Geohazzards and the publicees2.geo.rpi.edu/pe1geo1_08/SS/04_igneous-rocks.pdf · Igneous rocks are those formed from partially molten (melted) materials Partially molten rock is called

760 kA - last eruption

Geohazzards and the public

Igneous Rocks

Melting Earth Materials

Igneous rocks are those formed from partially molten

(melted) materials

Partially molten rock is called magma as it moves

through the Earth, and lava when it is extruded at the

surface.

From where does magma originate?

Partial melting of the mantle and crust due to

•Influxes of heat

•Decreasing pressure

•The movement of water or other fluids

Magma

Pressure & Temperature

Most materials

have positive

slopes between

their states

For rocks, we

need to consider

the solid-liquid

changes. Adding

water changes

melting

relationship

SiO2 and melting

Partial melting in S.W. Maine

When plates move away from each other, it

stretches and thins the crust, reducing

pressure on the mantle.

Tectonic thinning of the crust

Tectonic increase of

crustal thickness

Plume theory

Earth is hottest at its

core, the release of heat

may be convective -

mass transferred. Some

masses may move as

buoyant ductile bodies.

Diagram by J. Tarney

Thermochemical plumes

End of the ride… if such plumes exist, they would

become more or less stalled at the transition to

brittle mantle.


Four crystals growing

in melt. Note angular

faces typical of the 2/m

crystals as seen from

the 001 plane.

Planes (facets) result from

energy minimization along

a crystallographic plane -

depends on T, P, and X.

Hornblende

Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH, F)2

Crystal-liquid interfaces

On this phase

diagram, there

are two phases,

a solid and a

liquid.

The line

represents the

conditions

where both will

be present at

equilibrium

Crystallization

From Blackburn & Dennen,

1998

Growth interface

Crystal growth results from diffusion of components

to the crystal surface

Crystals can grow in

any medium - solids,

liquids, gasses,

supercritical fluids.

Liquids and fluids may

be melted rocks, C-O-

H or aqueous fluids, or

a mixture. All are

contingent on

component transport.

Diffusional growth

Reactive growth

New minerals may form from

recrystallization of reaction of

preexisitng grains.

Overgrowth, mantling, coronas

This is a diffusion driven process.

This may result from a change in

conditions or composition during

growth

Compositional zoning

Changes in physical

conditions or in

surrounding composition

may be reflected in

crystals

Ti variations as an

impurity in quartz (CL

image by D. Wark)

REE variations as an

impurity in zircon (BSE &

CL images by E.B.

Watson)

Bowen’s reaction series

Mineral names carry

a certain mystique -

rock names seem

more utilitarian

Igneous nomenclature“archaic” texture discrimination

boundary at 1mm.

An alkali feldspar and quartzdominant rock with an averagegrainsize >1 mm is a granite, <1mm is a rhyolite.

> 1mm Phaneritic

< 1 mm Aphanitic

Glassy - vitrophyric

Rhyolite

Dacite

Andesite

Basalt

Phonolite

Granite

Diorite

Tonalite

Gabbro

Anorthosite

Foid Syenite

After Strekeisen, 1978

Mean grain size < 1mmMean grain size > 1mm

Common rock names

SiO2

Quartz | Coesite | Stishovite

mineral.galleries.com

De minuscules cristaux de quartz ont une

disposition radiale autour de la coesite : ceci

montre que le quartz se forme au d師riment de la

coesite (LPA)

High birefringence Stishovite in coesite, synthetically grown

by J. Mosenfelder, CalTech.

Feldspars

Plagioclase Feldspar

Anorthite

(CaAl2Si2O8)

Alkali Feldspar

Albite (NaAlSi3O8)

Orthoclase (KAlSi3O8)

Images from mineral.galleries.com

Feldspathoids

Leucite (KAlSi2O6)

Nepheline (NaAlSiO4)

Sodalite (Na4Al3Si3O12Cl)

Image from mineral.galleries.com

dkimages

Grain size origins

Surface extrusion (volcanism)

forces molten material to cool

rapidly in the air. Grain sizes

are small.

Intrusion (plutonism)

forces molten material

to cool slowly deep

underground. Grain

sizes are large.

Cooling rapidly can limit the organization of atoms into

lattices. Fast = glass, slower = crystalline, slower still =

bigger crystals

From Blackburn & Dennen,

1998

Coarse: Dunite, Lherzolite, Harzburgite, Periodotite

Fine: Kimberlite

Minerals: these are rocks largely comprised of olivine

(isolated silicate) and pyroxene (single-chain silicate)

Origin: solidification of early Earth

Location: the mantle

Univ. North Carolina, Atlas of rocks,

minerals, and textures website

Ultramafic (very Mg-Fe rich) rocks

Classification

NesosilicatesOlivine

Forsterite Mg2SiO4

Fayalite Fe2SiO4


Inosilicates (singles)Pyroxene

Orthopyroxene - hypersthene

Enstatite Mg2Si2O6

Orthoferrosilite Fe2Si2O6

Clinopyroxene - Augite

Diopside CaMgSi2O6

Hedenbergite CaFeSi2O6




Kimberlite, lamprophyre

Volcanic ultramafic

rock. Very rare, and

sourced straight from

the mantle - eruption

rates must be

impressive. Some are

diamondiferous.

IUGS Crystaline

Coarse

crystaline rock

classification -

note that the

scheme

precludes the

coexistence of

foids and SiO2.

Fine grained

rock

classification -

compare with

the previous

slide.

This is a nice

diagram, but

the fine-grained

nature of these

rocks can

impede its use.

IUGS Fine grained

IUGS TAS

If you really want to classify a volcanic rock - powder

it and analyze its composition. How does this relate

to the previous slide? What of plutonic rocks?

Coarse: Gabbro, Anorthosite Fine: Basalt

Minerals: Largely olivine (isolated), pyroxene (single-

chain), and feldspar (framework).

Origin: Partial melting of the mantle

Oceanic crust, oceanic islands, the moon.

Univ. North Carolina, Atlas of rocks, minerals,

and textures website

AnorthositeAnorthosite

Univ. North Dakota, Plutonic Images

Mafic (Mg-Fe rich) Rocks

BasaltBasalt

GabbroGabbro

Univ. North Carolina, Atlas of rocks, minerals,

and textures website

If most of the heat needed for

melting is deep within the earth,

why are there volcanoes?

Magma is driven upwards.

Parameters - density, viscosity

Recall density = mass / volume

Viscosity is a measure of resistance to flow -

maple syrup is more viscous than water

Hogan et al., 1998

Ascent of Magmas

Kilauea, Hawaii

March, 1996

Hawaiian magmatism

Two plumes?

20-406Figure 20.19

A cross section of a shield volcano

Generally low Si

Low volatile

High rate

Mauna Kea, Hawaii

Pu’u Hulu

Mauna Loa, Hawaii

Pahoehoe

Viscosity and Lava

Compare water to maple syrup - the latter is many times

more viscous.

What makes for runny (less viscous) magma

Temperature

Viscocity is almost always reduced in hotter fluids

Lower silica content

Lower silica content means fewer tetrahedra sharing

oxygens - think about the minerals in mafic rocks

Volatile (H2O, CO2, HF, H2SO4) content

Many of these components work to depolymerize the

magma.

Viscosity - Resistance to flow

Fluorine and evolution

High -T crystalslargely cannotincorporate largeamounts ofcomponents like H2O,OH, F, Cl, and SO2.

“To many petrologists avolatile component…does just what onewants it to do.”

N.L. Bowen, 1928

Oceanic rifting

Example: the East African Rift

http://jsc.nasa.gov

STS-32

Continental Rift


Rifting - Birth of an ocean

Question 4: how does fluorine influence riftmagmatism?


Cartoon to left shows

some general attributes

of rift formation

Presumably, the tension

that drives rifting is not

well understood.

The minimum liquid fractions at each temperatureand pressure give the melting curve that we usefor mantle rocks. DJ DePaolo

By adding a small amount of water, thedegree of melting increases significantly.

With water - melting starts much deeper

Coarse: Granodiorite, Tonalite Fine: Andesite, Dacite

Minerals: Feldspar (framework), pyroxene (single

chain), amphibole (double chain), and quartz

(framework)

Origin: liquid fraction of crystallizing mafic liquids.

Location: Convergent tectonic settings

TonaliteTonalite

Intermediate rocks (less Fe-Mg, more Na-K-Si-Al)

Example: the Aleutians

Subduction: Island Arcs

Example: the Cascades

Subduction: Continental Arcs

Mount Saint Helens: Mount Saint Helens:

May 18, 1980 0832May 18, 1980 0832

Q: What is a

volcanic dome?

View from South Rim, June 1991View from South Rim, June 1991

Crater LakeCrater Lake

OregonOregon

Coarse: Granite

Fine: Rhyolite

Minerals: Feldspar and quartz (framework), with small

amounts of amphibole (double chain) and mica (sheet).

Origins: Partial melting of crustal rocks and liquid

fraction of crystallizing intermediate liquids.

Location: continental magmatism

GraniteGranite

Felsic (K, Na, and Si rich) rocks

Mount Scott Granite

An intergrowth ofquartz and feldspar

Likely result of too fewnucleation sites

Undercooling

Viscosity contrasts

Rapid diffusion

Photo by Mike Brown (UMD)

Metamorphic

materials that

exceed the

solidus for

the system

start to melt

Migmatite

Making granites?



RhyoliteRhyolite

VitrophyreVitrophyre

©Pam Gore, GPC

Felsic rocks

Pegmatite Pegmatites are

typically coarsely

crystalline - may

contain rare

minerals. Most

result from

extreme

fractionation.

Slow cooling

Low nucleation

Lots of H2O

Geohazzards and the publicees2.geo.rpi.edu/pe1geo1_08/SS/04_igneous-rocks.pdf · Igneous rocks are those formed from partially molten (melted) materials Partially molten rock is called

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