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 rock

groups (sedimentary and metamorphic).

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

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

• Compare and contrast the different types of igneous rock 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 plate tectonics plays in the formation and composition of magma and igneous rocks.

• 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 (lava 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, erosion, deposition, and lithification. Igneous rocks become metamorphic rocks through heating and/or compression.

Igneous Rocks and the Rock Cycle

Fig. 4.1

• Geologists study igneous rocks to better understand• Chemistry of igneous rocks• Cooling history• Composition of source rocks (and of Earth’s interior)• Transport of magma toward the surface• Volcanic eruptions• Plate tectonics

Igneous Rocks and the Rock Cycle

Magma Formation and TransportThree Main Steps of Formation1) Partial melting 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)• Sometimes: increase in pressure• Sometimes: removal of volatiles

Fig. 4.7

• Texture: size, shape, arrangement of crystals

Textures of Igneous Rocks

Coarse-grained Fine-grained

Glassy Porphyritic (phenocrystsin groundmass)

Figs. 4.4, 4.14

• 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

Textures of Igneous Rocks

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 and those that form quickly

Intrusive (plutonic) or extrusive (volcanic)

Textures of Igneous Rocks

Fig. 4.5

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

Igneous Rocks in the Field

Igneous Rocks in the Field

Fig. 4.10

• 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)

Classifying Igneous Rocks

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

• 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.

Evolution of Igneous Rocks

• 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.

Magmas of different compositions may mix together.

A magma can become more viscous as minerals with more complex structures become stable.

Figs. 4.16, 4.18, 4.19

• Eruption styles and volcanic structures are related to lava chemistry.• Mafic lavas

• 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

Volcanic Eruptions

Fig. 4.25

Volcanic Eruptions

Effusive eruptions of basalt are associated with seafloor spreading centers and oceanic hot spots.

Explosive eruptions are associated with convergent boundaries.

Figs. 4.33, 4.24

• Most volcanoes form along plate boundaries.• Tectonic environment affects magma composition and

volcano type.

Igneous Rocks and Plate Tectonics

Feature Boundary Type Lava Composition Volcano Types

Mid-Ocean Ridge Divergent (ocean-ocean)

Mafic Low linear shield

Volcanic Arc Convergent(subduction zone)

Intermediate Composite

Rift Volcanoes Divergent(continental rift)

Felsic to mafic Composite

Ocean Island Intraplate (not along a boundary)

Mafic Shield

Volcanism and Plate Boundaries: Igneous rocks are associated with divergent and convergent boundaries.

Igneous Rocks and Plate Tectonics

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.