GLG 101

Chapter 3

Igneous Rocks and Intrusive Igneous Activity

Introduction

Rock cycle – igneous rocks form molten rock cools and crystallizes (slides 20&235).

Magma – molten rock material originates from Earth’s mantle.

Volcanic eruptions - pyroclastic debris (ash & cinders) & lava flows.

Extrusive or volcanic igneous rocks form on surface of Earth lava solidifies.

Intrusive or plutonic igneous rocks form within Earth magma crystallizes at depth.

Texture

Texture: rate of cooling affects size and arrangement of crystals property called texture.

Aphanitic - fine grained texture, grains too small individual minerals distinguished unaided eye.

Phaneritic – coarse grained texture, individual minerals identified unaided eye.

Vesicular texture – voids left by gas bubbles that escape lava solidifies.

Glassy texture – rapid cooling lavas, ions not formed orderly crystalline structure.

Porphyritic texture – large crystals embedded matrix smaller crystals.

Composition

Igneous rocks - mainly composed silicate minerals.

Granitic rocks – approx 70% silica most abundant in continental crust.

Basaltic rocks – approx 50% silica rich in dark minerals

Bowen’s reaction series – minerals with higher melting point, crystallize first, example – olivine. Minerals lower melting point, crystallize much later, example – potassium feldspar and quartz.

Ultramafic composition – igneous rocks composed of earliest formed minerals (olivine & pyroxene).

Basaltic or mafic composition – igneous rocks composed mainly of pyroxene and calcium – rich plagioclase feldspar.

Andesitic or intermediate composition – igneous rocks composed of amphiboles and intermediate plagioclase feldspar.

Granitic or felsic composition – igneous rocks formed last crystals to crystallize, examples quartz, potassium feldspar and sodium rich plagioclase feldspar.

Classification of igneous rocks

Igneous rocks classified based on their texture and mineral composition.

Intrusive Igneous Bodies (plutons)

Dike – tabular discordant (not parallel layering in surrounding rock), intrusive structure (slide 380).

Sill – tabular concordant (parallel to layering in surrounding rock) intrusive structure (slide 381).

Laccolith – concordant intrusive structure, similar to a sill, central portion thicker and domed upward (slide 382).

Batholith – large discordant pluton with surface exposure greater than 100 square kilometers. (slides 383,384).

Volcanic neck – intrusive structure formed from magma solidified within throat volcano. Example: Ship Rock in New Mexico (Figure 3.10, p65).