption.
When volcanoes erupt, they eject mostly pyroclastic material
(broken rock, ash and dust) and/or lava. Two (2) common types
of lava are:
i. Pahoehoe (pronounced pah-hoy-hoy) – lava with a
relatively smooth skin with wrinkles which gives it a ropy
appearance (see Figure 4.5A, p. 93).
ii. aa (pronounced ah-ah) – rough, jagged blocks of lava with
sharp edges (see Figure 4.5B, p. 93).
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Igneous Activity (Volcanoes) and Plate Boundaries
Volcanoes usually occur in 3 main areas. Two of the areas are
along plate boundaries but one area occurs within plates (see pp.
188-119).
1. Along Convergent Plate Boundaries (Convergent Plate
Volcanism or Subduction Zone Volcanism) (see the top and
bottom diagram on p. 118). At convergent plate boundaries,
slabs of ocean crust descend into the mantle, creating an
ocean trench. The ocean crust eventually melts and the less
dense basaltic or andesitic magma moves slowly upward
toward the surface. The volcanoes produced lie in a linear or
slightly curved chain and are called continental volcanic arcs
if they lie on land and volcanic island arcs if they are formed
under the ocean and rise above sea level. Both arcs will lie
roughly parallel to the ocean trench (see Figure 4.35 p. 120).
Mount St. Helens is part of a continental volcanic arc. Most
volcanoes of this type occur in a ring surrounding the Pacific
Ocean called the Ring of Fire. The volcanoes in this area are
explosive because the oceanic crust that is subducted contains
water. See animation here.
2. Along Divergent Plate Boundaries (Divergent Plate
Volcanism or Spreading Center Volcanism) (see the top and
bottom diagram on p. 119). When two tectonic plates move
apart in the ocean, magma rises up into the rift valley created
and produces new seafloor on either side of the rift (seafloor
spreading). Material builds up on either side of the rift to
create an ocean ridge. Iceland was formed when the material
on either side of the rift rose above sea level. When two plates
move apart on a continent, a similar process occurs. The
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greatest volume of volcanic rock is produced at divergent
plate boundaries. See animation here and here.
3. Not Along Plate Boundaries (Intraplate Volcanism) (see the
left middle diagram on p. 118 and the right middle diagram on
p. 119). Volcanoes are formed here because the mantle under
the crust in this area is hotter than normal so the material
moves upward toward the crust and creates a mantle plume
(see Figure 4.36 A p. 122). When the magma (basaltic or
granitic) reaches the surface, a volcano called a hot spot is
formed (see Figure 4.36 B p. 122). The islands of Hawaii are
the result of hot spot volcanism, as is Yellowstone National
Park. See animation here and here.
Activity: Complete the following table.
Type of
Volcano
Silica
Viscosity
Content
Shield
Least
 50% 
Cinder

Composi
te

70%  Greatest
60% 
Gas Content
Eruption Style
Plate Boundary
Divergent,
possibly
intraplate (hot
spot)
1%  2%
 4%  6%
Intermediate
 3%  4%
Alternating quiet, freeflowing lava and violent,
explosive pyroclastic
material.
It turns out that volcanic eruptions don’t always build a cone or
shield. By far, most volcanic material comes from long, narrow
cracks in the crust called fissures, which extend deep into the
mantle. The lava which flows from these fissures can build up in
very thick, flat layers called lava plateaus (see Figure 4.24, p.
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108 and Figure 4.25, p. 109). Iceland, on the mid-Atlantic ridge,
was formed from eruptions from a fissure.
The Effects of Volcanic Activity
Short-term Effects
1. Material ejected from the volcano can block out sunlight.
2. Air travel disrupted
3. Create new land
4. Destroy property
5. Release gases into the air that increase acid precipitation.
Long-term Effects
1. Gases released contribute to global warming and climate
change.
2. Release gases into the air that increase acid precipitation.
3. Ejected material can enrich the soil.
4. Blocking out sunlight could lead to global cooling and ice
ages.
5. Plants relying on photosynthesis could die, disrupting food
webs and ecosystems.
Read pp. 122-124 to see how volcanoes can affect climate and
the composition of the atmosphere.
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Sample Exam Questions
1. Why is it difficult to predict volcanic eruptions?
a) Areas of volcanic activity often have long periods of dormancy
between eruptions.
b) You have no way to determine the nature of previous eruptions.
c) Volcanoes erupt without warning.
d) Volcanic eruptions are random.
2. Where do you find the most hazardous volcanoes?
a) Along a mid-oceanic spreading ridge.
b) At ocean hot spots.
c) Along convergent plate boundaries.
d) At continental fissure eruptions.
3. Match each volcano type with each statement.
___Hawaii
A. Shield
___Ash
B. Composite
___Built from lava flows.
C. Cinder
___Wide, not particularly steep.
___Convergent plate boundaries.
___Oceanic hot spots.
___Built from pyroclastics and lava flows.
4. From what type of lava are shield volcanoes usually formed?
5. How is magma produced at a subduction zone?
6. Volcanoes on continents usually have lavas that are more
silicic than those in hot spots. Why?
7. How can volcanic activity affect (1) the composition of the
atmosphere (2) climate?
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Activity: Describe the roles and responsibilities of any two
(2) of the following:
1. Structural geologist
2. Volcanologist
3. Seismologist
4. Geomorphologist
5. Geochemist
6. Geophysicist
7. Petrologist
8. Sedimentologist
Read p. 125, points 1-3, 6-8
Do #’s 1-4, 7, 9-11, 17,18, 23-29, 32, p. 126 text
Read pp. 158-172 for next day
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