Download view the Lecture Presentation

Up from the Inferno: Magma and
Igneous Rocks
Igneous Rocks



Solidified molten rock that freezes at high temp.

1,100°C to 650°C.

Temp depends on composition.
Earth is mostly igneous rock.

Magma – Subsurface melt.

Lava – Melt at the surface.
Volcanoes erupt magma.
Igneous Rocks

Melted rock can cool above or below ground.

Intrusive igneous rocks – Cool slowly underground.

Extrusive igneous rocks – Cool quickly at the surface.
Lava – Cooled liquid.
Pyroclastic debris – Cooled fragments.
Volcanic ash.
Fragmented lava.

Many types of igneous rocks.

All oceanic crust.

Most continental crust.
Sources of Heat


Sources of heat to the early
Earth:

Planetesimal and meteorite
accretion.

Gravitational compression.

Decay of radioactive
minerals.

Tidal pull of the Sun and
Moon.
The crust does NOT float on a
sea of molten rock.
Magma Formation

Magma forms in special settings that melt existing
rocks.

Partial melting occurs in the crust and upper mantle.

Melting is caused by…
 Decompression.
 Volatile
 Heat
addition.
transfer.
Magma Formation

Geothermal gradient – The Earth is hot inside.
 Crustal
temperature (T) averages 25°C / km of
depth.
 At

the base of the lithosphere T ~ 1280°C.
The geothermal gradient varies from place to place.
Magma Formation

Pressure release.

Base of the crust is hot enough to melt mantle rock.

Due to high pressure, the rock does not melt.

A drop in pressure initiates “decompressional melting.”
Pressure drops when hot rock
rises to shallower depths.
Addition of Volatiles

Volatiles lower the melting
T of a hot rock.

Common volatiles include
water and carbon dioxide.

Subduction introduces
water to the mantle,
melting rock.
Magma Formation

Heat transfer.
 Rising
magma carries
mantle heat with it.
 This
raises the T in
nearby crustal rock,
which then melts.
What is Magma Made of?

Magmas have three components (solid, liquid and gas).
 Solid
– Solidified mineral crystals are borne by the melt.
 Liquid
– The melt itself is comprised of mobile ions.
Dominantly Si and O; lesser Al, Ca, Fe, Mg, Na, and K.
Other ions present to a lesser extent.
 Different
mixes of elements yield different magmas.
What is Magma Made of?

Volatiles (Gases) – Magmas contain
abundant dissolved volatile gas.
Dry magma – Scarce volatiles.
Wet magma – To 15% volatiles.
Water vapor (H2O)
Carbon dioxide (CO2)
Sulfur dioxide (SO2)
Nitrogen (N2)
Hydrogen (H2)
Magma Compositions

There are four major magma types based on silica (SiO2)
percentage.

Felsic (feldspar and silica) 66 to 76% SiO2.

Intermediate 
Mafic (Mg and Fe-rich) 45 to 52% SiO2.

Ultramafic 52 to 66% SiO2.
38 to 45% SiO2.
Magma Compositions

Composition controls magma density, T, and viscosity.

The most important factor is silica (SiO2) content.
Silica-rich magmas are thick and viscous.
Silica-poor magmas are 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 1300°C)
Very low.
Magma Variation

Why are there different magma compositions?

Magmas vary chemically due to…

Initial source rock compositions.

Partial melting.

Assimilation.

Fractional crystallization.
Magma Variation

The source of the melt dictates the initial composition.

Mantle source – Ultramafic and mafic magmas.

Crustal source – Mafic, intermediate, and felsic
magmas.
Partial Melting

Upon melting, rocks rarely dissolve
completely.

Instead, only a portion of the rock
melts.

Silica-rich minerals melt first.

Silica-poor minerals melt last.

Partial melting, then, yields a silicarich magma.

Removing a partial melt from its
source creates…

Felsic magma.

Mafic residue.
Assimilation

Magma melts the country rock it passes through.

Blocks of country rock (xenoliths) fall into magma.

Assimilation of these rocks alters magma composition.
Magma Migration


Magma moves by…

Injection into cracks.

Melting overlying rocks.

Squeezed by overburden.
Low viscosity eases movement. Lower viscosity
from…

Higher T.

Lower Silica content.

Higher volatile content.
Magma Migration

Viscosity depends on T, volatiles, and silica.

Temperature:
Hotter – Lower viscosity
Cooler – Higher viscosity.

Volatile content:
More volatiles – Lower viscosity.
Less volatiles – Higher viscosity.

Silica (SiO2) content:
Less SiO2 (Mafic) – Lower viscosity.
More SiO2 (Felsic) – Higher viscosity.
Cooling Rates

Cooling rate – How fast does magma cool?

Depth: Deep is hot; shallow is cool.
 Deep plutons lose heat very slowly and cool slowly.
 Shallow flows lose heat rapidly and cool quickly.

Shape: Spherical bodies cool slowly; tabular bodies cool faster.

Ground water: Ground water removes heat.
Fractional Crystallization

As magma cools, early crystals settle by gravity.

Melt composition changes as a result.
 Fe,
Mg, and Ca are removed in early settled solids.
 Si,
Al, Na, and K remain in melt and increase in
abundance.
The original melt
As early-formed
Fractional Crystallization

Felsic magma can evolve from
mafic magma.
 Progressive
removal of mafics
depletes Fe, Mg, and Ca.
 Remaining
melt becomes
enriched in Na, K, Al, and Si.
Bowen’s Reaction Series

N. L. Bowen, in the 1920s, ran experiments with melts.

Settling removed elements from the remaining melt.

He discovered that minerals solidify in a specific series.

Continuous – Plagioclase changed from Ca-rich to Na-rich.

Discontinuous – Minerals that solidify in a narrow T range.
Olivine
Pyroxene
Amphibole
Biotite
Igneous Environments

Two major categories – Based on cooling site.

Extrusive settings – Cool at or near the surface.
Cool rapidly.
Chill too fast to grow big crystals.

Intrusive settings – Cool at depth.
Lose heat slowly.
Crystals grow large.

Most mafic magmas extrude.

Most felsic magmas don’t.
Tabular Intrusions

Tend to have a uniform thickness.

Can be traced laterally.

Two major subdivisions.

Sill – Parallels rock fabric.

Dike – Crosscuts rock fabric.
Tabular Intrusions


Dikes and sills modify invaded country rock.

They cause the rock to expand and inflate.

They thermally alter the country rock.
Dikes

Cut across preexisting layering (bedding or foliation).

Spread rocks sideways.

Dominate in extensional settings.
Tabular Intrusions


Sills

Are injected parallel to preexisting layering.

Are usually intruded close to the surface.
Both dikes and sills exhibit wide variability in...

Size.

Thickness (or width).

Lateral continuity.
Plutonic Activity

Most magma is emplaced
at depth in the Earth.


A large, deep, igneous
body is called a pluton.
Plutonic intrusions modify
the crust.

Push aside preexisting
rock.

Add new material.

Add heat.
Intrusive and Extrusive

Intrusive and extrusive
rocks commonly cooccur.

Magma chambers feed
overlying volcanoes.

Magma chambers may
cool to become plutons.

Many igneous
geometries are possible.
Intrusive and Extrusive

With erosion, progressively deeper features are
exposed.

Dikes.

Sills.

Laccoliths – Mushroom-shaped intrusions.
Igneous Textures


The size, shape, and arrangement of the minerals.

Interlocking – Mineral crystals fit like jigsaw puzzle pieces.

Fragmental – Pieces of preexisting rocks, often shattered.

Glassy – Made of solid glass or glass shards.
Texture directly reflects magma history.
Interlocking or crystalline texture
Glassy texture
Fragmental texture
Crystalline Igneous Textures

Texture reveals cooling history.

Aphanitic (finely crystalline).
Rapid cooling.
Crystals do not have time to grow.
Extrusive.

Phaneritic – (coarsely crystalline).
Slow cooling.
Crystals have a long time to grow.
Intrusive.
Crystalline Textures

Texture reveals cooling history.
 Porphyritic
texture – A mixture of coarse
and fine crystals.
Indicates a two-stage cooling history.
Initial slow cooling creates large
phenocrysts.
Subsequent eruption cools
remaining magma more rapidly.
Crystalline Textures

Pegmatitic

Coarse mineral crystals found in dikes.

Large crystals are not due to slow cooling.

Instead, pegmatites form from water-rich
melts.

Many unusual minerals are found in
pegmatites.

Some pegmatites are rich in prized
minerals.
Crystalline Textures

Glassy textures

Form by very rapid cooling of lava in water or air.

Glassy textures are more common in felsic magmas.

They often preserve gas bubbles (vesicles).

Underwater, basalt lava quenches into “pillows.”
Glassy Classification

Vesicular.
escaping volatiles cause frothing of lava
rapid cooling then solidifies the lava
producing rock with many vesicles
Fragmental Textures

Preexisting rocks that
were shattered by
eruption.

After fragmentation, the
pieces fall and are
cemented.
Fragmental Classification

aka Pyroclastic – Fragments of violent eruptions.
 Tuff
– Volcanic ash that has fallen on land and
solidified.
 Volcanic
breccia – Made of larger volcanic
fragments.
Igneous Activity Distribution

Igneous activity tracks tectonic plate boundaries.