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

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