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
Convergent plate boundary
• 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
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
• Granite is a common example
– Extrusive igneous rocks form when
magma solidifies at the Earth’s
surface (lava)
• Basalt is a common example
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 finegrained (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
Pegmatitic igneous rock
• 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
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
Si
TYPE
(%)
OTHER MAJOR
ELEMENTS
VISC.
OF
IGNEOUS ROCKS
PRODUCED
MAGMA
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 SOURCES
*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
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
CONTINUOUS SERIES
•
•
•
•
•
• CALCIUM
PLAGIOCLASE
• SODIUM
PLAGIOCLASE
OLIVINE
PYROXINE
AMPHIBOLE
BIOTITE MICA
MINERALS WITHOUT
Fe, Mg.
CONTD*AFTER BOTH SERIES COMPLETE, HIGH
– SILICA MINERALS FORM
EX:
K – FELDSPAR
MUSCOVITE
MICA
QUARTZ
HOW MAGMA CHANGES AS IT COOLS:
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
BASALTS-OCEAN & LAND
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
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
CHAPTER SUMMARY
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
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