Chapter 6

Magma and Igneous Rocks

Chapter 6

Outline• Igneous rocks

- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive

• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)

• Magma composition-4 major types of magma (based on silica content)

- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional

crystallization

• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism

Chapter 6

Chapter 6

Igneous Rocks

• Defined: rock solidified from the melt• Freezes at temperatures of 1,100degreesC

• Composition dependent

• Earth is mostly igneous rock• Magma – subsurface melt• Lava – melt at the surface• Magma erupts via volcanoes

Chapter 6

Igneous Rocks• Melted rock can cool above or below ground.

• Intrusive igneous rocks – Cool slowly underground• E.g. granite - yosemite

• Extrusive igneous rocks – Cool quickly at the surface

-E.g. basalt – hawaii

-lava

-flowing, cooling molten rock

Pyroclastic debris

-cooled rock frgaments

-e.g. ash, fragmented lava

Many types of igneous rocks!

Chapter 6

Outline• Igneous rocks

- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive

• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)

• Magma composition-4 major types of magma (based on silica content)

- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional

crystallization

• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism

Chapter 6

Chapter 6

Magma Formation

• Why does magma form?

• -earth is hot inside• Why?• 1. Earth formation• -planetesimal and

meteorite accretion• -differentiation• 2. Existence of

radioactive decay

Chapter 6

• Melting (partial) in

crust/upper mantle.

• Melting mechanisms:• 1 pressure release• 2 heat transfer• 3 volatile addition

Magma Formation

Chapter 6

Magma Formation• Earth is hot inside, thus a geothermal gradient.• -crustal temperature (T) increases 25degreesC/km depth• -base of crust T is 1,280degreesC• GEOTHERMAL GRADIENT VARIES FROM PLACE TO PLACE

Chapter 6

Magma Formation• A mechanism for melting: pressure release.

• Base of crust is hot enough to melt mantle rock• But due to high pressure, the rock does not• A drop in pressure initiates “decompression melting”• A B:

-Big change in pressure, little in

Temperature

Vocab:

-Solidus melting begins

-Liquidus No solid left

Chapter 6

• A mechanism for melting: Heat transfer.

Magma Formation

Chapter 6

Addition of Volatiles• A mechanism for melting: introduction of volatiles • -volatiles decrease rock melting temp• -water• -carbon dioxide• Adding volatiles to hot, dry rocks initiates melting• Important in subduction process

Chapter 6

What Is Magma Made of?

• Magmas have 3 components (solid, liquid, and gas).• Solid – solidified minerals carried by the liquid• Liquid – melt itself comprised of mobile ions

Mostly si and o; some ca, fe ,mg, al, na , k

Other ions to a lesser extent

Different mixes of elements

Chapter 6

What Is Magma Made of?

• Gas – volatiles dissolved in the melt.• Dry magma – no volatiles• Wet magma – up to 15% volatiles:

-water vapor (h2o)

-carbon dioxide (co2)

-sulfur dioxide (so2)

Chapter 6

Outline• Igneous rocks

- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive

• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)

• Magma composition-4 major types of magma (based on silica content)

- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional

crystallization

• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism

Chapter 6

Chapter 6

Types of Magma (composition)

• 4 major types based on % silica (SiO2).• Felsic (Feldspar and silica) 66 to 76% silica.• Intermediate 52 to 66% silica.• Mafic (Mg and Fe-rich) 45 to 52% silica.• Ultramafic 38 to 45% silica.

Chapter 6

Magma Composition -> Eruptive Style

• Composition controls density, T, and viscosity.• Most imp - is the silica (SiO2) content.

• Silica-rich magmas thick and viscous• Silica-poor magmas thin and flow easily (less viscous)

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

Chapter 6

Magma Composition Variation• Why various magma compositions?• Due to:

-intital source rock composition

-partial melting

-assimilation

-frctional crystallization

Chapter 6

Magma Composition Variation• Source rock dictates initial magma composition.

• Mantle source – ultramafic and mafic magmas• Crustal source – mafic, intermdiate, felsic magmas

Chapter 6

Partial Melting• Upon heating, silica-rich minerals melt first.• Thus, partial melting yields a silica-rich magma• Removing a partial melt from its source creates:• -felsic magma• -mafic residue left behind

Chapter 6

Assimilation• Magma melts the country rock it passes through• Assimilated materials change maga composition

Chapter 6

Magma Mixing• Different magmas may blend in a magma chamber

-result combines characteristics of both

-mixing often incomplete, resulting in blobs of one type suspended w the other

Chapter 6

Fractional Crystallization

• As magma cools, early formed crystals settle by gravity.• Melt compostion changes as a result

-fe, mg, ca is removed in early formed solids

-si, al, na, and k remain in melt

Chapter 6

Fractional Crystallization

• Felsic magma can evolve from mafic magma.

• By progressive removal of mafic minerals.

Chapter 6

Outline• Igneous rocks

- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive

• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)

• Magma composition-4 major types of magma (based on silica content)

- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional

crystallization

• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism

Chapter 6

Chapter 6

Magma Migration• Magma is less dense than rock, so it rises. • Magma moves by…

-injection into cracks

-melting overlying rocks

-pressure decrease with upward migration releases volatiles (bubbles), thereby decreasing density

Chapter 6

Magma Migration

• Viscosity depends on temp, volatiles, and silica.

• Temp: • Hot low viscosity (flows well)• Cooler high viscosity (flows poor)

• Volatile content: • More low vsc• Less high visc

• Silica (SiO2) content:

• Less (Mafic) low visc• More (Felsic) high visc

Chapter 6

Igneous Environments

• 2 major categories - based on cooling site.1. Extrusive settings – Cool at or near the surface.

-cool rapidity

-chill too fast-only small crystals form

2. Intrusive settings – Cool at depth.

-cool slowly

-crystals grow large

-most mafic magmas extrude

-most felsic magmas do not

Chapter 6

Extrusive Characteristics• 1. lava flows – sheets of cooled lava• 2. lava flows exit volcanic vents and flow outward• 3. lava cools as it flows, eventually solidifies• 4. low viscosity lava (basalt) can flow long distances

Chapter 6

Extrusive Characteristics• Explosive ash eruptions.

-high viscosity felsic magma builds up pressure

-violent eruptions yield huge volumes of volcanic ash

-ash can cover large regions

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Intrusive Characteristics• Intrusive rocks cool at depth, they don’t surface. • Magma invading colder country rock initiates…

-thermal heat metamorphism and partial melting.

-Inflates fractures, pushing rock aside

-incorporation of country rock fragments (xenoliths)

-hydrothermal (hot water) alteration

Chapter 6

Intrusive Characteristics• Intrusive contacts preserve evidence of high heat.

-baked zone: rim of heat altered country rock

-chill margin: magma at contact that cooled rapidly

Xenolith: country rock fragment in magma

-thermally altered

-magma cooled before xenolith

• Xenolith - Country rock fragment in magma.

Chapter 6

Intrusive Activity• Magma intrudes into rocks in 2 main ways:

-as planar, tabular bodies (dikes, sills)

-as balloon-shaped blobs (plutons)

Size varies widely: plutons can be massive

Chapter 6

Tabular Intrusions• Tend to have a uniform thickness. • Can be traced laterally.

2 subdivisions:

1. Sill – parallels rock fabric

2. Dike – crosscuts rock fabric

Chapter 6

Example Large Sill

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Plutons• Most magma is emplaced at depth within Earth.

-a large deep igneous body (blob) is called a pluton

Plutonic intrusions modify the crust

Chapter 6

Plutons Batholith• Plutons may coalesce to form

a larger batholith

-plutons are created at subduction zones

-magma generation may occur for 10s of myrs.

-long subduction history linked to large batholiths

Chapter 6

Intrusive and Extrusive• Intrusive & extrusive rocks commonly co-occur.• Magma chambers feed overlying volcanoes• Magma chambers can cool – become plutons• Many igneous geometries are possible • Dikes• Sills• Laccoliths• Plutons

Chapter 6

Influence on Landscape

• Deeper features are exposed by uplift and erosion.

-intrusive rocks are resistant to erosion

-intrusive rocks often stand above the landscape

Exposing intrusive rocks by erosion takes a long time

Chapter 6

Rate of Cooling

• How fast is heat lost? • Depth: Deep is hot, shallow

is cool.-deep plutons cool slowly-shallow flows cool rapidly

Shape: Surface to volume ratio.-spherical bodies cool slowly-tabular bodies cool faster

• Ground water.

Chapter 6

Igneous Textures • Size, shape, and arrangement of the minerals.

-glassy: solid glass or glass shards

-interlocking crystals: minerals that fit like jigsaw pieces

-fragmental: pieces of pre-existing rocks

-texture directly reflects magma history

Texture directly reflects magma history.

Chapter 6

Glassy Textures• Form by very rapid cooling of lava in water or air.

-basalts may quench into blobs of lava called pillows in water

Chapter 6

Crystalline Textures• Texture reveals cooling history.

• Aphanitic (small crystals – too hard to see).

-rapid cooling: extrusive

-crystals: no time to grow

Chapter 6

Crystalline Textures• Texture reveals cooling history.• Apharitic (small crystals – too hard to see)

-rapid cooling: extrusive

-crystals: no time to grow

• Phaneritic (large cryst• als – easy to see).• Slow cooling: intrusive• Crystals have a long• Time to grow

Chapter 6

Crystalline Textures• Texture reveals cooling history.

• Porphyritic – A mixture of coarse and fine crystals.

-indicates a 2-stage history

-initial slow cooling creates large phenocrysts

-subsequent eruption

Chapter 6

Igneous Rock Classification

• Based on composition and texture.• Composition (silica) – Felsic, intermediate, mafic, ultramafic.• Texture - Fine (aphanitic), coarse (phaneritic).

Type Aphanitic (fine) Phaneritic (coarse)

Felsic Rhyolite Granite

Intermediate Andesite Diorite

Mafic Basalt Gabbro

Chapter 6

• Composition.• Texture.• Grain size.

Igneous Rock Classification

Chapter 6

Igneous Activity Distribution• Igneous activity tracks tectonic plate boundaries.• Also in plate interiors – hot spots (Hawaii)

Chapter 6

Igneous Activity Distribution

• Igneous activity tracks tectonic plate boundaries.

-convergent boundaries: felsic igneous activity

-divergent boundaries: mafic igneous activity

-hot spots: mafic volcanic activity

-NOT at continental transform boundaries