Lab 5a: Igneous Rocks

Objectives:

- Relation of igneous rocks and plate tectonics

- The Rock Cycle

- Types of igneous bodies

- Review 8 rock forming minerals

- Textures, composition and color of igneous rocks

- Origin of igneous rocks

Introduction:

Rock Cycle: relation of three rock types

Overhead of Figure 4.1 – pg 78

Ign rx form when molten rock cools to a solid state, forming intergrown crystals or glass

Magma – below the earth’s surface

Lava – at the Earth’s surface

Sed rx: compaction and cementation of sediment

Meta rx: rocks that have undergone intense heat and pressure

Igneous Introduction:

Magma: under high pressure, less dense than confining rocks, causes it to rise

Intrusions: body of magma that pushes its way into the Earth’s crust

Overhead Figure 5.1 – pg 93

Types of intrusions: batholiths, sills, laccoliths, pipes, sheet, ring and radial dikes

Intrusive rocks (cooled in intrusions)

Extrusive rocks (extruded or violently ejected onto the surface through volcanoes)

Textures:

Texture: description of its constituent parts and their sizes, shapes and arrangement

Can be used to classify and infer origin of igneous rocks

Overhead of Figure 5.2– pg 94

Nucleation: initial formation of a microscopic crystal, to which other atoms bond

Glassy: volcanic glass

Aphanitic: fine-grained (<1mm)

Phaneritic: coarse-grained (1-10 mm)

Pegmatitic: very coarse-grained (>1 cm)

Porphyritic: two distinct textures, phenocrysts and matrix or groundmass

Porphyritic-aphanitic or Porphyritic-phaneritic

Vesicular: vesicles or gas-bubbles texture – scoria (sponge) and pumice (glassy)

Pyroclasts: rocky materials that have been fragmented and/or ejected

Ash (<2mm), lapilli or cinders (2-64mm), bombs or blocks (>64mm)

Pyroclastic texture: rocks composed of pyroclasts

Mineral Composition:

Eight rock forming minerals (Overhead of Figure 5.2)

Felsic (fel-feldspars and sic-silica): qtz, muscovite, potassium and plagioclase feldspar

Mafic (ma-magnesium and fic-iron): biotite, amphibole, pyroxene and olivine

Overhead of Figure 5.3 – pg 95

Color Index: percent by volume of mafic minerals in the rock

Classifying Igneous Rocks:

Felsic: 0-15% mafic minerals, generally light colored

Dominated by quartz and potassium feldspar

Intermediate: 16-45% mafic minerals, more light colored than dark

Dominated by potassium and plagioclase feldspar

Mafic: 46-85% mafic minerals, dark colored

Dominated by plagioclase and pyroxene

Ultramafic: 86-100% mafic minerals, dark to very dark colored

Dominated by pyroxene and olivine

Color is just approximation; you want to be able to identify the minerals present

Step 1: identify textures

Step 2: identify color index, and specific minerals and their relative abundances

Step 3: flowchart (Figure 5.2 and Figure 5.3)