Chapter 6
Up from the Inferno: Magma and Igneous Rocks
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Up from the Inferno:
Magma and Igneous Rocks
Prepared by
Ron Parker
Earlham College Department of
Geosciences
Richmond, Indiana
Igneous Rocks

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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
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.
Ash.
Fragmented lava.

There are many varieties
of igneous rocks.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Formation

Why does magma form?

Earth is hot inside. Why?
Planetesimal and meteorite accretion.
Gravitational compression.
Differentiation.
Radioactive mineral decay.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Formation
Partial melting in
crust / upper mantle.
 Melting is from…

Pressure release.
 Volatile addition.
 Heat transfer.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Formation

Geothermal gradient – Earth is hot inside.
Crustal temperature (T) increases 25°C / km with depth.
 At the base of the lithosphere T ~ 1,280 °C.


Geothermal gradient varies
from place to place.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
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.”

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Formation

Heat transfer.
Rising magma
carries mantle heat.
 This raises T in
crustal rock.
 Crustal rock melts at
lower T.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Addition of Volatiles

Volatiles cause rocks to melt at much lower T.
Water.
 Carbon dioxide.

Adding volatiles to hot, dry rocks initiates melting.
 Important subduction process.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
What Is Magma Made of?

Magmas have 3 components (solid, liquid, and gas).
Solid – Solidified minerals are carried by the liquid.
 Liquid – The melt itself is comprised of mobile ions.

Dominantly Si and O; lesser Ca, Fe, Mg, Al, Na, and K.
Other ions to a lesser extent.

Different mixes of elements yield different magmas.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
What Is Magma Made of?

Gas – Volatiles dissolved in the melt.
Dry magma – No volatiles.
Wet magma – To 15% volatiles.
Water vapor (H2O)
Carbon dioxide (CO2)
Sulfur dioxide (SO2)
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Compositions

4 major magma types based on % silica (SiO2).
Felsic (Feldspar and silica)
 Intermediate
 Mafic (Mg and Fe-rich)
 Ultramafic

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
66 to 76% silica.
52 to 66% silica.
45 to 52% silica.
38 to 45% silica.
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
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
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Variation
Why are there different magma compositions?
 Magmas vary chemically due to…

Initial source rock compositions.
 Partial melting.
 Assimilation.
 Fractional crystallization.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Variation

Source rock dictates initial magma composition.
Mantle source – Ultramafic and mafic magmas.
 Crustal source – Mafic, intermediate, and felsic magmas.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Partial Melting
Upon heating, silica-rich minerals melt first.
 Partial melting, then, yields a silica-rich magma.
 Removing a partial melt from its source creates:

Felsic magma.
 Mafic residue.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Assimilation
Magma melts the country
rock it passes through.
 Assimilated materials
change magma
composition.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Mixing
Different magmas may blend in a magma chamber.
 The result combines the characteristics of the two.
 Often magma mixing is incomplete, resulting in
blobs of one rock type suspended within the other.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Fractional Crystallization
As magma cools, early crystals settle by gravity.
 Melt composition changes as a result.

Fe, Mg, and Ca is removed in earlsettled solids.
 Si, Al, Na, and K remain in melt and increase.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Fractional Crystallization
Felsic magma can
evolve from mafic
magma.
 Progressive removal of
mafic minerals.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Migration
Magma is less dense than rock; it moves upward.
 Magma moves by…

Injection into cracks.
 Melting overlying rocks.
 Squeezed by overburden.

Pressure decrease with upward migration releases
volatiles (bubbles), thereby decreasing density.
 Viscosity controls migration ease.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Magma Migration

Viscosity depends on temp, 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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
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 do not.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Extrusive Characteristics
Lava flows – Sheets of cooled lava.
 Lava flows exit volcanic vents and flow outward.
 Lava cools as it flows, eventually solidifying.
 Low-viscosity lava (basalt) can flow long distances.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Extrusive Characteristics

Explosive ash eruptions.
High-viscosity felsic magma builds volcanic pressure.
 Violent eruptions yield huge volumes of volcanic ash.
 Ash can cover large regions.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Intrusive Characteristics
Intrusive rocks cool at depth, they don’t surface.
 Magma invading colder country rock initiates…

Thermal (heat) metamorphism and melting.
 Inflation of fractures pushing the rock aside.
 Incorporation of country rock fragments (xenoliths).
 Hydrothermal (hot water) alteration.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
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 zenolith could be assimilated.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Intrusive Activity

Magma intrudes into other rocks in 2 ways.
As planar, tabular bodies (dikes and sills), and
 As balloon-shaped blobs (plutons).


Size varies widely; plutons can be massive.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Tabular Intrusions
Tend to have a uniform thickness.
 Can be traced laterally.
 Two major subdivisions.

Sill – Parallels rock fabric.
 Dike – Crosscuts rock fabric.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Tabular Intrusions

Dikes and sills modify invaded country rock.
They cause the rock to expand and inflate.
 They thermally alter the country rock.


Dikes…
Spread rocks sideways.
 Dominate in extensional settings.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Tabular Intrusions

Sills
Lift entire landscapes skyward.
 Usually intruded near the surface.


Both dikes and sills exhibit wide variability.
Size.
 Thickness (or width).
 Lateral continuity.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Large Sill
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Plutonic Activity

Plutons may coalesce to
form a larger batholith.
Plutons are created at
subduction zones.
 Magma generation may
occur of over 10s of Ma.
 Long subduction history
linked to large batholiths.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Intrusive and Extrusive
Intrusive and extrusive rocks commonly co-occur.
 Magma chambers feed overlying volcanoes.
 Magma chambers may cool to become plutons.
 Many igneous geometries are possible.

Dikes.
 Sills.
 Laccoliths.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Influence on Landscape

Deeper features are exposed by uplift and erosion.
Intrusive rocks are more resistant to erosion.
 Intrusive rocks often stand above the landscape.


“Unroofing” takes long periods of geologic time.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Cooling Rates

Cooling rate – 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.
Ground water removes
heat.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Igneous Textures

The size, shape, and arrangement of the minerals.
Glassy – Made of solid glass or glass shards.
 Interlocking crystals – Minerals that fit like jigsaw pieces.
 Fragmental – Pieces of pre-existing rocks.


Texture directly reflects magma history.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Glassy Textures

Form by very rapid cooling of lava in water or air.
Quenching forms obsidian (volcanic glass).
 Basalts may quench into blobs of lava called pillows.
 Glasses may fragment explosively.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Crystalline Textures

Texture immediately reveals cooling history.

Aphanitic (finely crystalline).
Rapid cooling – extrusive.
Crystals do not have time to grow.

Phaneritic – (coarsely crystalline).
Slow cooling – Intrusive.
Crystals have a long time to grow.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Crystalline Textures

Texture immediately reveals cooling history.

Porphyritic – A mixture of coarse and fine crystals.
Indicates a 2-stage history.
Initial slow cooling creates large phenocrysts.
Subsequent eruption cools remaining magma quickly.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Other Textures

Pegmatitic - Exceptionally coarse mineral crystals.
From late stage crystallization of granitic magmas
 Many unusual minerals are found in pegmatites.

Made from ions that don’t easily fit into crystals.

Some pegmatites are rich in prized minerals.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Igneous Classification

Based upon composition and texture.
Composition – Felsic, intermediate, mafic, ultramafic.
 Texture - Fine (aphanitic), coarse (phaneritic).

Type
Aphanitic (fine)
Phaneritic (coarse)
Felsic
Rhyolite
Granite
Intermediate
Andesite
Diorite
Basalt
Gabbro
Very high
Very high (up to 1300°C)
Mafic
Ultramafic
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Glassy Classification

Glassy igneous rocks.

Obsidian – Volcanic glass from rapidly cooled lava.
Quenching – Lava flowing into water.
High-silica lavas – These can make glass without quenching.
Pumice – Frothy felsic rock full of vesicles; it floats.
 Scoria – Glassy, vesicular mafic rock.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Crystalline Classification
Composition.
 Texture.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Pyroclastic Classification

Pyroclastic – Fragments of violent eruptions.
Tuff – Volcanic ash that has fallen on land.
 Volcanic breccia – Made of larger volcanic fragments.
 Hyaloclasite – Fragments of lava exploded into water.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Igneous Activity Distribution

Igneous activity tracks tectonic plate boundaries.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
Igneous Activity Distribution

Igneous activity tracks tectonic plate boundaries.
Hot spots – Mafic volcanic activity.
 Convergent boundaries – Felsic igneous activity.
 Divergent boundaries – Mafic igneous activity.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks
This concludes the
Chapter 6
Up from the Inferno: Magma and Igneous Rocks
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 6: Up from the Inferno: Magma and Igneous Rocks