Chapter 4: Origin and Evolution of Igneous Rocks

Caption. (Figure X.X) Fig. 4.20 OBJECTIVES

• Describe how igneous rocks relate to the two other groups (sedimentary and metamorphic).

• Describe how forms and the factors that influence magma’s ascent toward the surface and its cooling history.

• Explain how produce a variety of igneous rocks with textures that vary according to the environment of their formation.

• Compare and contrast the different types of and explain the basis of their classification. OBJECTIVES

• Explain how the chemical composition and physical characteristics of magma evolve as magma cools.

• Identify the processes that cause volcanic eruptions and the various types of volcanoes and volcanic rocks that eruptions produce.

• Explain the role that plays in the formation and composition of magma and igneous rocks. Igneous Rocks and the

• Igneous rocks are one of the three main rock types (igneous, sedimentary, metamorphic).

• Igneous rocks form through the solidification of molten or partially molten rock ( on the surface or magma underground). • Magma and Lava: mixture of liquid rock, solid crystals, gases

• Igneous rocks constitute one stage of the rock cycle. Igneous Rocks and the Rock Cycle

Igneous rocks form when other rocks melt, or partially melt, and then solidify. Igneous rocks become sedimentary rocks through weathering, , deposition, and lithification. Igneous rocks become metamorphic rocks through heating and/or compression. Fig. 4.1 Igneous Rocks and the Rock Cycle

• Geologists study igneous rocks to better understand • Chemistry of igneous rocks • Cooling history • Composition of source rocks (and of ’s interior) • Transport of magma toward the surface • Volcanic eruptions • Plate tectonics Magma Formation and Transport Three Main Steps of Formation 1) of source rock • Increase in temperature • Decrease in pressure • Addition of volatiles 2) Transport of magma • Magma less dense than surrounding rock • Rises through fractures • May “digest” wall rock • May cause collapse of surrounding rock 3) Crystallization of magma • Generally: decrease in temperature (cooling) • Sometimes: decrease in pressure (decompression) Fig. 4.7 • Sometimes: increase in pressure • Sometimes: removal of volatiles Textures of Igneous Rocks • Texture: size, shape, arrangement of crystals

Coarse-grained Fine-grained

Figs. 4.4, Glassy Porphyritic (phenocrysts 4.14 in groundmass) Textures of Igneous Rocks • Two main types of igneous rocks: • Intrusive (aka Plutonic): turn solid underground • Extrusive (aka Volcanic): turn solid on the surface • Texture is a result of melting, transport, and crystallization history Texture History Igneous Rock Type/ Environment Coarse-grained Generally slow crystallization Intrusive (plutonic)

Fine-grained Generally fast crystallization Extrusive (volcanic)

Glassy Turn solid almost instantaneously Extrusive (volcanic)

Porphyritic Mixture of crystals that form slowly Intrusive (plutonic) or and those that form quickly extrusive (volcanic) Textures of Igneous Rocks

Fig. 4.5 Igneous Rocks in the Field

• Geologists study igneous rock formations in the field to better understand • Relative ages of formations • Partial melting and the source of igneous rocks • Magma transport • Crystallization history • Volcanic processes Igneous Rocks in the Field

Fig. 4.10 Classifying Igneous Rocks • Classified based on • Composition • Relative abundance of various minerals • Relative amounts of oxides • Texture (coarse, fine, glassy, porphyritic) • Field relationships • Four main groups based on mineralogy/chemistry (SiO2, Fe,

Mg) Group SiO2 content Fe, Mg content Color Felsic High Low Light-colored Intermediate Intermediate Intermediate Medium or speckled Mafic Low High Dark grey to black Ultramafic Very Low Very High Dark Green to Black Classifying Igneous Rocks

Fig. 4.13 Evolution of Igneous Rocks

• Magmas change over time. • Chemistry of the magma changes as crystals form. • Mineralogy changes as crystals react with magma. • Crystals settle out or separate from the magma (fractionation). • Magma mixes with other magmas. • Magma incorporates wall rock (assimilation). • Volatile content changes. • Temperature changes. • Viscosity changes. • Bowen’s Reaction Series • Different minerals are stable at different pressure and temperature conditions. • Crystals react with the remaining magma to form new minerals.

Fig. 4.15 A magma may fractionate through the settling of crystals.

A magma can become more Magmas of different viscous as minerals with more compositions may mix complex structures become together. stable. Figs. 4.16, 4.18, 4.19 Volcanic Eruptions • Eruption styles and volcanic structures are related to lava chemistry. • Mafic • More fluid • Less explosive eruptions • Low shield volcanoes and small cinder cones • Ropy lava, blocky lava, fissure vents, lava tubes, lava fountains, pillow lavas • Intermediate and felsic lavas • More viscous • More explosive eruptions • Large, steep composite volcanoes (stratovolcanoes) • Blocks, bombs, lapilli, breccia, tuffs, pumice, pyroclastic flows, lahars Fig. 4.25 Volcanic Eruptions

Explosive eruptions are associated with convergent boundaries.

Effusive eruptions of are associated with centers and oceanic hot spots. Figs. 4.33, 4.24 Igneous Rocks and Plate Tectonics • Most volcanoes form along plate boundaries. • Tectonic environment affects magma composition and type.

Feature Boundary Type Lava Composition Volcano Types Mid-Ocean Ridge Divergent (ocean- Mafic Low linear shield ocean) Convergent Intermediate Composite ( zone) Volcanoes Divergent Felsic to mafic Composite (continental rift) Ocean Island Intraplate (not Mafic Shield along a boundary) Igneous Rocks and Plate Tectonics

Volcanism and Plate Boundaries: Igneous rocks are associated with divergent and convergent boundaries. SUMMARY • Igneous rocks are one of the three main groups of rocks. • Igneous rocks can evolve from or into metamorphic and sedimentary rocks. • Magma forms from the melting of existing rocks. Magma rises as a result of its low density compared to surrounding rocks. • Igneous rocks are classified based on composition, from ultra- mafic (silica poor and iron rich) to felsic (silica rich and iron poor). • Igneous rocks are also classified based on texture, from glassy (no crystals) to fine-grained (small crystals) to coarse-grained (large crystals) and porphyritic (large crystals within a fine-grained matrix). The texture of an igneous rock reflects its cooling history. • Magma evolves from mafic to silicic as minerals crystallize within it. SUMMARY • Magma changes in temperature, viscosity, volatile content, and composition as it is transported toward the surface, mixes with other magmas, assimilates surrounding rock, and fractionates. • Eruption styles and volcanic structures are related to lava chemistry. • Mafic lavas generally result in gentle eruptions and low-profile volcanic structures. Intermediate and felsic lavas generally result in violent eruptions and steep volcanoes. • Many igneous rock formations formed along divergent and convergent plate boundaries, though some form within plates. • The composition of an igneous rock reflects its tectonic environment. • Geologists study igneous rocks to better understand the composition of Earth’s interior, igneous processes, and plate tectonics.