Home
Introduction
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Chapter 8
Chapter 9  
Chapter 10
Chapter 11
Chapter 12
Chapter 13
Chapter 14
Chapter 15
Chapter 16
Chapter 17
Chapter 18

Chapter 8 Igneous Activity

8-1    Nature of volcanic eruption:

Some are quiet flow and others violent explosion. How to cause?

  1. Viscosity:

    • When water and spaghetti sauce heated the latter produces burps. Why? The latter has too high viscosity. Gases can't escape easily.

    • Gas dissolves more at low T. As T increases, gases try to escape.

    • Epicenter: surface projection of focus.

  2. What controls the viscosity?

    • The composition of lava.

    • The temperature of lava.

      • Mefic: less SiO2, less viscous

      • Felsic: more SiO2, more viscous.

  3. Lava or pyroclastic:

    • Lava: when magma extruded out of surface. From low viscous material.

    • Pyroclastic: particles produced due to explosion. From high viscous material

  4. Types of lava flows:

    • Pahoehoe: ropy like. Picture below taken from Crater of the Moon National Monument. Relative low viscosity, high T.

    • AA: jagged shape. Fig. 8.4b. Relative high viscosity, low T.

  5. Type of pyroclastics:

    • Ash: particle small, often welded together to form welded tuff or pumice if more froth like.

    • Cinders: size of pea. Lapilli if walnut size

    • Block or bomb: size even bigger

  6. Gases: 1-5% of total weight. Water vapor, CO2, S, N2, etc. Stink smell is due to S. Corrosive.

8-2    Types of volcanoes:

  1. Shield volcanoes: like a shield, gentle slope, large in area, due to basaltic lava flow.

  2. Cinder cones: like a cone, steep slope, small area due to pyroclastic extrusion.

  3. Composite cones: inter-bedded between lava flows and pyroclastic deposits. Intermediate composition, larger, multi-phases, due to composition of magma changes.

8-3    Landform of volcanic activity:

  1. Volcanic peaks: like mount hood, etc.

  2. Calderas: empty of magma chamber below caused the rock to collapse. Fig. 8.15.

  3. Volcanic neck: rock in neck resist to weathering, e.g. ship rock. Fig. 8.19.

  4. Fissure eruption: at a greater area to produce lava plateau, e.g. Columbia basalt.

8-4    Intrusive activity:

Rocks often called plutonic rocks, cooling process s slow, more time for crystallization. Classified from shape and size:

  1. Batholith: larger area, coarse crystals over 100 km2.

  2. Laccolith: smaller, near 3-d, or lens like. Fig. 8.20.

  3. Sill: dish-like intrusive, parallel to beddings of sedimentary rocks, often has columnar jointing Fig. 8.21.

  4. Dike: sheet-like, but not parallel to the bed, often vertical.

8-5    Origin of magma:

  1. T: Melting temperature for basaltic rock is ~ 1000 degree, for granitic rock is ~ 750 degree. T increases as the depth increases.

  2. P: As P increases, the melting point increases. Less T needed to melt if P less.

  3. Partial melting:

8-6    Distribution of igneous activity:

  • Along the spreading center, i.e. along divergent boundaries, i.e., along Mid Ocean Ridge (MOR), where the farther away from the center, the older the rock. Fig. 8.24.

  • Along the subduction zone: where volcanic activity is on the continental side, if the subduction is under continents. Or 2 oceanic crusts meet, i.e., the Ring of Fire.

  • Interplate volcanism: considered by localized hot spots, could be within continental plate, e.g. Yellow Stone, or oceanic plates, i.e. Hawaii.

Homework:

  • Read chapter summary on p.241.

  • Use your own word to explain the key terms on page 242.

  • Answer the review questions on page 242.