Other factors:
1.  Magma composition depends on what you melt.
2.  Partial melting – as a rock begins to melt, the first parts that melt leave the scene of the crime
3.  Fractional crystallization – as a magma begins to crystallize, the first crystals drop out, changing the composition of the remaining magma
4.  Contamination occurs in several ways as magma migrates toward the surface.

<Be thinking how partial melting and fractional crystallization will affect the resulting magma, or make it different from its origins…>
 

Igneous Rocks:
A) Texture – crystal size and arrangement; mostly related to cooling history and less so to composition
B) Composition – minerals present; silica content control related to origin and history of the magma.

A) Texture (fabric of rock)
Main Control: Cooling Rate:
Slow cooling = Big crystals, Fast cooling  = Small.
Slow = intrusive rock, Fast = extrusive rock.
1. Phaneritic – Visible: crystals large enough to be readily visible to the naked eye.  Intrusive rock.

2. Aphanitic – Invisible: texture dominated by crystals too small to be visible to the naked eye. Extrusive rock.

3. Porphyritic – Mix of small and large: a few crystals are distinctly larger than the others.  Big ones are called phenocrysts (visible crystals), and the rest is called the groundmass or the matrix.  Groundmass may be either aphanitic or phaneritic.  Two cooling histories:  slow (phenocryst growth), then faster (groundmass growth).

4. Vesicular – Porous: bubbles exist in the rock where the rock formed with gas bubbles in the lava.  Extrusive rocks only.  Scoria = dark, vesicular basalt chunks, usually bombs or from aa flows.  Pumice = light, vesicular felsites (intermediate to high-silica rocks) with much smaller bubbles, because the lava is so highly viscous.  Pumice is usually formed as bombs.
5.  Glassy – Not crystalline, but a volcanic glass: Volcanic texture that usually forms from quenching (very rapid cooling) of lava.  Nearly all ash and pumice is glassy, the tops of fresh basalt flows are glassy, and obsidian is glass.  Extrusive rocks only.

6.  Pegmatitic – Very large crystals: Phaneritic texture with large crystals (minimum of 1cm but may be decimeters across).  Pegmatites are typically felsic (high silica).  Usually form as a rhyolitic magma with a high water vapor content that cools intrusively (typically found in large veins).

7.  Pyroclastic  – Rock made of pyroclasts: Tuff (ash) and breccia (coarser grained pyroclasts) are the common pyroclastic rocks.  Extrusive only, can be considered a “sedimentary” igneous rock (pyroclasts as transported sediments, in a way).

Repeat: 3 sizes of pyroclasts: Bombs, Cinders, Ash.
Ash  = sand size or smaller (down to dust)
Cinders  = gravelish size up to hand-size
Bombs are big enough to do you in.
Tuff is a rock formed of ash plus cinders, even bombs.

B) Composition :  Eight very important silicate minerals for igneous rocks:
1. Quartz

2, 3. Feldspars:
    * Orthoclase (also called potassium feldspar, or K-feldspar: KAlSi3O8)
    * Plagioclase (two types: Na-plag - NaAlSi3O8 and Ca-plag  - CaAl2Si2O8).  These are endmembers of a continuous series of feldspar minerals with ionic substitution of Al for Si in the tetrahedrons and Na or Ca to make up the charge balance.

4, 5. Micas:
    * Muscovite
    * Biotite

6. Amphibole - hornblende (most commonly)

7. Pyroxene - augite (most commonly)

8. Olivine
 

Here is a table that makes some sense of these:

Non-ferromag minerals (low Fe + Mg) (high-silica)	Non-ferromags (low Fe + Mg) (int-low silica)	Ferromag minerals (high Fe + Mg) (int-low silica)
Quartz		Biotite
K-feldspar	Na-plagioclase	Amphibole
Muscovite	Ca-plagioclase	Pyroxene
		Olivine

Ferromags: short for ferromagnesian minerals, those silicate minerals that are both enriched in Fe (iron) and Mg (magnesium), and relatively low in silica content.  Note that Ca-plagioclase is low in silica (has a lot of Al replacing Si), but is not a ferromag because it is without Fe and Mg.

Felsic rock: 0-15% ferromag minerals
Intermediate: 15-40% ferromags
Mafic: 40-95% ferromags  (Mafic = Mg + Fe)
Ultramafic: 95-100% ferromags

Igneous Rock Names and Composition Chart:
( Oops! Note:  Change "Peridotite" in the Aphanitic row to "Komatiite (rare)")

Intrusive Rock Bodies:  Plutonic Rocks, form Plutons

See diagrams in the textbook that picture these:

Batholiths (BIG >100 km2 area) -  batholiths represent very large magma chambers that cooled into plutonic rocks.
Stocks (smaller than 100 km2 area) – stocks are smaller magma chambers, commonly connected by feeders (necks, dikes) to nearby batholiths.
Dikes (vertical) – vertical fractures that have filled with intrusive (plutonic) rock.  Fissure eruptions leave dikes beneath them.  May be very thin (cm) to many meters across.
Sills (horizontal) – horizontal plutons where vertically-rising magma was able to fill sideways between layers of rock (usually sediments).  Range from very local and thin to extensive and many meters thick.
Necks (vertical pipes) – feeders to volcanoes with simple craters on top.
Laccoliths – (underground domes) – an intrusion with the approximate shape of an irregular mushroom cap; forms by upward doming of a growing sill.
 

- Magma chambers grow upward by slowly fracturing and intruding the surrounding rocks, with blocks falling in and melting (assimilating their composition into the magma).  Unmelted or partially melted blocks that survive and are found in igneous rocks are called xenoliths (foreign rocks)

Extra credit: memorize this definition of a little known type of pluton…
CACTOLITH: A quasi-horizontal chonolith composed of anastamosing ductoliths, whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an akmolith or ethmolith.
-Hunt, USGS Professional Paper 228, 1954
 
 
 
 
 
 
 

How Magma Cools Down

Rule: Silica-poor minerals crystallize before (...at higher temperatures than...) silica-rich minerals.

Example: Basaltic magma
Composition (future minerals):  Ol, Px, Ca-plag, Am
First minerals:  Olivine (the most silica poor)
Next:  Pyroxene and Ca-plagioclase
Last:  Amphibole (not much, usually)
 
 
 
 

Example: Rhyolitic magma
Composition (future minerals):  Qtz, K-spar, Musc, Bio, Na-plag
First minerals: Na-plagioclase, Biotite
Next: K-spar, Muscovite
Last: Quartz
 
 
 
 

Quiz Question: explain cooling history of a gabbro with each olivine crystal having an abraded rim encrusted with pyroxene crystals.

<draw>
 
 
 

(answer: olivine grows first, the remaining magma becomes relatively enriched in silica, now pyroxene becomes the mineral in equilibrium for growth, scavenges ions from the olivine crystals and encrusts it)

Quiz Question: explain cooling history of a diorite with “zoned plagioclase” crystals.
<draw>  (Ca-rich plagioclase in the middle, surrounded by Na-rich plag on the outside).
 
 
 

Hint-- plagioclase: CaAl2Si2O8 <==> NaAlSi3O8.
(which is more silica-poor?)
(answer: Ca-plag grows first, the remaining magma becomes more enriched in silica, and plagioclase with progressively more Na [and thus less Al and more Si] crystallizes, forming a compositionally “zoned” crystal.)

Quiz Question:   Does a magma become more silica-rich or more silica-poor during crystallization?
(answer: rich)
 

A guy named Bowen did some experiments about 70 years ago to figure this all out.  How do you tell (in a lab) which minerals form first from a magma as it cools?
(answer: melt some rock to make a magma, cool it very slowly to a certain temperature and keep it there for a long time, long enough for some crystals to grow, then QUENCH the magma abruptly to make a glass around the phenocrysts that have grown. Simulates making a porphyritic texture in glass.  Then cut it and examine it under the microscope to see which crystals grow under what conditions, with which type of starting magma compositions.)

 Bowen’s Reaction Series:  (see diagram and box of text in the book)

Left side              Right side
Olivine                  Ca-plagioclase
Pyroxene              (Ca,Na)-plagioclase
Amphibole            (Na,Ca)-plagioclase
Biotite                   Na-plagioclase
          K-feldspar
          Muscovite
           Quartz
 

Temp?  Si-content?  Color?  Ferromag vs. Non-fmg?
Basaltic vs Andesitic vs Rhyolitic?  Magma viscosity?

Important notes or caveats: You CAN'T get all these minerals from one magma.  A basaltic magma will only generate olivine, pyroxene, Ca-plag and maybe a little bit of amphibole.  Period.  You still can't get blood from a turnip.  Bowen himself said that this is useful only in a general way, for very typical sorts of magmas.  He also said (very paraphrased) that another way to look at it is that they are arranged like relatives at a picnic. Ones that are close to each other on the list can commonly be seen sitting at the picnic table together or playing together, but the farther away two minerals get on the list, the less likely you are to find them together in a rock.  THUS, it is very common to find rocks with olivine and pyroxene, fairly common to find olivine and amphibole, reasonable to find olivine with biotite, rare to find olivine with K-feldspar or muscovite, and nearly impossible to find an igneous rock that contains both olivine and quartz that crystallized from the same magma.

Let’s get tricky now.....
- Magma of one composition can yield igneous rocks of several compositions
- Magma of different compositions can be derived from melting one parent rock type

* Fractional Crystallization *.
(draw one volcano with linked magma chambers)
 
 
 
 
 
 

Basaltic magma, partially crystallizes: __________
Leftover magma => enriched in Si, depleted in Fe+Mg = an Andesitic magma.
This Fractional Crystallization process creates a new magma that is more Silica-rich than the original.

* Partial Melting * - Bowen’s Backwards!
Oceanic crust:
Minerals:
First to melt:

Andes Mountains:
Minerals:
First to melt:

Mantle Peridotite:
Minerals:
First to melt:

This Partial Melting process creates a new magma that is more Silica-rich than the original rock.

*** Know This (or be able to figure it out):
Products of Typical 10-30% partial melting...
- Partially melt a Mantle Peridotite (ol + px)  =>   Basalt (Hawaii, MOR)
- Partially melt a Wet Basalt (px + pg + am)  =>  Andesite (Andes, Cascades)
- Partially melt an Andesite (pg+px+am+bio) =>  Granite (Yellowstone)

Repeat Summary:
Fractional Crystallization makes magma more silica rich than before.
Partial Melting makes magma more silica rich than parent rock.

=== === === ===
Global Distribution of Igneous Activity:

Ocean ridges (site of sea floor spreading)
 => Basaltic magma, forms due to partial melting of peridotite in the rising mantle, makes new ocean crust at the divergent boundary.
 

Ocean Hot spots (upwelling plumes of mantle peridotite: Hawaii and Iceland are examples)
 => Basaltic magma, makes ocean island chains with one volcano at the head of the chain, dead volcanoes along the plate where it has moved past the hot spot head.

Subduction of Ocean Lithosphere:
A. Oceanic Island Arcs : ocean-ocean subduction
 => Andesitic magma, from wet melting where the subduction zone carried water beneath another oceanic plate.  Water reduces the melting temperature of rocks, allowing them to melt at lower temperatures.
 - ex. Aleutians, Japan, Philippines, “Ring of Fire”
 - some basaltic lava erupts here, too, from melting of peridotite.
 

B. Continental Arcs  : ocean-continent subduction
 =>  Andesitic magma, from wet melting where the subduction zone carried water beneath another continental plate
- ex. Andes, Cascades, Central America
- some rhyolitic (where the lower crust heats up and melts) and basaltic magmas, too
 

Continental Rifting
 => Basaltic magma, mostly, due to partial melting of peridotite in the rising mantle.
- some andesitic and rhyolitic magma, too, from where the lower crust heats up and melts

Continental Hot Spots
 => Rhyolitic magma, mostly
 - ex. Yellowstone National Park
- some basaltic lava too (from rising peridotite in the mantle) and andesitic magma, too (from where the lower crust heats up and melts).