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Igneous Rocks and the Rock Cycle

Igneous Rocks and the Rock Cycle . I. rock. Rock - any naturally formed aggregate ( mixture) minerals of usually formed into a hard mass. A rock usually contains many different minerals (ex: granite), but a few rocks consist of only one type of mineral (limestone, sandstone).

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Igneous Rocks and the Rock Cycle

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  1. Igneous Rocks and the Rock Cycle

  2. I. rock Rock - any naturally formed aggregate (mixture) minerals of usually formed into a hard mass. A rock usually contains manydifferent minerals (ex: granite), but a few rocks consist of only onetype of mineral (limestone, sandstone).

  3. II. Three major groups of rocks • Igneous Rocks (“Ig Rx) - formed by the coolingand hardening of magma(hot molten rock from the interior of the earth). • Sedimentary Rocks (“Sed Rx”) - formed by compaction & cementing of loose sediments (rock fragments, shell fragments, mud, sand, etc.)-usually layered • Metamorphic Rocks (“Meta Rx”) - formed when existing rocks are alteredby heat, pressure and/or chemical reaction (“heated and squeezed”).

  4. III. James Hutton Scottish geologist; developed the Theory of Uniformitarianism (Theory of Uniform Processes): “The present is the key to the past.” (The way processes work on earth today is the way they’ve always worked - rivers, glaciers, volcanoes, etc.). Hutton & Charles Lyell developed the concept of geologic time: the earth is billions of years old, and some rock layers took millionsof years to form.

  5. Igneous Rocks • 2 Major Types of Igneous Rocks - (determined by wherethey form) • Extrusive Rocks(Volcanic) - magma cools quicklyat or nearthe surface (as lava); rocks arefine-grained, glassy or full of holes • Intrusive Rocks (Plutonic) - magma cools and hardens slowly deep below the surface; coarse-grained (large visible crystals)

  6. II. Magma vs. Lava A.Magma is moltenor partially melted rock from the asthenosphere (weak layer) of the upper mantle. Magma meltsits way upward through the overlying rocks. B. Lava forms when magma finds a volcanic pipe and reaches a surface vent hole while it is still liquid. Magma can collect for years in magma chambers fairly close to the surface before erupting or it can flow non-stop up the pipe to erupt. Magma is frothy and bubbly. As magma rises to the surface to become lava, it: 1) melts and mixes with overlying rocks, which are usually higher in silica 2) loses a lot of its dissolved gasses, which bubble out into the air

  7. II. Magma vs. Lava c. Lava forms smooth, ropy, billowy pahoehoe(paw-hoy-hoy) lava when it’s hot and runny, and blocky, jagged aa(ah-ah) lava when its cooler and more viscous (thick). When lava cools quickly, gasbubbles are trapped inside to form scoria or pumice (Pumice can be found floating as large rafts down rivers following eruptions.)

  8. III. Magma has three basic chemical compositions (“what” they are made of) • Sialic - high in Silica and Aluminum. They are lighterin color and form very viscous(thick, sticky) lava which flows slowly. Ex: Granite, Rhyolite lava

  9. III. Magma has three basic chemical compositions (“what” they are made of) 2) Mafic - high in Magnesium and Iron (Ferric ion), but low insilica. They are darkerin color and form lava which flows easily. Ex: Basalt lava and Gabbro

  10. III. Magma has three basic chemical compositions (“what” they are made of) 3) Ultra-mafic - almost nosilicaat all; form in deepest part of crust; often crumbly and weathered when exposed at the surface; Ex: Peridotite

  11. III. Igneous Rock Textures give visible evidence of “how”the rock formed. Textures depend on the sizeof the crystal, not how the rock feels to the touch

  12. Size of mineral crystals depends on: 1) How quicklythe magma cools 2) %of dissolved gassesin the magma 3) The amount of waterin the magma 4) Composition - Mafic minerals cool first and tend to be more regular in shape. Quartz isthe last mineral to cool and tends to be very irregular in shape.

  13. Phaneritic– coarse -grained crystals (all about the same size/visible tothe naked eye) which formed slowly. (usually plutonic rocks)

  14. Aphanatic – fine -grained crystals (too small to see without a hand lens) which formed slowly(usually volcanic rocks).

  15. Glassy (Amorphous) - nocrystalstructure at all; formed by a sudden chilling of lava (Ex: Obsidian)

  16. Vesicular - filled with holes(vesicles) caused by trapped gasbubbles; Ex: small holes w/glassy rock - pumice; large holes - scoria

  17. Porphyritic - large crystals (phenocrysts) surrounded by a fine - grained ground mass (background rock)

  18. Pyroclastics (tephra)- layered rocks formed from hot ashdeposits (tuff) or larger bombs(ejecta) thrown from a volcano (“fiery sediments”)

  19. Pegmatitic- the last minerals to cool in a magma chamber form in magma which is very fluid due to a large amount of water. As a result, granite pegmatites contain huge crystals of sialic minerals (up to 30’ long!)

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