Igneous rocks and Volcanoes

what are igneous rocks

Rocks that solidify from magma

• Extrusive/volcanic = outside crust

  • Fast cooling

  • Lava = cooled liquid

  • Pyroclastic debris = cooled ash, fragmented lava

  • Smaller/fine grains

• Intrusive/plutonic = inside crust

  • Slow cooling

  • Larger/course grains

• Earth is mostly igneous rock

magma vs lava

• Magma erupts from volcanoes -> extrusive rock

○ Magma = molten material (melt) below ground

○ Lava = molten material (melt) above ground

Intrusive vs extrusive igneous rock

Intrusive

Extrusive

Course grained Phaneritic rock >1mm Slow cooling Porphyritic rock -> phenocrysts Two stage cooling history Pegmatitic HUGE crystals 1cm-1m Super slow cooling Generally Clear/glassy grains = quartz Biotite = shiny black Amphibole = dull black Plagioclase feldspar = white K feldspar = pink E.g. granite, diorite

Fine grained Porphyritic rock -> groundmass Two stage cooling history Fine groundmass is formed from extrusive rock Course phenocrysts come form exploding intrusive rock that mix with extrusive rock Overall form extrusive rock Aphanitic rock <1mm Fast cooling Can have vesicles -> if explosive Glassy -> extreme rapid cooling = amorphous structure Generally higher silica composition from pyroclastic debris rocks E.g. rhyolite, andesite

relationship between mineral composition and viscosity

• Silica ∝ Viscosity ∝ Explosivity

  • Low silica = low viscosity

• Less silica = more other elements make up composition

  • Low silica/High MgO = mafic e.g. Basalt -> mostly calcium, magnesium, iron

  • High silica/Low MgO = felsic e.g. rhyolite, granite -> mostly silica

• More silica = more structured material

• SiO4 molecules make up structured lattices in silicates -> silica tetrahedra

• When silicates melt

  • Silica tetrahedron are still connected

  • Structured liquid

  • Liquid is sticky/more viscous

• When less silica present in material

  • Silica tetrahedron aren't connected to each other

  • When material melts, the structure breaks -> less chained tetrahedra

  • The tetrahedra can move past each other

  • Runny/less viscous

• Presence of water in magma = break up silica tetrahedron chains = reduces viscosity

relationship between temp and viscosity

Indirectly proportional

• Higher temperature = runnier

• Lower temperature = thicker

viscosity vs explosivity

Viscosity determines the explosiveness of an eruption

• Higher viscosity magma = more explosive volcanoes

• Thick magma traps gas inside = pressure builds = it bursts out

• Also influences volcanic landform

Lower volcano slope = less silica = less explosive

Higher volcano slope = more silica = more explosive

Silica impact on explosivity and melting temperature of rock

High silica (felsic)	Low silica (Mafic)
High viscosity Less eruptive volcanoes Explosive More pyroclastic debris, vesicles High viscosity traps gasses Low density Low melting temperature	Low viscosity More eruptive volcanoes Less explosive Runny flow eruptions, less pyroclastic debris Higher density Higher melting temperature

what is partial melting

when half of crystal grains melt and the other half stay solid

• Decrease in pressure

• Addition of volatile/explosive material

• Input of heat from core

  
  

partial melting: decrease in pressure

Decrease in pressure = decrease density in rock= lowers melting point

Less pressure closer to surface

Increase in elevation in crust while temp stays the same = causes mantle to start to melt

Happens when material is removed or crust stretches

partial melting: addition of volatiles

Water/CO2 flushing through lower crust

These cause rocks to melt at lower temperatures = volatiles lowers melting point

Wet rocks = lower melting temperature

partial melting: Input of heat from core

Magma plume/travelling up of magma

Heat transfers into surroundings = higher temperature = melting

Note: rocks can remain solid at really high temperature because if there is also high pressure the rock will remain denser and solid

why is there magma

Due to earth's hot internal temperature

• Planetesimal and meteorite accretion

  • Rocks from outer space slamming into earth

• Gravitational compression

  • Converts gravitational energy into heat

  • Core is still hot from creation of the earth + is still radiating heat out

• Differentiation processes

  • Some are exothermic

• Radioactive mineral decay

  • Release heat

  • Rock is a poor conductor = even a small amount of decay can lead to a large build up of heat

forms of magma

• solid

  • Solidified minerals are carried by the liquid.

• Liquid

  • The melt itself is comprised of mobile ions.

  • Dominantly Si and O; lesser Ca, Fe, Mg, Al, Na, and K.

  • Other ions to a lesser extent.

• Gas

  • Responsible for explosivity

  • Volatiles dissolved in the melt

  • Dry magma – No volatiles.

  • Wet magma – To 15% volatiles.

    • Water vapor (H2O)

    • Carbon dioxide (CO2)

    • Sulfur dioxide (SO2)

partial melting process

Process

1. Increase in temp

2. Melts on grain boundary

3. Little pockets of molten material form outside grain

4. As temperature increases, more of grain melts and gets smaller

5. Until material melts entirely (rare)

 

As melting increases = lower silica content

• High silica materials contribute more to partial melt

Impact of where partial melting takes place

If partial melting takes place in mantle = basaltic material made

If partial melting takes place in crust = intermediate materials made

• E.g. Andesite, diorite

• High partial melting = more silica compositions form

what is Fractional crystallisation

Modification of magma in magma chambers

• As magma cools, = crystals form by gravity

○ Low in silica content

○ Removes a fraction of the crystal

• Removal of crystal minerals as magma cools causes melt composition to change

○ Goes from mafic to felsic

• Magma moves from basaltic eventually to rhyolitic

  • Starting materials are removed in at the bottom of magma chamber -> separated from the melt

  • Fe, Mg, and Ca

  • Low silica material at bottom of chamber

• Si, Al, Na, and K remain in melt and increase

  • More silica incorporates in crystals overtime

  • Overtime mafic materials are removed by settling outside magma chambers

  • Higher silica material at top of chamber

what is Bowen's Reaction series

Determine what minerals are present in different magma types and the order in which they form

• High temperature withstanding minerals form first = are first to settle and be removed from melt -> generally the more mafic materials

• As composition changes = different minerals form (e.g. amphibole, pyroxene)

• Biotites start to form mid way through

• Quartz and high silica materials form last

bowens reactions series: discontinuous

(left side): Dark minerals form in a sequence as temperature decreases

1. Olivine (mafic)

2. Pyroxene

3. Amphibole

4. Biotite (felsic)

The final mineral: Quartz is last mineral to form

• If quartz is a phenocryst in a porphyritic rock, it does NOT FORM before the groundmass since in the Bowen's reaction series it is the last to form

• Means it must explode out into aphanitic rock

bowens reactions series: continuous

(right side): Light minerals progressively change from calcium to sodium plagioclase as temperature decreases

where do mafic rocks form

• Mid ocean ridges -> diverging plates

• Oceanic hot spots

  • Form field volcanoes

  • Less explosive since less silica

where do felsic rocks form

• Volcanic arcs along subduction zones of converging plates

  • On overriding continental plates

  • They have strato/composite volcanoes

  • More explosive since more silica

• Continental hot spots

where do mafic + felsic rocks form

continental rifts

• Stretching of lithosphere causes decompression melting = makes mafic magma

• Partial melting continental crust (rich in silica) = makes felsic magmas

basaltic magmas

• viscosity

• volcano

• eruption style

Low silica, low viscosity -> runny lava

• Lava runs off easily to form flat sheets

• Form low/gentle slopes

  • 2-6°

•  Less silica = mafic rocks → olivine, pyroxene

 

Landforms: Shield volcanoes

• The volcanoes along Hawaiian islands

 

Eruption Style

• Quiet

• Gas escapes easily

• Effusive = Less explosive

• More frequent eruptions

• Still dangerous when lava meets water -> steam explosion

intermediate magmas

• viscosity

• volcano

• eruption style

Moderate to high silica content and viscosity -> thicker lava

• Lava flows slowly and less far = steeper slopes

  • 15-30°

• Makes Intermediate materials → amphibole, biotite, plagioclase

• More felsic rocks

 

Landform: Composite/Stratovolcanoes

• Santorini, Mount St Helens

 

Eruption style

• More explosive due to trapped gas

• Pyroclastic flows (deadly), ash fall, lateral blasts, dome collapse.

felsic magmas

• viscosity

• volcano

• eruption style

High silica content and viscosity -> honey lava

• Really thick, high pressure lava = very explosive

• Massive eruption empties magma chamber

• Central part of volcano collapses = forms large depression

 

Landforms: Calderas

• E.g.  Long Valley

 

Eruption style

• High silica = Very viscous → gas retention → extreme explosivity

• Catastrophic vertical + lateral blasts. 

• Large‑scale pyroclastic flows + tuff travelling tens of kilometres

• Can cause tsunamis and major earthquakes

• Generally less frequent eruptions

Igneous Textures + crystal size

Appearance of material reflects magma history and cooling

• The size, shape, and arrangement of the minerals.

  • Glassy – Made of solid glass or glass shards

  • Interlocking crystals – Minerals that fit like jigsaw pieces.

  • Fragmental – Pieces of pre-existing rocks.

• Rapidly cooled over years/decades = small crystals

• Cooling over thousands of years = big crystals

  • Materials at a depth stay hotter for longer = slower cooling

two types of igneous textures + sub categories

Glassy	Rapid cooling in water or air Quenching from blobs of lava Glasses mostly from explosive fragments
Crystalline	Aphanitic Fast cooling Extrusive No time for crystal growth   Phaneritic Slow cooling Intrusive Time for crystal growth   Porphyritic Course  phenocrysts cool slow Fine groundmass the forms from fast cooling -> due to subsequent eruption   Pegmatitic Extremely course grained 1cm to 1m Very unusual