Igneous rocks

 

rock cycle

 

 

Igneous rocks

Igneous rocks form when magma (molten rock) cools down and CRYSTALLIZES.

How does magma form?

Probable causes of melting:

1. Geotermal gradient: (gg)

The rate at which temperature increases with increasing depth beneath the surface.

In an average it is ~25C/KM on Earth.

However it is not the same everywhere

Much higher in active volcanic region, and lower at plate interiors.

Beneath the volcanic region to melt the rocks there is no need for larger depth (50C/KM.). However beneath a plate interior, much larger depth is necessary for melting (17C/KM.).

 

2. HOT MANTLE PLUMES

Hot spots in the crust (with very high local geothermic gradient), usually above hot mantle plums (Hawaii) probable Yellowstone park.

3.FRICTION

Rocks griding past rocks may generate heat as well. (along tectonic zones).

 

4.HEAT TRANSFER

Heat from magma body may pass through country rocks and heat them. crystallization o f magma releases lots of heat.

 

MELTING TEMPERATURE

CONTROLLING FACTORs

       - PRESSURE

                   Melting temperature increases with increasing pressure.

Pressure on the other hand is increasing with increasing depth, meaning, if a rocks would melt at the surface at a certain temperature would need highertemperature to melt in the mantle.

          -Presence of water under pressure

If enough gas especially water vapor is present under high pressure, the melting temperature will be lower.

                   ex. water pressure in granite lowers the melting                                              tempr. from 900 to 650c

          -MIXING MINERALS

    Will lower the melting temperatures.

How magmas with different composition evolve?

          Differentiation

As the magma starts to crystallize the forming solid crystals separate out, gradually changing the composition of the remaining magma.

(like the whole milk differentiates into cream and skim milk).

A scienctist named Bowen had a high temperature laboratory, where he constructed a series of experiments, and figured out the order of crystallization. He collected different igneous rocks, and melted them together.

He figured, that the minerals crystallize first have higher melting temperatures, than those which would crystallize as last.

CRYSTALLIZATION BEGINS ALONG TWO BRANCHES

Discontinuous Branch

1. Olivine with very high Fe, Mg. after olivine crystallized the remaining liquid has less Fe, Mg, and more Si.

2.magma further cooles down, till it reaches the melting temp. of pyroxene, and it begins to crystallize. some pyroxenes forming from previous olivine reacting with the melt, others crystallizing from the melt, till the temperature reaches the melting temp. of amphibole

3. Pyroxene reacts with the melt and it rearranges into amphibole

4. The next step is biotite

Any magma left after biotite, has no Mg, and Fe

CONTINOUES BRANCH

CA, NA FELDSPARS

Plagioclase feldspars

The Ca rich plagioclase starts to crystallize, with the highest melting temperature. As the temperature cools, more and more Na incorporates into the structure. This branch is crystallizing till all the Na and Ca is gone.

If any more melt is left, than it has lots of Si and K and Al

                   Potassium feldspar if the magma is dry

                   muscovite if water pressure high

the last phase to crystallize if there is still melt is pure silica

                                      QUARTZ

Bowen used his experiment to prove, that every kind of igneous rock could form from a single magma called parent magma (by differentiation).

In reality there is differentiation, but rare .

Most of the time there are different magmas. magmas with low or high SiO2 content. SiO2 content is very important.

       UltraMAFIC: Very low SiO2

       MAFIC: LOW SiO2 LESS THAN 45%

       INTERMEDIATE:45-60% SiO2

       FELSIC:MORE THAN 60% SiO2

 

WE KNOW, THAT THE MANTLE HAS VERY LOW SIO2 CONTENT,

AND THE CONTINENT HAS HIGH SIO2

HOW  magma with DIFFERENT composition can form??

ALONG MID-OCEANIC RIDGES WE HAVE MAFIC MAGMA FORMING

 

BUT THINK WHAT HAPPENES ALONG SUBDUCTION ZONES WHAT DO WE HAVE?

       1, OCEANIC/OCEANIC CRUST + SEDIMENTS (MOSTLY              KAOLINITE (HIGH SIO2 CONTENT) AND LOTS OF                   SEAWATER + LOW SIO2 OCEANIC                     CRUST===INTERMEDIATE MAGMA,LOWER MELTING TEMP. BEC. OF THE WATER

       2. COLLISION, OR OCEANIC +CONTINENTAL

              FELSIC CONTINENTAL CRUST, + SEAWATER,

+KAOLINITE======FELSIC MAGMA

Assimilation

Magma form somewhere else intruding and melts some of the country rocks

Partial melting

When the temperature is not high enough to melt the whole rock, the minerals with the lowermost melting temperature will melt, resulting in the formation of more felsic magma than the original composition.

VISCOSITY

Viscosity tells us how the magma is able to flow, and helps us to know how violent an eruption will be.

Viscosity depends on:

       1. temperature

higher the temperature the larger the ability of the magma to flow (less viscous) like water

       2.SiO2 content of the magma

       The more SiO2 the magma has the higher the viscosity will be. In the melt more and more SiO2 will result in more complex huge silicate structures, and it slows down the flow ability (honey)

Depth of crystallization

If the magma will not reach the surface   INTRUSIVE

If it gets to the surface     EXTRUSIVE

HOW DO WE KNOW THE DIFFERENCE:

INTRUSIVE: SLOW COOLING ----------LARGE CRYSTALS

EXTRUSIVE:FAST COOLING -----------SMALL CRYSTALS

          extrusive molten magma is LAVA

TEXTRURE OF IGNEOUS ROCKS

Phaneritic

The characteristic intrusive texture

 Each crystal is visible with naked eyes

Extrusive texture types:

1. Porphiritic (mixed)

  Some crystals are visible, but most in the so called matrix is invisible for the naked eye.

2. Aphanitic

  Very few tiny crystals, most everything is invisible in the matrix.

3. glassy (hyaline)

 

4. vescicular

Volcanic glass with gas bubbles

5. Pyroclastic

Classification of igneous rocks 

 1. Felsic group

  Intrusive version: Granite

   Texture: Phaneritic              

   Composition: Quartz, K-feldspar, Biotite, Amphibole

    Extrusive versions: 1. Ryolite

    Texture: Aphanitic             

   Composition: Same as granite,

   Visible: few tiny feldspar, Biotite, Amphibole crystals

    Extrusive versions: 2. Obsidian

    Texture: Glassy

     Composition: Same as granite,

      No visible minerals   

    Extrusive versions: 3. Pumice

     Texture: Vesicular     

       Composition: Same as granite, no visible minerals

2. Intermediate group

  Intrusive version: Diorite

   Texture: Phaneritic              

   Composition: Na-plagioclase, Biotite, Amphibole

    Extrusive versions: Andesite

    Texture: Porphiritic              

   Composition: Same as Diorite

   Visible: Some Biotite, Amphibole crystals  

3. Mafic group

  Intrusive version: Gabbro

   Texture: Phaneritic              

   Composition: Ca-plagioclase, Olivine, Pyroxene, Amphibole

    Extrusive versions: 1. Basalt

    Texture: Aphanitic              

   Composition: Same as gabbro

   Visible: few tiny olivine

    Extrusive versions: 2. Porphiritic basalt

    Texture: Porphiritic

     Composition: Same as gabbro

      Visible mineral: pyroxene   

    Extrusive versions: 3. Scoria

     Texture: Vesicular     

       Composition: Same as gabbro, but no visible minerals

4. Ultramafic group

  Intrusive version: Dunite

   Texture: Phaneritic              

   Composition: Olivine, Pyroxene

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Occurence of igneous rocks

 

Most magma is implaced at depth and never come to the surface. They are called

                             PLUTONS

we can study them after erosion

 

Dike discordant

Sill:tabular plutons formed along sedimentary layers (Palisades sill) Their depth is shallow, they often very similar to volcanic rocks. (basaltic most of the time)

 

 

Laccoliths

More viscous magma makes it, but similarly shallow as sill. lens shape

 

batholiths

Hoge intrusive bodies,  Idaho batolith is 40,000Km2

(granitic)    

 

 

Volcanic activity

repeate viscosity

Property

basaltic

andesitic

rhyolitic

viscos.

least (low SIO2)

med.

med. SiO2

highest

high SiO2

tendency to form lavas

high

med

low

 

tend. to form pyroclastic

low

med

high

density

high

med

low

 

melt temp.

high (1200-1400)

 

med

low (600-900)

Minerals

Ca-feldspar, olivine, pyroxene

Na-feldspar, amphibole, pyroxene

 

 

 

 

 

Lava flows

5% OF MAGMAS ARE VOLATILE (GAS) MOSTLE WATER VAPOR, OR N

Basaltic flows are very fluid: they form stream like fluid flow of 10-300 m/hours

PAOHEHOE (PAH-HOY-HOY) FORM A SKIN THAT WRINKLES

AA (AH-AH) JAGGED BLOCKS WITH SHARP EDGES (SLOW)

LAVE TUBES (INSULATION)

PILLOW LAVAS UNDER WATER, NO2

LAPILLI VOLANIC PYROCLASTICS OF A SIZE OF A VALNUTS

BLOCKS

BOMB

 

 

 

 

VOLCANO

CENTRAL VENT

STEEP-WALLED DEPRESSION     CRATER

                                                    CALDERA >KM

 

BLOW OUT___ VISCOUS MAGMA PLUGGED INTO VENT AND LATER EXPLODING

 

 

 

SHIELD(BASALTIC) VERY GENTLE SLOPE

HAWAII

FLOOD BASALTS

CINDER CONES (INTERMED) VERY STEEP SLOPE OF PYROCLASTITE

30-40 OF THE SLOPE

PARICUTIN (MEXICO) 1943

 

COMPOSITE CONES

STARTOVULCANO

ST. HELENE

VESUVIO (POMPEI)