Igneous Rocks Lab

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Last Modified: 08/20/09
Lecture Notes

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Igneous rocks are formed when molten rocks solidify (crystallize).


Topics of Discussion for Igneous rocks: (just click on the topic to jump to that section)


Magma vs. Lava

MAGMA - is a silicate melt that contains crystals and dissolved volatiles

MAGMAS when cooled form PLUTONIC (also termed INTRUSIVE) igneous rocks

LAVAS when cooled form VOLCANIC (also termed EXTRUSIVE) igneous rocks

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Magma Viscosity

Different magmas have different viscosities. Viscosity is the property of a substance to internally resist flow.

Why do magmas have different viscosities?

Why are some eruptions from volcanoes explosive while others aren't?

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Classification of Igneous Rocks

Igneous rocks are CLASSIFIED according to their mineral composition and texture.

Minerals form as molten rock crystallizes. A generalized order of crystallization of minerals is shown in the Bowen's Reaction Series. As the magma cools, first olivine, then pyroxene, then amphobole, etc. will be crystallized (assuming the proper elements are available). Please note that this is a very simplified progression, but still useful to help us discern some trends.

THE BOWEN'S REACTION SERIES

· Shows a generalized crystallization order of minerals as a magma cools.

GENERALITIES down the reaction series (from top to bottom):


Terms related to Mineral Composition

FELSIC rocks - typical colors include: white, pink, and colorless

MAFIC rocks - typical colors include: black and gray

ULTRAMAFIC rocks - typical colors include: olive green and black

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Terms related to Texture

Crystal Size:

Porphyry (p. 108): Describes an igneous rock where coarse crystals (called PHENOCRYSTS) and fine grained crystals (termed GROUNDMASS) exist together in an igneous rock.

Pegmatite: Describes an igneous rock where ALL crystals are pegmatitic (> 1 cm) in size.

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Texture: An indicator of Cooling History

If your igneous rock sample displays...

What if I have both fine-grained and coarse-grained crystals in a rock (a Porphyry)?

It indicates that the magma underwent two cooling histories. Rapid cooling can only occur at the surface of the earth (such as in a volcanic eruption). Slow cooling (millions of years) can only occur in an insulated cavity deep within the earth. Therefore, the coarse-grained crystals must have formed first deep within the earth and then those crystals (and the surrounding liquid rock) was ejected at the earth's surface--at which time the remaining fluid cooled quickly.

Textures and Cooling Histories of Volcanic and Plutonic Igneous rocks

Since lava cools quickly at the earth's surface, VOLCANIC rocks will have a texture of: no crystals, fine-grained crystals, or a mixture of fine-grained and coarse-grained (porphyry).

Conversely, since magma cools very slowly deep within the earth, PLUTONIC rocks will have a texture consisting of coarse-grained crystals or pegmatitic crystals.

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Simplified Igneous Rock Identification Table:

   

Felsic

Mafic

Ultramafic

Volcanic

Glass

Obsidian; Pumice

     

Fine-grained

Rhyolite

 

Basalt; Scoria

 

Fine & Coarse

 

Andesite Porphyry

   

Plutonic

Coarse-grained

Granite

 

Gabbro

Peridotite

Pegmatitic

Granite Pegmatite

     

NOTE: On this chart, blue names are the igneous rock types. Italicized rocks indicate the presence of vesicles (remnant gas bubbles).

Example on how to use this table:

Let's say you find a black and gray igneous rock in which all of the grains are small, but easily seen with the naked eye. The gray and black colors are indicative of a mafic rock mineralogy (plagioclase = gray, augite/hornblende = black). The fact that you can make out individual mineral grains indicates a coarse-grained texture. Using the chart, locate the Mafic column and find where it intersects the Coarse-grained row to get the rock name.  From the chart, we determine that this rock is called GABBRO (a Plutonic rock).

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TERMS RELATED TO PLUTONIC ROCKS (See Fig. 6-3, p. 111):

TERMS RELATED TO VOLCANIC ROCKS

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LANDFORMS (See Table 6-2, p. 114)

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PYROCLASTIC ROCKS

Tuff: explosively formed, fine grained volcanic rock welded together

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