Lecture 4 Igneous Rocks

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Lecture 4 Igneous Rocks

• Three Main Rock Types
• Formation of Igneous Rocks
• Where does magma come from?
• The nature of volcanic eruptions
• Volcanic products
• Intrusive rock bodies
• Igneous rock textures
• Igneous rock classification
• Engineering considerations of igneous rocks

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• Three Main Rock Types
• Rocks are divided into three main types depending
  on their origin
• Igneous rocks are cooled from a molten state.
• Sedimentary rocks are deposited in a fluid medium
  (usually water).
• Metamorphic rocks are formed from preexisting
  rocks by heat and pressure.

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• Formation of Igneous Rocks
• magma igneous rocks form from the cooling of
  molten (or partially molten) rock materials
  called magma, which consists liquid, dissolved
  gas, and crystals.
• lava magma that reaches Earth's surface
• extrusive or volcanic igneous rocks that form
  when molten rock solidifies at the surface
• intrusive or plutonic igneous rocks that form at
  depth.

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• W. W. Norton

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Fig. 6.01b

• J. D. Griggs/U.S. Geological Survey

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• Where does magma come from?
• The Earth's crust and mantle are composed of
  solid, not molten, rock. So what is the source of
  magma that produces igneous activity? Magma is
  generated in the lower crust and upper mantle (at
  depths of 50 to 200 km also) by (1) raising the
  temperature, (2) reducing the pressure, (3)
  adding water. Thus, magma forms in distinctive
  tectonic settings, generally related to plate
  boundaries.
• The temperature increases with depth (on average
  by 20-30 degrees centigrade per kilometer in the
  upper crust known as the geothermal gradient). At
  convergent plate boundaries as the oceanic crust
  descends into the mantle, it is heated and
  dehydrates. The fluid reduces the melting
  temperature to cause melting.
• At divergent boundaries as hot mantle rock
  ascends, it moves to zones of lower pressure to
  trigger melting even without additional heat.

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Melting of rocks from the heat of rising magma
and decompression

• W. W. Norton

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The Earths geotherm and melting curve for mantle
rock (peridotite). A rock rises up from A to B
will start to melt (known as decompression
melting).

• W. W. Norton

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Melting by addition of water and volatiles.

• W. W. Norton

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• The addition of water and volatiles decreases the
  melting temperature of rocks. (W.W. Norton)

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• At divergent boundaries, hot mantle rock ascends
  from the ashenosphere and moves to zones of lower
  pressure as the overlying lithosphere splits and
  moves apart. This reduced pressure triggers
  melting even without additional heat.

• At convergent boundaries, as the oceanic crust
  descends into the mantle, it is heated and
  dehydrates. The fluid reduces the melting
  temperature to cause melting.

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• Explosive Mt. St. Helens eruption 1980. (Photo by
  Austin Post of USGS)

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• Why some eruptions are explosive and some are
  "quiet"?
• The primary factors are viscosity and dissolved
  gas content. The viscosity depends on temperature
  and silica content. The lower the temperature or
  the higher the silica content, the greater the
  viscosity. Very fluid basaltic (low silica)
  magmas allow expanding gases to migrate easily
  out of the vent, making the eruptions less
  violent.
• Viscosity describes resistance to shear during
  the motion of a fluid.

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• Volcanic products
• Lava flows pahoehoe flows, and aa flows.
• pahoehoe smooth, ropy surface, from low
  viscosity flows
• aa rough, jagged blocks, from high viscosity
  flows.
• Lava tubes lava conduits
• Pyroclastic materials ash, dust, pumice, bombs.

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• (Left) pahoehoe flows. (Right) aa
  flows.

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• Corrections for the first paragraph of West p.38
  The correct one should be
• ... Pahoehoe forms ropy features on the surface
  but aa forms a rough jagged blocky surface.
  Pahoehoe shows a relatively smooth surface. This
  illustrates that pahoehoe is the more fluid of
  the two lavas and it yields thinner individual
  flows.

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• View of an active lava tube as seen through the
  collapsed roof. (Photo by Jeffrey B. Judd, USGS).

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• Intrusive rock bodies Igneous intrusions are
  masses of rock formed when magma cools beneath
  the surface (generally called plutons). They are
  classified according to their sizes, shapes, and
  relationships to the rock they have invaded
  (known as the country rock). Important intrusive
  rock bodies include
• batholiths large exposures (over 100 km2) of
  intrusive rock. Batholiths typically form in the
  deeper zones of mountain belts and are exposed
  after considerable uplift and erosion, e.g.
  Sierra Nevada batholith.
• stocks small plutons with an exposure area of
  less than 10 km2.
• dikes narrow, tabular intrusive bodies. They are
  quite common. All dikes are discordant, cutting
  across preexisting structure (such as bedding
  planes). Dikes are related to the fractures
  caused by magma intrusions on the country rocks.
• sills tabular intrusive bodies formed when magma
  is injected along the bedding planes of layered
  rocks.
• laccoliths lens-shaped intrusive bodies between
  sedimentary beds that formed when the injection
  of magma arches up the overlying strata.

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• Basic igneous structures. A) relationship between
  volcanic and intrusive igneous activities. B)
  intrusive igneous structures. C) A stock and
  batholith are exposed after uplifting and erosion.

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Fig. 6.11de

• Paul Hoffmann

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• Igneous rock texture
• The texture of an igneous rock describes the
  overall size, shape, and arrangement of of its
  constituents. Texture reveals a great deal about
  the environment in which the igneous rock formed.
  The most important factor contributing to the
  texture of igneous rocks is the rate at which
  magma cools.

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• Types of igneous rock texture
• glassy texture no distinct grains, conchoidal
  fracture, produced by rapid cooling.
• aphanitic (fine-grained) texture grains not
  discernible with the naked eye, relatively rapid
  cooling at the surface (Greek phaneros --
  visible).
• phaneritic (coarse-grained) texture grains
  discernible with naked eye, slow cooling below
  the surface.
• porphyritic texture grains of two distinct
  sizes, representing two stage cooling
• pyroclastic texture composed of rock fragments
  ejected by volcanic eruptions.
• pegmatitic texture composed of unusually large
  crystals (gt 1cm). Pegmatites are formed at the
  last cooling stage of magmatic liquid crystals
  are able to grow in the fluid (water) rich final
  melt.

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(UL) Aphanitic, (UR) phaneritic, (LL)
porphyritic, (LR) glassy
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The uniformity of the granite pluton makes
carving possible. (S. Marshak)
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• Igneous rock classification
• Igneous rocks are classified on the basis of
  texture and mineral composition. The major kinds
  of igneous rocks are granite, diorite, gabbro,
  rhyolite, andesite, and basalt.
• A general relationship between color and mineral
  composition can be used Dark color generally
  indicates ferromagnesians light color generally
  indicates high silica content.

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• The classification of common igneous rocks is
  based on texture and composition. The print size
  of the rock names is proportional to their
  relative abundance at the Earths surface.

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Granite at the pavement of the Alma Mater Statue.
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• Engineering considerations of igneous rocks
• An engineering siting must consider 1) rock
  types and distribution 2) type of alteration
  after formation (tectonic fracturing,
  weathering).
• Unaltered intrusive igneous rocks are generally
  suitable for most engineering projects because of
  the tight interlocking network of mineral
  crystals.
• Problems for extrusive rocks The water-bearing
  capacity is much greater than intrusive rocks,
  making them unsuitable for reservoir or tunnel
  construction. Extrusive rocks with pyroclastic
  materials are much weaker.