Chapter 3- Igneous Minerals and Rocks
                               Physical Geology Illustrated Vocabulary                                                      
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Volcanoes and Igneous Rocks 
Roger Weller, geology instructor
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mahogany obsidian

Chapter 3
copyright 2018-Roger Weller
last edited: 3/18/18

Igneous Rock Table  (shows relationships)


-is a sodium-rich plagioclase feldspar; often white,
a property from which the name is derived.

-albite, cleavelandite 



-is a group of rock-forming silicate minerals, rich in iron, magnesium, and calcium.                                                            
Crystals are often elongated.




-is a gas pocket in an igneous rock that has been filled in with a secondary mineral such as calcite
or quartz.

-basalt, amygdaloidal 

-basalt, amygdaloidal with thomsonite 



-is a fine grained igneous rock intermediate in composition between rhyolite and basalt. 
It is  named after its common occurrence in the Andes mountains of South America.

-andesite, porphyritic 
-assorted views of andesite


Apache tears

-are rounded, dark, translucent obsidian nodules enclosed in perlite. 
These glassy nodules are often tumble polished for jewelry..

-obsidian, Apache tears 
-tumble polished Apache tears

-perlite and Apache tear 


aphanitic texture

-is also known as fine grained.  This igneous texture refers to very fine grained igneous rocks such
as basalt and rhyolite.

-basalt, fresh broken surface 

-rhyolite, light gray colored 



-is a fine-grained equivalent of granite often in the form of a vein. 
As a granite body cools, fractures open up and the remaining magma fills these fractures, cooling
relatively quickly to form the aplite veins.

-aplite vein exposed on Mt.Whitney-1

-aplite vein exposed on Mt.Whitney-2

-aplite vein in decomposed granite

-occurs as an upward moving magma melts the rocks above it and thus adds the elements of these
rocks to its composition.


-is the most common member of the pyroxene group of silicate minerals.  It is usually black or
blackish green in color.

-augite cleavage
-augite cleavage-2


basic igneous rock

-is an igneous rock rich in dark-colored silicate minerals.

-basalt, variety scoria 

-gabbro with labradorite 



is a dark, fine-grained (aphanitic) volcanic rock.
a quick guide to basalt   (a photo tour)


-is a black mica.



Bowen’s reaction series-continuous series

-describes how the composition of developing feldspar crystals changes as the magma cools. 
The first plagioclase feldspar crystals to form at high temperatures are calcium-rich (anorthite)
and the last crystals to form are sodium-rich (albite). During the entire crystallization process the
already formed feldspar crystals chemically react with the magma.





Bowen's reaction series-discontinuous series

-is part of Bowen's Reaction series that describes the sequence in which the ferromagnesian
crystallize as magma cools. The first member of the discontinuous series to for is olivine,
followed by pyroxene, then amphibole, and finally biotite.





Bowen's reaction series-residual, low temperature silicates
-These are the last silicate minerals to crystallize out of a cooling magma.
-muscovite (white mica)
-orthoclase feldspar

chill zone
-occurs at the outer surface of an igneous pluton where it is in contact with cold country rocks. 
The magma cools faster along its outer surface, producing a finer grain texture than the rest of the
intrusion which cools more slowly.
-chill zone


coarse grained

-is a descriptive term for an igneous rock texture in which the mineral grains in the rock are visible
to the naked eye.

-rough granite 

-diorite, polished

country rock
-is the pre-existing rock into which a magma body intrudes.

-country rock


-is the process of disorganized atoms and molecules in a solution or melt coming together to form
a solid with an organized, repeating geometrical pattern of atoms.

crystal settling
-As a magma body cools and starts to crystallize, the heavier (denser) crystals that are forming sink
to the bottom of the magma chamber, thereby altering the composition of the remaining magma.
A good example is where olivine crystals settle within a basaltic magma.
-crystal settling



-is a green pyroxene mineral.




-is a coarse grained igneous rock containing mostly albite (white) feldspar and hornblende.





ferromagnesian silicate

-is a silicate minerals rich in iron and magnesium, such as olivine, augite, hornblende, and biotite.






fine grained

-is an igneous rock texture in which the mineral grains are smaller than what the naked eye can
distinguish; also called aphanitic.

-basalt, fresh broken surface 

-rhyolite, light gray colored 



-is a dark, coarse grained igneous rock.

-gabbro with labradorite 


-is a solid in which the atoms are all in a chaotic distribution and not crystallized.

-black obsidian



-is a highly metamorphosed rock that is just one step away from becoming a granite.  
This metamorphic rock is often described as looking like a layered or lineated granite.





-is a coarse grained igneous rock made up of feldspar, mica, and quartz.

-rough granite 

-polished granite 
-polished granite-2

-feldspar- Opal's Pals



-is a coarse grained igneous rock midway in composition between granite and diorite.

-close up view of granodiorite on Mt. Whitney

gravitational magmatic segregation
-Under the influence of gravity, the denser components of a magma sink downwards and the
lighter components rise.
-gravitational magmatic segregation


-is the mass of small crystals that surround the phenocrysts in a porphyry.
-groundmass with phenocrysts 



-is the most common member of the amphibole family of silicate minerals. 
Usually hornblende forms dark brown, elongated crystals in igneous rocks such as diorite.


igneous petrologist
-is a geologist  who studies igneous rocks.


igneous rocks

-are rocks formed as magma cools and solidifies.

-black obsidian 

-basalt with olivine 

-rough granite 
-igneous rocks



-is a calcium-rich plagioclase feldspar, often identified by its iridescent schiller.



mafic rock

-is an igneous rock rich in iron and magnesium silicates.

-basalt with olivine 

-basalt with fresh broken surface 

magma mixing
-occurs when two or more magma bodies underground come into contact with each and mix
together creating a magma with a new composition.

magmatic differentiation
-occurs as a magma body cools and crystallizes.  The minerals that crystallize first remove certain
elements from the magma, thus changing the composition of the remaining magma.



-is also known as white mica; it is valuable because of its transparency and high electrical
insulating property.

-muscovite crystal 
-muscovite in pegmatite
-mica book



-is volcanic glass.

-black obsidian-1 

-black obsidian-2 

-black obsidian-3 



-is an iron magnesium silicate that is usually yellow-green in color. 
This mineral is one of the first minerals to form as a basaltic magma starts to cool and crystallize.



orthoclase feldspar

-is a potassium feldspar.

-orthoclase, salmon colored 


partial melting

-occurs as a subducted slab of crust moves down a subduction zone. 
Those minerals with the lowest melting temperatures melt first.
-partial melting



-is a very coarse grained igneous rock. 
The crystals within a pegmatite are larger than one half inch.



Pele's hair
If the wind is blowing strongly during an eruption at Kilauea crater in Hawaii,
the molten lava is drawn out into glass fibers like fiberglass.
-Pele's hair

Pele's tears
-are small droplets of lava that cool quickly in the air from eruptions
at Kilauea crater in Hawaii.
-Pele's tears


-is a volcanic glass with high water content that is riddled with concentric cracks.

-perlite and Apache tear 


perthitic structure

-is a wood-like pattern of small, short bands of a translucent feldspar embedded in a slightly
more transparent feldspar. This type of structure is often used to identify orthoclase feldspar. 
This structure is produced by an exsolution process as an orthoclase crystal crystallizes and
then slowly cools.

-perthitic structure-1 
-perthitic structure-2 


phaneritic texture

-is coarse grained, a term applied to igneous rocks whose constituent grains are large enough to
see with the naked eye.

-rough granite-1 

-rough granite-2 



-are the large crystals within a porphyry.




-is a form of volcanic glass that is in the process of breaking down (devitrifying).



plagioclase feldspars

-are the calcium and sodium rich feldspars.





plagioclase twinning

-On a cleavage surface of plagioclase feldspar, light is reflected off of a series of straight-edged,
narrow strips on cleavage surface. The cleavage surface actually consists of two slightly different
crystal orientations which formed as the plagioclase crystal slowly cooled. Light is being reflected
off one set of micro-cleavage surfaces from one of the crystalline orientations.  If a plagioclase
specimen is tipped again slightly with respect to the light source, the other set of micro-cleavage
surfaces will then reflect light.

-plagioclase-polysynthetic twinning 
-albite-plagioclase twinning  

plutonic (intrusive) rocks
-are coarse-grained igneous rocks that have cooled slowly underground.

polysynthetic twinning

-See the definition for polysynthetic twinning.

-plagioclase-polysynthetic twinning   


porphyritic texture

-is an igneous rock texture that is a combination of aphanitic (fine grained) and phaneritic (coarse
grained) textures.




-is an igneous rock containing two different sizes of crystals.

-porphyry, rough 

-porphyry, cut 
-porphyry-Opal's Pals



-is a very light-weight silicic volcanic rock consisting of a glass foam. 
Pumice is so light weight that it can float on water.

-pumice, close up-1 

-pumice, close up-2 

-pumice, various colors 
-pumice-Opal's Pals
-pumice as an abrasive tool-1
-pumice as an abrasive tool-2

-are the hot, fiery (molten) fragments tossed out of a volcano.
-volcanic bombs of various sizes



-is a group of single chain silicate minerals. 
Augite, a member of this silicate family of minerals, is a major constituent of basalt.





-is one of the most common minerals on the Earth, consisting of silicon and oxygen. 
This mineral is number 7 on Mohs scale of hardness. 
Quartz occurs in many different forms and may be crystalline, cryptocrystalline, or massive.

-quartz, variety rock crystal 

-quartz, variety amethyst 

-quartz, variety smoky quartz 



-is an aphanitic (fine grained) pastel-colored volcanic rock.

-rhyolite, light gray colored 

-rhyolite, tan colored 

-are bands of melted xenoliths that have not been completely mixed into the magma.
-schlieren-Opal's Pals



-is a very vesicular form of basalt; dark-colored with many small gas bubble holes.

-basalt, variety scoria 

-refers to the size and shape of the constituents of the rock: coarse-grained, fine-grained, glassy,
vesicular, fibrous, etc.



-is an igneous concretion, usually round, formed in conjunction with obsidian. 
These concretions start hollow within a obsidian flow and may either stay hollow and become
geodes, or they may fill with agate or opal.  The scientific term for type of structures is lithophysa




ultramafic igneous rock

-is made almost entirely of dark-colored iron and magnesium silicates. 
Peridotite is an ultramafic rock made of olivine (peridot).


vesicular basalt

-is a fine grained, dark igneous rock with gas bubble holes.

-basalt with fresh broken surface 
-vesicular basalt 
-vesicular basalt



-is a gas bubble hole in a volcanic rock.
-vesicles in basalt

volcanic ash
-is finely powdered volcanic matter.
-volcanic ash from Mount Saint Helens eruption-  10x  30x

volcanic (extrusive) rocks
-are fine grained or glassy igneous rocks that have been erupted from volcanoes.

Weller's spaghetti theory of magma
-explains how the constituents of magma (silica, cations, and dissolved gases)
affect the viscosity of a magma.
-Weller's spaghetti theory of magma *


-is a baked inclusion of country rock surrounded by an intrusive igneous rock.

-xenolith from Mt. Whitney-1

-xenolith from Mt. Whitney-2
-xenolith-Opal's Pals