3.5 ignous (instrusive & extrusive)

8/17/2019 3.5 Ignous (Instrusive & Extrusive)

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Magma and Igneous

RocksRock: A coherent, naturally occurring, aggregate of mineralsor glass

Geologists distinguish three main types of rocks

1- I gneous

Rocks that form by the freezing or solidification of melt

2- Sedimentary

Rocks that form by the cementing of grains or fragments of pre-existing rocks, or by the precipitation of minerals out of a solution

3- Metamorphic

Rocks that form when pre-existing rocks change due to temperature or pressure,

and/or as a result of squashing or shearing.


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Thin Sections

• To study rocksin detail,geologists cutthin slices of

rock so that theyare translucent

• Geologists can

then look atthem through petrologicmicroscopes

Geologists cut rocks with a rotating saw

and then grind them into thin sections

A hand sample of granite A magnified thin section of granite


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Igneous Rocks- The Basics• Solidified molten rock

(which freezes at hightemp).

– 1,100°C to 650°C.

– Depends on composition.

• Earth is mostly igneousrock.

– Magma: Subsurface melt.

– Lava: Melt at the surface.

• Magma erupts via

volcanoes.


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Igneous Rock Types• In general, there are

two basic types ofigneous rocks

– Extrusive/Volcanic: Igneous rocks that

form due to thefreezing of meltsabove the surface ofthe Earth

• Includes rocks madeof volcanic ash

(pyroclastics)

– Intrusive/Plutonic: Form by freezing ofmelts below the

surface of the Earth.


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Formation of Magma•

Remember that the tectonic plates don’t really float on aliquid asthenosphere, rather the asthenosphere is a ductile

solid and is only melted in specific locations.

• Most magma/lava is not 100% liquid. – Magma/Lava is made of many compounds, all of which have

different melting temps. Analogy: a slushy or frozen margarita

– Only a few percent of liquid is required to make a melt.

• Other than a rise in temperature, what causesmelting of rock within the Earth?Melting happens because of:

– Decrease in pressure (decompression)

– Addition of volatiles (H2O, CO2, etc…)

– Heat transfer from rising magma


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The Earth gets hotter

with increasing depth

due to primordial heatand radioactive decay of

elements near the core.

The rate at which

temperature increases

with depth is called thegeothermal gradient , or

geotherm

Liquids have no

organized structure, so to

melt a rock, the mineral bonds must be broken

(animated gif of atoms)

Melting due to Decompression

The geotherm of the Earth


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At depth, confining

pressure prevents atoms

from breaking free ofcrystals

Solidus: The temperature

when a rock first begins

to melt

Liquidus: The

temperature where the

last solid particle melts

The asthenosphere cools

only slightly as it rises

(convection) because it is

a good insulator (high

specific heat)

The solidus and liquidus of peridotite (ultramafic mantle rock)

Melting due to Decompression


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Melting due to the Addition of Volatiles

• Volatiles: A substance that can easily change into a gas at relatively low

temperatures (H2O, CO2, etc…).

• The addition of volatiles at depth (mainly H2O) seeps into rocks andhelps break bonds (aids in melting).

• Analogy: Think of putting salt onto ice to lower the melting temperature.

Likewise, adding water to rocks changes the melting point of rocks just

like adding salt to water.


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Melting due to the Addition of Volatiles

• The addition of H2O into basalt, for example,

drastically changes its

melting temperature

• In this case, basalts at60km depth beneath the

continents could begin to

melt only if they were

volatile rich.

The geotherm beneath a continent and the solidus

of wet and dry basalt

D e p t h

( k m )


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Melting Due To Heat Transfer

• Melting can also occur

when rising bodies ofhot material essentially

bake the nearby rock

• Analogy: Think of

pouring hot fudge intoice cream. The hot fudge

transfers heat to the ice

cream and melts it


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What is Magma Made of ?• All magmas contain Si and O

– Upon cooling, bond together into si l icon-oxygen tetrahedrons

• More silica (i.e. felsic), more viscous (harder to flow, thicker)• Also contain varying amounts of other elements like Na, K, Al,

Ca, Mg, Fe, etc…

• Dry magmas – no volatiles

•Wet Magmas

– up to 15% volatiles• Volatile content strongly effects the viscosity (ability to flow)

– More volatiles, less viscous (easier to flow or more fluid)


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Types of Magma - CompositionLike rocks, not all magma is made of the same stuff

• We divide magmas into groups by their composition – Felsic (Silicic): 66-76% Silica (SiO2)

• Most viscous, Least dense (~2.5 gm/cm3), melting point 650-800oC

– Intermediate: 52-66% SiO2

– Mafic: 45-52% SiO2, lots of MgO, FeO, and Fe2O3

– Ultramafic: 38-45% SiO2, abundant MgO, FeO, and Fe2O3• Least viscous, Most dense (~3.5 gm/cm3), melting point up to 1300oC I

n c r e a s i n g S i O 2

I n c r e a s i n g F e , M g


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Magma Compositions• Composition controls density, T, and viscosity.

– Most important is the content of silica (SiO2).• Silica-rich magmas are thick and viscous.

• Silica- poor magmas and thin and “runny.”

– These characteristics govern eruptive style.

Type Density Temperature Viscosity

Felsic Very low Very low (600 to 850°C) Very High: Explosive eruptions.

Intermediate Low Low High: Explosive eruptions.

Mafic High High Low: Thin, hot runny eruptions.

Ultramafic Very high Very high (up to 1,300°C) Very low


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Bowen’s Reaction Series • In order to understand the melting and solidifying of magma we need to understand

Bowen’s reaction series. – Bowen figured this out by melting rocks in an oven, lettingthem cool, and watching what minerals crystallized

• This series outlines the order in which minerals form in a cooling melt• Also applies in reverse order to rocks that are partially melted

• Discontinuous series (different minerals form) and Continuous series (Plagioclase only)

• So, a melt gets less mafic as it cools; In heating, the first minerals to melt are felsic.


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Why are Magmas so Variable in Composition?

Di ff erences in M agma composition occur due

to 5 main reasons…

1. Different source rock compositionsmelt a felsic rock = felsic magma

2. Magma mixingmix felsic magma with mafic magma

= intermediate magma

3. Partial melting

4. Assimilation

5. Fractional crystallization


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Partial Melting• Most magmas are not 100% liquid

– Commonly 2-30% melt; called a crystal mush

• According to Bowen’s reaction series, rocks that are partiallymelted become more mafic, because the silica-rich felsic mineralsare melted first.

• The melted part of the partial melt is thus more felsic than theremaining rock.

The felsicmineral,

quartz, is a

common

cement in

many rocks


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Assimilation

• As magma sits in its chamber,

it may incorporate minerals

from the surrounding wall rock

– Called assimilation

• Occurs when wall rocks fall

into the magma and melt

(stoping) or when the magma

partially melts minerals from

the wall rock

• Degree of assimilation depends

on composition of wall rock,

temp of magma, amount of

H20 present, amount fractures

in and strength of the wall

rock, and residence time


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Stoping & Xenoliths• Stoping: The process of incorporating chunks of wall rock into a magma body

• Xenolith: A non-melted chunk of wall rock incorporated into a magma body

– May have a very different composition than the magma


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Xenolith

• A xenolith ingranite in theMojave desert

• Usually recognized because they mayhave a differenttexture (grain size)and compositionthan the rest of therock


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Magma Movement• If magma did not move, no extrusive/volcanic rocks would

ever have formed

• Magma rises because: – hotter and less dense than the surrounding rock and therefore

buoyantly rises.

– the weight of the overlying rock (lithostatic pressure) literallysqueezes the magma out.

• Analogy: Think of stepping on a tube of toothpaste to force it out, or mud squishing through your toes when you step in a puddle

• Viscosity affects a magma or lava’s ability to flow

– Controlled by:• Temperature (high temp - low viscosity)

• Volatile content (more volatiles – less viscous)

• Silica content – silica tends to form silica-oxygen tetrahedrons that bond with each other to make long chains that ultimately resist flow(more silica – more viscous)


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Extrusive Igneous Rock Environments• Explosive eruptions generally occur

when source magma is:

– High in silica (felsic-intermediate)

– Low temp

– High in volatiles

• These volcanoes form

– Lava domes

– Ash clouds and ash flows

• Ef fusive eruptions generally occur when

source magma is:

– Low in silica (mafic)

– High temp

– Low in volatiles

• These volcanoes form

– Fluid lava flows

– Fire fountains (if volatiles), lava tubes

Hawaii

Cascades NW USA


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Intrusive Igneous Rock Environments• Magma rises by percolating between grains and/or by forcing open cracks in the

subsurface

• The magma that doesn’t reach the surface of the Earth cools into intrusiveigneous rocks

– Country rock or wall rock: The pre-existing rock that magma intrudes into

– I ntr usive contact: The boundary between the igneous intrusion and the wall rock

• Tabular intrusions: Dike, Sil l, L accoli th (pseudo-tabular, or sheet-like)

• Non-tabular intrusions: Pluton, Batholi th, Stock

Mt.

Rushmore is

carved out of

a granitic

igneous

intrusion


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• Dikes: igneousintrusions that cut

across layering, i.e.discordant

• Sills: igneousintrusions that

follow layering, i.e.concordant

Dikes and Sills

Dik i h Si N d B h li h


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Dikes in the Sierra Nevada Batholith

• Near Ruby Lake, CA @ ~12,000 ft


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• Laccolith: a dome-like sill that bends the layers above it into a

dome shape

Laccoliths


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Non-Tabular Intrusions: Plutons

• Pluton: Irregular blob-shaped

discordant intrusions that rangein size from 10’s of m, to 100’s

of km

• Batholith: A pluton that is 100 km2

in surface exposure

• Stock: A pluton that is


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The Sierra Nevada Batholith

h Si d h li h


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The Sierra Nevada Batholith

• At ~100 Ma the

west coast of theUS, was a

subduction zone

with numerous

volcanoes

• The magma

chambers cooled

and the rocks

above were

eroded away

leaving a large

batholith

exposed.

Eff t f I t i


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Effects of Intrusions

• Dikes form in regions of

crustal stretching

• Sills may cause uplift at the

surface of the Earth

Eff t f I t i


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• Dikes form in regions of

crustal stretching

• Sills may cause uplift at the

surface of the Earth

La Sal Mountains, Utah were uplifted by a laccolithScotland was stretched during the Cenozoic

Eff t f I t i


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• Plutons disrupt the

surrounding layers of rockand may cause crustal

stretching above

• Plutons grow by stoping:

opening cracks andassimilating xenolithic

blocks in the melt

C li f M d L


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Cooling of Magma and Lava

• Magma cools forseveral reasons – Removal of

volatiles

– It rises to a coolerlocation and hastime to cool

• Cooling dependsvery much on thegeometry (surfacearea) of theintrusion.

Tabular-shape = fastcooling

Spherical shape =slow cooling

– Cooling times varyfrom days minutes

to millions of years

I T


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Igneous Textures• Glassy Texture : A solid mass of glass

or tiny crystals surrounded by a glass

matrix – Matrix: the smaller stuff in a rock

(relative term)

• I nter locking Texture (Phaneritic) :Rock made of interlocking crystals thatgrew as the melt solidified. Commonlycalled crystall ine igneous rocks

– Crystals fit together like pieces of a puzzle

I T t


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Igneous Textures

• Fragmental Texture :

Volcanic rocks that are madeof various types of fragments

that form from volcanic

eruptions.

– Fragments can be:• Crystals

• Xenoliths (from volcano walls)

• Glass

A Welded Tuf f – white specks are fragments, grey is ash

VolcanicBreccia –

angular pieces

of fragments

entrained in the

eruption

Crystalline Igneous Rocks


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Crystalline Igneous Rocks

Glassy Igneous Rocks


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Glassy Igneous Rocks

• Obsidian: Mass of solid felsic glass; conchoidal fracture

• Tachylite: mafic, bubble-free mass of >80% glass (very rare)

• Pumice: glassy felsic volcanic rock that contains abundant open

pores called vesicles (lt grey to tan in color). Occasionally lessdense than water (it floats!)

– Vesicle: a open space left over from a gas bubble in a lava or magma

• Scoria: glassy mafic volcanic rock with abundant vesicles(>30%). Grey, black, or red in color.

– Typically has larger and rounder vesicles than pumice

Fragmental Igneous Rocks


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Fragmental Igneous Rocks

{ Rocks blasted out of volcanoes…commonly called pyroclastic rocks }

• Tuff: Fine-grained rock, composed of lithified volcanic ash and/or fragmented

lava and pumice. Formed from ash fall from the air, or from hot material that

avalanches down the side of a volcano.

– If material is still very hot (gooey) it may get squished upon landing and weld with

other particles forming a welded tuf f

• Volcanic Breccia: Large angular chunks of material from either volcanic

debris flows (blocky lava flow) or air fall (bombs).

• Hyaloclasite: formed when lava erupts under ice of water and cools so quickly

that it shatters into fragments that weld or cement together.

Where Does Igneous Activity Occur?


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Where Does Igneous Activity Occur?

Most volcanoes occur at plate boundaries or Hot Spots

Most subaerial (above sea level) occur in volcanic arcs

Subduction-related volcanic arcs are responsible for “the ring of fire”

Subduction and Volcanism


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Subduction and Volcanism Subduction creates

volcanism

1- The down-going slab haslots of volatiles (e.g. H2O).

At depth, these volatiles are

heated and are squeezed

from the rock and migrate

into the asthenosphere

above the plate.2- The addition of volatiles,

as we now know, changes

the melting point of rocks

and causes the

asthenosphere to melt above

the sinking plate.3- The sinking plate may

partially melt too, but most

melting occurs in the

asthenosphere above the

slab.

H t S t d V l i


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Hot Spots and Volcanism

• There are many hot spots throughout the world, includingHawaii and Yellowstone.

• Many pacific islands are or were hot spots

Large Igneous Provinces (LIPs)


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Large Igneous Provinces (LIPs)

• LIP: a region of particularly voluminous eruptions of magma/lava

– May be a consequence of a super plume

Flood Basalts


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Flood Basalts

• Flood basalts are a type of LIP that emits a large amount of basalt flows.

• The Columbia River flood basalts (above) and the DeccanTraps (India) are two examples.

Formation of Igneous Rocks at Mid Ocean Ridges


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Formation of Igneous Rocks at Mid Ocean Ridges

• ~70% of the Earth’s surface(including the underwater

surface) is oceanic crust, somost igneous rocks form at midocean ridges

• Mid Ocean Ridge lavas arecompositionally similar toOceanic Hot Spots (basalt,mafic)

• Underwater flows for PillowBasalts

– Pillows have glassy outer rim andmore crystalline center

3.5 ignous (instrusive & extrusive)

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