Igneous Rocks

Summary

1. The Rock Cycle

2. Formation of Igneous Rocks

3. Classification of Igneous Rocks

The Rock Cycle

Igneous Rock

Solidification

Magma

Partial Melting

Mantle Rock Fig 3.1

Geological Materials

Transformation Processes

Formation of Magma

How are rocks melted? 1. Heating 2. Depressurization 3. Partial Melting

Where do rocks melt? Subduction zones (Silicic

and Intermediate)

Mantle Plumes or Hot Spots

Partial Melting Different rocks melt

at different temperatures.

Depending on the temperature, some rocks in the mixture may be liquidified and others might not.

Types of Igneous Rock

Extrusive Fine grained rock

that cooled quickly on the Earth’s surface.

Intrusive Course grained

rock that cooled slowly under the surface.

This is the most common type of rock.

Also describe as Plutonic

See Kehew, Fig 3-6

Classification of Igneous Rocks

These classes of rock are based on the type of magma that cooled to create the rock.

These are describe in terms of temperature, composition, viscosity, and pH level.

Silicic (a.k.a, felsic) Magmas Cool (<700oC) Viscous (sticky, doesn’t flow easily) Gaseous (steam of H2O and C02)

Silicic Rocks Usually intrusive, course-grained, Silicic (Granite) to Intermediate

(Diorite). If extrusive, fine-grained rocks

formed by explosive volcanoesRhyolite or Andesite Volcanoes

Silicic Rocks

Intrusive Silicic Igneous RockE.g., Granite (Silicic, Phaneritic):

Crystallized (Solidified) Silicic Magma Poor in: Fe, Mg,

Ca, (<20%) Rich in: Silica

(>70%)

Quartz

Na Plagioclase

Biotite

Mafic Rocks

Mafic Magmas Hot (>1000oC) Non-Viscous (runny, flows easily) “Dry” (no H2O or C02)

Mafic Rocks Usually Extrusive, Fine-grained,

Mafic (Basalt) rock forms oceanic crust, Shield Volcanoes and Basalt Floods

If Intrusive, course-grained mafic rocks are formed Gabbro.

If intrusive, Dikes and Sills more common

See Kehew, Fig 3-40

Mafic Volcanism, Hawaii

Mantle Hot Spot Volcano

See Kehew, 3-8

Hawaii

10.3 12.0

20.6

Midway 27.2 19.9 43.4

42.4

48.1 55.2

56.2 59.6

0-5

Current PlateMotion

HawaiiA long chain of inactive volcanoes Island ages, millions of years

Shield Volcanoes

Mafic Magma Low-viscosity Non-explosive eruptions Gentle slopes Covering large areas

Types of Eruptions (Mafic Volc.)

Types of Eruptions Lava floods Lava fountains Fissure eruptions

Rock Textures (Table 3-1)

Aphanitic Porphyritic Vesicular Glassy (Obsidian)

Surface Textures (Extrusive, Mafic Rocks)

Ahah (Rubbley) Mostly solid when

flowing Pahoehoe (Ropey)

mostly liquid when flowing)

Mafic Sill: Intruded between layers Mafic magma is less

viscous and hotter so Does not form plutons

but Cuts along layers (Sills)

or even across layers (Dikes) Also Baked Zones

of adjacent country rock and Chill Zones within the intrusion

Igneous Rock ClassificationIntrusive (Plutonic)

Extrusive (Volcanic)

Min

eral

Per

cen

tag

e

Continental Crust Oceanic Mantle Crust

Igneous Rock Classification Silicic Intermediate Mafic

Granite Diorite Gabbro

Rhyolite Andesite Basalt

(Porphyritic)

Intr

usi

veE

xtru

sive

1200

oC

1000

o

Bowen’s Reaction SeriesTwo series of minerals formed during crystallization of magma

Intrus. Extrus.

Gabbro Basalt

Diorite Andesite

Granite Rhyolite

Low Silica Magma

High Silica Magma

Fra

me-

D

oubl

e

S

ingl

e

Isol

ated

wor

k

S

heet

Cha

in

Cha

in

Temperature ofCrystallization

750

o

Bowen’s Reaction Series

Illustrates the relationship between the cooling magma and the crystallization of the minerals contained in the rock.

Rocks on the right side of this chart are rich in calcium and sodium

Rocks on the left side represents iron-rich minerals. They cool and create quartz.

Terminology

Felsic – silicate minerals, magmas and rocks enriched in lighter elements for oxygen, aluminum, sodium, and silicon.

Aphanitic – dark-coloured rock where the grains are so fine they cannot be seen by the naked eye.

Terminology

Mafic – iron and magnesium enriched minerals are found in these rocks. They are dark in colour.

Phaneritic – Grains in the rock can be seen by the naked eye.

Types of Igneous Rocks

Granite Acidic Very pale Coarse-grained Intrusive, felsic Can be pink to dark

gray.

Rhyolite

Extrusive form equivalent to Granite

Fine grained. Make up Mt Fuji.

Andersite

Intermediate rock Less quartz that

granite, thus has a darker colour.

Both sizes of grains. Not as acidic as

Granite. Found in the Andes Extrusive

Diorite

Coarse grained Grey to dark gray in

colour Intrusive.

Basalt

Black to gray in colour (dark)

Fine-grained Mafic Found in Ireland

(Giants Causeway) and Hawaii.

Basic Extrusive

Gabbro

Dark Basic Intrusive Coarse-grained Part of the oceanic

crust

Peridotite

Ultrabasic Rare and part of the

mantle Course grained Green in colour

Obsidian

Glassy (volcanic glass)

Can be green or black

Extrusive

Pumice

Glassy (Frothy) Light to dark in colour

depending on its impurities.

Solidified foam Extrusive

Homework

P. 106 #1-3 P. 113 #1-4

Rocks in the Collection

1. Grantie

2. Rhyolite

3. Andesite

4. Obsidian

5. Pumice

6. Basalt

7. Gabbro

8. Anorthosite

9. Diorite

10. Scoria

11. Syenite

12. Peridotite