Igneous activity and plate tectonics

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 EAR

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

:

DISTRIBUTION OF MAJOR

CONTINENTAL IGENOUS ROCKS

MOLTING ROCKS AND CRYSTALLIZING

MAGMA

MINERALS – BONDS BROKEN- BECOMES MAGMA

– CHEMICAL COMPOSITION OF MAGMA CHANGES

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

•PEGMATITES: IGNEOUS ROCKS WITH EXCEPTIONALLY LARGE

CRYSTALS (QUARTZ, MICA, FELDSPAR ARE COMMON)

CRYSTALS ARE SMALL

APHANITIC TEXTURE – ROCKS WITH AHANITIC STRUCTURE ARE CALLED EXTRUSIVE ROCKS

COOLING FOLLOWED ABRUPTLY BY RAPID COOLING

*VOLCANIC GLASS: WHEN LAVA SUDDENLY COOLS,

NO TIME TO FORM CRYSTALS TEXTURE IS GLASSY.

RICH LAVA COOLS VERY RAPIDLY

GAS, COOL VERY QUICKLY

Igneous Rock Textures

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

OTHER MAJOR ELEMENTS

VISC OF

MAGMA

IGNEOUS ROCKS PRODUCED

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

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

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

ULTRAMARIC

(PERIDOTITE)

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

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

*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

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

• SODIUM PLAGIOCLASE

*AFTER BOTH SERIES COMPLETE, HIGH

– SILICA MINERALS FORM

CONTD-HOW MAGMA CHANGES AS IT COOLS:

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

MAGMA, THEY APPEAR IN THE SOLIDIFIED ROCK AS DISTINCT

BODIES – XENOLITHS

PLUTONS: MAGMA THAT COOL UNDERGROUND FORM

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:

SEMINARY RIDGE TOPOGRAPHIC RIDGE

SILLS AND LAVA FLOWS

LACCOLITH

PLATE TECTONICS AND IGNEOUS ROCKS:

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

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

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