The Three Groups of Rock

Rock itself is both construction materials (e.g. for fill and reclamation materials) or part of engineering structure (e.g. in cutting of slope, stability of foundation and excavation of tunnel). It is formed thousands of years through the volcanic activity (volcanism) with source of Earth internal heat (magma and lava flow). According to its manner of formation, rocks are divided into three major groups – Igneous rock, Sedimentary rock, and Metamorphic rock.

The diagram below represent on how the rock was formed:

Terms as above:

• Crystallization happens when magma or lava cools, and solidifies into igneous rock forms.
• Lithification defined as the transforming sediment to sedimentary rocks which involved of compaction and/or cementation. * Note: Compaction – from weight of overlaying sediment, reduce pore spaces and volume. Cementation – consists of chemically precipitated minerals in pore spaces that bind the particles; e.g calcium carbonate (CaCO3), silicon dioxide (SiO2).
• Melting of rock from beneath Earth’s surface to form magma.
• Metamorphism defined as the process when transformation of pre-existing rocks (igneous, sedimentary, metamorphic rock). Examples: Granite to Gneiss, Shale to Slate, and Slate to Schist.
• Weathering defined as the process of breaking down rock that is transported and deposited as sediment.

Igneous Rock
Igneous rock is the magma solidifies by crystallizing into a mosaic of minerals. There are two types:

• Intrusive igneous rocks – magma solidifies below the surface of the earth, may later be exposed at the surface when the cover rocks are eroded away.
• Extrusive igneous rocks – form from the magma which is extruded onto the earth surface to create a volcano.

The main composition of igneous rock – silica minerals (olivine, pyroxene, feldspars, quartz). Igneous textures often referred to size, shape and arrangement of mineral grains. The engineering properties of igneous contribute by its strength, porosity and deformation behavior.

Igneous Rock samples as above:
(1) OBSIDIAN: Volcanic grass formed from quick-cooling lavas.
(2) PUMICE: Light frothy lava containing so many air cells that is floats readily on water.
(3) SCORIA: Volcanic slag formed like Pumice but having fewer and larger cavities.
(4) BASALT: Very fine-grained and dark colored rock.
(5) RHYOLITE (FELSITE): Very fine-grained and light colored rock.
(6) RHYOLITE PORPHYRY: Small crystals of Quartz and Feldspar.
(7) BIOTITE GRANITE: Feldspar, Quartz and black Biotite Mica.
(8) PEGMATITE: Coarse-grained granite, contains Quartz and Feldspar.
(9) SYENITE: Looks like granite, but no Quartz.
(10) DIORITE (TRAP ROCK): Medium and dark colored rock.
(11) GABBRO: Coarser grained dark colored rock (than Diorite).
(12) PERIDOTITE: Usually occurs in dikes or in pipe-like intrusions.

Sedimentary Rock
Sedimentary rocks are either termed as clastic or non-clastic, as determined by their formation. Clastic sedimentary rocks are formed from the compaction and cementation of rock minerals and/or organic matter. Whereas, non-clastic sedimentary rocks are the product of a chemical process – typically the precipitation of salts dissolved in groundwater.

Sedimentary rocks – composed of sediment (derived by weathering process, eroded from weathering site, transported elsewhere and deposited as a loose aggregate of particles) . Sediments accumulate at earth’s surface – contain evidence of past events, e.g environment of particles deposited, mechanisms involved in transported and fossils to study geologic past.

Sedimentary rock considered an important resource in economic – coal, petroleum, and sources of iron.

Materials accumulating as sediment have two principle sources:

• Detrital rock – made up of solid particles from parent material by mechanical and chemical weathering.
• Chemical rock – derive from material that is carried in solution – does not remain dissolved in water, some is precipitate to form chemical sediments.

Sedimentary Rock samples as above:
(13) CHERT: Dense and extremely fine crystalline (cryptocrystalline) rock.
(14) SILTSTONE: Thinly bedded clastic rock contains quartz and silt size grains.
(15) COQUINA: Formed of shell and shell fragments (fossil debris).
(16) QUARTZ CONGLOMERATE: Typical conglomerate with quartz gravel and quartz sand composed.
(17) RED SANDSTONE: Consolidated sand contains iron.
(18) ARGILLACEOUS (CLAYEY) SANDSTONE: Consolidated of fine sand and clay.
(19) ARGILLACEOUS SHALE: A solidified clay.
(20) BITUMINOUS SHALE: Black shales separate many coal beds.
(21) SHELL LIMESTONE: Composed of very many fossil shells.
(22) DOLOMITIC LIMESTONE: Composed of the mineral Dolomite.
(23) TRAVERTINE: Cave roofs water drips formed stalactites with deposit lime.
(24) GYPSUM: Composed of mineral Gypsum (very soft) and manufactured as plaster.

Metamorphic Rock
Geological metamorphism is the process of a rock changing composition and texture by heat and pressure (without melting). This can be achieved through increases in temperature from the underlying magma rising and extreme overburden pressure or tectonic plate movement. During metamorphism, new minerals are formed and mineral grains are sheared creating a foliated texture. Foliation is where platy minerals develop a parallel orientation.

There are three main types of metamorphism, these are:

• Dynamic Metamorphism, where an increase in pressure in the dominant cause
• Thermal or Contact Metamorphism, where an increase in temperature is the dominant cause. This is most likely due to heat created by magma intrusions
• Regional Metamorphism, where increases in temperature and pressure generated on a regional scale. This is most likely due to the heat created by magma intrusions and the high stresses developed during the formation of mountains / hills or tectonic plate.

Agent of metamorphic rocks:

• Heat – ­ rate of chemical reactions which produce minerals different from original rock, and from intrusive magma / result of deep burial.
• Pressure – from weight of overlying rocks.
• Fluid activity – may contain ions in solution, ­ rate of chemical reactions ® enhance metamorphism.

Metamorphic Rock samples as above:
(25) BIOTITE GNEISS: Originally granite and transformed by pressure.
(26) GRANITOID GNEISS: Was formerly a granite.
(27) SCHIST: Dark and light colored micas or chlorite, gives glistening appearance.
(28) GARNETIFEROUS MICA SCHIST: Numerous garnet crystals.
(29) HORNBLENDE SCHIST: Formerly dark colored igneous rock.
(30) QUARTZITE: Was once a sandstone and transformed by pressure.
(31) RED SLATE: Once a red clay or red shale.
(32) GRAY SLATE: Formerly a black mud or shale containing organic matters.
(33) WHITE MARBLE: White and sugary-textured marble.
(34) PINK MARBLE: Marble were once limestone.
(35) SERPENTINE: Dark green rock formed by metamorphism of peridotites.
(36) SOAPSTONE: Soft rock with slippery feel and composed largely of talc.

With that, these are the three types of rocks by origin that formed our world. This knowledge required and related to Geology and Rock Mechanics in civil engineering. Credit and reference from Ward’s Natural Science.

Read More Constructive Posts:

• 2 Category of Rock Testing
• Rocks in the Forms of Minerals
• Rock Mechanics in Civil Engineering

Tagged as: Rock and Testing

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