CHAPTER 5:Igneous Rock

Rock of Ages granite quarry, Barre, Vermont

What important

natural resources are

found in igneous rock?

Steven Earle

Stawamus Chief, Squamish BC

Massive granite

Mafic dyke (~5 m across)

Igneous Rock

• Igneous rock is formed when molten, or partially molten, rock solidifies.

• Extrusive– Lava and Pyroclastic Debris– Extruded at surface or at very– shallow levels and cooled quickly

• Intrusive– Magma crystallized slowly within

the crust.– No exposure to the cool

atmosphere.– Plutonic – intrusive igneous rock

cooled slowly at great depth within crust or mantle.

Igneous Rocks (two types)

Basalt is Extrusive

Granite is Intrusive

What is the key textural difference between these two rocks?

Why Study Igneous Rocks?

• All rocks on Earth have evolved from the first igneous rocks through Igneous Evolution

• Provide information about the composition of the mantle • Absolute age can be obtained through Radiometric Dating• Features are characteristic of a specific tectonic environment• Host important mineral deposits (metal ores)• Numerous other critical economic uses• Millions of people are endangered by volcanoes

Decompression melting

• Convection brings hot rock to shallower depths

• Rifting lowers pressure• Lower pressure causes a

lower melting point• Igneous rock is formed

through a process ofcrystallization and magma differentiation

Why does a lower pressure lower the melting point?

Partial Melting

• Partial Melting– Silica-rich compounds melt before

other compounds.

• Magma Differentiation– Four types:

• Magma mixing• Crystal settling• Wall-rock assimilation• Magma migration

Bowen’s Reaction Series

• Describes the order and process of magma crystallization

Bowen’s Reaction Series Animation

How Are Igneous Rocks Named?

• Texture– Texture depends on crystal size– Phaneritic:

• Slow cooling (or in the presence of water), larger crystals• Coarse texture• Intrusive or Plutonic

– Aphanitic:• Rapid cooling , smaller crystals• Finer texture• Extrusive or Volcanic

– Other textures are Glassy, Pyroclastic, Vesicular, and Porphyritic• Composition

– Minerals at the TOP of Bowen’s• Dark in color• Mafic to Ultramafic • Iron and Magnesium are dark.

– Minerals at the BOTTOM of Bowen’s• Light in color• Felsic• Sodium plagioclase, potassium plagioclase, and quartz are light

– Intermediate composition between these

How is Texture Estimated?

• Texture is estimated using visual grain size• (depends on crystallization

history)

What has to happen for a porphyritic texture to form in an igneous rock?

How is Composition Estimated?

• Composition is estimated using visual color– Can be misleading

Mafic vs. Felsic

• Mafic minerals crystallize early and felsic minerals crystallize late in magma

Minerals at the BOTTOM of Bowen’s Reaction Series:• Light in color

• Intermediate to Felsic• Iron- and magnesium-poor and relatively light in weight

Minerals at the TOP of Bowen’s Reaction Series:• Dark in color

• Mafic to ultramafic • Iron- and magnesium-rich and relatively heavy

Bowen’s Reaction Series

What is the significance of the word “reaction” in the Bowen Reaction Series?

What does olivine react with to produce pyroxene?

Igneous Rock Naming

• Igneous rocks are named on the basis of their texture and composition

Igneous Rock Naming(continued)

• The igneous rock diagram shows the range of mineral proportions of the various types of igneous rocks

What are the approximate mineral proportions (in %) of the rocks indicated by the two dashed lines?

Classification System for Igneous Rocks

• The classification system for igneous rocks is based on the following:

– Intrusive and extrusive igneous rocks with the same chemical composition contain the same minerals

– Silica content decreases as iron and magnesium content increases

– Potassium and sodium content decreases as silica content decreases

Seven Common Types of Igneous Rock

1) Graniteintrusive

2) Rhyoliteextrusive

Felsic Composition

Seven Common Types of Igneous Rock

3) Dioriteintrusive

4) Andesiteextrusive

Intermediate Composition

Seven Common Types of Igneous Rock

Mafic Composition

5) Gabbrointrusive

6) Basaltextrusive

Seven Common Types of Igneous Rock

Ultramafic Composition

7) Peridotiteintrusive

Igneous Evolution

• All rocks on Earth have evolved from the first igneous rocks• All Igneous Rocks Result from Magma Differentiation

Tectonic Processes

• Igneous rock evolves as a product of tectonic processes and is a major component of Earth’s crust

What is the mechanism that leads to partial melting above a subduction zone?

Identify the magma source for each type of volcano. How is the magma source responsible for the rock composition?

Basalt Formation (Spreading Centers)

• High heat flow creates Hydrothermal (hot water) Vents (black smokers)• Oceanic crust consists of layers of igneous rock

– Gabbro crystallizes at the base – Sheeted dikes of basalt extend above the gabbro– Pillow basalt, glassy fragments and metallic sulfide deposits (at black smokers)

form at the sea floor

Basalt Formation (Hot Spots)

• Intraplate sites of active Plutonism and Volcanism

• Large Igneous Provinces of Flood Basalts form above mantle hot spots

• Plumes of anomalously hot mantle rock underly active hot spots. Mantle plumes, which are solid rock, not magma, rise slowly through the mantle (but faster than the rate of mantle convection) due to their positive buoyancy with respect to the surrounding rock.

• Basalt is also present at many subduction-related volcanoes, but typically only in minor amounts.

Modern sea-floor pillow basalt

40 mya pillow basalt on Vancouver Island

Steven Earle

Igneous Intrusions

• Igneous intrusions occur in a variety of sizes and shapes.

Subduction Zones and Spreading Centre

• Subduction zones and spreading centers are areas in which magma is formed as a result of partial melting, plutonism, and volcanism.

What igneous environment is not represented in this image?

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