Wikipedia for Geological mapping
Rock Cycle:
Igneous and metamorphic rocks can become physically ground into small pieces by wind, water and
abrasion (rock slides, faulting) and then compacted/cemented to form sedimentary rocks. (mechanical
breakdown, deposition, sedimentation)
Igneous and sedimentary rocks can be heated and squeezed (pressure) to form metamorphic rocks. This
usually occurs over large areas. (regional metamorphism)
Low grade metamorphic rocks (rocks metamorphosed with low temperature and pressure) can be
cooked and squeezed more intensively into a metamorphic rock of higher grade. The texture and
mineralogy will change so as to change the rock’s appearance.
The original rock type, the temperature (at least 200C), pressure and duration all together determine
which metamorphic rock will result. (metamorphic facies)
Any of the three types of rocks can be drawn deep into the earth (subduction) and mixed with magma.
They melt in with the magma and may later form plutons of solid rock below surface (plutonic rocks),
intrude into cold rock that has cracks (faults) to form dikes or intrude parallel to bedding planes to form
sills (intrusive rocks). They can also ooze out as lava or explode out as a volcanic eruption (volcanic
rocks). Thick lava explodes out with gases while thin lava flows like a river.
The above processes taken together are called the “rock cycle”. (rock cycle)
All three rock types, when intruded by very hot fluids or magma, will produce a left and right border
zone of special high Temperature/ low Pressure metamorphic rocks (skarn, hornfels). These zones are
called “chilled margins” and their width varies depending on the size of the intrusion and the type of
rock that was intruded. (contact metamorphism)
Unique minerals that can only form at very high temperature and low pressure are found in these
border zones, some of which have economic value. Even a young granite magma intruding older granite
can lead to interesting mineralogy!
Hot fluid containing silica (quartz), water, sulfur, calcium, iron and many other elements plus gases in
solution can move quietly through rock cracks, cooling to form quartz veins. (metasomatism,
hydrothermal veins, hydrothermal ore deposits). These veins can be “barren”( quartz and maybe some
calcite) or contain a variety ofminerals. The heat and chemistry of the fluid may alter the surrounding
rock (wall rock alteration). Potassic alteration looks pink or red. Sericitic alteration is a thin layer of shiny
looking mica. Chlorite alteration looks dark green while epidote alteration looks a bright apple green.
These alteration minerals can sometimes be with visible or microscopic gold.
Note: Steam under great pressure can be much hotter than 100C and can carrymany chemical
components in solution. Thehigh temperature and pressure allows solution or suspension. As the fluids
cool, components come out of solution as metals and minerals.
Igneous rocks
Igneous rock is formed through the cooling of magma or lava. Igneous rock may form below the surface
as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can come from
partial melts of pre-existing rocks in a planet's mantle or crust.
Typically, the melting is caused by one or more of three processes: an increase in temperature, a
decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described,
most of them having formed beneath the surface of Earth's crust. (Geochemistry)
In some special circumstances they host important mineral deposits (ores): for example, tungsten, tin,
and uranium are commonly associated with granites and diorites, whereas ores of chromium and
platinum are commonly associated with gabbros. (metallogenic ore deposits)
Feldspars, quartz OR feldspathoids, olivines, pyroxenes, amphiboles and micas are all
important minerals in the classification of these rocks. All other minerals present are called
accessory minerals. Igneous rock types are separated on the basis of the type of feldspar present,
the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or
magnesium minerals present. Rocks containing quartz (silica) are silica-oversaturated. Rocks
with feldspathoids are silica-undersaturated. Feldspathoids can never be with quartz in the same
rock.
Igneous rocks which have crystals large enough to be seen by the naked eye are called
phaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking,
phaneritic implies an intrusive origin; aphanitic an extrusive one.
An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is
termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable
size before the main mass of the magma crystallizes as finer-grained, uniform material.
Extrusive igneous rocks begin forming as a result of the partial melting of rocks within the
mantle and crust. They cool faster than intrusive igneous rocks and so are fine grained. The
magma which is brought to the surface through fissures or volcanic eruptions solidify at a faster
rate. Hence such rocks are smooth, crystalline and fine grained. They are also called volcanic
rocks. Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava
plateaus. The melted rock is called magma. It rises because it is less dense than the rock from
which it was created. When magma reaches the surface it is called lava.The volume of extrusive
rock erupted annually by volcanoes varies with plate tectonic setting.
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divergent boundary: 73%
convergent boundary (subduction zone): 15%
hotspot: 12%.
Magma which erupts from a volcano behaves according to its viscosity, determined by
temperature, composition, and crystal content. (Water has low viscosity and so it flows easily,
honey and oil are higher viscosity)
High-temperature magma, most of which is basaltic in composition, behaves in a manner similar
to thick oil and can look like rope when cold. Intermediate composition magma such as andesite
tends to form cinder cones of intermingled ash, tuff and lava, and may behave like cold honey or
even rubber when erupted. Felsic magma such as rhyolite is usually erupted at low temperature
and is up to 10,000 times more viscous than basalt. This commonly erupts explosively, and
rhyolitic lava flows typically are of limited extent and have steep margins, because the magma is
so thick. Felsic and intermediate magmas can have explosions driven by release of dissolved
gases—typically water but also carbon dioxide. Explosively erupted pyroclastic material is called
tephra and includes tuff, agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms
ash tuff deposits which can often cover vast areas.
Because lava cools and crystallizes fast, the rock is fine grained. If the cooling has been very
rapid the resulting rock may be mostly glass. (obsidian).
Because the minerals are mostly fine-grained, it is hard to name some volcanic rocks by hand
lens. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be
determined by examination of thin sections of the rock under a microscope, so only an
approximate classification can usually be made in the field.
Intrusive rocks
Formed from magma that cools and solidifies within the crust of a planet. Surrounded by preexisting rock (called country rock), the magma cools slowly, and as a result these rocks are
coarse grained. The mineral grains in such rocks can generally be identified with the naked eye.
Intrusive bodies of rock are named by the shape and size and also its relation to the other
formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths,
sills and dikes. The magma cools slowly forming coarse textured rocks such as granite, gabbro,
or diorite.
The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite.
When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's
surface.
Deep intrusions are called abyssal or simply plutons. Shallow ones near the surface are termed
hypabyssal. Hypabyssal rocks (shallow intrusions) are less common than plutonic and volcanic rocks
and often form dikes, sills, laccoliths, lopoliths, or phacoliths.
The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartz
diorite. Generally light colored, coarse-grained plutons of these compositions are referred to as
granitoids because with a hand lens all of the above rocks look the same..
Texture
Gabbro specimen showing phaneritic texture; Rock Creek Canyon, eastern Sierra Nevada,
California; scale bar is 2.0 cm.
Main article: Rock microstructure
Texture is used for the naming of volcanic rocks but for complete and accurate naming,
chemical information must also be known. The texture of volcanic rocks, including the size,
shape, orientation, and distribution of mineral grains and the intergrain relationships, will
determine whether the rock is termed a tuff, a pyroclastic lava or a simple lava.
Texture is less important in classifying intrusive rocks as many minerals will be visible with a
hand lens. The texture is usually phaneritic. Textural terms can be used to differentiate different
intrusive phases of large plutons, for instance porphyritic margins to large intrusive bodies,
porphyry stocks and aplitic (sugary) texture (subvolcanicdikes) (apophyses).
REMEMBER: Mineralogical classification is used most often to classify plutonic rocks.
Chemical classifications are preferred to classify volcanic rocks, with phenocryst species
used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".
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see also List of rock textures and Igneous textures
Basic classification scheme for igneous rocks on their mineralogy. This is not an exact method
because the classification of igneous rocks also depends on other components than silica, yet in
most cases it is a good first guess.
Chemical classification
Total alkali-silica content (TAS diagram) for volcanic rock classification used when modal or
mineralogical data is unavailable:
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felsic igneous rocks containing a high silica content, greater than 63% SiO2 (examples
granite and rhyolite)
intermediate igneous rocks containing between 52 – 63% SiO2 (granodiorite/diorite)
(andesite /dacite)
mafic igneous rocks have low silica 45 – 52% and typically high iron – magnesium
content (example gabbro and basalt)
An idealized mineralogy (the normative mineralogy) can be calculated from the chemical
composition, and the calculation is useful for rocks too fine-grained or too altered for
identification of minerals that crystallized from the melt.
Mineralogical classification
For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important
criterion is the phenocryst species, followed by the groundmass mineralogy. Often, where the
groundmass is aphanitic, chemical classification must be used to properly identify a volcanic
rock. This can only be done in a special laboratory.
Mineralogic contents – felsic versus mafic
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felsic rock, highest content of silicon, with predominance of quartz, alkali feldspar and/or
feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light
coloured, and have low density.
mafic rock, lesser content of silicon relative to felsic rocks, with predominance of mafic
minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro)
are usually dark coloured, and have a higher density than felsic rocks.
For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least
via microscope, the mineralogy is used to classify the rock. The following table is a simple
subdivision of igneous rocks according both to their composition and mode of occurrence.
Mode of occurrence Felsic
Composition
Intermediate Mafic Ultramafic
Intrusive
Granite Diorite
Gabbro Peridotite
Extrusive
Rhyolite Andesite
Basalt Komatiite
Essential rock forming silicates
Felsic
Intermediate Mafic
Ultramafic
Coarse Grained Granite Diorite
Gabbro Peridotite
Medium Grained
Diabase
Fine Grained
Rhyolite Andesite
Basalt
Komatiite
For a more detailed classification see QAPF diagram.
Example of classification
Granite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica
and predominately quartz plus potassium-rich feldspar plus sodium-rich plagioclase) and a
phaneritic texture.
Sedimentary rocks
Deposition
Sedimentation
Metamorphic rocks
Metamorphic grade
Metamorphic facies
Alteration
Metamorphism
Contact metamorphism
quartz veins
Gold
Metasomatism
Hydrothermal ore deposits
Panning
Mineral paragenesis
Mineral associations
Geology of Cambodia
Topics in Geology
Coal and Coal formation
Granite
Granular and phaneritic in texture.Mainlyquartz, mica, and feldspar. Occasionally some individual crystals
(phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A
granitic rock with a porphyritic texture is called a granite porphyry or porphyritic granite.
Granites can be pink to gray in color, depending on their chemistry and mineralogy.Granites have at
least 20% quartz. Granite differs from granodiorite because at least 35% of the feldspar in granite is alkali
feldspar. This gives many granites a pink color. Outcrops of granite tend to form tors and
rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills,
formed by the metamorphic aureole or hornfels.
The average density of granite is 2.65 -> 2.75 g/cm3 and the melting temperature is 1215 - 1260 °C.
Granitoid is a general, descriptive field term for light-colored, coarse-grained igneous
rocks. Petrographic examination with microscope is required for identification of specific types of
granitoids.
When a granitoidhas noplagioclase the rock is referred to as alkali granite. When a granitoid contains
less than 10% orthoclase, it is called tonalite; pyroxene and amphibole are common in tonalite. A granite
containing both muscovite and biotite micas is called a two-mica granite. Two-mica granites are typically
high in potassium and low in plagioclase. The volcanic equivalent of plutonic granite is rhyolite.
Granite forms a major part of continental crust. Granite often occurs as relatively small, less than 100 km²
stock masses (stocks) and in batholiths that are often associated with orogenic mountainranges.
Small dikes of granitic composition called aplites are often associated with the margins of
granitic intrusions. In some locations, very coarse-grained pegmatite masses occur with granite.
Granite has intruded into the crust of the Earth during all geologic periods, but most of it is
of Precambrian age. Granitic rock is widely distributed throughout the continental crust and is the most
abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.
Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that have
compositions at or near a eutectic point (or a temperature minimum on a cotectic curve). Magmas will
evolve to the eutectic because of igneous differentiation, or because they represent low degrees of partial
melting.
Fractional crystallisation serves to reduce a melt in iron, magnesium, titanium, calcium and sodium, and
enrich the melt in potassium and silicon - alkali feldspar (rich in potassium) and quartz (SiO2), are two of
the defining constituents of granite.
This process operates regardless of the origin of the parental magma to the granite, and regardless of its
chemistry. However, the composition and origin of the magma which differentiates into granite, leaves
certain geochemical and mineral evidence as to what the granite's parental rock was. The final
mineralogy, texture and chemical composition of a granite is often distinctive as to its origin. For instance,
granite which is formed from melted sediments may have more alkalifeldspar, whereas a granite derived
from melted basalt may be richer in plagioclase feldspar. It is on this basis that the modern "alphabet"
classification schemes (QAPF diagram) are based. Granite has a slow cooling process which forms larger
crystals.
Diorite
Grey to dark grey intermediate intrusive igneous rock composed principally
of plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. It may contain small
amounts of quartz, microcline and olivine.
Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory minerals. It can also be black
or bluish-grey, and frequently has a greenish cast. Diorite with little hornblende and other dark minerals is
called leucodiorite. Whenolivine and more iron-rich augite (a pyroxene mineral) are present, the rock
grades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makes the rock
type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassiumfeldspar) is
present at greater than ten percent the rock type grades into monzodiorite or granodiorite. Diorite has a
medium grain size texture, sometimes porphyritic.
Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge.
Diorite results from partial melting of a mafic rock above a subduction zone. It is commonly produced
in volcanic arcs, and in cordilleran mountain building such as in the Andes Mountains as large batholiths.
The extrusive volcanic equivalent rock type is andesite. Diorite is a relatively rare rock and much harder
than granite.
Gabbro
A group of dark, coarse-grained, intrusive mafic igneous rocks (chemically equivalent to the volcanic rock
basalt). They are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools
into a crystalline mass.
The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by
basalt magmatism at mid-ocean ridges.
Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole,
and olivine (olivine gabbro when olivine is present in a large amount).
The pyroxene is mostly clinopyroxene; small amounts of orthopyroxene may be present. If the amount of
orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then
a norite. Quartz gabbros are also known to occur and are probably derived from magma that was oversaturated with silica. Essexites represent gabbros whose parent magma was under-saturated with silica,
resulting in the formation of the feldspathoid mineral nepheline. (Silica saturation of a rock can be
evaluated by normative mineralogy). Gabbros contain minor amounts of magnetite, ilmenite,
and ulvospinel.
Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained
equivalents of gabbro are called diabase (dykes and sills)or microgabbro. Gabbro may be extremely
coarse grained to pegmatitic, but usually equigranular in texture. May be porphyritic at times, especially
when plagioclase oikocrysts have grown earlier than the groundmass minerals.
Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and
plagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene and
plagioclase.
Gabbroic rocks often contain valuable amounts of chromium, nickel, cobalt, gold, silver, platinum,
and copper sulfides.
Basalt
A common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or
very near the surface of a planet or moon. By definition, basalt is an aphanitic (microscopic grain size)
igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least
65% of the feldspar is in the form of plagioclase. Basalt is usually grey to black in colour, but rapidly
weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals.
Basalts are mostly calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent.
Accessory minerals: iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite.
These oxide minerals cancause some basalts to be slightly magnetic. Weathered, rusty basalts will have
no magnetism.
Rhyolite
Andesite
Dacite
Pegmatite
Sandstone
Limestone
Shale
Siltstone
Mudstone
Greywacke
Quartzite
Marble
Gneiss
Slate
Schist
Phyllite
Skarn
Hornfels
Amphibolite
Pyroxenite
Rock forming minerals
Apatite
Calcite
Dolomite
Quartz
Gypsum
Olivene
HIGH GRADE METAMORPHICS
Garnet
Wollastonite
Sillimanite
Kyanite
Staurolite
Andalusite
Cordierite
PYROXENES
Diopside
Augite
Enstatite
AMPHIBOLES
Tremolite
Actinolite
Hornblende
Riebeckite
Glaucophane
MICAS
Muscovite
Phlogopite
Biotite
Lepidolite
Spodumene
ZEOLITES
Stilbite
Natrolite
FELDSPARS
Orthoclase
Microcline
sanidine
Albite-Anorthite series (Plagioclase)
ACCESSORY MINERALS
Corundom (Ruby, Sapphire)
Diamond
Spinel
Rutile
Zircon
Tourmaline
Aragonite
Humite
Chloritoid
Antigorite
Pyrophyllite
Montmorillonite
SULFIDES
Pyrite
Galena
Sphalerite
Chalcopyrite
Arsenopyrite
Molybdenite
ALTERATION MINERALS
Chlorite
Epidote
Sericite
Hematite
Limonite
Leucoxene
Economic minerals
Bauxite
Fluorite
Talc
Allanite
Rhodonite
Pyrolusite
Titanite (sphene)
Ilmenite
Magnetite
Scheelite
Bornite
Monazite
Magnesite
Vermiculite
Barite
Celestite
Kaolinite