Magma & Igneous Rocks

Magma and Igneous Rocks

Rock: A coherent, naturally occurring, aggregate of minerals

or glass

Geologists distinguish three main types of rocks

1- Igneous

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.

Thin Sections

• To study rocks in detail, geologists cut thin slices of rock so that they are translucent

• Geologists can then look at them through petrologic microscopes

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

Igneous Rocks- The Basics • Solidified molten rock

(which freezes at high

temp).

– 1,100°C to 650°C.

– Depends on composition.

• Earth is mostly igneous

rock.

– Magma: Subsurface melt.

– Lava: Melt at the surface.

• Magma erupts via

volcanoes.

Igneous Rock Types • In general, there are

two basic types of igneous rocks

– Extrusive/Volcanic: Igneous rocks that form due to the freezing of melts above the surface of the Earth

• Includes rocks made of volcanic ash (pyroclastics)

– Intrusive/Plutonic: Form by freezing of melts below the surface of the Earth.

Formation of Magma • Remember that the tectonic plates don’t really float on a

liquid 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 causes melting of rock within the Earth? Melting happens because of:

– Decrease in pressure (decompression)

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

– Heat transfer from rising magma

The Earth gets hotter

with increasing depth

due to primordial heat

and radioactive decay of

elements near the core.

The rate at which

temperature increases

with depth is called the

geothermal 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

At depth, confining

pressure prevents atoms

from breaking free of

crystals

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

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 and

helps 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.

Melting due to the Addition of Volatiles

• The addition of H2O into

basalt, for example,

drastically changes its

melting temperature

• In this case, basalts at

60km 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

Dep

th (

km

)

Melting Due To Heat Transfer

• Melting can also occur

when rising bodies of

hot material essentially

bake the nearby rock

• Analogy: Think of

pouring hot fudge into

ice cream. The hot fudge

transfers heat to the ice

cream and melts it

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

– Upon cooling, bond together into silicon-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)

Types of Magma - Composition Like 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

Incr

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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

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, letting them 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.

Why are Magmas so Variable in Composition?

Differences in Magma composition occur due

to 5 main reasons…

1. Different source rock compositions melt a felsic rock = felsic magma

2. Magma mixing

mix felsic magma with mafic magma

= intermediate magma

3. Partial melting

4. Assimilation

5. Fractional crystallization

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 partially melted become more mafic, because the silica-rich felsic minerals are melted first.

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

The felsic

mineral,

quartz, is a

common

cement in

many rocks

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

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

Xenolith

• A xenolith in granite in the Mojave desert

• Usually recognized because they may have a different texture (grain size) and composition than the rest of the rock

Fractional Crystallization

Bowen’s reaction

series is an example

of fractional

crystallization

• Not all minerals crystallize at

the same temperature – This is

fractional crystallization

• As magmas cool, they become

more felsic.

• Mafic minerals crystallize first

and are more dense than the

melt, so they sink to the bottom

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) literally squeezes 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)

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

• Effusive 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

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 intrusive

igneous rocks

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

– Intrusive contact: The boundary between the igneous intrusion and the wall rock

• Tabular intrusions: Dike, Sill, Laccolith (pseudo-tabular, or sheet-like)

• Non-tabular intrusions: Pluton, Batholith, Stock

Mt.

Rushmore is

carved out of

a granitic

igneous

intrusion

• Dikes: igneous intrusions that cut across layering, i.e. discordant

• Sills: igneous intrusions that follow layering, i.e. concordant

Dikes and Sills

Dikes in the Sierra Nevada Batholith

• Near Ruby Lake, CA @ ~12,000 ft

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

dome shape

Laccoliths

Non-Tabular Intrusions: Plutons

• Pluton: Irregular blob-shaped

discordant intrusions that range

in 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 <100 km2 in

surface exposure

The Sierra Nevada Batholith

The Sierra Nevada Batholith

• At ~100 Ma the

west coast of the

US, was a

subduction zone

with numerous

volcanoes

• The magma

chambers cooled

and the rocks

above were

eroded away

leaving a large

batholith

exposed.

Effects of Intrusions

• Dikes form in regions of

crustal stretching

• Sills may cause uplift at the

surface of the Earth

Effects of Intrusions

• 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 laccolith Scotland was stretched during the Cenozoic

Effects of Intrusions

• Plutons disrupt the

surrounding layers of rock

and may cause crustal

stretching above

• Plutons grow by stoping:

opening cracks and

assimilating xenolithic

blocks in the melt

Cooling of Magma and Lava

• Magma cools for several reasons – Removal of

volatiles

– It rises to a cooler location and has time to cool

• Cooling depends very much on the geometry (surface area) of the intrusion.

Tabular-shape = fast cooling

Spherical shape = slow cooling

– Cooling times vary from days minutes to millions of years

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)

• Interlocking Texture (Phaneritic): Rock made of interlocking crystals that grew as the melt solidified. Commonly called crystalline igneous rocks

– Crystals fit together like pieces of a puzzle

Igneous Textures

• Fragmental Texture:

Volcanic rocks that are made

of various types of fragments

that form from volcanic

eruptions.

– Fragments can be:

• Crystals

• Xenoliths (from volcano walls)

• Glass

A Welded Tuff – white specks are fragments, grey is ash

Volcanic

Breccia –

angular pieces

of fragments

entrained in the

eruption

Crystalline Igneous Rocks

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 less

dense 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

{ 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 tuff

• 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?

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 Subduction creates

volcanism

1- The down-going slab has

lots 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.

Hot Spots and Volcanism

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

• Many pacific islands are or were hot spots

Large Igneous Provinces (LIPs)

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

– May be a consequence of a super plume

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 Deccan Traps (India) are two examples.

Formation of Igneous Rocks at Mid Ocean Ridges

• ~70% of the Earth’s surface (including the underwater surface) is oceanic crust, so most igneous rocks form at mid ocean ridges

• Mid Ocean Ridge lavas are compositionally similar to Oceanic Hot Spots (basalt, mafic)

• Underwater flows for Pillow Basalts

– Pillows have glassy outer rim and more crystalline center