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:

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.

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.

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:

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.

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.

slow cooling - few large crystals produced by growth rate greater than nucleation rate
rapid cooling - many small crystals produced by nucleation rate greater than growth rate
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)

high silica melts are viscous and crystallize at low temperatures (<850ûC). Ions have difficulty migrating through liquid and organizing into crystals
low silica melts are fluid (low viscosity) and have high temperatures (850û-1200ûC). Ions easily migrate through liquid and organize into crystals
higher silica content, the higher the viscosity
(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

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.

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.

Vitric or glassy texture indicates rapid cooling or quenching of the magma, best exemplified by obsidian or high-silica (rhyolite) glass.

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.

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).

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.

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 Li-mica, 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.

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.

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.

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.

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, water-rich magmatic fluids.

In addition to composition, pyroclastic rocks are further subdivided according to fragment size and type:

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.
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:

Geometry of intrusion:
size
shape
Relationship to surrounding rocks:
concordant or boundaries parallel to layering in surrounding rocks
discordant or boundaries cut across layering in surrounding rocks

Tabular Plutons
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.
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 .
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
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
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

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.

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.
Mechanisms of Magma Emplacement
Batholith are found in the roots of mountain systems, and have been suggested to form by:

Granitization - an exxtreme type of metamorphism by which country rock is altered to granitic composition by ion-rich solutions. Contact with country rock is gradational

Forceful Injection - Country rock is deformed and forced aside as viscous magma slowy rises buoyantly. Magma may also move upward by stoping or detaching and engulfing pieces of the country rock. Xenoliths are unmelted remnants of the surrounding rock found in the upper parts of intrusive bodies. Contact with country rock is sharp. Most batholiths show characteristics consistent with an injection emplacement.