Download Ch 6 volcanoes

Volcanoes
Processes that Form Magma in the
Crust and Upper Mantle
• Increase in temperature
• Pressure-Release Melting: decrease in
confining pressure lowers the melting
point of rock
• Addition of water lowers the melting
point of rock
Fig. 8-2, p.171
Environments of Magma
Formation
• Spreading centers associated with
divergent boundaries
• Subduction zones associated with
convergent boundaries
• Mantle plumes
Pressure-release melting at spreading
center
Fig. 8-7, p.173
Fig. 8-6, p.173
Figure 4.27
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
rise up because it is less dense
Forming Igneous Features and Landforms
Fig. 8-15, p.179
Fig. 8-16, p.180
Figure 4.24
Figure 4.26
Volcanoes
• Types of Volcanoes
• Shield volcano
– 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
Volcanoes
• Types of Volcanoes continued
• Cinder cone
– Built from ejected lava (mainly cinder-sized)
fragments
– Steep slope angle
– Rather small size
– Frequently occur in groups
Sunset Crater – a cinder
cone near Flagstaff, Arizona
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Volcanoes
• Types of volcanoes continued
• 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 interbedded lava flows and
layers of pyroclastic debris
Figure 4.1a
A composite volcano
Figure 4.7
Figure 4.20
Plate Tectonics and Magma Generation
Tectonic Settings and Volcanic Activity
Formation of Crater Lake