Chapter Three Chapter Three

Igneous Activity and Plate Tectonics

The Rock Cycle

• A rock is a naturally formed, consolidated material usually composed of grains of one or more minerals

• The rock cycle shows how one type of rocky material gets transformed into another– Representation of how rocks are

formed, broken down, and processed in the geosphere

– Arrows indicate possible process paths within the cycle

CHAPTER – 3 IGNEOUS ROCKS

*IGNEOUS ROCKS: ROCKS THAT COOLED AND FIRE

CRYSTALLIZED DIRECTLY FROM MOLTEN ROCK, EITHER

AT THE SURFACE OR DEEP UNDERGROUND

*MAGMA: MOLTEN ROCK WITHIN THE EARTH

*LAVA: WHEN MAGMA REACHES EARTH’S SURFACE

*MOST IGNEOUS PROCESSES ARE HIDDEN FROM VIEW

*REGIONAL EROSION EXPOSES ANCIENT IGNEOUS EVENTS

:

DISTRIBUTION OF MAJOR CONTINENTAL IGENOUS ROCKS

MOLTING ROCKS AND CRYSTALLIZING

MAGMA

• MAGMA FORMATION: HEATED UNGERGROUND MINERALS – BONDS BROKEN- BECOMES MAGMA

– CHEMICAL COMPOSITION OF MAGMA CHANGES

• MAGMA COOLING AND CRYSTALLIZATION: AS COOLING PROGRESSES, DIFFERENT MINERALS

CRYSTALLIZE

 

EFFECT ON THERMAL ENERGY

The Rock Cycle and Plate Tectonics

• Magma is created by melting of rock

above a subduction zone

• Less dense magma rises and cools

to form igneous rock

• Igneous rock exposed at surface

gets weathered into sediment

• Sediments transported to low areas,

buried and hardened into sedimentary rock

• Sedimentary rock heated and squeezed at depth to form metamorphic rock

• Metamorphic rock may heat up and melt to form magma

Convergent plate boundary

CLASSIFICATION OF IGNEOUS ROCKS:

• TEXTURE: SIZE AND SHAPE OF MINERAL CRYSTALS CRYSTAL GROWTH DURING COOLING

• MINERAL CONTENT:CHEMICAL COMPOSITION COOLING HISTORY

• TEXTURE: RATE AT WHICH MAGMA OR LAVA COOL WHEN 100 – 1000 YRS FOR COOLING

TIME TO GROW LARGER CRYSTALS CRYSTALS CAN BE

VISIBLY SEEN PHANERTIC TEXTURE • INTRUSIVE ROCKS (OR PLUTONIC ROCKS) SLOW COOLING OCCURS WHEN MAGMAS INTRUDE PREEXISTING

SOLID ROCKS

Igneous Rocks

• Magma is molten rock• Igneous rocks form when magma

cools and solidifies– Intrusive igneous rocks form when

magma solidifies underground• Granite is a common example

– Extrusive igneous rocks form when magma solidifies at the Earth’s surface (lava)

• Basalt is a common example

Granite

Basalt

•PEGMATITES: IGNEOUS ROCKS WITH EXCEPTIONALLY LARGE CRYSTALS (QUARTZ, MICA, FELDSPAR ARE COMMON)

•EXTRUSIVE OR VOLCANIC ROCKS: WHEN ROCKS SOLIDIFY QUICKLY,

CRYSTALS ARE SMALL

APHANITIC TEXTURE – ROCKS WITH AHANITIC STRUCTURE ARE CALLED

EXTRUSIVE ROCKS

•PORPHYRITIC STRUCTURE: LARGER AND SMALLER GRAINS – SLOW

COOLING FOLLOWED ABRUPTLY BY RAPID COOLING

*VOLCANIC GLASS: WHEN LAVA SUDDENLY COOLS,

NO TIME TO FORM CRYSTALS. TEXTURE IS GLASSY.

*PUMICE: FORMS WHEN HIGHLY GASEOUS, SILICA –

RICH LAVA COOLS VERY RAPIDLY

*OBSIDIAN: VERY Si-RICH LAVAS CONTAINING LESS

GAS, COOL VERY QUICKLY 

Igneous Rock Textures• Texture refers to the size, shape and

arrangement of grains or other constituents within a rock

• Texture of igneous rocks is primarily controlled by cooling rate

• Extrusive igneous rocks cool quickly at or near Earth’s surface and are typically fine-grained (most crystals <1 mm)

• Intrusive igneous rocks cool slowly deep beneath Earth’s surface and are typically coarse-grained (most crystals >1 mm)

Coarse-grained igneous rock

Fine-grained igneous rock

Special Igneous Textures• A pegmatite is an extremely coarse-grained

igneous rock (most crystals >5 cm) formed when magma cools very slowly at depth

• A glassy texture contains no crystals at all, and is formed by extremely rapid cooling

• A porphyritic texture includes two distinct crystal sizes, with the larger having formed first during slow cooling underground and the small forming during more rapid cooling at the Earth’s surface

Pegmatitic igneous rock

Porphyritic igneous rock

IGNEOUS COMPOSITION

 • MAGMA O2, Si, Al, Fe, Ca, Mg, Na, K, S.

• DISSOLVED GASSES WATER

VAPOR, CO2, SO2.

• SILICATES ARE THE MAJOR

CONSTITUENTS OF IGNEOUS ROCKS

Igneous Rock Identification• Igneous rock names are based on texture (grain size) and

mineralogic composition

• Textural classification– Plutonic rocks (gabbro-diorite-granite) are coarse-grained and cooled

slowly at depth

– Volcanic rocks (basalt-andesite-rhyolite) are typically fine-grained and cooled rapidly at the Earth’s surface

• Compositional classification– Mafic rocks (gabbro-basalt) contain abundant dark-colored

ferromagnesian minerals

– Intermediate rocks (diorite-andesite) contain roughly equal amounts of dark- and light-colored minerals

– Felsic rocks (granite-rhyolite) contain abundant light-colored minerals

CLASSIFICATION OF IGNEOUS ROCKS AND MAGMAS

COMPOSITION   TYPE

Si   ( % )

OTHER MAJOR ELEMENTS

VISC. OF   MAGMA

IGNEOUS ROCKSPRODUCED

FELSIC >65 Al, K, Na HIGH ~ 600 – 800 0C

INTERMEDIATE 55-65 Al, Ca, Na, Fe, Mg MEDIUM ~ 800 – 1000 0C

MAFIC (BASALT) 45 - 55 Al, Ca, Fe, Mg LOW ~ 1000 – 1200 0C

ULTRAMARIC (PERIDOTITE)

< 40 Al, Ca, Fe, Mg VERY LOW > 1200 0C

IGNEOUS ROCK - CHART

ULTRA MAFIC IGNEOUS ROCKS: * < 40% Si. * EX: PERIDOTITE & KOMATITE * OCCURRENCE: RARE AT EARTH’S SURFACE

MAFIC IGNEOUS ROCKS: * 45 – 55 % Si. * EX: BASALT & GABBRO * OCCURRENCE: COMMON ON OCEAN FLOORS AND CONTINENTS

INTERMEDIATE IGNEOUS ROCKS: * 55 – 65 % Si. *EX: ANDESITE & DIORITE * OCCURRENCE: ABUNDANT VOLCANIC ROCK. FELSIC IGNEOUS ROCKS: * > 65% Si. * EX: GRANITE & RHYOLITE * OCCURRENCE: COMMON ON CONTINENTS 

CREATION OF MAGMA: * PARTIAL MELTING

 

WHEN ROCKS MELT TO PRODUCE MAGMA PARTIAL MELTING

DIFF. MELTING POINT

EX: ALBITE = 1118 0C

ANORTHITE = 1553 0C

 

* MELTING OF ROCKS DEPENDS ON

 

HEAT

PRESSURE

AMOUNT OF H2O IN THE ROCKS.

 

• THERMAL ENERGY

*HEAT: SOURCE OF HEAT IN THE INTERIOR

  * DECAY OF RADIOACTIVE ISOTOPES *RESIDUAL FROM EARTH’S FORMATION *FRICTIONAL HEAT FROM PLATE MOTION  *HIGH PRESSURE: THE IONS AND ATOMS IN A CRYSTALLINE SOLID

CLOSER TOGETHER – HIGH TEMP IS REQUIRED TO VIBRATE, WEAKEN,

AND BREAK THEIR BONDS.

*AS PRESSURE INCREASES, THE TEMPERATURE AT WHICH ROCKS MELT INCREASES  EX: Na – FELDSPAR ALBITE MELTS AT 1118 0C AT 100 KM PRESSURE IS 35, 000 TIMES HIGHER–MP 1440 0C

 

HEAT SOURCES

GEOTHERMAL GRADIENT

MELTING TEMPERT.-DRY

MELTING TEMPERT.-WET

FLUIDITY AND VISCOSITY OF MAGMA: MAGMA RISES BECAUSE

 

IF IT IS LESS DENSE THAN SURROUNDING ROCK

EXPANDING GASES DRIVE IT UPWARD

IT IS SQUEEZED UPWARD BY SURROUNDING ROCKS

 

VISCOSITY: FLUID RESISTANCE TO FLOW

     A) INCREASES WITH DECREASING TEMPERATURE

     

B) MINERAL (SILICA) CONTENT INCREASES VISCOSITY VALUE.

CRYSTALLIZATION OF MAGMA:

*MINERALS MELT AT THE SAME

TEMPERATURE AT WHICH THEY CRYSTALLIZE

FIRST TO MELT LAST TO CRYSTALLIZE

* AT EACH STAGE OF COOLING,  CRYSTAL/LIQUID RATIO CHANGES 

OCEANIC PLATE SUBDUCTS

MAGMA MIXING

BOWEN’S REACTION SERIES:

A)     BOTH MAFIC AND FELSIC ROCKS CAN CRYSTALLIZE FROM AN

ORIGINALLY MAFIC MAGMA

B)      EARLY – FORMING CRYSTALS REMAINING IN CONTACT WITH

THE STILL – LIQUID MAGMA REACT WITH IT TO EVOLVE INTO

DIFFERENT MINERALS

BOWEN’S REACTION SERIES

Bowen’s Reaction Series• Minerals crystallize in a predictable

order, over a large temperature range

• Discontinuous branch– Ferromagnesian minerals (olivine,

pyroxene, amphibole, biotite) crystallize in sequence with decreasing temperature

– As one mineral becomes chemically unstable in the remaining magma, another begins to form

• Continuous branch– Plagioclase feldspar forms with a

chemical composition that evolves (from Ca-rich to Na-rich) with decreasing temperature

EARLY-FORMING CRYSTALS

MAGMA & EARLY FORMING CRYSTALS

SILICATE MINERALS CAN CRYSTALLIZE FROM MAFIC MAGMAS

TWO WAYS:

DISCONTINUOUS SERIES

 • OLIVINE • PYROXINE

• AMPHIBOLE • BIOTITE MICA• MINERALS WITHOUT Fe,

Mg.  

CONTINUOUS SERIES

• CALCIUM PLAGIOCLASE

• SODIUM PLAGIOCLASE

*AFTER BOTH SERIES COMPLETE, HIGH

– SILICA MINERALS FORM EX: K – FELDSPAR MUSCOVITE MICA QUARTZ 

CONTD-

HOW MAGMA CHANGES AS IT COOLS:

1.

CRYSTALS CAN * REMAIN SUSPENDED AND REACT WITH MAGMA

* SINK

* BE PLASTERED TO THE WALLS OR CEILING OF THE MAGMA *BE FILTERED OUT AS MAGMA FLOWS ELSEWHERE OTHER MAGMA CRYSTALLIZATION PROCESSES: * OTHERS – ASSIMILATION OF ROCK BODIES

* MAGMA MIXING

INTRUSIVE ROCK FORMATION:

• RISING MAGMA MAY FORCE OVERLYING ROCKS TO BULGE

UPWARD RESULTING ROCK APPEARS AS A DOOMED INTRUSION

WITHIN OTHER ROCKS THIS STRUCTURE IS KNOWN AS DIAPIR

 

• XENOLITHS: WHEN PREEXISTING ROCK IS ASSIMILATED IN A

MAGMA, THEY APPEAR IN THE SOLIDIFIED ROCK AS DISTINCT

BODIES – XENOLITHS

PLUTONS: MAGMA THAT COOL UNDERGROUND FORM

PLUTONS

CONCORDANT PLUTONS:

DISCORDANT PLUTONS:

TABULAR PLUTONS

PARALLEL TO THE PREEXISTING ROCK LAYERS – CALLED “SILLS”

CUT ACROSS THE PREEXISTING LAYERS – CALLED “DIKES” MASSIVE SIZE BATHOLITHS

RELATIVELY THIN

TABULAR PLUTONS 1. DIKES

2. SILLS – DISTINGUISHED FROM EXTRUSIVE FLOWS BY  A)EVIDENCE OF HEATING OF ADJACENT ROCK

SURFACES

B)EVIDENCE OF INCLUSIONS OF COUNTRY ROCK IN BOTH UPPER & LOWER SILL SURFACE

C)LACK OF VESICLES ( HOLES FROM GAS BUBBLES ) ON UPPER SURFACE

D)LACK OF WEATHERING OF LARGE SURFACE.

PLUTONIC IGNEOUS FEATURES

BATHOLITHS AND LARGE PLUTONS: 1.        LACCOLITHS

2.        LAPOLITHS   3.        BATHOLITHS  A)DEFINITION

B)EXAMPLES

C)TEXTURE 

SEMINARY RIDGE TOPOGRAPHIC RIDGE

SILLS AND LAVA FLOWS

LACCOLITH

PLATE TECTONICS AND IGNEOUS ROCKS:

A) THE ORIGIN OF BASALT & GABBROS  1) INTRODUCTION  * UPPER MANTLE LACKS LIGHT ELEMENTS

*DEEPER MANTLE POSSESSES SOME LIGHT ELEMENTS

*PRESENCE OR ABSENCE OF LIGHT ELEMENTS IN GABBRO &

BASALT IDENTIFIES SOURCE OF PARENT MAGMA

Igneous Activity and Plate Tectonics

• Igneous activity occurs primarily at or near tectonic plate boundaries

• Mafic igneous rocks are commonly formed at divergent boundaries

– Increased heat flow and decreased overburden pressure produce mafic magmas from partial melting of the asthenosphere

• Intermediate igneous rocks are commonly formed at convergent boundaries

– Partial melting of basaltic oceanic crust produces intermediate magmas

Igneous Activity and Plate Tectonics

• Felsic igneous rocks are commonly formed adjacent to convergent boundaries

– Hot rising magma causes partial melting of the granitic continental crust

• Intraplate volcanism– Rising mantle plumes can produce

localized hotspots and volcanoes when they produce magmas that rise through oceanic or continental crust

– Hawaii is an example

PLATE SETTINGS & BASALTS

2) OCEANIC BASALTS  a) MORBS FROM UPPER MANTLE

b)OIBS ( OCEAN ISLAND BASALTS) FROM DEEPER MANTLE 

3) CONTINENTAL BASALTS   a) COMPOSITION VARIES WIDELY

b) BASALTS NEAR CONTINENTAL RIFTS FROM DEEP MANTLE

c)BASALTS NEAR SUBDUCTION ZONES FROM UPPER MANTLE 

BASALTS-OCEAN & LAND

ANDESITE & DIORITE ORIGIN

B) ORIGIN OF ANDESITES & DIORITE

1. PROXIMITY TO SUBDUCTION ZONES

2. FACTORS IN FORMATION  a) WATER CONTENT b) ASSIMILATION OF COUNTRY ROCKS c) OCEANIC SEDIMENTS

C) ORIGIN OF RHYOLITES & GRANITES 1. NEARLY ALL FOUND ON CONTINENTS

2. DERIVE FROM PARTIAL MELTING OF LOWER CONTINENTAL CRUST

3. EXIST NEAR MODERN OR ANCIENT SUBDUCTION ZONES 

GEOLOGY AT A GLANCE

 1. MAGMA, LAVA, DIFFERENCE BETWEEN THEM

2. PROPERTIES TO IDENTIFY IGNEOUS ROCKS

3. FACTOR(S) THAT GOVERN ROCK TEXTURE

4. APHANITIC, PHANERTIC, PLUTONIC, PORPHYRITIC

5. MAJOR ELEMENTS IN IGNEOUS ROCKS

6. CLASSIFICATION OF IGNEOUS ROCKS – BASIS

7. FASTEST COOLING RATE RESULTS

CHAPTER SUMMARY

8. EXAMPLES OF APHANITIC ROCK, PHANERTIC ROCK

9. WHEN ROCKS MELT UNDER LOWER TEMPERATURE?

10.FACTORS THAT CONTROL MELTING POINT OF A MINERAL.

11. WHAT IS BOWEN’S REACTION SERIES?IT EXPLAINS

WHAT?WHAT IS DISCONTINUOUS SERIES?

12. DIKE, SILL, BATHOLITHS, XENOLITHS

13. OCEANIC CRUST – BASALTS & GABBROS

14. WHAT TYPES OF MAGMA ASSOCIATED WITH WHAT BOUNDARIES