IGNEOUS ROCK

Felsic rich igneous lunar rock collected

from the lunar

highlands of the moon by

Apollo 16 astronauts.

Rock Classification

ROCKS

IGNEOUS-most abundant-primary rocks-source is magma

or lava

SEDIMENTARY-thin veener in Oceanic

and Continental Crusts

-secondary rocks

METAMORPHIC-proportion is similar to

that of Igneous rocks

-change of forms of Ig. And Sed. Due to Temperature, Pressure and Chemical Fluids

Rock

Cycle

Origin: Igneous Rock

“Fire-formed rocks“ Crystallize from molten material:

IGNEOUS ROCKS: The rocks formed through volcanic action OR The rocks which are derived from a molten mass “magma or lava”

Magma – molten rock below the Earth's surfaceLava – molten rock that erupts onto the Earth's surface through a volcano or crack (fissure)

SOURCE OF IGNEOUS ROCKS:

Magma: Molten mass comprising most abundant elements in earth – Si, Al, Fe, Ca, Mg, K, H & O. Where the SiO2 is most abundant amongst all.

Temperature of Magma- 10400 to 12000 C

VOLCANO: A gap in the Earth’s Crust where molten rocks and other material escape onto the Earth’s surface

Origin: Igneous Rock

Formation of Magma

How are rocks melted? 1. Heating 2. Depressurization 3. Increase water content 4. Increased silica content

Where do rocks melt? Subduction zones (Felsic and Intermediate) Mantle Plumes (“Hot Spots”) not only at Divergent Boundaries

MaficHot andHigh Pressure

Hot andLow Pressure

Cooling Rates of Magma

Cooling rates influence the texture if the igneous rock:

Quick cooling = fine grains Slow cooling = coarse grains

Magma

Different magmas have different viscosities. Viscosity is the property of a substance to internally resist flow. Example: Pine Sap or Molasses (High Viscosity) vs. Water (Low Viscosity) The HIGHER the viscosity, the more resistance to flow!!

Igneous textures:

Glassy AphaniticPhaneriticPorphyriticVesicularPyroclastic

Igneous rocks are classified on their texture and their composition.

Igneous textures:

Glassy - instantaneous cooling Obsidian = volcanic glass

Igneous rocks are classified on their texture and their composition.

Aphanitic - fine grain size

(< 1 mm); result of quick cooling

Igneous rocks are classified on their texture and their composition.

Basalt Rhyolite Andesite

Phaneritic - coarse grain size; visible grains

(1-10 mm); result of slow cooling

Igneous rocks are classified on their texture and their composition.

Granite Diorite Gabbro

Porphyritic- Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling.

Igneous rocks are classified on their texture and their composition.

Vesicular - contains tiny holes called vesicles which formed due to gas bubbles in the lava or magma. Very porous. May resemble a sponge. Commonly low density; may float on water.

Igneous rocks are classified on their texture and their composition.

Pyroclastic or Fragmental - pieces of rock and ash come out of a volcano and get welded together by heat.

Igneous rocks are classified on their texture and their composition.

Tuff - made of volcanic ash Volcanic breccia - contains fragments of fine-grained igneous rocks that are larger than ash.

BROAD CLASSIFICATION OF IGNEOUS ROCKS

Volcanic rocks(Extrusive rocks)-Lava or Magma flows-Pyroclastic flows

Intermediate rocks(Hypabyssal rocks) Plutonic rocks

(Intrusive rocks)-dykes, sills, batholiths, laccoliths etc.

•Volcanic (Extrusive)Volcanic igneous rocks form at the earth's surface as lava cools.

•Plutonic (Intrusive)Plutonic igneous rocks form deep underground where magma cools slowly.

Igneous Rocks Are Subdivided Into Two Classes

Igneous Rock Classification

Intrusive (Plutonic)

Extrusive (Volcanic)

Min

eral

Per

cen

tag

e

Continental Crust Oceanic Mantle Crust

Intrusive Igneous RockE.g., Felsic -Granite (Phaneritic):

Crystallized (Solidified) Felsic Magma Poor in: Fe, Mg, Ca, (<20%) Rich in: Silica (>70%)

Quartz

Na Plagioclase

Biotite

Extrusive vs. Intrusive (Mafic Rocks)Mafic Magmas Hot (>1000oC) Non-Viscous (runny, flows

easily) “Dry” (no H2O or C02)

Mafic Rocks Usually Extrusive, Fine-grained, Mafic (Basalt) rock forms oceanic

crust, Shield Volcanoes and Basalt Floods

If Intrusive, course-grained mafic rocks are formed Gabbro.

If intrusive, Dikes and Sills more common

Igneous Rock Classification Felsic Intermediate Mafic

Granite Diorite Gabbro Rhyolite Andesite Basalt

(Porphyritic)

Intr

usi

veE

xtru

sive

Bowen’s Reaction SeriesMinerals form as molten rock crystallizes. A generalized order of crystallization of minerals is shown in the Bowen's Reaction Series. As the magma cools, first olivine, then pyroxene, then amphobole, etc. will be crystallized (assuming the proper elements are available).

Bowen's Reaction Series has two branches. They are:

1.Discontinuous reaction series, from olivine to biotite,. 2.Continuous reaction series, from Ca plagioclase to Na plagioclase.

 KINDS OF IGNEOUS ROCK F

elsi

cM

afic

Inte

rmid

Bowen’s Reaction Series

The discontinuous reaction series involves the dark-colored ferromagnesian minerals:

1.olivine 2.pyroxene 3.amphibole 4.biotite.

The continuous reaction series involves the plagioclase feldspars. Plagioclase feldspars are an example of a "solid solution series", exhibiting gradations in chemical and physical properties. Chemically, this series consists of two "end members":

1. Na plagioclase (NaAlSi3O8), the sodium "end member", and 2. Ca plagioclase (CaAlSi2O8), the calcium "end member".

BOWEN'S REACTION SERIES

Shows a generalized crystallization order of minerals as a magma cools. GENERALITIES down the reaction series (from top to bottom): Easier to melt minerals The more complex the silicate mineral Higher the viscosity of the magma More resistant the mineral is to chemical weathering Lower the rock's final density

FLUIDITY OF MAGMA

Fluidity or Viscosity of magma depends on content (%) of Silica

Silica Rich (felsic)-known as Acidic magma-More viscous, so do not spreads and piles up at one place (trap gas easily)

Silica poor (mafic)-Known as Basic magma-Less viscous, moves faster and occupies larger area (release gas easily)

However, the viscosity of magma is considerably influenced by temperature too. When temperature is less- magma is more viscous (thicker) and when temperature is high - less viscous (thinner).

Volcanic Eruption of Magma

When magmas reach the surface of the Earth they erupt from a vent.  They may erupt explosively or non-explosively.   Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas).Usually begin with fire fountains due to release of dissolved gases Produce lava flows on surface Produce Pillow lavas if erupted beneath water

Eruption of Magma Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas).

Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure

High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface.

Bursting of bubbles fragments the magma into pyroclasts and tephra (ash).

Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.

Eruption of Magma

Tephra that falls from the eruption column produces a tephra fall deposit.

If eruption column collapses a pyroclastic flow  may occur, wherein gas and tephra rush down the flanks of the volcano at high speed.  This is the most dangerous type of volcanic eruption.  The deposits that are produced are called ignimbrites.

Eruption of Magma

Lateral blasts and debris avalanches occur when gas is released suddenly by a large landslide or debris avalanche taking out part of the volcano

Eruption of Magma

Plutons

Plutons = Igneous rocks cooled at depth. Name comes from Greek god of the underworld - Pluto.

Pluton features

Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude.

Discordant means that they cut across preexisting structures. They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth.

Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude.  Sills usually are fed by dikes, but these may not be exposed in the field.

Pluton features

Pluton features

Laccoliths are somewhat large intrusions that result in uplift and folding of the preexisting rocks above the intrusion.  They are also concordant types of intrusions.

Batholiths are very large intrusive bodies, usually so large that there bottoms are rarely exposed.  Sometimes they are composed of several smaller

intrusions.

Pluton features

Stocks are smaller bodies that are likely fed from deeper level batholiths.  Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed.

Methods of Igneous IntrusionMethods of intrusion

Melting - crystallization

Stoping - xenoliths

Injection

Igneous Intrusion