Download Chapter 4 - Volcanoes

Volcanoes
The Nature of
Volcanic Eruptions
• Viscosity (resistance to flow)
determines the “violence” or
explosiveness of a volcanic eruption
• Factors which determine viscosity
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
Temperature - Cooler magmas
are more viscous
• A volcano’s
eruptions may get
more explosive over
time, as magma in
chamber cools
down
• Example: Crater
Lake (formerly Mt.
Mazama)
Magma Composition and
Viscosity
• Granitic/andesitic lavas
have greater silica
(SiO2) content and are
more viscous
– Convergent plate
volcanism
• Basaltic lavas have
less(SiO2) content and
are less viscous
– Divergent plate
volcanism
– Intraplate (hot spot)
volcanism
Dissolved Gases Content and
Viscosity
• Gases expand within a
magma as it nears the
Earth’s surface due to
decreasing pressure
• The violence of an
eruption is related to
how easily gases
escape from magma
• “Wet” magma (oceanic
subduction) has
significant gas content
Materials extruded from
a volcano
• Lava Flows
– Pahoehoe lava
(resembles a
twisted or ropey
texture)
– Aa lava (rough,
jagged blocky
texture)
Figure 4.5a
Figure 4.3
Materials extruded from
a volcano
• Dissolved Gases: Mainly water vapor
and carbon dioxide
• Pyroclastic materials – “Fire
fragments”
– Ash and dust - fine, glassy fragments
– Cinders – slightly larger than ash
– Pumice - porous rock from “frothy” lava
– Blocks and bombs – larger discrete pieces
of lava
Shield volcanos
• Broad, slightly
domed-shaped
• Composed primarily
of basaltic lava
• Generally cover large
areas
• Produced by mild
eruptions of large
volumes of lava
• Mauna Loa on Hawaii
is a good example
Cinder Cones
– Built from ejected
lava (mainly
cinder-sized)
fragments
– Steep slope angle
– Rather small size
– Frequently occur
in groups
Composite cone (Stratovolcano)
– Most are located adjacent to the
Pacific Ocean (e.g., Fujiyama, Mt. St.
Helens).
– Large, classic-shaped volcano
(1000’s of ft. high & several miles
wide at base).
– Composed of lava flows alternating
with large quantities of pyroclastic
flow deposits.
Figure 4.1a
Figure 4.1b
A composite volcano
Figure 4.7
Size comparison of volcano types
Formation of Crater Lake
Partial Melting and Magma
Formation
• Formation of Basaltic magmas
• Most originate from partial melting of ultramafic rock in
the mantle
• Basaltic magmas form at mid-ocean ridges by
decompression melting or at subduction zones
• Formation of Granitic magmas
• Basaltic magma pools beneath granitic continental rock
and melts it, forming granitic magma
• Granitic magma often does not reach the surface, but
instead forms intrusive rocks at depth.
How Magma Rises
Formation of Plutons from Granitic
Magma
•
Formation of Granitic magmas
• Basaltic magma pools beneath granitic continental rock
and melts it, forming granitic magma
• Granitic magma often does not reach the surface, but
instead forms intrusive rocks at depth.
• Pluton – a large mass of intrusive rock
• Most plutons are granitic in composition
• Granitic magma forms at base of continental crust and
rises up because it is less dense than the solid crust
Forming Igneous Features and Landforms
Fig. 8-15, p.179
Fig. 8-16, p.180
Figure 4.24
Figure 4.26
Figure 4.20
Figure 4.21
Plate Tectonics and Magma Generation
Figure 4.27
Tectonic Settings and Volcanic Activity