Igneous activity and plate tectonics

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

OTHER MAJOR
ELEMENTS

(%)

VISC.

OF

IGNEOUS ROCKS
PRODUCED

MAGMA

TYPE

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