Chapter 8: Volcanoes
and Other Igneous Activity
I. Volcanic eruptions
A. Factors that determine the violence of an eruption
1. Composition 2. Temperature 3. Dissolved
B. Viscosity of magma
1. Viscosity is a measure of a material's resistance to
flow
2. Factors affecting viscosity
a. Temperature (hotter magmas are less viscous)
b. Composition (silica content)
1. High silica B high viscosity (e.g., rhyolitic lava)
2. Low silica B more fluid (e.g., basaltic
lava)
c. Dissolved gases (volatiles)
1. Mainly water vapor and carbon dioxide
2. Gases expand near the surface
3. Provide the force to extrude lava
4.
Violence of an eruption is related to how easily gases escape from magma
a. Easy escape from
fluid magma
b. Viscous magma
produces a more violent eruption
II. Materials associated with volcanic
eruptions
A. Lava flows
1. Basaltic lavas are more fluid
2. Types of lava
a. Pahoehoe
lava (resembles braids in ropes)
b. Aa lava (rough, jagged blocks)
B. Gases
1. One to 5 percent of magma by
weight
2. Mainly water vapor and carbon
dioxide
C. Pyroclastic
materials
III. Volcanoes
A. General features
1. Conduit, or
pipe caries gas-rich magma to the surface
2. Vent, the surface opening
(connected to the magma chamber via a pipe)
3. Crater
a. Steep-walled depression at
the summit
b. Caldera (a summit
depression greater than 1 km diameter)
4. Parasitic cones
5. Fumaroles
B. Types of volcanoes
1. Shield volcano
a. Broad, slightly domed
b. Primarily made of basaltic
(fluid) lava
c. Generally large
d. Generally produce a large
volume of lava
e. e.g.,
2. Cinder cone
a. Built from ejected lava
fragments
b. Steep slope angle
c. Rather small size
d. Frequently occur in groups
3. Composite cone (or stratovolcano)
a. Most are adjacent to the
b. Large size
c. Interbedded
lavas and pyroclastics
d. Most violent type of
activity
e. Often produce nuée ardente
1. Fiery pyroclastic flow made of hot gases infused with ash
2. Flows down sides of a
volcano at speeds up to 200 km (125 miles) per hour
f. May produce a lahar, a type of mudflow
IV. Intrusive igneous activity
A. Most magma is emplaced at depth
B. An underground igneous body is called
a pluton
C. Plutons are
classified according to
1. Shape
a.
Tabular (sheetlike)
b. Massive
2. Orientation with respect to the
host (surrounding) rock
a. Discordant B cuts across existing structures
b. Concordant B parallel to features such as sedimentary strata
D. Types of igneous intrusive features
1. Dike, a tabular, discordant pluton
2. Sill, a tabular, concordant pluton
3. Laccolith
a. Similar to a a
sill
b. Lens shaped mass
c.
Arches overlying strata upward
4. Batholith
a. Largest intrusive body
b. Often occur in groups
c. Surface exposure 100+
square kilometers (smaller bodies are termed stocks)
d. Frequently form the cores
of mountains
VI. Origin of magma
VII. Plate tectonics and igneous activity
A. Global distribution of igneous
activity is not random
1. Most volcanoes are located on
the margins of the ocean basins (intermediate,
andesitic composition)
2. Second group is confined to the
deep ocean basins (basaltic lavas)
3. Third group includes those found
in the interiors of continents
B. Plate motions provide the mechanism
by which mantle rocks melt to form magma
1. Convergent plate boundaries
a. Deep-ocean trenches are
generated
b. Descending plate partially
melts
c. Magma slowly rises upward
d. Rising magma can form
1.
Volcanic island arcs in an ocean
a. Basaltic
composition
b. e.g. the
2. Continental volcanic
arcs
a. Andesitic or rhyolitic
composition
b. e.g.
2. Divergent plate boundaries
a. The greatest volume of
volcanic rock is produced along the oceanic ridge system
1. Lithosphere pulls apart
2. Less pressure on
underlying rocks
3. Partial melting occurs
4. Large quantities of
fluid basaltic magma are produced
3. Intraplate
igneous activity
a. Activity within a rigid
plate
b. Plumes of hot mantle
material rise
c. Form localized volcanic
regions called hot spots
1. Associated with
2. Columbia Plateau in the northwestern