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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
Copyright © 2006 Pearson Prentice Hall, Inc.
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
MagmaRiseppt.html MagmaRiseppt.html
Formation of Crater Lake