Igneous Rocks Lab

Last Modified: 08/20/09

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 Viscosity
• Classification of Igneous Rocks
• Terms relating to Mineral Composition
• Terms relating to Texture
• Texture: An indicator of Cooling History
• Simplified Igneous Rock ID table
• Terms related to Plutonic and Volcanic Rocks
• Volcanic Landforms
• Pyroclastic Rocks

Magma vs. Lava

• Molten rock located within the Earth is called MAGMA.
• Molten rock that emerges at the Earth's surface is called LAVA.

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

• What are silicates? (p. 67) Silicates are a group of minerals that contain Si and O (in the form of Si-O tetrahedra in which 4 O atoms surround each Si atom).
• Why are there crystals? Each mineral has a different melting point. More complex silicates (such as the framework silicates--quartz, feldspar, etc.) have lower melting points than simple silicates (such as olivine) generally speaking.
• What are volatiles? Substances that are gases at low temperatures--chiefly water.

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.

• Example: Pine Sap or Molasses (High Viscosity) vs. Water (Low Viscosity)
• The HIGHER the viscosity, the more resistance to flow!!

Why do magmas have different viscosities?

• As a lava cools, the viscosity increases as the lava starts to solidify.
• The more complex the silicates melted in the magma, the higher the viscosity.

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

• The more viscous a magma, the more volatiles it can hold.
• The more volatiles within a magma, the more explosive it becomes!

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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):

• Easier to melt minerals
• The more complex the silicate mineral
• Higher the viscosity of the magma
• More resistant the mineral is to chemical weathering
• Lower the rock's final density

Terms related to Mineral Composition

• FELSIC: Igneous rock rich in light colored minerals such as feldspar (K feldspar/plagioclase)
• MAFIC: Igneous rock rich in dark colored ferromagnesian minerals (augite, hornblende), but with abundant feldspar (plagioclase)
• ULTRAMAFIC: Igneous rock composed chiefly of dark colored ferromagnesian minerals (especially olivine & pyroxene)

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:

• Fine Grained: Can't see individual crystals with the naked eye
• Coarse Grained: Can see crystals with naked eye (>2 mm)
• Pegmatitic: exceptionally coarse grained crystals (approximately 1 cm or larger)

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...

• No Crystals (GLASS) it indicates EXTREMELY RAPID COOLING (seconds, minutes)
• Fine-Grained Crystals it indicates RAPID COOLING (hours, days, weeks)
• Coarse Crystals it indicates SLOW COOLING (thousands to millions of years)
• Pegmatitic Crystals it indicates SLOW COOLING (thousands to millions of years) and the presence of water

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:

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):

• Sill: Sheetlike igneous body injected parallel to the local structure
• Dike: Sheetlike igneous body injected across the local structure
• Batholith: Enormous body of granitic rock (>100 sq. km) exposed at the surface due to erosion

TERMS RELATED TO VOLCANIC ROCKS

• Columnar Joint: polygonal columns developed in basalt as it cools and shrinks
• Vesicles: remnant gas bubbles in a volcanic rock

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

• Caldera: Very large collapse depression (means "kettle"); usually filled with water
• Composite Volcano: Contains alternating layers of lava and pyroclastic debris
• Fissure Flow: Lava comes from extended fissure instead of a vent
• Shield Volcano: Large, gently sloping volcano due to basalt (low viscosity) flows
• Cinder Cone: Small, steeply sloping volcano made up of pyroclastic debris

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PYROCLASTIC ROCKS

Tuff: explosively formed, fine grained volcanic rock welded together

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