Assignments

1. Understanding Earth: Chapter 4 - Igneous Rocks: Solids from Melts
2. Earth Revealed: movie. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view #13 Volcanism and #14 Intrusive Igneous Rocks
3. Lab: Intrusive Rocks (pdf)

Terms and concepts: Igneous, intrusive, extrusive, hypabyssal, plutonic, pluton, effusive, pyroclastic, crystalline, texture, glassy, aphanitic, phaneritic, magma, viscosity, eruptive style, effusive, pyroclastic, decompression melting, felsic, intermediate, mafic, ultramafic, granite, basalt, obsidian, peridotite, Bowen's Reaction Series, geothermal gradient

Igneous activity above and below the Earth:

• Intrusive: Magma is injected into rocks beneath the earth's surface where it cools.
  • Hypabyssal: shallow
  • Plutonic: deep
  • Any intrusive igneous rock body, whether hypabyssal or plutonic, is called a pluton
• Extrusive: Magma is extruded onto the earth's surface by lava flows or explosive eruptions
  • Pyroclastic: aerial (explosive)
  • Effusive: surface flow
  In most instances the origin of an igneous rock, that is whether it's intrusive or extrusive, can be determined by its texture.

Terms and concepts

Magma: Molten material, typically a silica melt, created by the melting of rock. Magma that has reached the surface is called lava. Unlike water, when magmas crystallize several minerals form, each according it's temperature of formation (freezing/melting temperature).

Magma viscosity: A magma's resistance to flow. Some magmas are quite fluid while others flow as stiff pasty masses.

• Viscosity is controlled by:
  • Magma composition: magmas with a higher silica (Si0₂) content are more viscous because of the polymerization of SiO₄ tetrahedra
  • Temperature: High temperature inhibits bond formation and reduces viscosity
  • Water and gas content: reduce viscosity.

    The most important of these factors is silica content

Texture: refers to the size, shape, and occurrence of minerals and other components (for example, vesicles and glass fragments) in a rock.

• Igneous textures are a function of
  • cooling rate: the longer the rate of cooling the larger the crystals
  • magma viscosity: high viscosity inhibits crystal formation
  • Views some textures of igneous rocks in hand sample and thin section (Duke)
• Crystalline textures: contain minerals
  • Phaneritic: composed of visible crystals
  • Aphanitic: composed of microscopic crystals
• Non crystalline(glassy) textures: don't contain minerals
  • massive glass
  • highly vesicular glass textures: scoriaceous (dark), pumiceous (light and frothy)
  • pyroclastic: fragmental rock composed of ash, lampilli, or bomb

Types of magmas (lavas)

Ultramafic (ultrabasic)

• Composition: <45% SiO₂ and rich in Fe and Mg. Ferromagnesium minerals dominant
• Magma is generally to dense to reach the surface
• Most common occurrence: Mantle, rarely penetrates the upper crust

Mafic (basaltic, basic)

• Composition: 45-52 % SiO₂ and rich in Fe, Mg and Ca: Ferromagnesium minerals and plagioclase feldspar dominant.
• Properties: Dense and fluid--less viscous that more silica-rich magmas
• Eruptive style: Eruptions produce thin fast-moving lava flows and pyroclastic fountains.
  • Least dangerous.
• Most common occurrence: Oceanic and continental rift zones and hot spots

Intermediate (andesitic)

• Composition: 53-65 % SiO₂ with intermediate amounts of Na, K, Fe, Mg, Ca. Contains near equal amounts of ferromagnesium and non-ferromagnesium minerals.
• Properties: Moderately viscous
• Eruptive style: Lava erupts as flows and pyroclastic explosions.
  • Dangerous.
• Most common occurrence: volcanic arcs (along subduction zones)

Felsic/Silicic (rhyolitic-dacitic, acidic)

• Composition: >65 % SiO₂ and relatively rich in Na and K. Non ferromagnesium minerals dominant.
• Properties: most viscous and least dense
  • Lava ejected in the form of thick pasty flows and violent pyroclastic explosions.
• Eruptive style: Very explosive pyroclastic eruptions (high gas content) and thick pasty flows (low gas content).
  • Most violent.
• Most common occurrence: Continental rift zones, continental hot spots and continental volcanic arcs.

Classification of Crystalline - Graphic Version

World-Wide distribution of igneous activity and mechanism of magma formation

Introductory concepts:

• Igneous minerals have different temperatures of formation.  Minerals crystallize from a magma or melt from a rock according to their freezing/melting temperature. See Bowen's reaction series  (lynn Filtcher)
• partial melting: Occurs when minerals having lower melting temperatures are selectively melted from a rock. (e.g. pyroxene in a peridotite is melted and higher temperature minerals such as olivine are are left behind)
• A magma produced by partial melting is always more felsic than the parent rock. For example partial melting of a peridotite produces basaltic magma, and partial melting of a mafic rock produces a more intermediate (andesitic) magma.

Factors that promote melting

• mixtures melt at lower temperatures (e.g. at sea level olivine melts at 1890°C, whereas olivine+pyroxene+feldspar melt at 1200°C)
• Decreasing pressure (decompression): caused by thinning crust or flow into shallower crustal regions
• addition of water decreases the melting temperature of rocks
• addition or transfer of heat

Distribution of volcanism (Isand Arc, Oceanic and Continental volcanics, USGS CVO)

• Hotspots: Upwelling of hot mantle rock (peridotite) into a region of lower pressure promotes partial melting melting.
  • Hotspots Mantle Thermal Plumes (United States Geological Survey): A comprehensive site by the USGS discussing the origin and driving processes of hotspots and the people behind the theory.
  • Mantle Plumes
• Continental rift zones and oceanic spreading centers: Partial melting of the mantle is initiated by a decrease in pressure as the over lying crust is thinned and the hot upwelling rock is brought to regions of lower pressure.
• Subduction zones: Introduction of water into the mantle by subduction of hydrated rocks initiates partial melting. Mafic magma may differential or mix with felsic magma produced by melting of continental rocks.

Additional Online Resources

• online Characteristics of Igneous Rocks ( Fundamentals of Physical Geography, 2001, Michael J. Pidwirny, Dept. of Geography, Okanagan University College, British Columbia, Canada,
• Explore Online: Igneous Rocks, 2000, Lynn S. Fichter, Dept. of Geology and Environmental Science, James Madison University, Harrisonburg, Virginia 22807
• ***Igneous Rocks, Bowen's Reaction Series, (pdf) Lynn Fichter, James Madison University, Harrisonburg, VA
• Igneous Rock Tour, University of California at Long Beach (CSULB)
• Bowen's reaction Series: AskGeoMan

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