Rock Classification

Practically all the material of the Earth is in the form of minerals such as quartz, mica, calcite, feldspar, olivine, kaolin, etc. They are inorganic chemical compounds with constant chemical composition and definite physical and chemical properties. Minerals are crystal compounds made up of one or more elements. Rocks are aggregates of minerals; e.g.,

granite contains the minerals quartz, feldspar and mica

slate consists of quartz, feldspar, mica and ferromagnesians.

Rocks vary in their chemical and physical features depending on the proportion of the different minerals present. Consequently the type of soil formed and the nutrients it contains will be influenced by the nature of the original rocks.

Depending on their mode of origin, all rocks fall into three distinct groups: igneous, sedimentary and metamorphic.

1) Igneous Rocks

All those rocks which originated by the cooling and solidification of hot molten magma are igneous rocks.

They include:

a)Volcanic rocks (extrusive) - extruded onto the earth's surface. Because of their rapid cooling, these rocks are finely crystalline eg basalt or glassy eg obsidian.

Basalt

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Obsidian(http://geology.about.com/od/more_igrocks/ig/obsidian/obsidianrind.-183.htm)

b) Hypabyssal rocks (shallow intrusive) - these rocks are found at intermediate depth. They have cooled slower than the volcanic rocks and their structure is medium grained (often isolated larger crystals in a glassy ground mass). The most common type around Goulburn is dolerite or diabase.

dolerite(http://en.wikipedia.org/wiki/Diabase#mediaviewer/File:Polished_Diabase.jpg )

c) Plutonic rocks (deep intrusive) - these rocks are found at great depth. They have cooled slowly resulting in coarsely grained crystals.GraniteGranite is made of large mineral grains (which is where its name came from) that fit tightly together. It always consists of the minerals quartz and feldspar, with or without a wide variety of other minerals (accessory minerals). The quartz and feldspar generally give granite a light color, ranging from pinkish to white. That light background color is punctuated by the darker

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accessory minerals. Thus classic granite has a "salt-and-pepper" look. The most common accessory minerals are the black mica biotite and the black amphibole hornblende. Almost all granite is igneous (it solidified from a magma) and plutonic (it did so in a large, deeply buried body or pluton). The random arrangement of grains in granite—its lack of fabric—is evidence of its plutonic origin. Some granite like rocks can also be formed as a result of extreme metamorphism.

Granitehttp://geology.about.com/od/more_igrocks/a/granite.htm

Igneous rocks are further classified as:

acidic - light coloured rocks with a high content of silica (over 65% silica),

intermediate - medium coloured rocks with a medium (between 55 - 65%) silica content,

basic - dark coloured rocks with a silica content less than 55%.

(2) Sedimentary Rocks

These are formed by the accumulation and consolidation of: fragments of rock produced . by weathering of pre-existing rocks plant and animal remains or chemical compounds

Sedimentary rocks may be classified as:

(a) Mechanically formed, e.g. conglomerates, sandstones, shales.

These rocks originate from the transportation and deposition of rock fragments produced by weathering and erosion of pre-existing rocks. During this erosion by wind and water, the fragments are mechanically sorted into varying sizes and deposited in lakes (Lacustrine

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deposits), seas (Marine deposits) or on land (Alluvial deposits) giving rise to sediments of varying grain size. These sediments are consolidated, usually under low pressures, and bonded with chemical cements and become sedimentary rocks.

(b) Organically formed, e.g. , coal, limestone, chalk, phosphate rock, petroleum oil.

These rocks are formed when plant and animal remains accumulate in sufficient quantity and are consolidated.

(c) Chemically formed, e.g., Bauxite, laterites. Formed by precipitation of chemical compounds.

Some Sedimentary Rock Types

Conglomerate - rounded fragments (pebbles) of other rocks embedded in soft cement Rocks containing angular fragments are called breccia. Very coarse grained.

Conglomerate rock with rounded fragments(http://geology.about.com/od/more_sedrocks/ig/conglomerates/congl_tumey.htm)

Breccia with angular fragments

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http://geology.about.com/od/rocks/ig/sedrockindex/rocpicbreccia.htm

Sandstone - sand grains in soft cement. Different sandstones depending on the nature of the cementing material. Coarse grained.

Bundanoon sandstone (http://www.bsint.com.au/oursandstone/color.php )Shale - fine silt or clay showing bedding, fine grained.

Leaf fossil embedded in shale (australianmuseum.net.au)

Limestone - compact but soft, consisting of calcium carbonate. Sometimes shows fossils.

http://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/limestone.html

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Sedimentary rocks may be laid down in layers (some centimetres thick) beds or strata (very thick). When there is a difference between two consecutive beds it is called stratification.

Like igneous rocks, the sedimentary rocks have different resistances to weathering. The conglomerates and sandstones weather very slowly. The shales and mudstones act as "storehouses" of finer weathered material, especially clays. Sedimentary rocks are often poorly supplied with minerals - the more valuable minerals were lost in the previous weathering cycle during which the rocks were formed.

3) Metamorphic Rocks

These rocks originate by profound alteration of pre-existing igneous or sedimentary rocks by high temperatures, great pressures and chemical change. The causes of metamorphism may arise from: the weight of overlaying sediments;

pressures exerted by the faults and folding of earth's crust. e.g., shale becomes slate

heat from cooling magma e.g. quartz becomes quartzite and limestone becomes marble.

Some Metamorphic Rocks

Quartzite - very hard, compact, granular or glassy mass, pale yellow to fawn, almost completely quartz derived from sandstone or chert.

Quartzitehttp://geology.about.com/od/rocks/ig/metrockindex/rocpicquartzite.htm#step-heading

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Slate - medium, compact, fine grained, good cleavage (i.e., splitting along definite planes), black to green to purple.

Slate(http://geology.about.com/od/more_metrocks/ig/metamorphics/slate.htm#step-heading)

Marble - crystalline - squarish interlocking grains, pure white marbles composed of calcite, other minerals will give marble its specific colour. May also be derived from dolomite.

(http://geology.about.com/od/rocks/ig/metrockindex/rocpicmarble.htm#step-heading)

Hornfels -very hard, compact, fine or coarse mass of mineral grains, variable chemical composition, formed from granite.

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Hornfels(http://geology.about.com/od/rocks/ig/metrockindex/rocpichornfels.htm#step-heading)

The Rock Cycle Rocks, like mountains, do not last forever. The weather, running water, and ice wear them down. All kinds of rocks become sediment. Sediment is sand, silt, or clay. As the sediment is buried it is compressed and material dissolved in water cements it together to make it into sedimentary rock. If a great amount of pressure is exerted on the sedimentary rock, or it is heated, it may turn into a metamorphic rock. If rocks are buried deep enough, they melt. When the rock material is molten, it is called a magma. If the magma moves upward toward the surface it cools and crystallizes to form igneous rocks. This whole process is called the Rock Cycle.

http://www.minsocam.org/msa/collectors_corner/id/rock_key.htmFor an animated view of the rock cycle see: http://www.learner.org/interactives/rockcycle/diagram.html

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Rock WeatheringMost soils are formed from weathered rocksRock weathering, is a series of processes that break up rock and soil minerals, modify or destroy their chemical and physical characteristics and carry away the soluble products and solid particles.

The three main types of weathering are:

(1) Physical - disintegration.

(2) Chemical - decomposition.

(3) Biological.

All three of these processes are interconnected. They do not work in isolation.

(1) Physical Weathering

This method involves the breaking up of rocks and minerals without any chemical change. Their surface area is increased and so becomes more susceptible to chemical attack.

(a) Changes in Temperature

Minerals in rocks have different expansion and contraction properties. With changes in temperature, the minerals in the rocks cause the rocks to split, and the cracks formed provide centres for further physical and chemical changes. The weathering of rocks in desert soils is predominantly of this type. An everyday example of this effect is the cracking of large concrete slabs if expansion joints are absent.

(b) Frost Action

This action depends on the occurrence of cracks and joints in rocks. During winter, these become filled with water and as ice is formed in these cracks great pressures are exerted causing the rocks to split further. (Water increases about 1/11 in volume when it becomes ice.) What happens when a sealed water-filled glass bottle is put in a freezer?

(c) Glacial movement - Abrasive Action

The movement of glaciers can move large rock masses breaking them down into gravel. Intense grinding results in the formation of rock flour or loess.

(d) Water - Abrasive Action

The action of water flowing over rocks in streams and rivers wears them away and will transport weathered particles long distances. Rocks are eroded away by wave action.

(e) Wind

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Wind acts as a weathering agent by':

rolling particles over surface, and lifting-sand and silt into the atmosphere and carrying them some distance. The blast of sand grains transported by wind erodes away exposed rocks.

Desert soils and soils of the alpine regions am predominantly the result of physical weathering. Soils derived from physical processes are often termed skeletal soils.

(2) Chemical Weathering

Chemical weathering involves alterations to the chemical structure of rock particles. Minerals may disappear, partially or wholly, and a new material is formed which differs markedly from the original mineral. Water is a fundamental requirement for chemical weathering, because the weathering reactions are ionic and ions exist only in solution. An ion is an electrically charged particle formed when a neutral atom loses or gains an electron. If an electron is lost the particle becomes positively charged and is called a cation; If the particle gains an electron it is called an anion and is negatively charged.Chemical weathering is essentially the movement and replacement of ions from one mineral to another. When ions leave a mineral or are replaced by smaller ions, the structure of the mineral may collapse, releasing more ions.

There are five main means of chemical weathering. They are involved processes and so will only be dealt with very briefly.

a) Hydration Hydration is the chemical combination of water with a particular mineral. This is often accompanied by an increase in the volume of the mineral crystal and this in turn may cause the rock to crack. Hydration may affect the colour of a new mineral;

e.g., red haematite yellow limonite

(iron compound) (iron compound)

b) Oxidative processes These processes affect the ferrous (iron) and sulphur compounds in minerals. As a result of oxidation the rocks weaken and easily crumble (e.g., the rusting of steel is an oxidative process).

c) Hydrolysis Hydrolysis is the most important process of chemical weathering. Water is a reactant in a chemical reaction producing a hydroxide of some kind. It could be said that water acts as a very weak acid.

d) Carbonation. Carbon dioxide dissolved in water produces a weak acid, carbonic acid (H2C03). Although it is a weak acid, it occurs in great quantities and acts continuously over extensive time periods.

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greatly assisting decomposition. End products formed from carbonic acid action are carbonates (CO3

2-) and bicarbonates (HCO3-)

Other weak inorganic and organic acids (e.g., sulphuric acid, phosphoric acid and nitric acid) formed from decaying organic matter and atmospheric storms may carry out similar reactions.

e) Solution Chemical weathering takes place at the surface of minerals. Water may dissolve and remove many of the simple products of weathering, exposing fresh faces and allowing the weathering processes to continue.

The rate of chemical weathering is influenced by:

The presence of water. Temperature - chemical reactions usually proceed faster at higher temperature. Surface area of the mineral. The finer the material, the greater the surface area and the greater the rate of weathering. '

When a layer of soil is formed over the parent rock, further weathering is predominantly chemical. Thus the presence of warm water greatly influences further weathering. In hot humid climates soils are formed much more quickly and are dee per than soils formed in cool climates.

(3) Biological Weathering

The presence of vegetation, soil micro-organisms and macro-organisms affect the weathering of minerals by:

Action of plant roots, especially trees In widening cracks and possibility of splitting cracks.

Production of carbon dioxide in respiration.

Furnishing material for soil organic matter. As this decomposes weak acids are produced, aiding the weathering process.

The flaking-off effect of small fragments of rock caused by the alternate wetting and drying of lichen or mosses, etc.

Transport of Weathered Material

Whilst most soils are derived from the parent material below the soil profile some soils are formed from transported parent material.The principal means of transporting weathered material are:

Gravity – On steeper slopes loose material tends to shift downhill and accumulate at the break of slope. Colluvial soils are formed from this material..

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Running water - carries the weathered material in solution, suspension (fine particles), saltation (larger particles) and traction (rolling of pebbles along bed of stream). Alluvial soils are formed from the deposition of water borne material.

Moving Ice – glaciers. The soils formed from glaciation are called Till soils.

Wind. - Aeolian soils are formed from wind borne deposits. They may also be called loess soils. Many Riverina soils are aeolian.

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