CHAPTER 5: Igneous Rock

Rock of Ages granite quarry, Barre, Vermont What important natural resources are found in igneous rock?

Stawamus Chief, Squamish BC Mafic dyke (~5 m across) Massive granite Steven Earle

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

Igneous Rocks (two types) 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. 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 of crystallization and magma differentiation Why does a lower pressure lower the melting point?

Partial Melting Partial Melting Magma Differentiation 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 top of Bowen’s Reaction Series: Dark in color Mafic to ultramafic Iron- and magnesium-rich and relatively heavy Minerals at the bottom of Bowen’s Reaction Series: Light in color Intermediate to Felsic Iron- and magnesium-poor and relatively light in weight

Bowen’s Reaction Series What does olivine react with to produce pyroxene? What is the significance of the word “reaction” in the Bowen Reaction Series?

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 Felsic Composition 1) Granite intrusive 2) Rhyolite extrusive

Seven Common Types of Igneous Rock Intermediate Composition 3) Diorite intrusive 4) Andesite extrusive

Seven Common Types of Igneous Rock Mafic Composition 5) Gabbro intrusive 6) Basalt extrusive

Seven Common Types of Igneous Rock Ultramafic Composition 7) Peridotite intrusive

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 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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