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