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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.
Granite
Granite 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.
Granite
http://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
deposits), seas (Marine deposits) or on land (Alluvial deposits) giving rise to sediments of
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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
http://geology.about.com/od/rocks/ig/sedrockindex/rocpicbreccia.htm
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 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.,

heat from cooling magma e.g. quartz becomes quartzite and limestone becomes marble.
shale becomes slate
Some Metamorphic Rocks
Quartzite - very hard, compact, granular or glassy mass, pale yellow to fawn, almost
completely quartz derived from sandstone or chert.
Quartzite
http://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.htm
For an animated view of the rock cycle see:
http://www.learner.org/interactives/rockcycle/diagram.html
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Rock Weathering
Most soils are formed from weathered rocks
Rock 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
Wind acts as a weathering agent by':
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 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.
greatly assisting decomposition. End products formed from carbonic acid action are
carbonates (CO32-) and bicarbonates (HCO3-)
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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..
 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.
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
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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