Download Chapter 3 Igneous Rocks What are Rocks?

Chapter 3
Igneous Rocks
What are Rocks?
Rock: any naturally formed, firm and coherent aggregate of minerals that
constitutes part of the Earth. Despite considerable diversity, we can
classify all rocks into three types, according to how they formed:
1. Igneous rocks (from Latin "ignis" meaning "pertaining to fire") are
formed by cooling and solidification of molten rock material and
typically represented by an interlocking aggregate of silicate
minerals.
2. Sedimentary rocks (from Latin "sedimentum" meaning "settle") are
formed from particles of pre-existing rocks by cementation or other
processes at the Earth's surface.
3. Metamorphic rocks (from Greek "meta" meaning "change" and
"morpho" meaning "form") are formed within the Earth's crust by
solid-state transformation of pre-existing rock (igneous,
sedimentary or even metamorphic) as a result of high temperature,
high pressure or both.
Within each group, rocks share common origins, but do not necessarily
look alike. Difference are clue to how and where a rock formed.
What are Igneous Rocks?
Early study of igneous rocks was shrouded in controversy. In an attempt
to organize rocks into a simple easily understood system, the 18th century
german mineralogist Abraham Werner proposed that all rocks were
precipitated in layers from a universal sea. Active volcanoes were
explained by burning of subterranean coal beds. Because he was highly
respected by his peers, Werner's theory, called Neptunism, gained wide
acceptance and was not questioned. Neptunism, however, had a number
of problems: e.g., the volume of rock assumed to have been precipitated
was much greater than could have been in solution.
Eventually Werner's theory was disproven by studies of volcanic rocks.
The geological community came to accept Plutonic theory, the belief that
igneous rocks originate as molten rock material deep in the Earth. The
new theory got its name from the Greek god of the underworld, Pluto.
Magma and Lava
Magma is the term used to describe naturally occurring molten rock
material beneath the Earth's surface. Mobility of this liquid within the
Earth is controlled by its physical properties, density and viscosity. Being
a liquid, it is less dense than solid rock and thus, tends to rise buoyantly
within the Earth as long as it is lighter than the surrounding or country
rocks. Lava represents hot streams or sheets of magma that flow over the
Earth's surface. Two types of igneous rocks can form from magmas:
1. Intrusive (Plutonic) igneous rocks are produced by cooling and
crystallization of magma beneath the Earth's surface. The resulting
igneous body may represent solidification of a magma chamber or
reservoir in which magma would have been stored during
movement toward the surface.
2. Extrusive (Volcanic) igneous rocks are produced rapid cooling and
crystallization of magma on the Earth's surface. Volcanoes
represent the vents from which molten silicate material, solid rock
debris, and gases escape from the subsurface. The volcanic
products may be coherent (lava) or fragmented (pyroclastic)
material. Pyroclasts are produced when the erupting magma is torn
apart by violent explosions within the volcanic vent.
Magmas are composed of the major elements (O, Si, Al, Mg, Fe, Ca, Na
and K) that form the Earth. The dominant component of most magmas is
silicon dioxide (silica), which constitutes 35-79% of the liquid. Magmas
are grouped into compositional categories based on silica content:
ultramafic (245% silica), mafic (45-52% silica), intermediate (53-65%
silica), and felsic or silicic (>65% silica).
Origin of Magma
Geophysical studies demonstrate that except for the outer core, the Earth's
interior is solid. Thus, there must generally be insufficient internal heat
generation to melt pre-existing rock. Nevertheless, magmas demonstrate
that melting must occur, although it is probably incomplete or partial
melting rather that complete melting of solid rock material. The necessary
melting conditions are present along divergent plate boundaries and
above subduction zones. Melting always produces a magma that is more
silica- and alkali-rich than its source rock.
Crystallization of Magma
Magma is molten rock material and dissolved gases (e.g., water, carbon
dioxide, and various sulfur gases including H2S and SO2). Magma
temperatures range from 700û to 1200ûC. Cooling magma begins to
solidify through crystallization of minerals and release of gases and
hydrothermal fluids. The crystallized minerals may be carried along in
the rising magma as suspended solids (crystals).
N.L. Bowen (1922) demonstrated that a mafic parent magma can produce
intermediate to felsic rocks as a result of progressive crystallization.
Bowen determined the sequence of mineral crystallization from a basaltic
magma, and showed that two different types of reaction occur between
crystalline solids and the magma as it cools. Continuous reaction series
minerals react with the melt to form new crystals with a different
composition but a constant atomic structure. Discontinuous reaction
series minerals react with the melt to form new crystals with both a new
composition and a new atomic structure.
If the crystallizing minerals are continuously removed from contact with
the magma, the magma composition will change from basalt through
andesite to rhyolite. Generation of rhyolite (granite) from a basaltic liquid
would require ninety percent solidification of the parental magma.
However, geological observations indicate that there are ten times more
granitic than mafic plutons, suggesting the granitic magmas must also be
generated in other ways.
Separation of earlier-formed minerals, called fractional crystallization or
crystal fractionation, can occur due to (a) crystal (gravitational) settling
due to density differences with the magmatic liquid, (b) filter pressing or
compaction of the crystal-liquid mush, and (c) differential flow. The
separated crystals may settle and accumulate to form cumulate igneous
rocks at the bottom of a magma chamber.
Magma may undergo additional compositional changes during ascent
from its source region. The rising magma may react with and partially or
completely melt the surrounding crustal rocks, incorporating elements
that were originally present in the wall rocks. Partially melted inclusions
of country rock in many magmas attest to such crustal contamination or
assimilation. Different composition magmas within the same magmatic
plumbing system may also intermingle or mix during ascent, such that
magma mixing produces a hybrid magma of intermediate composition.
Texture of Igneous Rocks
Texture refers to variations in the sizes and shapes of mineral grains in a
rock, and the type of relationships between the grains. Texture is
determined by:
(1) Rate of Cooling - a primary control on texture, determines the
relative rate of crystal nucleation and growth.
1. slow cooling - few large crystals produced by growth rate
greater than nucleation rate
2. rapid cooling - many small crystals produced by nucleation
rate greater than growth rate
3. quench - glass produced where ions have no time to organize
into
crystals
(2) Magma composition and Temperature - control magma density
and magma viscosity (or its internal resistance to flow)
1. high silica melts are viscous and crystallize at low
temperatures (<850ûC). Ions have difficulty migrating
through liquid and organizing into crystals
2. low silica melts are fluid (low viscosity) and have high
temperatures (850û-1200ûC). Ions easily migrate through
liquid and organize into crystals
3. higher silica content, the higher the viscosity
4. (d) higher temperature, the lower the viscosity
(3) Gas content of magma - High gas content reduces viscosity,
leading to larger crystals even at low temperatures.
Igneous Rock Textures
1. Phaneritic texture is where individual mineral grains (crystals) are
visible with the naked eye. The coarse-grained texture indicate
slow cooling, and is typical of intrusive rocks.
2. Aphanitic texture is where individual mineral grains (crystals) can't
be seen with unaided vision. The fine-grained texture indicate rapid
cooling, and is typical of volcanic rocks.
3. Vitric or glassy texture indicates rapid cooling or quenching of the
magma, best exemplified by obsidian or high-silica (rhyolite) glass.
4. Vesicular texture describes an aphanitic rock characterized by
preservation of cavities (vesicles) originally filled by escaping
gases. Highly vesicular basalts (low-silica magma) are called
scoria, whereas highly vesicular rhyolite (high-silica magma) is
known as pumice.
5. Porphyritic texture describes a rock, known as a porphyry, in
which large crystals (phenocrysts) are surrounded by a fine-grained
matrix (groundmass). The texture indicates non-uniform cooling
(slow cooling followed by a period of rapid cooling).
6. Pyroclastic texture denotes a rock made up of broken volcanic
particles (pyroclasts) that are fused by heat or cemented together
by finer grained material into a rock. The term is derived from
"pyro" meaning "fire" and "Klastos" meaning broken.
7. Pegmatitic texture indicates that the igneous rock is characterized
by an extremely coarse-grained texture. Abnormally large (ª1 cm)
crystals (locally containing rare metals such as Li, Be, or Ta in Limica, beryl or tantalum oxides) are formed from water-rich
magmatic solutions (hydrothermal fluids).
Igneous Rock Classification
Igneous rocks are grouped on the basis of texture and mineral
assemblage. Compositional categories based on silica content also apply
to magma types that cool to form different types of volcanic rocks:
komatiite (ultramafic), basalt (mafic), andesite (intermediate) and rhyolite
(felsic). If the same magma cools to form intrusive igneous rock, the
corresponding rock names are peridotite (komatiite), gabbro (basalt),
diorite (andesite) and granite (rhyolite). Differences in magma
composition are reflected by the mineral assemblage, or variety and
abundance of different minerals, in an igneous rock.
1. Ultramafic rocks include peridotite (olivine-pyroxene rock), dunite
(olivine rock) and pyroxenite (pyroxene rock). These rocks are
predominantly intrusive in nature throughout Earth history.
Volcanic equivalents, known as komatiites, primarily existed
during early Earth history (22.0 Ba), and there are no known
modern examples. The Earth's mantle is thought to be composed of
peridotite.
2. Mafic rocks are black, dark gray or dark green in color, and
composed primarily of olivine, feldspar (calcium plagioclase) and
pyroxene. Basalt, aphanitic but locally porphyritic or vesicular
mafic volcanic rock, is the most abundant igneous rock of the
Earth's crust, forming the ocean floor and volcanic oceanic islands.
Gabbro is the phaneritic intrusive equivalent of basalt, and
composes the deeper ocean crust.
3. Intermediate rocks are medium-gray color, and composed of
amphibole and feldspar (intermediate plagioclase) together with
some pyroxene and biotite. Andesites, locally porphyritic, are
intermediate volcanic rocks found in volcanic chains on continental
margins and in island arcs above subduction zones. Diorite is the
phaneritic intrusive equivalent of andesite, and comprises many of
the batholiths found associated with subduction processes.
4. Felsic rocks are light-colored, locally glassy (obsidian), and
composed of quartz and potassium feldspar with minor sodium
plagioclase, biotite and amphibole. The volcanic rock, rhyolite,
characterizes continental volcanoes and is typically associated with
extremely explosive volcanic activity. The explosive nature of
rhyolite volcanism reflects the magma's high viscosity and gas
content relative to mafic or even intermediate magmas. Granite is
the phaneritic intrusive equivalent of rhyolite, and comprises many
of the batholiths found within continental crust. Pegmatite is an
extremely coarse-grained granite, that forms from residual, waterrich magmatic fluids.
In addition to composition, pyroclastic rocks are further subdivided
according to fragment size and type:
1. tuff is a pyroclastic volcanic rock consisting of broken crystals and
pieces of volcanic glass less than 2 mm in diameter. Welded tuffs
occur where particles were hot enough to fuse together after
coming to rest.
2. volcanic breccia is a pyroclastic volcanic rock composed of
consolidated, angular volcanic particles greater than 2 mm in
diameter.
Intrusive Igneous Rock Bodies
Magmas crystallized beneath the Earth's surface form intrusive bodies of
igneous rock known as plutons. The term pluton (after the Greek god
Pluto) refers to any igneous intrusion regardless of size, shape or
composition of the magma. Classification of plutons is based on:
1. Geometry of intrusion:
Size
shape
2. Relationship to surrounding rocks:
Concordant or boundaries parallel to layering in surrounding
rocks
discordant or boundaries cut across layering in surrounding
rocks
Tabular Plutons
1. A sill is a concordant body, few cm to >1 km thick, produced when
magma is injected between layers of older sedimentary or volcanic
rock, and are generally composed of intermediate to basic
composition magma.
2. A dike is a discordant body, few cm to >100 m thick, produced
when magma is injected along fractures in surrounding rock layers.
Dikes ftypically form from magmas of basic to granitic
composition. Ring and Radial dikes are discordant bodies having
either a concentric (circular) or radial distribution; develop above a
large subsurface intrusive body (batholith or stock) or adjacent to
volcanic pipes or necks (see below).
3. A lopolith is a spoon-like shaped concordant body similar to a sill
except the floor and roof sag downward. The intrusions are
generally magma of intermediate to basic composition.
Massive Plutons
1. A laccolith represents magma that pushes overlying rock layers
upward to form a condordant, mushroom-shaped, sill-like body,
typically comprising magma of intermediate to granitic
composition
2. A batholith is a discordant magma body with exposed surface area
of more than 100 square kilometers; typically consists of multiple
intrusions. Batholith are usually magma of granitic composition
with minor intermediate varieties
3. A stock is a discordant magma body with exposed surface area of
less than 100 square kilometers; may represent exposed portion of
a much larger intrusion. It is usually magma of granitic
composition with minor intermediate varieties.
4. Volcanic pipes and necks are discordant bodies that represent the
upper part of the conduit that connects the volcanic vent (crater)
with an underlying magma source (magma chamber or reservoir).
Volcanic necks are erosional remnants of magma that solidified in
the pipe or conduit.