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.
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)
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.
A composite volcano Figure 4.7
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.
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