**ROCKS\
ROCK**

**by**

**Igneous rocks** (etymology from Latin ignis, fire) are rocks formed by
solidification of cooled magma (molten rock), with or without
crystallization, either below the surface as intrusive (plutonic) rocks
or on the surface as extrusive (volcanic) rocks. This magma can be
derived from partial melts of pre-existing rocks in either the Earth\'s
mantle or crust. Typically, the melting is caused by one or more of the
following 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 formed beneath the surface of the
Earth\'s crust. These have diverse properties, depending on their
composition and how they were formed.

## Geologic significance

Igneous rocks make up approximately ninety-five percent of the upper
part of the Earth\'s crust, but their great abundance is hidden on the
Earth\'s surface by a relatively thin but widespread layer of
sedimentary and metamorphic rocks.

Igneous rocks are geologically important because:

1.  their minerals and global chemistry give information about the
    composition of the mantle, from which some igneous rocks are
    extracted, and the temperature and pressure conditions that allowed
    this extraction, and/or of other pre-existing rock that melted

2.  their absolute ages can be obtained from various forms of
    radiometric dating and thus can be compared to adjacent geological
    strata, allowing a time sequence of events

3.  their features are usually characteristic of a specific tectonic
    environment, allowing tectonic reconstitutions (see plate tectonics)

## Morphology and setting

In terms of modes of occurrence, igneous rocks can be either intrusive
(plutonic), extrusive (volcanic) or hypabbysal.

### Intrusive igneous rocks

Intrusive igneous rocks are formed from magma that cools and solidifies
within the earth. Surrounded by pre-existing 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 rocks can also be classified according to the shape
and size of the intrusive body and its relation to the other formations
into which it intrudes. Typical intrusive formations are batholiths,
stocks, laccoliths, sills and dikes. The extrusive rocks often produce
lava flows.

### Extrusive igneous rocks

Extrusive igneous rocks are formed at the Earth\'s surface as a result
of the partial melting of rocks within the mantle and crust.

The melt, with or without suspended crystals and gas bubbles, is called
magma. Magma rises because it is less dense than the rock from which it
was created. When it reaches the surface, magma extruded onto the
surface either beneath water or air, is called lava. Eruptions of
volcanoes into air are termed *subaerial* whereas those occurring
underneath the ocean are termed *submarine*. Black smokers and mid-ocean
ridge [basalt](http://en.wikipedia.org/wiki/Basalt) are examples of
submarine volcanic activity.

The volume of extrusive rock erupted annually by volcanoes varies with
plate tectonic setting. Extrusive rock is produced in the following
proportions:[^\[1\]^](http://en.wikipedia.org/wiki/Igneous#cite_note-0%23cite_note-0)

1.  divergent boundary: 73%

2.  convergent boundary (subduction zone): 15%

3.  hotspot: 12%

### Hypabbysal igneous rocks

Hypabbysal igneous rocks are formed at a depth in between the plutonic
and volcanic rocks.

## Classification

Igneous rocks are classified according to mode of occurrence, texture,
mineralogy, chemical composition, and the geometry of the igneous body.

In a simplified classification, 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 in
composition) are silica-oversaturated. Rocks with feldspathoids are
silica-undersaturated, because feldspathoids cannot coexist in a stable
association with quartz.

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.

### Texture

Main article: Rock microstructure

Texture is an important criterion for the naming of volcanic rocks. 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.

However, the texture is only a subordinate part of classifying volcanic
rocks, as most often there needs to be chemical information gleaned from
rocks with extremely fine-grained groundmass or from airfall tuffs,
which may be formed from volcanic ash.

Textural criteria are less critical in classifying intrusive rocks where
the majority of minerals will be visible to the naked eye or at least
using a hand lens, magnifying glass or microscope. Plutonic rocks tend
also to be less texturally varied and less prone to gaining structural
fabrics. Textural terms can be used to differentiate different intrusive
phases of large plutons, for instance porphyritic margins to large
intrusive bodies, porphyry stocks and subvolcanic dikes (apophyses).
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\".

see also List of rock textures and Igneous textures

### Chemical classification

Igneous rocks can be classified according to chemical or mineralogical
parameters:

Chemical: total alkali-silica content (TAS diagram) for volcanic rock
classification used when modal or mineralogic data is unavailable:

acid igneous rocks containing a high silica content, greater than 63%
SiO2 (examples granite and rhyolite)

intermediate igneous rocks containing between 52 - 63% SiO2 (example
andesite and dacite)

basic igneous rocks have low silica 45 - 52% and typically high iron -
magnesium content (example gabbro and basalt)

ultrabasic igneous rocks with less than 45% silica. (examples picrite
and komatiite)

alkalic igneous rocks with 5 - 15% alkali (K2O + Na2O) content or with a
molar ratio of alkali to silica greater than 1:6. (examples phonolite
and trachyte)

Note: the acid-basic terminology is used more broadly in older
(generally British) geological literature. In current literature
felsic-mafic roughly substitutes for acid-basic.

Chemical classification also extends to differentiating rocks which are
chemically similar according to the TAS diagram, for instance;

Ultrapotassic; rocks containing molar K2O/Na2O \>3

Peralkaline; rocks containing molar (K2O + Na2O)/ Al2O3 \>1

Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 \<1

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. For instance, normative quartz classifies a
rock as silica-oversaturated; an example is rhyolite. A normative
feldspathoid classifies a rock as silica-undersaturated; an example is
nephelinite.

### History of classification

In 1902 a group of American petrographers proposed that all existing
classifications of igneous rocks should be discarded and replaced by a
\"quantitative\" classification based on chemical analysis. They showed
how vague and often unscientific was much of the existing terminology
and argued that as the chemical composition of an igneous rock was its
most fundamental characteristic it should be elevated to prime position.

Geological occurrence, structure, mineralogical constitution, the
hitherto accepted criteria for the discrimination of rock species were
relegated to the background. The completed rock analysis is first to be
interpreted in terms of the rock-forming minerals which might be
expected to be formed when the magma crystallizes, e.g., quartz
feldspars, olivine, akermannite, feldspathoids, magnetite, corundum and
so on, and the rocks are divided into groups strictly according to the
relative proportion of these minerals to one another.\[2\] \[3\]

**Sedimentary rock** is one of the three main rock groups (the others
being igneous and metamorphic rock). Rock formed from sediments covers
75-80% of the Earth\'s land area, and includes common types such as
chalk, limestone, dolomite, sandstone, conglomerate and shale.\[1\]

Sedimentary rocks are classified by the source of their sediments, and
are produced by one or more of;\
clastic rock formed from fragments broken off from parent rock, by
weathering in situ or erosion by water, ice or wind, followed by
transportation of sediments, often in suspension, to the place of
deposition; biogenic activity; or precipitation from solution.

The sediments are then compacted and converted to rock by the process of
lithification.

## Formation

Sedimentary rocks are formed because of the overburden pressure as
particles of sediment are deposited out of air, ice, wind, gravity, or
water flows carrying the particles in suspension. As sediment deposition
builds up, the overburden (or \'lithostatic\') pressure squeezes the
sediment into layered solids in a process known as lithification (\'rock
formation\') and the original connate fluids are expelled. The term
diagenesis is used to describe all the chemical, physical, and
biological changes, including cementation, undergone by a sediment after
its initial deposition and during and after its lithification, exclusive
of surface weathering.

Sedimentary rocks are laid down in layers called beds or strata. That
new rock layers are above older rock layers is stated in the principle
of superposition.There are usually some gaps in the sequence called
unconformities. These represent periods in which no new sediments were
being laid down, or when earlier sedimentary layers were raised above
sea level and eroded away.

Sedimentary rocks contain important information about the history of
Earth. They contain fossils, the preserved remains of ancient plants and
animals. Coal is considered a type of sedimentary rock. The composition
of sediments provides us with clues as to the original rock. Differences
between successive layers indicate changes to the environment which have
occurred over time. Sedimentary rocks can contain fossils because,
unlike most igneous and metamorphic rocks, they form at temperatures and
pressures that do not destroy fossil remains.

The sedimentary rock cover of the continents of the Earth\'s crust is
extensive, but the total contribution of sedimentary rocks is estimated
to be only 5% of the total. As such, the sedimentary sequences we see
represent only a thin veneer over a crust consisting mainly of igneous
and metamorphic rocks.

## Classification

Sedimentary rocks are classified into three groups. These groups are
clastic, chemical precipitate and biochemical or biogenic.

### Clastic

Clastic sedimentary rocks are composed of discrete fragments or clasts
of materials derived from other rocks. They are composed largely of
quartz with other common minerals including feldspar, amphiboles, clay
minerals, and sometimes more exotic igneous and metamorphic minerals.

Clastic sedimentary rocks, such as breccia or sandstone, were formed
from rocks that have been broken down into fragments by weathering,
which then have been transported and deposited elsewhere.

Clastic sedimentary rocks may be regarded as falling along a scale of
grain size, with shale being the finest with particles less than 0.002
mm, siltstone being a little bigger with particles between 0.002 to
0.063 mm, and sandstone being coarser still with grains 0.063 to 2 mm,
and conglomerates and breccias being more coarse with grains 2 to 263
mm. Breccia has sharper particles, while conglomerate is categorized by
its rounded particles. Particles bigger than 263 mm are termed blocks
(angular) or boulders (rounded). Lutite, Arenite and Rudite are general
terms for sedimentary rock with clay/silt-, sand- or
conglomerate/breccia-sized particles.

### Organic

Outcrop of Ordovician oil shale (kukersite), northern Estonia. Organic
sedimentary rocks contain materials generated by living organisms, and
include carbonate minerals created by organisms, such as corals,
mollusks, and foraminifera, which cover the ocean floor with layers of
calcite which can later form limestone. Other examples include
stromatolites, the flint nodules found in chalk (which is itself a
biochemical sedimentary rock, a form of limestone), and coal and oil
shale (derived from the remains of tropical plants and subjected to
pressure).

### Chemical

Chemical sedimentary rocks form when mineral solutions, such as sea
water, evaporate. Examples include the evaporite minerals halite (rock
salt) and gypsum.

## Economic and scientific relevance

Sedimentary rocks are economically important in that they can easily be
used as construction material because they are soft and easy to cut. For
example, the White House in Washington DC is made of sandstone. In
addition, sedimentary rocks often form porous and permeable reservoirs
in sedimentary basins in which petroleum and other hydrocarbons can be
found (see Bituminous rocks).

The shape of the particles in sedimentary rocks has an important effect
on the ability of micro-organisms to colonize them. This interaction is
studied in the science of geomicrobiology. One measure of the shape of
these particles is the roundness factor, also known as the Krumbein
number after the geologist W. C. Krumbein.

Metamorphic rock is the result of the transformation of a pre-existing
rock type, the protolith, in a process called metamorphism, which means
\"change in form \".The protolith is subjected to heat and pressure
(temperatures greater than 150 to 200 °C and pressures of 1500
bars\[1\]) causing profound physical and/or chemical change. The
protolith may be sedimentary rock, igneous rock or another older
metamorphic rock. Metamorphic rocks make up a large part of the Earth\'s
crust and are classified by texture and by chemical and mineral
assemblage (metamorphic facies). They may be formed simply by being deep
beneath the Earth\'s surface, subjected to high temperatures and the
great pressure of the rock layers above. They can be formed by tectonic
processes such as continental collisions which cause horizontal
pressure, friction and distortion. They are also formed when rock is
heated up by the intrusion of hot molten rock called magma from the
Earth\'s interior.

Some examples of metamorphic rocks are gneiss, slate, marble and schist.

Metamorphic minerals

Metamorphic minerals are those that form only at the high temperatures
and pressures associated with the process of metamorphism. These
minerals, known as index minerals, include sillimanite, kyanite,
staurolite, andalusite, and some garnet.

Other minerals, such as olivines, pyroxenes, amphiboles, micas,
feldspars, and quartz, may be found in metamorphic rocks, but are not
necessarily the result of the process of metamorphism. These minerals
formed during the crystallization of igneous rocks. They are stable at
high temperatures and pressures and may remain chemically unchanged
during the metamorphic process. However, all minerals are stable only
within certain limits, and the presence of some minerals in metamorphic
rocks indicates the approximate temperatures and pressures at which they
were formed.

Foliation

The layering within metamorphic rocks is called foliation (derived from
the Latin word folia, meaning \"leaves\"), and it occurs when rock is
being compressed from one direction to form a recrystallizing rock. This
causes the platy or elongated crystals of minerals, such as mica and
chlorite, to grow with their long axes perpendicular to the direction of
the force. This results in a banded, or foliated, rock, with the bands
showing the colors of the minerals that formed them.

Rocks that were subjected to uniform pressure from all sides, or those
which lack minerals with distinctive growth habits, will not be
foliated. Slate is an example of a very fine-grained, foliated
metamorphic rock, while phyllite is coarse, schist coarser, and gneiss
very coarse-grained. Marble is generally not foliated, which allows its
use as a material for sculpture and architecture.

Types of metamorphism

Contact metamorphism

Contact metamorphism is the name given to the changes that take place
when magma is injected into the surrounding solid rock (country rock).
The changes that occur are greatest wherever the magma comes into
contact with the rock because the temperatures are highest at this
boundary and decrease with distance from it. Around the igneous rock
that forms from the cooling magma is a metamorphosed zone called a
contact metamorphism aureole. Aureoles may show all degrees of
metamorphism from the contact area to unmetamorphosed (unchanged)
country rock some distance away. The formation of important ore minerals
may occur by the process of metasomatism at or near the contact zone.

Regional metamorphism

Regional metamorphism is the name given to changes in great masses of
rock over a wide area. Rocks can be metamorphosed simply by being at
great depths below the Earth\'s surface, subjected to high temperatures
and the great pressure caused by the immense weight of the rock layers
above. Much of the lower continental crust is metamorphic, except for
recent igneous intrusions. Horizontal tectonic movements such as the
collision of continents create orogenic belts, and cause high
temperatures, pressures and deformation in the rocks along these belts.
If the metamorphosed rocks are later uplifted and exposed by erosion,
they may occur in long belts or other large areas at the surface. The
process of metamorphism may have destroyed the original features that
could have revealed the rock\'s previous history. Recrystallization of
the rock will destroy the textures and fossils present in sedimentary
rocks. Metasomatism will change the original composition.