1. Igneous Rock Types
- formed by crystallization of molten rocks called magma
- Classified based on:

A. Texture- primarily crystal size.

Intrusive - coarse-grained, slow cooling at depth; "plutonic rocks" (plutons, batholiths, stocks, dikes, sills).

Extrusive - fine-grained, rapid cooling at or near surface; "volcanic rocks" (lavas, pyroclastic rocks)

B. Chemical and mineral composition (SiO2 varies from 45 to 70 wt%)

Felsic- mostly quartz, K- and Na-feldspars, muscovite; high SiO2; light- colored; low-T (~700-800°C), high-viscosity. Granite, Rhyolite.

Mafic- mostly Fe-Mg rich olivine and pyroxene, Ca-rich feldspars; low SiO2; dark-colored; high-T (~1100-1200°C), low-viscosity. Basalt, Gabbro.

Intermediate- Na-rich feldspars, amphibole, biotite, minor quartz.  Granodiorite, Dacite, Diorite, Andesite.

Ultramafic - Mg-rich olivine and pyroxenes. Peridotite, Komatiite.

• Magma forms when rocks melting T° is exceeded; melting T° depends on rock composition and conditions of T and P.
• Rocks are multicomponent systems, thus they melt over a range of T°'s. Minerals that melt at the lowest T melt first and produce a partial melt. Partial melts can have very different composition than a completely melting rock (ie., basalt from the mantle). Partial melts rise and coalesce to form magma chambers.
• Increasing P causes melting T to increase. Explains why most of crust and mantle are solid. Partial melting may be induced by lowering P rapidly (decompression melting).
• Composition: Rocks with minerals that crystallize at low-T also melt at low T. Water lowers the melting T significantly.
• Diversity of igneous rocks was first explained by Magmatic Differentiation (Bowen, 1928). Process by which a uniform parent magma evolves into daughter magmas of varied composition. Occurs via fractional crystallization. (gravity settling or by compaction/deformation). Bowen's Reaction Series.  Modern theories are more complex but incorporate Bowen's theories.  Important concepts are: (1) partial melting of various source rock compositions (2) dynamic magma chamber processes

• Mid-Ocean ridges (Divergent Boundaries) Mafic; oceanic crust.
• Subduction Zones (Convergent Boundaries) Mafic to Felsic; Batholiths; crustal growth.
• Mantle Plumes (Intraplate Hotspots) Mafic

Prof. Gorring
GEOS 112 PHYSICAL GEOLOGY
Sept. 16-17, 1998

A. Basaltic-  high T° (~1100°C), dark, low viscosity, far traveling (10's of km).

• Flood basalts- immense plateaus, fluid-lava, on flat terrain, 100's m thick.
• Pahoehoe - smooth, ropey flows, dissolved gases, forms tubes.
• Aa - blocky, jagged flows, degassed lava.
• Pillow lavas - underwater cooling, spheriodal blocks, like toothpaste.

B. rhyolitic - low T° (800-1000°C). light, high viscosity, local, forms domes.

• Pyroclasts- material ejected, classed by size. Ash to house-size; tuffs; breccias.
• Pyroclastic Flow - hot mix of ash, gas, and dust that travels near surface, flows down flanks of volcanoes up to 200 km/hr; most dangerous volcanic hazard.

2. VOLCANIC LANDFORMS and eruptive styles
- Shield Volcanoes - very large, convex, broad, "shield-shaped" cone, thousands of very fluid-lavas (mostly basalt), typical at hotspots; Mauna Loa, Kilauea.

- Cinder Cones - small, concave cones, made of layers of cinders, commonly basaltic

- Composite Volcanoes - large, concave, steep-sided, alternating lavas flows and pyroclastic deposits, andesitic, erupt explosively, subduction zones; Mt. St. Helens.

- Volcanic Domes - small, steep-sided domes, viscous rhyolitic magma, usually plugs the vent of composite cones after explosive eruption, periodically collapse or explode.

- Calderas - large collapse structures; emptying of large, shallow magma chamber during violent eruptions. Occur on shield and composite volcanoes. Very dangerous.  Yellowstone; Long Valley

- Phreatic Eruption - magma in contact with water (ground, sea, lake, ice); can be very explosive; Krakatoa 1883.

- Fissure Eruptions - large volumes of lava from linear cracks; shield volcanoes; mid-ocean ridges; Iceland; flood basalts.

- Lahars (mudflows) - warm mix of wet volcanic debris; moves rapidly in stream valleys; melting glacial ice or rain on recent pyroclastic deposits. Dangerous.

 
3. TEXTONIC SETTING OF VOLCANOES
- Spreading-Zones (Divergent Boundaries)
Hot mantle rising in convection cells causes partial melting of the upper mantle. This generates large volumes of basaltic magma that collects in large, narrow, wedge- shaped magma chambers beneath the ridge axis and erupts to form new oceanic crust.

- Subduction Zones (Convergent Boundaries)
The subduction of oceanic lithosphere generates magma along narrow zones below the overiding plate to form volcanic arcs. Magmas are largely produced in the mantle 'wedge' above the subducting plate as fluids released from subducted sediments and altered oceanic crust lowers the melting T of this part of the mantle. Partial melting of the subducted sediment and oceanic crust also may contributes to subduction zone magmas. Further processing occurs in crustal magma chambers before eruption. Compositions range from basaltic to rhyolitic. Dioritic to granitic plutons form from the magma that is nor erupted. Major process of continental crustal growth.

Ocean-Ocean = more basaltic to andesitic
Ocean-Continent = more andesitic to rhyolitic (although basalts do erupt)

- Intraplate Volcanism (Hotspots; mantle plumes)
Narrow conduits of very hot rising mantle generates extremely large volumes of basaltic magmas by decompression melting of the mantle. Can erupt on continents or in the ocean basins. Mostly basalts. Continental areas can erupt felsic rocks.

Flood Basalts - Siberian Traps (250 Ma); Deccan (66 Ma) due to "superplumes" or plume heads; normal hotspot chains are the plume tails.

 
4. VOLCANIC HAZARD PREDICTION
- Land use planning - identifying hazardous areas, geologic mapping

- Monitoring Techniques - ground deformation, seismicity, gas emmisions can give adequate short-term warnings (Pinatubo, 1991; Rabaul, 1994)

- Benefits of Volcanism - produces fertile soil; hydrothermal energy, precious metals