Rocks

• Igneous rocks originate from the cooling of the hotter material, magma or lava.
• On the basis of depth of cooling, igneous rocks can be plutonic, hypabyssal or volcanic.
• Depth of cooling determines the texture in deep-seated condition; this is coarsely crystalline; fast cooling on the surface leads to fine-grained texture; and in between at moderate depth, phenocrystalline.
• On the basis of chemical composition, igneous rocks are divided into acid, intermediate, basic and ultrabasic.
• On the basis of mineralogical composition igneous rocks are divided into mafic (composing Mg and Fe) and felsic (composing feldspar and silica).

Classification of Igneous rocks

Geological occurrence	Volcanic	Hypabyssal	Plutonic
Grain Size	Cryptocrystalline or Glassy	Medium Grained	Phaneritic
Silica Percentage		Common Rocks Types
> 66%& acid	Rhyolite,      Dacite	Microgranite  Microgranodiorite	Granite  Granodiorite
52-66%& Intermediate	Trachite,  Andesite	Microsyenite  Microdiorite	Syenite      Diorite
44-52%&Basic, Mafic	Basalt	Dolerite	Gabbro
<44%&Ultrabasic, Ultramafic	Rare	Rare	Peridotite

 

Sedimentary rocks are composed of rock fragments, weathering products, organic material, or precipitates; and most are deposited in beds or layers. They compose only about 8 per cent of the Earth’s crust, although they cover about 75 per cent of the surface.

Lithification is the transformation of sediment into a rock.

Bedding of sedimentary rocks records the layers in order of deposition, with the oldest at the bottom.

Graded bedding is formed by larger fragments settling faster than smaller fragments; bottom is indicated by the larger fragments.

Cross bedding, another current feature, also shows top and bottom.

Clastic rocks are composed of fragments or minerals broken from any type of pre-existing rock; elastic rocks are generally sub-divided by fragment size, not by composition, except for some sandstone types.

Conglomerate-fragments over 2 mm; may be subdivided into roundstone or sharpstone conglomerates (breccia) if fragments rounded or angular, respectively.

Sandstone—fragments between 1/16 and 2 mm. Some compositional types are quartzose sandstone (mainly quartz), arkosic sandstone or arkose (over 20 per cent feldspar), and graywacke (poorly sorted clay or chloritic matrix).

Non-clastic rocks are formed by chemical or biologic precipitation and by accumulation of organic material.

Limestone is composed of calcite, in some cases from chemical precipitation but usually of biologic origin. If composed mainly of fossils, it is called coquina.

Chalk is soft, white limestone, formed by accumulation of the calcium carbonate shells of microscopic animals.

Dolomite is formed by replacement of calcite with dolomite.

Chert is fine-grained silica; some precipitated directly from seawater, most from accumulation of silica shells of organisms.

An evaporite is a deposit formed by evaporation of seawater. The precipitation sequence is calcite, dolomite, gypsum, rock salt, and lastly potassium and magnesium salts. Gypsum and rock salt (halite) are the main rocks formed this way.

Diatomite is soft, white rock, composed of silica remains of microscopic plants.

Coal is formed by accumulation of plant material.

Porosity is a measure of the amount of void space in sediment; permeability is a measure of the inter-connections of the void spaces.

Shale, limestone, and sandstone make up over 99 per cent of all sedimentary rocks. Shale is most abundant because of the abundance of feldspars.

Metamorphism involves changes in mineral assemblage and rock texture occurring in the solid state as a result of changes in temperature and pressure.

Mineral assemblages and rock textures change continually as temperature and pressure change.

Mechanical deformation, recrystallisation and chemical reactions are the processes that affect rock during metamorphism.

The presence of intergranular fluid greatly speeds up metamorphic reaction.

Heat, given off by bodies of intrusive igneous rock, causes contact metamorphism and creates contact metamorphic aureoles.

Thermal or contact metamorphic rocks, called hornfels, generally are found at margins of intrusive igneous bodies.

Regional metamorphic rocks occupy large areas and reveal much uplift and erosion after their formation. They are usually foliated (the new minerals have preferred orientation because of recrystallisation under stress). The most common are gneiss and schist.

Cataclastic metamorphic rocks result from breaking and grinding without much recrystallisation.

Textural groups are (1) foliated—directional or layered aspect, and (2) non-foliated-homogeneous or massive.