Download Lecture 5. Igneous Rocks

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Igneous Rocks
and Intrusive
Igneous Activity
The 3 types of rocks:  Sedimentary  Metamorphic Marble
 Igneous 1
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SEDIMENTARY ROCKS
Come from rocks sediments
Fossils
Siltstone
METAMORPHIC ROCKS
Gneiss
Rocks have been changed from a pre-existing rock,
by high-pressure/high-temperature
(Metamorphism by high pressure)
(Metamorphism by high temperature)
Marble
Sedimentary
Igneous
Metamorphic
Sedimentary
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IGENEOUS ROCKS
Are made of volcanic material (lava or magma)
Or Volcanic
Granite
Or Plutonic
The Rock Cycle Igneous
Rock
High Temp. Sediments Pressure and
Metamorphic
Rock
Sedimentary
Rock
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Igneous rocks  Igneous rocks make up large parts of the continents and all the oceanic crust ‐ indicate plate boundaries and hot spot activity 4
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Composition of Magma  Silica is the primary constituent of magmas  Magmas are differentiated based on the relative proportions of silica, iron, and magnesium.  Felsic – silica rich, iron and magnesium poor  Intermediate – intermediate between felsic and mafic  Mafic – silica poor, iron and magnesium rich Viscosity – Resistance to Flow  Viscosity is controlled primarily by temperature and composition (silica and iron content).  Felsic magma Mafic magma – high viscosity low viscosity  Pyroclastic materials  Lava flow MORE EXPLOSIVE !!!!!
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Bowen’s reaction series  Bowen’s reaction series describes the sequence of mineral crystallization in a cooling magma.  There are two branches in the reaction series:  Discontinuous series. ‐ produces ferromagnesian minerals  Continuous series ‐ produces a variety of plagioclase feldspars Igneous Rock Textures  Minerals begin to crystallize from magma and lava after small crystal nuclei form and grow.  Two broad groups of igneous rocks are:  Volcanic (extrusive) Rapid cooling = = Aphanitic texture =mineral grains are not easily visible  Plutonic (intrusive) Slow cooling = = Phaneritic texture: mineral grains that are easily visible  Rocks with more complex cooling histories are characterized by porphyritic textures. Mineral grains with different sizes. 6
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Crystals vs. Sediments (Granit = Igneous intrusive) (Conglomerate = sedimentary) Intrusive Igneous Bodies ‐ Plutons  Plutons are bodies of igneous rock which have been intruded in country rock or have formed in place far beneath the surface. Geo-inSight 1., p. 100
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Volcanic pipes and necks
 Volcanic pipes are magma‐filled, cylindrical feeder channels beneath volcanoes.  Pipes can become volcanic necks with deep erosion. Geo-inSight 3., p. 100
Laccoliths
Geo-inSight 6. and 7., p. 101
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Dikes and sills
 Dikes are discordant features (meaning they cut across layering in the country rock)  Sills are concordant (parallel to the rock layers). Batholiths and Stocks
 Batholiths are plutons that have more than 100 km² in area of exposure.  Stocks are somewhat smaller plutonic bodies. 9
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Examples Igneous Extrusive Rocks
Obsidian is usually black in color though it can also be
red or have a greenish tint. It is a dense volcanic glass,
usually composed of rhyolite, rich in iron and
magnesium. Obsidian is formed when the lava cools so
quickly that crystals do not have time to grow. Obsidian
fractures with very sharp edges. It was used by Stone
Age cultures for making knives, arrowheads, and other
tools where sharp edges are important.
Pumice is light and porous. It forms during explosive
eruptions. Pumice is full of holes caused by expanding
volcanic gases. It is composed of volcanic glass and
minerals, and can form in all types of magma: basalt,
andesite, dacite, and rhyolite.
Examples Igneous Extrusive Rocks
Andesite is a gray to black volcanic rock. It is generally
erupted from stratovolcanoes as thick lava flows. It can
also generate strong explosive eruptions to form
pyroclastic flows.
Basalt Lava - Basalt is a hard, black volcanic rock.
Less than ½ of the weight of basalt is silica (SiO2).
Because of basalt's low silica content, it has a low
viscosity (resistance to flow). This enables basaltic lava
to flow quickly and allows volcanic gases to escape
without explosive events.
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