Igneous rocks:Igneous rocks: Some definitions

Molten rockMolten rock

when magma reaches Earth’s surface it is called lavalava

in the Earth is magmamagma

Magma is buoyant, rises to surface,& sometimes breaks through

An igneousigneous rock is formed when magma orlava cools and solidifies

Why should we care?

Igneous rocks make up bulk of Earth’s crust

Earth’s mantle is basically one huge igneous rock

Important rocks economically

Striking landscape features

Igneous rocks that form

- at the surface are volcanicvolcanic (extrusive)(extrusive)

Igneous rocks that form

- deep down are plutonicplutonic (intrusive)(intrusive)

Plutonic rocks

Magma bodyMagma body

To see them, they must be uplifted to surfaceAnd softer surrounding rock eroded away

Plutonic rocks

For us to ever see them, they must be uplifted to the surfaceAnd softer surrounding rock eroded away

Plutonic igneousPlutonic igneousrock example:rock example:

El CapitanYosemite Nat. Park,California

As magma cools, atoms arrange in anorderly crystal structure

crystallizationcrystallization

Note: different crystal sizes

Coarse-grained igneous rock

Close up of crystals

Note the variety of crystal sizes

TYPES OF MAGMA

1. granite ------ rhyolite

“coarse” “fine” phaneritic

aphanitic

silica (SiO2) richfeldspar and quartz

2. basalt---------gabbro “fine” “coarse” aphanitic phaneritic

iron – magnesium (Fe – Mg) richferromagnesian minerals,

plagioclase (Ca, Na feldspar)

Minerals to Rocks:Minerals to Rocks:Major Rock GroupsMajor Rock Groups

IGNEOUS SEDIMENTARY METAMORPHIC

Igneous Rock - Molten rock that has Igneous Rock - Molten rock that has cooled and crystallized into sold rock.cooled and crystallized into sold rock.

• Crystal size determined by Crystal size determined by how fast the rock cooled. Due how fast the rock cooled. Due to how crystals grow.to how crystals grow.– Long time = large crystalsLong time = large crystals– short time = small crystalsshort time = small crystals

• Course grain - crystals are Course grain - crystals are easily seen with naked eye.easily seen with naked eye.

• Fine grain - crystals < 1mm, Fine grain - crystals < 1mm, but can still be seen without but can still be seen without magnification.magnification.

• Glassy - no crystals can be Glassy - no crystals can be seen without magnification.seen without magnification.

Forms and Structures of Forms and Structures of Igneous RocksIgneous Rocks

Forms and Structures of Forms and Structures of Igneous RocksIgneous Rocks

Intrusive Intrusive StructureStructure

ss

Extrusive Extrusive StructuresStructures

Cone-sheet,Cone-sheet,Ring DikesRing Dikes

Lens-shaped Lens-shaped IntrusionsIntrusions

Sheet Sheet IntrusionsIntrusions

SubjacentSubjacent PlutonsPlutons Volcanic Volcanic

ConeConeLava PlateauLava Plateau

Columnar Columnar JointingJointing

VesiculaVesicular r

structurstructureePillow lavaPillow lavaAmygdaloidal Amygdaloidal

structurestructure

Ropy lavaRopy lava

DikesDikes SillsSills

Block lavaBlock lava

LaccolithLaccolith

PhacolithPhacolith

DiapirsDiapirs

BatholithBatholith

StockStock

LopolithLopolith

IgneousRocks

Intrusives and Extrusives

Igneous Rock StructuresIgneous Rock Structures

IntrusiveGranite

(large crystals)

Intrusive Igneous RocksIntrusive Igneous Rocks• Also called PlutonicAlso called Plutonic• Rocks form inside Rocks form inside

the earth. Granite is the earth. Granite is the most common. the most common.

• Cools slowly - Large Cools slowly - Large crystals (course crystals (course grain)grain)

• Magma - molten rock Magma - molten rock inside the earth. inside the earth.

Intrusive Igneous Rocks Intrusive Igneous Rocks ExposedExposed

Shiprock California Granite Outcrop, Yosemite

Subjacent PlutonsSubjacent PlutonsSubjacent PlutonsSubjacent Plutons BathoilthBathoilth

Large, irregular, major deep seated intrusive Large, irregular, major deep seated intrusive masses of very large size, elongated parallel to masses of very large size, elongated parallel to major tectonic regime.They have an outcrop major tectonic regime.They have an outcrop areas greater than100km2areas greater than100km2

StockStock Is similar to batholiths, with surface area less Is similar to batholiths, with surface area less

than 100 km2. They have steeply plunging than 100 km2. They have steeply plunging contact with no visible floor. Boss is a term contact with no visible floor. Boss is a term applied to stocks of circular sectionapplied to stocks of circular section

DiapirsDiapirs are bodies of rock and/or magma that are are bodies of rock and/or magma that are

buoyant and moves upwards, piercing rocks buoyant and moves upwards, piercing rocks above them.They appear as ballon like bodies above them.They appear as ballon like bodies intruded and displaced the country.intruded and displaced the country.

ExtrusiveBasalt

(small crystals+

glass)

Block Lava(subareial lava flows, result when volatile in the magma are boiled off in the vent before eruption

Pillow Lava(Submarine lava flows, exhibit a pillow structure that consists of isolated pillowed shaped masses piled one upon another

The Structures of LavasThe Structures of Lavas

Ropy Lava(lava flows with less loss of volatiles show contorted snaky folds)

Vesicular Lava(Lava flows with gas cavities or vesicles which are trapped during solidification, eg. Scoria, Pumice,…)

Amygdaloidal structure(When vesilcles are later filled with secondary minerals such asCalcite, chlorite, chalcedony,..)

Columnar Jointing(fractures that separate polygonal, pencil-like cooling structuresSeveral centimeters across.)

Extrusive Igneous RocksExtrusive Igneous Rocks• Also called volcanicAlso called volcanic• Rocks form at the Rocks form at the

earth’s surface. earth’s surface. Known as BasaltKnown as Basalt

• Cools rapidly - small Cools rapidly - small crystals (glassy)crystals (glassy)

• Lava - molten rock Lava - molten rock at the earth’s at the earth’s surfacesurface

Extrusive Igneous Rocks Extrusive Igneous Rocks form near volcanoesform near volcanoesMt. Pinatubo

Hawaii

Lava flow at Volcanoes National Park, Hawaii

Fig. 4.16Fig. 4.16

Fig. 4.17Fig. 4.17

Vesicular structure

Obsidian - extrusiveObsidian - extrusive

Glassy texture (no defined crystals)Glassy texture (no defined crystals)

Fig. 4.12Fig. 4.12

Pumice - extrusive withvesicles

Fig. 4.28Fig. 4.28

Fig. 4.30Fig. 4.30

Pillow Structure

Fig. 4.29Fig. 4.29