Op ch06 lecture_earth3, igneous rocks

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.



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.



Magma Formation


Why does magma form?


Earth is hot inside. Why?
Planetesimal and meteorite accretion.
Gravitational compression.
Differentiation.
Radioactive mineral decay.



Magma Formation
Partial melting in
crust / upper mantle.
 Melting is from…


Pressure release.
 Volatile addition.
 Heat transfer.




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.



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.”




Magma Formation


Heat transfer.
Rising magma
carries mantle heat.
 This raises T in
crustal rock.
 Crustal rock melts at
lower T.




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.




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.



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)



Magma Compositions


4 major magma types based on % silica (SiO2).
Felsic (Feldspar and silica)
 Intermediate
 Mafic (Mg and Fe-rich)
 Ultramafic



66 to 76% silica.
52 to 66% silica.
45 to 52% silica.
38 to 45% silica.


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



Magma Variation
Why are there different magma compositions?
 Magmas vary chemically due to…


Initial source rock compositions.
 Partial melting.
 Assimilation.
 Fractional crystallization.




Magma Variation


Source rock dictates initial magma composition.
Mantle source – Ultramafic and mafic magmas.
 Crustal source – Mafic, intermediate, and felsic magmas.




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.




Assimilation
Magma melts the country
rock it passes through.
 Assimilated materials
change magma
composition.




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.




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.




Fractional Crystallization

Felsic magma can
evolve from mafic
magma.
 Progressive removal of
mafic minerals.




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.




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.



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.




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.




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.




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.




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.




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.



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…
Spread rocks sideways.
 Dominate in extensional settings.




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.




Large Sill



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.



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.




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.




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.



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.



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.



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.




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.



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.



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.



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



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.




Crystalline Classification
Composition.
 Texture.




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.




Igneous Activity Distribution


Igneous activity tracks tectonic plate boundaries.



Igneous Activity Distribution


Igneous activity tracks tectonic plate boundaries.
Hot spots – Mafic volcanic activity.
 Convergent boundaries – Felsic igneous activity.
 Divergent boundaries – Mafic igneous activity.




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.


Op ch06 lecture_earth3, igneous rocks

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