Igneous Rocks
Review: The Rock Cycle
High Temperature
Low Temperature
Figure 1- The Rock Cycle
-3 main types of rocks: igneous, sedimentary & metamorphic
Definition: Igneous rock -rock that forms by the crystallization of molten magma
Definition: Magma -a body of molten rock found at a depth
-igneous undergo weathering
Definition: Weathering -the disintegration and decomposition of rock at or near the surface of the Earth
-the weathered material is picked up, transported and deposited by agents of erosion
Definition: Erosion -the incorporation and transportation of material by a mobile agent, such as water, wind or ice
Definition: Sediment -loose particles created by weathering and erosion of rock, by chemical precipitation from
solution in water, or from the secretions of organisms, and transported by water, wind or ice
-sediment is deposited in beds and undergo lithification to form sedimentary rock
Definition: Lithification -the process of converting sediment to solid rock
-if sedimentary rock is buried deep in Earth or involved in orogenesis, great pressures and heat will form
metamorphic rock
-metamorphic rock can then form igneous rock (magma) under increased pressure and heat
-tectonic forces also increase heat and pressure
-“shortcuts” in rock cycle model:
 igneous rock may be transformed directly to metamorphic rock (high temp. & pressure)
 exposed metamorphic or sedimentary rock can be weathered and eroded to later form new sedimentary
rock
-rock cycle first proposed by James Hutton
-very little known about rock formation
-later plate tectonics used to explain cycle
-not all geologic processes included in rock cycle  most magma comes from partial melting of mantle (not
recycled rock)
Figure 2 - The Rock Cycle as it relates to the Theory of Plate Tectonics
Igneous Rocks:
-form when magma cools and solidifies
-molten rock originates down to 200 km beneath the surface
-magma consists mostly of 8 elements of silicate minerals (silicon, oxygen, aluminum, sodium, potassium, calcium,
iron, magnesium)
-also contains volatile gases (like water vapor & carbon dioxide) that are kept in magma due to surrounding rock
-magma is less dense than surrounding solid rock it works its way toward surface
-when magma breaks through surface  volcanic eruption
-explosions produced by gases escaping as pressure lessens near the surface
-sometimes magma is blocked and surface water seeps into magma  huge explosions
Definition: Lava –magma that has reached the surface
2 main types of igneous rocks
 extrusive (volcanic) – refers to igneous rocks that are formed at the surface, possibly by volcanic eruption
 intrusive (plutonic) – refers to igneous rocks which are formed below the surface very slowly; they reach the
surface by uplifting or exposed through the years of erosion of overlying rock -rocks
that form when lava
Definition: Crystallization -the formation and growth of a crystalline solid from a liquid or gas
-cooling causes silicate minerals to form
3 factors influence crystallization:
a. rate of cooling
-when magma cools slowly, large crystals form (no space restrictions)
-when magma cools rapidly, small crystals form (space restrictions)
b. amount of volatile gases (water vapor, carbon dioxide)
c. chemical composition of magma
-these factors cause the texture and mineral composition of igneous rocks to vary greatly
Definition: Texture – size, shape & distribution of particles that make up a rock
-texture indicates information about environment in which rock was formed
Definition: Aphanitic Texture – texture of igneous rocks in which crystals are too small for individual minerals to
be seen with unaided eye
-many aphanitic rocks have holes left by gas bubbles that escape when magma solidifies (holes are called vesicles)
-these rocks formed in upper parts of lava flow
Figure 3 - Scoria, showing vesicles
Definition: Phaneritic Texture – texture of igneous rocks in which crystals are roughly equal in size and large
enough so that individual minerals can be seen with unaided eye
-formed when large masses of magma cool and solidify slowly
Figure 4 - aphanitic texture (left); phaneritic texture (right)
Definition: Porphyritic Texture – texture of igneous rocks that shows two different crystal sizes
-large crystals are called phenocrysts
-large crystals surrounded by smaller crystals called groundmass
-rocks with this texture are called porphyry
Figure 5 - porphyritic texture
Definition: Glassy Texture – texture of igneous rocks that contain no crystals
-example: obsidian
-results from rapid cooling
Figure 6 - glassy texture
Definition: Pyroclastic Texture – texture of igneous rocks resulting from the combination of individual rock
fragments that are ejected during a volcanic eruption
Figure 7 - pyroclastic texture
Bowen’s Reaction Series:
-in early 1900’s, Norman Bowen discovered that as magma cools in lab, minerals with higher melting points
crystallize before minerals with lower melting points
-olivine (1st mineral to form) will react with
remaining magma to form pyroxene
-pyroxene then react with magma to form
amphibole
-this reaction will continue until last mineral, biotite,
is formed
-left branch of Bowen’s reaction series 
discontinuous reaction series (each mineral has
different crystal structure)
-right branch called  continuous reaction series
(calcium-rich [plagioclase] feldspar react with sodium
to become more sodium rich)
-during last stage of crystallization, muscovite and
potassium (orthoclase) feldspar are formed
-if any magma remains, it is high in silicon and forms
quartz
Definition: Magmatic Differentiation – process of developing more than one rock type from a common magma
Definition: Crystal Setting – process of separating solid and liquid parts of magma during crystallization
-occurs if earlier-formed crystals are denser than liquid portion & settle to bottom of magma
-occurs frequently in dark silicate minerals, like olivine and pyroxene
-these minerals can solidify at bottom of magma chamber OR escape chamber and have very different chemical
composition than parent magma
Definition: Assimilation – process by which composition of magma can change due to foreign material (ie.
inclusion)
Definition: Magma Mixing – process where one magma body overtakes and mixes with another
Naming Igneous Rocks:
1)
1st minerals to crystallize in magma: calcium feldspar, pyroxene and olivine (high in
iron/magnesium/calcium & low in silicon)
-basalt is a common rock with this chemical composition
-rocks like basalt are called basaltic
-aka. mafic
-darker in color due to iron content and very dense
2)
last minerals to crystallize: potassium feldspar & quartz
-granite is a common rock with this chemical composition
-rocks like granite are called granitic
-aka. felsic
3)
minerals that have a composition between basalt and granite are andesitic
-amphibole and potassium feldspars are common andesitic minerals
-possible to have igneous rocks that share properties of different groups
-example: granodiorite has mineral composition between granitic and andesitic rock
Granitic Rocks:
a. Granite
-best known of all igneous rocks
-commonly used for tombstones, monuments, building stones
-phaneritic rock
-composed of 25-35 % quartz and over 50 % potassium/sodium feldspar
-often produced by orogenesis
-very resistant to weathering and erosion
-found in core of eroded mountains
b. Rhyolite
-volcanic equivalent of granite, relatively uncommon
-composed of mainly light silicates
-color pink to light gray
-aphanitic rock, contains glass fragments and vesicles
-sometimes contains phenocrysts of quartz or potassium feldspar
c. Obsidian
-dark, glassy rock
-forms when silica-rich lava cools very quickly
-glassy texture due to high silicon content
-dark color due to presence of metals
d. Pumice
-volcanic rock with glassy texture
-usually found with obsidian
-forms when large amounts of gas escape through lava to produce a gray,
frothy mass
-some samples show vesicles
-other samples show fine shards of intertwined glass
-some pumice will float on water
-flowlines sometimes visible (indicating movement before solidification)
Andesitic Rocks:
a. Andesite
-medium gray, volcanic rock
-aphantic rock
-sometimes shows porphyritic texture
b. Diorite
-phaneritic, intrusive rock
-looks similar to gray granite
-composed of sodium feldspar, amphibole, and a little biotite
-has “salt & pepper” appearance
Basaltic Rocks:
a. Basalt
-very green to black
-aphanitic texture
-volcanic rock composed of pyroxene, calcium feldspar and a little olivine
& amphibole
-can be porphyritic  contains small, light-colored calcium feldspar
phenocrysts OR glassy olivine phenocrysts embedded in dark groundmass
-most common extrusive igneous rock
-many volcanic islands composed mainly of basalt
b. Gabbro
-intrusive equivalent of basalt
-very dark green to black
-composed of pyroxene and calcium feldspar
-main rock that makes up oceanic crust
Pyroclastic Rocks:
a. Tuff
-composed of tiny ash-sized fragments which were later cemented together
b. Welded Tuff
-formed when ash particles remained hot enough to fuse
-consists of mostly glass shards
-frequently contains obsidian or other rock fragments
c. Volcanic Breccia
-composed of particles larger than ash