The term petrogenesis refers to the means by which a rock or mineral deposit is formed. Three different modes of formation are common; these consist of igneous activity, metamorphism, and sedimentary processes. Mineral deposits may thus occur in igneous, metamorphic or sedimentary environments and also in vein environments.
Igneous rocks are formed by the
solidification of magma or molten rock. Igneous rocks are
classified according to whether they formed from magma which cooled at
depth within the earth's crust or from magma which erupted out of a
volcano or vent at the earth's surface. Intrusive igneous rocks,
or rocks which solidify deep within the earth, are called plutonic
rocks. Extrusive rocks, in contrast, form from magma which erupts
at the earth's surface before cooling. Extrusive igneous rocks are termed
volcanic rocks.
Igneous rocks may also be classified
according to their texture, a characteristic defined by the size
and shape of the grains of which they are composed. The size of the
crystals in an igneous rock depend largely on the rate of cooling of the
magma which formed the rock. Plutonic rocks tend to have large crystals
because magma trapped at depths within the earth cools slowly. Magma
which is ejected by a volcano, on the other hand, tends to cool quickly.
Volcanic rocks therefore tend to contain only very small crystals.
One final means of classifying igneous
rocks is by composition. Rocks are divided into two classes
according to whether they contain a greater proportion of iron and
magnesium or of potassium, sodium, and calcium. Igneous rocks which
contain a high percentage of iron and magnesium tend to possess a dark
color and are said to be of basaltic or mafic composition.
In contrast, igneous rocks which contain a greater percentage of
potassium, sodium, or calcium tend to have a lighter color and are said to
be of granitic or felsic composition. Rocks with an
intermediary composition are termed andesitic.
Metamorphism describes the set
of solid state processes which transform one type of rock into another.
Any type of rock, whether igneous, metamorphic, or sedimentary, may be
metamorphised. Metamorphic changes occur mainly in mineral structure and
texture; changes in chemical composition may also take place.
Metamorphism typically occurs at the high temperatures and pressures which
are present deep within the earth's crust. The process is defined to
occur in the solid state and melting of the rock may therefore not occur.
Any process during which complete melting of the rock does occur is
considered to be of igneous rather than metamorphic nature.
Metamorphic processes may be divided
into two categories according to the area of their extent. Regional
metamorphism consits of geologic processes which act over wide areas
and cause metamorphic changes in vast expanses of rock. In contrast,
local metamorphism affects only relatively small areas. Local
metamorphism occurs when an igneous body or pluton intrudes into and heats
a surrounding body of rock. For this reason local metamorphism is also
called contact metamorphism.
Different types of metamorphism may
alternately be categorized according to the agents which are active during
each process. Stress, pressure, and shock are causative agents of
dynamic metamorphism, while thermal metamorphism is due
merely to the influx of heat from a nearby igneous intrusion. In
contrast, hot, volatile fluids play a significant role in hydrothermal
metamorphism. Hydrothermal metamorphism takes place when volatile
solutions percolate into and react with a host rock. Both the heat of
the intrusive igneous body and the reactive fluids serve to catalyze
metamorphic reactions in the host rock. The intrusive magma and the
associated volatile fluids may introduce previously absent elements into
the reaction process; the chemical composition of the host rock may
therefore be extensively altered during hydrothermal metamorphism.
A third possible method of classifying
metamorphic processes is whether or not previously absent elements are
introduced into the system. Isochemical metamorphism consists of
the recrystallization of previously present elements into new species of
minerals. Such minerals may differ structurally from the original
minerals, but will contain only those elements which were originally
present. In contrast, allochemical metamorphism introduces new
elements into the system during the reaction process and the resultant
mineral species contain elements not previously present. Dynamic and
thermal metamorphism are thus types of isochemical metamorphism, whereas
hydrothermal metamorphism is an allochemical process.
Veins are mineral deposits which
form when a fracture or fissure within a larger body of rock is
filled with new crystalline material. Veins are believed to form when
aqueous solutions migrate through fissures in rock and deposit minerals
onto the fissure walls.
Most veins form as new mineral species
are precipitated onto rock walls but leave the wall rock unaltered. In
such cases minerals fill the original crack or fissure but do not extend
into the host rock, and the boundary betwen host rock and newly deposited
vein minerals remains clearly delineated. Because the mineral species
which compose the veins were precipitated by hot water, vein deposits of
this nature are a type of hydrothermal deposit and the mineral material of
such veins is chemical sedimentary rock. However, occasionally the rock
wall which contains the vein undergoes alteration. Portions of the host
rock may dissolve or react chemically with the circulating volatile
fluids. In this case the boundary between vein deposit and original rock
wall will be unclear. Such veins are termed hydrothermal replacement
deposits. Hydrothermal replacement deposits are a form of
hydrothermal metamorphism.
Sediment includes all
uncemented, particulate matter which accumulates at the earth's surface.
Sediments are derived from the erosion of preexisting rocks, by
precipitation from aqueous solution, or from the skeletal remains of
organisms, and are transported by flowing water, wind, and moving
glaciers. Sedimentary rock is formed when loose sediments are
collected together, compacted, and cemented into rock with the aid of
heat, pressure, and chemical agents.
The term diagenesis is used to
portray the processes which result in the conversion of loose sediment
into sedimentary rock. These changes include recrystallization;
lithification, or compaction and cementation; and precipitation.
Compaction is a physical change which occurs as layers of sediment
accumulate in one area and the lowest layers are gradually compressed by
the weight of the layers above. Cementation, in contrast, is a chemical
change during which aqueous solutions seep through the sediment, filling
spaces and cementing individual particles together. Sedimentary
diagenesis takes place at relatively shallow depths and low temperatures.
Sedimentary rocks may be classified
according to their texture. Such rocks are categorized according to
whether they are clastic or nonclastic. Rocks of clastic
texture exhibit a mass of discrete particles and grains which are cemented
together but retain their individual shape, whereas rocks which are
nonclastic do not contain visible, macroscopic grains or particles.
Sediments are derived by the two
different processes of mechanical and chemical weathering. These two
different means of derivation provide another method for the
classification of sedimentary rocks. Detrital sedimentary rock
contains particles and sediment which were derived by mechanical means.
The materials which compose such rocks were ground down by mechanical
processes such as the passage of wind, the movement of glaciers, and
splitting due to freezing and thawing. These materials were then
transported to the site of deposition by mechanical means such as wind,
flowing water, or travelling glaciers. Few chemical changes occur in the
mineral material of which detrital sedimentary rocks are composed.
Chemical sedimentary rock, on the other hand, contains only
substances which were derived by chemical weathering processes. The
constituent materials of chemical sedimentary rock were dissolved in
water, transported to the deposition site in the form of aqueous
solutions, and finally precipitated.
Magma is molten rock which originates deep within the earth. Magma is formed as rock melts at the intense temperatures and pressures present at great depths within the earth. Igneous rocks form when magma rises towards the earth's crust, cools, and solidifies. The name igneous is derived from ignius, a Latin term meaning 'fire'. Most of the rocks which compose the earth, moon, and other planets are igneous in origin.
It is hypothesized that the earth's
crust and mantle are composed mainly of solid rather than molten rock.
It is also estimated that rock within the lower crust and upper mantle
maintains a temperature of 1200° to 1400° Celcius. This
temperature is very close to the melting point of rock. With the aid of
added heat, volatile agents, or a change in pressure, melting may occur.
One potential source of heat is
friction in subduction zones as one crustal slab is subsumed by another.
Another is the introduction of heat by mantle rock rising from yet greater
depths. Either of these may serve to introduce the heat necessary in
order to initiate partial melting of the mantle rock. Because an increase
in pressure causes an increase in the melting temperature of rock, rock
may also undergo a solid to liquid phase transition if the pressure of its
environment drops. This process is termed decompression melting. Rock
which contains a significant portion of volatile fluids such as water
vapor (H2O) and gaseous carbon and sulfur
dioxide (CO2 and
SO2)
has a lower melting point than rock which lacks such volatiles. If
volatiles are introduced into previously solid rock, melting may be
induced. Frictional heat, externally introduced heat, volatile fluids,
and the reduction of pressure may thus all serve as agents which initiate
the partial melting of mantle rock and the formation of magma.
The Ascent and Evolution of Magma
When a magma is less dense than the
surrounding mantle rock, the rock exerts an upward force on it. Due to
this buoyant force the magma then rises from its place of origin deep
within the crust. Both the viscosity, qualitatively defined as the
internal resistance to fluid flow, and the density of a magma determine
how fast it rises. A large silica content tends to increase the viscosity
of a magma. In contrast, the presence of volatile agents such as
H2O, CO2 and
SO2 tends to decrease viscosity. An increase in
pressure will decrease the viscosity of a magma but will increase its
density.
An abundance of silica
(SiO2) tends to increase the occurrance of
polymerization, or the formation of silica chains. A more
polymerized magma possesses a greater viscosity and therefore flows more
slowly towards the surface of the earth. On the other hand, the presence
of volatile fluids such as H2O,
CO2 and SO2 may
decrease the viscosity of a magma and thereby speed its flow. Because
magma is much more compressible than crystalline substances, its density
rises at high pressures whereas its viscosity decreases. A change in
pressure may therefore either speed or slow the magma's ascent towards the
surface.
Magma is not typically a simple liquid.
Instead, it is usually a mixture of molten rock, solid crystals, and
dissolved gases. Most magmas carry volatile agents such as
H2O, CO2 and
SO2. As a magma rises through the earth's
crust, its chemical composition may undergo a series of changes.
Crystallization may remove some elements from solution and increase the
concentration of those which remain. Melting of the enclosing rock may
introduce into the solution elements which were previously absent.
Through processes such as these a magma will change in composition as it
rises through the crust.
Minerals may crystallize within the
magma and then drop out of solution under the effect of gravity;
alternately, crystals which are less dense than the magma may float to the
upper portions of the enclosing chamber. Crystals may form along the
enclosing rock walls and be left behind as the magma ascends through the
crust. Components of the magma may exsolute, crystallize, and be carried
along as the magma rises. All such crystallization changes the
constituency of the magma by removing certain elements from solution and
increasing the concentration of those which remain.
The magma may also break off portions
of the wall rocks through which it passes and transport them towards the
earth's surface. Minerals from the rock wall may be melted and
assimilated into the magma. Both of these possibilities may introduce to
the solution elements which had previously been absent. Minerals which
formed earlier may continue reacting with the still-fluid portion of the
magma to form new substances. Through processes such as crystallization
and melting of surrounding rock a magma may change dramatically in
composition as it rises through the crust. A wide variety of igneous
rocks may thus originate from a single parent magma.
Plutonic versus Volcanic Rocks
Igneous rocks are classified according
to whether they formed from magma which cooled slowly at great depths
within the earth's crust or from magma which erupted at the earth's
surface. Intrusive igneous rocks, or rocks which solidify while
buried deep within the earth, are called plutonic rocks. This term
was derived from the name of Pluto, Greek god of the underworld. In
contrast to intrusive or plutonic rocks, extrusive rocks form from
lava, or magma which erupts at the earth's surface before cooling.
Extrusive igneous rocks are termed volcanic rocks, after Vulcan,
the Roman god of fire. Intrusive rocks crystallize slowly at high
temperatures and pressures whereas extrusive rocks crystallize at
atmospheric pressure and cool quickly.
The size of the crystals which compose
an igneous rock depend largely on the rate of cooling of the magma which
formed the rock. In a magma which cools quickly, large crystals do not
have time to form, and the resultant rock will be glassy or contain only
tiny crystals. In contrast, a magma which pools at great depths and
looses its heat slowly may develop crystals which are quite large.
Once a lava erupts at the earth's
surface it tends to loose heat very rapidly to its surroundings. Because
they usually cool quickly, volcanic rocks are typically of glassy texture
or contain only very tiny crystals. Plutonic rocks, in contrast, solidify
from magmas which pool at great depths within the earth's crust. At such
depths the surrounding rock wall is already at a relatively high
temperature. Plutonic rocks therefore tend to loose heat only very slowly
to the rock wall which surrounds them. Large plutons - which may possess
a diameter on the order of 100 kilometers - can take hundreds of thousands
of years to solidify. Because such rocks cool very slowly, they tend to
form large crystals. Plutonic rocks are observed at the earth's surface
due to erosion or tectonic upheaval long after crystallization.
The term texture describes the size and shape of the grains which compose a rock as well as the manner in which they are arranged. Igneous rocks may be classified according to their texture. Volcanic rocks cool and crystallize quickly from magma which has been extruded upon the earth's surface. During such rapid cooling large or complex crystalline structures do not have time to form. Volcanic rocks therefore tend not to contain visible crystals or else to contain only tiny ones. The texture of such rocks is termed glassy or aphanitic. Plutonic rocks, in contrast, cool and crystallize slowly at great depths and may thus contain large crystals. The texture of these rocks is described as phaneritic. Porphyritic texture describes a rock which contains a few large crystals embedded in an aphanitic matrix.
A pegmatite is a body of
plutonic rock which contains unusually large crystals. Crystals measuring
thirty centimeters across are common. Quartz crystals weighing thousands
of kilograms and mica crystals of three meters or more in diameter have
been found in association with pegmatites.
Pegmatites often extend outwards as
veins or dikes from larger masses of plutonic rock. The formation of a
pegmatite is probably directly related to the formation of this larger
plutonic body. As the magma within the main chamber cools, minerals
crystallize and exsolute, increasing the concentration of volatiles such
as water vapor (H2O), boron (B),
fluorine (F), chlorine (Cl), and phosphorous
(P) in the remaining magma. Higher concentrations of such
volatiles increase the fluidity and lower the viscosity of the magma which
remains, which may then escape the main chamber and penetrate into the
surrounding rock as a vein or dike. In this more fluid magma, ions
possess an unusually high mobility, and atypically large crystals may
develop as the magma cools.
Pegmatites are occasionally rich in
molybdenum, lithium, cesium, uranium, and titanium-bearing minerals.
Gemstones such as garnet, tourmaline, beryl, and topaz have been found in
pegmatite environments.
The Composition of Igneous Rocks
The chemical composition of a magma at
the time when it cools determines the identity of the minerals which
crystallize from the magma and therefore the identity of the resultant
igneous rock. The most prevalent component of magma by weight is
typically silica (SiO2). However, magma also
contains in varying quantities ions of all the other elements (aluminum,
Al, iron, Fe, calcium, Ca,
sodium, Na, potassium, K, and magnesium,
Mg) which compose the bulk of the earth's crust.
As a magma cools its constituent
elements bond to form two different types of silicate minerals. These two
types of silicates are divided according to which metallic elements they
contain. The ferromagnesian silicates are rich in iron and
magnesium, although they may also contain sodium or calcium. These
include olivine (Mg2SiO4,
Fe2SiO4), the minerals of the
pyroxene group - enstatite,
Mg2(Si2O6), hypersthene,
Fe2(Si2O6), diopside,
CaMg(Si2O6), and hedenbergite,
CaFe(Si2O6) - and the minerals of
the amphibole group - anthophyllite,
(Mg,Fe)7(Si8O22)(OH)2,
tremolite,
Ca2Mg5(Si8O22)(OH)2,
actinolite
(Ca2Fe5(Si8O22)(OH)2),
and glaucophane,
Na2Mg3Al2(Si8O22),
as well as biotite mica
(K(Mg,Fe)3(AlSi3O10)(OH)2).
In contrast, the nonferromagnesian silicates contain potassium,
sodium and calcium rather than iron or magnesium. These include quartz
(SiO2), the minerals of the feldspar group
(orthoclase, K(AlSi3O8), albite
Na(AlSi3O8), and anorthite
(Ca(Al2Si2O8)) and
muscovite mica
(KAl2(AlSi3O10)(OH)2).
Igneous rocks which contain a high
percentage of the ferromagnesian silicates tend to possess a dark color.
In contrast, those igneous rocks which contain a greater percentage of
nonferromagnesian silicates tend to have a lighter color.
Igneous rocks can be divided into two
classes according to their proportional content of ferromagnesian and
nonferromagnesian silicates. Igneous rocks composed mainly of dark,
ferromagnesian silicates are said to be of basaltic composition.
They are also called mafic rocks, the word mafic being derived from
the first syllables of magnesium and ferrum, or iron.
Because of their iron content such rocks tend to be both denser and darker
in color than those rocks composed mainly of nonferromagnesic silicates.
Igneous rocks which are composed mainly of the light-colored,
nonferromagnesian silicates such as quartz and feldspar are said to be of
granitic composition. Granitic rocks are also called felsic
rocks, the word felsic being derived from the initial syllables of
feldspar and silica (or quartz). Such rocks tend to contain
a relatively greater percentage of silica
(SiO2); typically this is about 70% by mass.
Rocks with an intermediary composition are termed andesitic after
the volcanic rock andesite.
Carbonatite is a unique type of
intrusive igneous deposit which is rich in calcium carbonate (calcite,
CaCO3) and minerals of the carbonate class.
Calcium carbonate is typically burned out of igneous materials by intense
heat; the preponderance of calcite found in carbonatite therefore makes it
a very unusual type of igneous formation. Carbonatite deposits may form
as plugs, dikes, sills, or veins when carbonatite magma intrudes into host
rock formations. Carbonatites are only observed to occur in continental
plates and do not occur in oceanic plates or at plate boundaries.
Carbonatite magma differs greatly from
the more prevalent silicate magma, and the two different types of magma
are immiscible. Carbonatite rarely contains more than 10% silica by mass
and may contain much less. Such magmas possess a very low viscosity
because the lack of silica prevents extensive silicate polymerization.
It has been shown experimentally that at a temperature of around 600°
Celsius a magma possessing a high carbon dioxide
(CO2) content will divide into separate,
immiscible silicate and carbonatite magmas.
One possible method of carbonatite
formation is that a parent magma originating in the mantle underneath a
continental crust rises until it reaches the boundary between crust and
mantle. The magma may then be of a higher density than the crustal plate
and may be detained. Ferromagnesic silicates may crystallize out at these
high temperatures although non ferromagnesic silicates remain liquid.
Portions of the crustal plate, which contains plentiful carbonates, may
also melt and be incorporated into the magma. When the ferromagnesic
silicates are removed from the magma its density decreases due to the
newly lessened relative concentration of iron and magnesium. The magma
may then rise into the crust until it reaches zones of lower temperature
(around 600° Celsius) where it may separate into silicate and
carbonatite magmas.
Carbonatites contain atypically high
concentrations of rare earth elements such as titanium
(Ti), vanadium (V), manganese
(Mn), iron (Fe), copper (Cu),
zinc (Zn), yttrium (Y), zirconium
(Zr), niobium (Nb), molybdenum
(Mo), barium (Ba), lanthanum
(La), cesium (Ce), samarium
(Sm), europium (Eu), lead
(Pb), thorium (Th), and uranium
(U). The elements sulfur (S), phosphorous
(P), and fluorine (F) are also frequently
present. High concentrations of magnetic materials may result in
observations of unusual magnetic phenomena; atypically high concentrations
of radioactive elements may produce unusual levels of radioactivity in the
vicinity of a carbonatite.
Calcite
(CaCO3), dolomite
(CaMg(CO3)2), and ankerite
(CaFe(CO3)2) are the most common
minerals found in carbonatites. The carbonates strontianite
(SrCO3), and magnesite
(MgCO3); the sulfides pyrite
(FeS2), molybdenite
(MoS2), galena (PbS),
chalcopyrite (CuFeS2), and sphalerite
(ZnS); the oxides ilmenite
(FeTiO3), hematite
(Fe2O3), rutile
(TiO2), and zircon
(ZrSiO4); the sulfate barite
(BaSO4); the phosphates monazite
((Ce,La)PO4), and fluorapatite
(Ca5F(PO4)3); and the
halide fluorite (CaF2) are other species which
are often present.
Metamorphism describes the set
of solid state processes which transform one type of rock into another.
Any type of rock, whether igneous, metamorphic, or sedimentary, may be
transformed into a metamorphic rock.
Metamorphism occurs when the
protolith, or parent rock, is either subjected to temperatures and
pressures greater than those at which it formed or else is chemically
altered by incident fluids. The metamorphic process occurs as the
minerals of the original rock become unstable at the new set of
temperatures and pressures or upon the inclusion of previously absent
chemical substances. During metamorphism, mineral and rock deposits
change and adapt until equilibrium with the new environmental conditions
is reached.
Agents of metamorphism are heat,
volatile fluids, and pressure or stress. Heat capable of catalyzing a
metamorphic reaction may be supplied by nearby igneous intrusions, the
latent heat of crystallization of a nearby solidifying magma, or by the
geothermal gradient within the earth's crust. Volatile fluids often act
as a catalyst, reactant, or product during metamorphism. Such fluids are
typically a mixture of H2O,
CO2 and SO2 and may be
derived from meteoric groundwater, seawater, or a nearby igneous
intrusion. Pressure and stress applied to rock may also induce
metamorphism.
Metamorphic changes occur mostly in
mineral structure and texture. Molecular structure may be altered and the
rearrangement of chemical components may occur during recrystallization.
Changes in chemical composition due to reactions involving aqueous
solutions may also take place. Mineral grains may be physically deformed
or broken. The identity of the resultant metamorphic rock depends not
only on the chemical composition of the parent rock and that of any
incident fluids but also on the temperature and pressure at which the
metamorphism took place.
Metamorphism is a process which
typically occurs at the high temperatures and pressures present deep
within earth's crust. The delineation between sedimentary diagenesis and
metamorphism is defined to occur at about 200° Celsius. Changes in
mineral form which occur at temperatures lower than 200° Celcius are
thus defined as sedementary diagenesis; changes which take place at higher
temperatures are considered to be metamorphic. Metamorphism is defined to
occur in the solid state, so that melting of the rock may not take place.
The high temperature limit of metamorphism is therefore given by the
melting temperature of the rock in question. Any process in which
temperatures are great enough that complete melting of the host rock takes
place is considered to be igneous rather than metamorphic.
Metamorphic processes may be divided
into two categories according to the area of their extent. Regional
metamorphism may affect vast expanses of rock, whereas local
metamorphism will affect only relatively bounded areas.
Different types of metamorphism may
alternately be categorized according to the agents active during each
process. Thermal metamorphism is due merely to the influx of heat
from a nearby igneous intrusion. In contrast, highly reactive volatile
fluids play a significant role in hydrothermal metamorphism.
Stress, pressure, and shock are causative agents of dynamic
metamorphism.
A third possible method of
classification of metamorphic processes is whether or not previously
absent elements are introduced into the reaction process. The process of
isochemical metamorphism consists of the recrystallization of
previously present elements into new species of minerals. Such minerals
may differ structurally from the original minerals, but will contain only
those elements which were originally present. In contrast,
allochemical metamorphism integrates elements introduced into the
system during the reaction process into the newly formed mineral species.
It is apparent that thermal and dynamic metamorphism are types of
isochemical metamorphism because they introduce no new elements into the
reacting system, while hydrothermal metamorphism is a variant of
allochemical metamorphism and may radically change the chemical
composition of the system.
Regional metamorphism describes geologic processes which act over wide areas and cause metamorphic changes in vast expanses of rock. This type of metamorphism is also called dynamothermal metamorphism. Mountain ranges may be composed of large bodies of rock which were shaped and formed by regional metamorphism.
Regional metamorphism occurs in
subduction zones where one crustal plate slides under another. At plate
boundaries and in places of collision, large quantities of rock may be
subsumed and subjected to the greater temperatures and pressures found
deep within the earth. This type of regional metamorphic process is
called orogenic metamorphism.
Regional metamorphism often occurs
beneath midoceanic ridges and trenches. As one oceanic plate slides
beneath another or as two plates draw apart, seawater may seep into crust
rocks. Both seawater and water escaping from upwelling magmas may
catalyze metamorphic processes.
Burial metamorphism occurs in
large basins which progressively fill with sedimentary material. The rock
underneath the basin may sink as the basin fills, experiencing increasing
pressures as subsequent levels of sediment are deposited over the top.
Metamorphism typically begins at depths of about 8 kilometers where
temperatures may be from 100° to 200° Celsius.
Local metamorphism occurs in an area of much smaller scale than that of the great bodies of rock affected by regional metamorphism. Local metamorphism occurs when an igneous body or pluton intrudes into and affects the surrounding rock. For this reason local metamorphism is also called contact metamorphism.
The process of thermal
metamorphism, also called pyrometamorphism, takes place when an
intrusive igneous body heats the surrounding rock. Metamorphism then
occurs as heat inflow from the nearby igneous source catalyzes
recrystallization of previously present rock.
In cases of thermal metamorphism no
new elements are introduced into the set of available elements. Instead
new minerals form from those elements which were previously part of the
system. For this reason thermal metamorphism is a type of isochemical
metamorphism. For example, when a pure limestone
(CaCO3) is heated by an intruding igneous body,
it will recrystallize into pure marble (CaCO3).
However, if impurities such as quartz (SiO2),
clay (kaolinite,
Al4(Si4O10)(OH)8)
or iron oxide (hematite, Fe2O3, or
magnetite Fe3O4) are present, new
mineral species may be created. Quartz may combine with limestone to form
wollastonite (Ca(SiO3)); the aluminum in clay
may enter into the reaction and form corundum
(Al2O3), spinel
(MgAl2O4), and garnet (almandine,
Fe3Al2(SiO4)3).
Strain metamorphism occurs when the host rock is subjected to stress; this frequently takes place along fault lines. At low temperatures and pressures the rock may be pulverized; at higher temperatures and pressures ductile flow may occur, and rocks may elongate, fold and flow. Confining non-directional pressures may initiate metamorphism which produces a more dense, compact rock. Shock metamorphism due to extremely high temperatures and pressures of very short duration may occur when initiated by a meteorite impact. Dynamic metamorphism is a type of isochemical metamorphism because no new elements are introduced into the host rock.
Hydrothermal metamorphism, also
called metasomatism, may take place across wide regions of rock,
thereby constituting a variant of regional metamorphism. It may
alternately may take place in a limited, localized area and constitute a
variant of local metamorphism.
Hydrothermal metamorphism takes place
when hot, volatile solutions percolate into and react with the protolith,
or the original rock. The heat of the intrusive igneous body and the hot
volatile fluids serves to catalyze metamorphic reactions in the host rock.
The incident fluids enhance ion mobility in the system and are highly
reactive. Both the intrusive magma and the associated volatile fluids may
introduce elements which were not present in the protolith into the
reaction process. The incident volatile fluids may also dissolve and
remove elements originally abundant in the host rock. The chemical
composition of the host rock may be extensively altered during the process
of hydrothermal metamorphism. Hydrothermal metamorphism is therefore a
type of allochemical metamorphism.
The hot, volatile fluids responsible
for hydrothermal metamorphism may be derived from several sources. Such
fluids may consist of volatiles escaping directly from the intrusive
magma; they may also be composed of meteoric groundwater heated by the
igneous intrusion or by the geothermal gradient. High temperatures and
pressures can expel water from hydrated minerals such as gypsum, talc,
chalk, and clay. These sources of fluid are typically associated with
local metamorphism. When an oceanic plate is subsumed at a boundary
between two plates, water may be transported along with it and may
subsequently take part in any metamorphic processes which occur.
Seawater may also seep into the crust as two oceanic plates move apart.
Such sources of volatile fluids are associated with regional metamorphism
and the movement of crustal plates.
Much hydrothermal metamorphism occurs
at the boundaries of oceanic plates. Plates which are moving apart allow
seawater to percolate through the oceanic crust. As the seawater
migrates, it heats and reacts with the host rock. Large quantities of
metals such as iron, cobalt, nickel, silver, gold, and copper are
dissolved from the crust. When the hot, metal-laden fluid later comes
into contact with cold seawater, sulfide and carbonate minerals
precipitate to form deposits of metal ores. The copper ore which has been
mined on Cyprus for several thousand years is thought to have been
deposited in this way.
The presence of previously unavailable
elements and the catalyzation provided by heat and volatile fluids enables
a wide variety of mineral species to form during hydrothermal
metamorphism. Many of these species contain elements not originally
present in the host rock. The process of hydrothermal metamorphism thus
produces a greater variety of minerals than may be formed by thermal
metamorphism alone. Silicates which are found in hydrothermal metamorphic
deposits include quartz, garnet, wollastonite, olivine, topaz, and
tourmaline. The halide fluorite may be present. Sulfide ores such as
pyrite, chalcopyrite, sphalerite, and molybdenite and oxides such as
magnetite, hematite, spinel, and corundum may also occur.
Local hydrothermal metamorphic
deposits may grade into hydrothermal vein deposits. A vein forms when
hot, mineral-bearing fluids deposit new materials along the walls of a
preexisting fissure or crack in the host rock. In the case where minerals
merely exsolute from the aqueous solution to crystallize along the walls
of the fissure, leaving the wall rock intact and unchanged, no
metamorphism is involved. However, the minerals of the fissure walls
often react with the passing volatile fluids. Some may be dissolved and
transported away; some may recrystallize or crystallize as new mineral
species. The formation of this type of vein deposit, called a
hydrothermal replacement deposit, does constitute hydrothermal
metamorphism.
Isochemical and Allochemical Metamorphism
Isochemical metamorphism requires that no new elements are introduced into the system and that the relative concentrations of previously present elements remain constant throughout the metamorphic process. Isochemical metamorphism thus involves no injection or leaching out of new elements by magma or volatile fluids. In contrast, allochemical metamorphism involves changes in chemical composition due to the arrival of new substances or the removal of previously present ones. Thermal metamorphism and dynamic metamorphism are types of isochemical metamorphism, whereas hydrothermal metamorphism typically implies allochemical metamorphism and the introduction of previously absent elements.
Several different textures are observed in metamorphic rocks.
Veins are mineral deposits which
form when a preexisting fracture or fissure within a host rock is filled
with new mineral material. The deposition of minerals is typically
performed by circulating aqueous solutions. Many ore deposits of economic
importance occur in veins.
Vein deposits are believed to form
when aqueous solutions carrying various elements migrate through fissures
in rock and deposit their burden onto the fissure walls. Hot, rising water
escaping from cooling igneous plutons may deposit minerals as it ascends
through the crust. As heated magmatic waters rise, the temperature and
pressure of their environment drop and minerals exsolute and crystallize.
Meteoric ground water may also percolate down through the earth's crust,
dissolving surface minerals and gaining heat from the geothermal gradient
or from nearby igneous intrusions. At greater depths the dissolved
substances may precipitate and crystallize along the walls of the fissures
and cavities through which the water travels.
Most vein deposits are formed as new
mineral species are precipitated onto rock walls which themselves remain
unaltered. In such cases mineral deposits fill the original crack or
fissure in the host rock but do not extend into the host rock itself.
The boundary betwen host rock wall and deposited vein minerals therefore
remains clearly delineated. Vein deposits of this nature are a type of
hydrothermal deposit because the mineral species which compose the
veins were precipitated by hot waters. However, sometimes the preexisting
rock wall which contains the vein undergoes alteration. Portions of the
host rock may either dissolve and be transported away or else react
chemically with the circulating volatile fluids or the newly formed
mineral species. In this case the boundary between vein deposit and
original rock wall will be unclear. If most of the mineralization process
occurs within the space once occupied by unaltered wall rock then the vein
is termed a hydrothermal replacement deposit. A hydrothermal
replacement deposit occurs when hot circulating aqueous solutions replace
the original rock with new mineral species. This typically occurs in more
soluble rocks such as limestone. Hydrothermal replacement deposits are a
form of hydrothermal metamorphism or metasomatism.
Hydrothermal deposits are categorized
according to the depth and temperature at which they formed.
Hypothermal deposits are formed at great depths and high
temperatures; mesothermal deposits at intermediate depths and
temperatures; and epithermal deposits at the shallowest depths and
relatively low temperatures.
Some mineral species crystallize
mainly at preferred temperatures and pressures. Because the temperatures
and pressures are different for each type of hydrothermal deposit, each
has a different, characteristic set of associated minerals.
Secondary Replacement Deposits
When exposed to meteoric ground water
containing dissolved oxygen and carbon dioxide, primary ore minerals near
the surfaces of vein deposits may be altered to secondary minerals.
Primary sulfide minerals such as pyrite, chalcopyrite, galena, and
sphalerite are in particular vulnerable to oxidization and alteration.
Pyrite (FeS2) oxidizes to goethite
(FeO(OH)) or melanterite
(FeSO4•7H2O), galena
(PbS) oxidizes to anglesite
(PbSO4) and cerussite
(PbCO3), and sphalerite (ZnS)
alters to hemimorphite
(Zn4(Si2O7)(OH)2•(H2O))
and smithsonite (ZnCO3). If the presence of
carbon dioxide is sufficient, then copper sulfide minerals may alter to
the copper carbonates azurite
(Cu3(CO3)2(OH)2)
and malachite
(Cu2CO3(OH)2) and the
oxide cuprite (Cu2O) as well as native copper
(Cu). Just as primary sulfides alter to secondary
sulfates, arsenides or phosphides may oxidize to produce secondary
arsenates or phosphates.
Oxidized pyrite and copper sulfides
produce sulfuric acid (H2SO4) and
ferric sulfate (FeSO4) which in turn react with
copper to form soluble copper sulfate (CuSO4).
Copper sulfate may be transported downwards as meteoric groundwaters
percolate through the rock. As the copper sulfate moves downwards it
reacts with the sulfide minerals which it encounters. These are altered
to produce secondary copper sulfides such as chalcocite
(Cu2S). The secondary copper sulfides may then
in turn oxidize, continuing the downward migration of metal through the
vein environment.
The upper zone of a vein in which
secondary replacement occurs may become barren and leached and is
referrred to as the oxidized zone. Delicate crystalline materials
may be found in the oxidized zone because the primary minerals have been
removed, providing the room necessary for extensive crystals to form.
The zone of oxidation is typically fairly shallow; it tends to extend from
the surface to the water table. Near the water table a zone of secondary
enrichment may form; in this region copper which seeped down from the
higher primary regions will be concentrated. Large chalcocite 'blankets'
are occasionally found in zones of secondary enrichment. Below the
secondary regions only primary minerals are found.
Sediments include all
uncemented, particulate materials which accumulate at the earth's surface.
Such materials are derived from the erosion of preexisting rocks by
mechanical or chemical weathering processes, by precipitation from aqueous
solution, or from the skeletal remains of organisms. Sediments are
transported by the movement of water, wind, and glaciers. Sedimentary
rock is formed when volumes of loose sediments are collected together,
compacted, and cemented into rock with the aid of heat, pressure, and
cementing agents.
Only five percent of the earth's crust
by volume is estimated to consist of sedimentary rocks. However, at least
two thirds of the surface area of the earth is covered by a thin layer of
this type of rock. The processes by which sedimentary rocks are formed
are surface phenomena driven by surface waters and weather.
The term diagenesis is used to
portray all changes which result in the conversion of loose sediment into
sedimentary rock. These changes include recrystallization,
lithification and precipitation. Sedimentary diagenesis takes
place at depths of less than a few kilometers within the earth's crust and
occurs at temperatures of less than 200° Celsius. Changes in rock
which occur at greater depths or temperatures are defined to be
metamorphic rather than sedimentary.
Recrystallization is the means
by which a less stable mineral species may take on the form of a
chemically identical but more stable mineral species. In order to do this
a mineral will dismantle its crystalline lattice and rearrange its
chemical constituents. The chemical composition of the system does not
change during recrystallization although the crystalline structure does
change. One example of this process is provided by the two calcium
carbonate species calcite and aragonite, both of which possess the
chemical formula CaCO3. The shells of marine
organisms are composed of aragonite; volumes of these shells may
accumulate to produce large deposits of sediment. Because aragonite is
less stable than calcite, when these deposits are buried and thereby
subjected to increased temperatures and pressures the aragonite
recrystallizes to adopt the structure of calcite. Calcite remains stable
in more extreme conditions than does aragonite, and is the main
constituent of the familiar sedimentary rock called limestone.
The methods by which unconsolidated
particles of sediment are transformed into sedimentary rock are termed
lithification. Lithification includes the two processes of
compaction and cementation. Compaction is a physical change
which occurs as layers of sediment accumulate in one area. Gradually the
lowest layers are compressed by weight of the layers above; the amount of
pore space between individual grains of sediment is reduced, and water
trapped within the pore spaces is forced out. Cementation, in contrast,
is a chemical change. During cementation elements carried by aqueous
solutions which seep through the sediment precipitate into the pore spaces
between grains, filling these spaces and cementing the individual
particles together. The most prevalent cementing minerals are calcite
(CaCO3), silica
(SiO2), and iron oxide
(Fe2O3). The presence of iron, which
is a strong pigmenting agent, tints rocks cemented with iron oxide orange,
red, or red-brown.
Not all sedimentary rocks undergo
lithification; some are crystalline and originate as masses of interlocked
crystals. Certain limestone deposits, which form when deposits of
aragonite recrystallize into calcite, offer examples of crystalline
sedimentary rock.
Detrital and Chemical Sedimentary Rock
Sediments and the particulate mineral
matter which compose sedimentary rocks are derived by the two different
means of mechanical and chemical weathering. Sedimentary
rocks may in turn be classified according to which of these two means
resulted in their formation. Detrital sedimentary rock contains
particles and sediment which were derived by mechanical means, while
chemical sedimentary rock contains only substances which were
derived by chemical weathering processes. The materials which compose
detrital sedimentary rocks were ground down by mechanical processes such
as wearing by wind, grinding by glaciers, and splitting due to cyclic
freezing and thawing. These materials were also transported to the site
of deposition by mechanical means such as wind, suspension in moving
water, or travelling glaciers. These are compacted and cemented together;
few chemical changes occur in the mineral material of which the rocks are
composed. The constituent materials of chemical sedimentary rock, in
contrast, were dissolved in water, transported to the deposition site in
the form of aqueous solutions, and finally precipitated. Attack by
rainwater and atmospheric carbon dioxide (CO2)
may dissolve parent rock and allow it to be transported to the site of
deposition. The precipitation of chemical sedimentary rocks may then be
caused by either inorganic or organic agents. Inorganic processes include
evaporation whereas organic processes occur when water-dwelling organisms
extract calcite from their environment and in turn excrete a calcareous
exoskeleton.
Sandstone provides an example of a
familiar detrital sedimentary rock while limestone is perhaps the most
well-known chemical sedimentary rock. Sandstone is so named because it is
composed of sand-sized grains. Because the mineral quartz
(SiO2) is quite stable and durable, it forms the
most prominent constituent of most sandstones. Less stable or robust
minerals often disintegrate during the mechanical weathering and
collection processes which result in detrital sedimentary rocks.
Limestone is an example of a chemical
sedimentary rock which may be formed through either inorganic or organic
processes. Limestone
stalactites (CaCO3) offer an example of
inorganic precipitation. Such stalactites are formed as calcium-carbonate
bearing water drips from an overhead projection in a cave. (The greek
word stalaktos means 'dripping' or 'trickling'.) A minute quantity
of calcium-carbonate is precipitated from each water droplet, slowly
building up the pendulous stalactite formation. Many larger limestone
deposits, on the other hand, are formed through organic precipitation.
In particular this occurs when small organisms such as corals extract
calcite from their ocean environment and secrete a calcareous
extoskeleton. A colony of corals may over time achieve a massive, solid
limestone reef.
Chemical sedimentary minerals which
precipitate when evaporation removes the water in which they are dissolved
are termed evaporites. These minerals include halite
(NaCl), sylvite (KCl) and gypsum
(CaSO4•2H2O).
The quality of texture also
plays a role in the classification of sedimentary rocks. With respect to
texture, sedimentary rocks are categorized according to whether they are
clastic or nonclastic. The word clastic is derived from the
Greek term klastos, which means 'broken'. Rocks of clastic texture
exhibit a mass of discrete particles and grains which are cemented
together but retain their individual shape. All detrital sedimentary
rocks display clastic texture, while some chemical sedimentary rocks
display clastic texture and others display nonclastic texture. Chemical
sedimentary rocks such as halite (NaCl) and sylvite
(KCl) which are deposited when seawater evaporates possess
a nonclastic texture. Rocks such as calcite, which is formed as aragonite
recrystallizes, typically have a nonclastic texture composed of
interlocking crystals.