Metamorphism and metamorphic rocks

Posted on April 26, 2008 by Rashid Faridi

Introduction

The word metamorphism comes from Greek and means ‘change of form’. Thus, metamorphic rocks are pre-existing rocks whose mineralogy and/or texture has been changed by processes within the Earth. Metamorphic rocks form because of changes in temperature and depth of burial within the Earth without actual melting of the rocks taking place. The changes that affect these rocks occur in the solid state.

The cratonic regions of all continents are made up almost exclusively of metamorphic rocks and most of the oldest rocks on Earth are therefore metamorphic. Fold mountain belts such as the Alps, Himalayas and Andes contain large amounts of metamorphic rocks which were deformed by folding, faulting and thrusting and intrusion of granitic magmas. The fact that most metamorphic rocks are deformed indicates that metamorphism and tectonism (e.g. deformation) occur together.

Within New South Wales, most metamorphic rocks occur west of the Sydney Basin, from Oberon outwards to Broken Hill. The Lachlan Fold Belt of New South Wales comprises many metamorphic sequences, along with numerous granitic intrusions. The Broken Hill region of far western New South Wales is composed almost exclusively of metamorphic rocks. The oldest regions in Australia (e.g. the Pilbara and Yilgarn cratons of Western Australia) are likewise composed almost exclusively of high temperature metamorphic rocks and metamorphosed granites.

Metamorphism is also strongly associated with many ore deposits. This is because metallic elements (such as lead, zinc, copper) are particularly mobile during metamorphism, especially when fluid is involved. This mobility is both physical (where the ore minerals actually flow) and chemical (where the original ore minerals breakdown into their components and then move in solution in the metamorphic fluid and precipitate elsewhere). Examples of this include the famous Broken Hill deposit of far western New South Wales.

 Metamorphic processes

 Metamorphism is an internal process of the Earth and occurs as a result of changes in temperature and/or pressure. This is because most minerals are only stable at particular temperatures and pressures, so changes in these result in the formation of new minerals (those stable at the new temperatures and pressures). Other than water, few other components (these include fluorine, boron) enter or leave during metamorphism. No overall chemical changes occur, the original chemical constituents are just re-adjusted during recrystallisation and the texture of the rock changes, usually becoming coarser-grained.

The important factors that produce metamorphic changes are

  • pressure deep within the crust
  • temperature
  • strain (shape and volume changes as a result of stress during deformation)
  • fluid activity (pressure due to fluids in pore spaces within the original rocks)

All of these factors vary widely and result in different types of metamorphism depending upon which factor is dominant. The three main types of metamorphism are:

  • Contact (also known as thermal) metamorphism: this is produced by high temperature, low pressure, low strain and variable fluid pressure. It is normally produced by igneous rocks such as granites intruding into older colder rock sequences within a few kilometres of the Earth’s surface.
  • Regional metamorphism: this is produced by variable temperature, high pressure, variable strain and variable fluid flow and is normally created by tectonic processes within the Earth at variable depths.
  • Dynamic metamorphism: this is produced by variable strain, variable pressure, variable temperature and high fluid pressure and normally occurs in active fault zones.

Contact and dynamic metamorphism are usually restricted to localised areas whereas regional metamorphism affects large areas of the crust, sometimes over tens of thousands of square kilometres. Contact metamorphism occurs as zones a few hundred metres wide around large igneous intrusions while dynamic metamorphism is restricted to fault and thrust zones only a few tens of metres thick. All three types of metamorphism can overlap.

 Compositional groups

Metamorphic rocks can also be classified according to the chemical composition of the original source rock (protolith). Using this system, there are five main groupings:

  • Pelitic: formed from aluminous sedimentary rocks, commonly shales and mudstones. These contain abundant micas. Common metamorphic minerals in these rocks include andalusite, kyanite, sillimanite, cordierite and staurolite.
  • Quartzo-feldspathic: formed from quartz-rich sandstones or felsic igneous rocks. These are characterised by high silicon and low iron and magnesium. They usually contain quartz, feldspars and micas – much like their original protoliths.
  • Calcareous: formed from limestones and dolomites. These are composed of a large variety of calc-silicate minerals such as grossular garnet, wollastonite, tremolite, forsterite and calcite.
  • Basic: formed from basaltic and andesitic igneous rocks, and impure marly sediments with significant calcium, aluminium, magnesium and iron. Chlorite, actinolite and epidote are common at low grades whereas hornblende, diopside, Ca-plagioclase and almandine garnet are common at medium to high grades.
  • Magnesian: formed from peridotites, serpentinites and impure dolomites. Talc, olivine, chlorite, tremolite and brucite are the most common minerals produced.

Metamorphic grades, zones and facies

The term metamorphic grade is used to give a relative measure of the intensity of metamorphism in a particular area. A high grade rock is one that has been formed at relatively high temperature and/or pressure and a low grade rock at relatively low temperature and/or pressure. Many low-grade metamorphic rocks that were originally derived from mudstones and shales (initially wet, fine-grained sediments) contain hydrated minerals such as micas and chlorites. With increasing metamorphic grade, water is driven-off and all minerals that were initially hydrated become anhydrous phases. All high grade rocks are composed solely of anhydrous minerals.

A progressive metamorphic sequence in an area or region may be subdivided in the field into metamorphic zones with each zone representing a different metamorphic grade. A metamorphic zone is characterised by the appearance of a distinctive index mineral.

Index minerals are those that are stable under the temperature/pressure (often referred to as P-T) conditions of a particular metamorphic grade. For example, in low-grade rocks, the mineral chlorite is the index mineral for the chlorite zone. With increasing metamorphic grade, biotite starts to form and is the index mineral for the biotite zone. Lines on a map that separate different metamorphic zones are termed isograds, meaning lines of equal grade. However, there are exceptions as metamorphic zones are characterised by particular metamorphic minerals and this is dependent on the original protolith. For example, when a quartzose sandstone (essentially composed of 100% quartz) undergoes metamorphism, no new minerals can form and the quartz grains just recrystallise. This also applies to pure limestones, where the calcite just recrystallises into coarser grains and the rock becomes a marble.

The term metamorphic facies is defined as a set of metamorphic mineral assemblages where there is a constant relationship between the mineral assemblage and rock composition. The term is a mineralogical one, incorporating several mineral assemblages or rock types formed under the same broad P-T conditions. That is, the rocks of the same chemical composition have the same mineral assemblage if they belong to the same metamorphic facies. This concept is used to give a broad classification of the P-T conditions of metamorphism.

Contact metamorphism

At shallow depths within the crust (usually less than 6 km) the heat sources responsible for contact metamorphism are bodies of hot magma (e.g. igneous intrusions) which raise the temperature of the surrounding rocks. These thermal affects are usually restricted to the contact zones of the intrusions, hence the term contact metamorphism. However, sometimes hot fluids are released from the intrusions and penetrate the enclosing rocks along fractures and produce contact metamorphic zones. Determining factors governing the extent of contact metamorphism are the size of the intrusion and its temperature. Basic magmas are much hotter than acid magmas and hence will have a greater thermal effect. Also, a large intrusion contains much more heat than a small dyke-like body and its effect on the surrounding country rocks will be much greater and more widespread.

Country rocks surrounding large, hot bodies of magma are heated, initiating mineral reactions and forming new minerals. Rocks adjacent to thin dykes and sills are simply baked and hardened and do not experience any great mineralogical and/or textural changes. Large plutons give rise to contact aureole zones within which the country rocks are thermally metamorphosed, with those closest to the plutons experiencing more heat than those further away (hence they have a higher metamorphic grade). As large plutons take millions of years to cool down, the surrounding country rocks also stay hot for tens of thousands of years allowing chemical reactions to continue to completion.

As pelitic rocks (e.g. shales and mudstones) contain many different minerals and elements, many new minerals can form when they are metamorphosed. New minerals grow at progressively higher temperatures and thus, pelitic rocks are the most useful in determining the metamorphic zones in contact metamorphic assemblages. ‘Spotted rocks’ are common in the outer-most metamorphic zone around intrusions and resemble the surrounding country rocks except for the presence of patches of iron oxide and/or graphite. In the middle zone, porphyroblasts (that is, coarse-grained crystals) of the metamorphic minerals andalusite and cordierite appear. The inner or hornfels zone is composed of a hard, splintery fine to medium-grained rock with the constituent minerals forming an interlocking mosaic which is termed ‘granoblastic’.

Common metamorphic minerals produced from pelitic rocks include:

  • almandine garnet
  • andalusite
  • biotite
  • chlorite
  • chloritoid
  • cordierite
  • corundum
  • microcline
  • muscovite
  • sillimanite
  • spinel

For carbonate rocks, pure limestones simply undergo recrystallisation of the constituent calcite forming marble. Impure carbonate rocks contain many more elements and therefore, many new metamorphic minerals can form. Metamorphosed impure carbonate rocks are commonly termed skarns. With impure carbonate rocks, the carbon dioxide produced by the breakdown of calcite is removed from the system and calc-silicate rocks are formed. In some cases, the metamorphic fluid contains other volatiles such as boron and fluorine, resulting in the formation of different mineral assemblages.

Common metamorphic minerals produced from carbonate rocks include:

  • diopside
  • epidote
  • grossular garnet
  • hornblende
  • scapolite
  • tremolite
  • vesuvianite
  • wollastonite

In basic rocks, initially chlorite, albite and epidote are produced at relatively low temperatures. With increasing temperature, amphiboles and then pyroxenes form, sometimes with olivine and spinel.

The metamorphic facies produced by contact metamorphism in order of increasing grade are as follows:

  • Albite epidote hornfels
  • Hornblende hornfels
  • Pyroxene hornfels
  • Sanidinite

Dynamic metamorphism

Dynamic metamorphic rocks are restricted to narrow zones adjacent to faults or thrusts. The high shear stresses associated with faults and thrusts crush the adjacent rocks. The rise in temperature is produced by frictional heat generated within the fault zone. The high shear stresses may be short-lived or long-lived depending on the activity of the fault or thrust. Dynamic metamorphism involves high shear stress, high pressure, high strain, high fluid partial pressure and variable temperature. In many instances, water plays a fundamental role.

Crushed rocks in fault zones are known as fault breccias which consist of angular fragments of the country rock in a matrix of crushed or powdered rock, cemented by quartz and/or calcite. Fluids move easily along fault zones between grain boundaries and through cracks and fissures. These fluids are able to transport large amounts of silica, carbonates and other minerals in solution.

Pseudotachylite is a fault-zone rock which is black and glassy. It usually occurs as narrow dykes and veins and forms by frictional melting of the country rock. Mylonites are partially recrystallised rocks with a pronounced foliation that are produced by intense shearing during large-scale movements along faults and thrusts. The different rock types produced by dynamic metamorphism vary with depth from the surface as, with increasing depth, both the surrounding pressure and temperature increase.

Regional metamorphism

Most metamorphic rocks occur in fold mountain belts or cratonic areas. Such rocks cover large areas of the Earth’s crust and are therefore termed regional metamorphic rocks. They arise by the combined action of heat, burial pressure, differential stress, strain and fluids on pre-existing rocks. The resulting rocks are always deformed (as a result of the differential stress) and commonly exhibit folds, fractures and cleavages. Large amounts of granitic intrusions are also associated with regional metamorphic rocks. The most common regional metamorphic rocks are slates, schists and gneisses. Regional metamorphism covers a wide range of temperature and pressure conditions from 200° C – 750° C and 2 kbar – 10 kbar (or 5 km – 35 km depth).

There are three metamorphic facies within regional metamorphosed rocks, which from lowest to highest grade are:
  • Greenschist: can be further divided into chlorite and biotite zones. The term greenschist gets its name from the rocks themselves as many rocks of this facies are grey-green in colour and have a schistose (parallel arrangement of platy minerals) texture.
  • Amphibolite: can be further divided into the garnet and staurolite zones. The term amphibolite gets its name from the most common constituent minerals of this facies, minerals of the amphibole group.
  • Granulite: can be further divided into the kyanite and sillimanite zones. The term granulite reflects the most common texture of these rocks – granular.

High Pressure regional metamorphism

In some parts of the world, geologically young (Cenozoic and Mesozoic) fold mountain belts contain sequences of metamorphosed fine-grained sedimentary rocks and basic volcanic rocks that contain unusual blue amphiboles. These rocks are commonly schistose, may have a characteristic blue colour, and are termed blueschists. These form at low temperature but high pressure conditions in the collision zones of subducting slabs.

When subducting oceanic slabs are dragged down to depths exceeding 50 kilometres, the basalt is metamorphosed at very high pressures to form a dense rock with the same bulk chemical composition but different mineralogy (dominantly pyroxene and garnet) and texture. These rocks are called eclogites.

Retrogressive metamorphism

Many metamorphic rocks contain evidence of retrograde mineral changes, that is, alteration of higher grade minerals into lower grade ones. Many of these changes involve hydration and are the result of a decrease in temperature and increase in the activity of water. Retrograde metamorphism is normally produced by repeated regional metamorphism where a lower grade episode is superimposed on a higher grade one. Most retrogressive events are probably just a consequence of the metamorphic system cooling down after peak metamorphism has been reached (i.e. the system has to cool down with time and as the region undergoes uplift with time, both pressure and temperature are dramatically reduced). The secondary minerals produced during retrogressive metamorphism generally occur as fibrous fringes around, inclusions within, and pseudomorphous grains after, the higher grade metamorphic minerals. A good example of retrogressive metamorphism is the occurrence of serpentinites. These form by generally low temperature hydration of ultramafic rock (containing minerals composed chiefly of magnesium and iron), commonly at subduction zones.

Metamorphic belts

Regional metamorphism occurs over wide areas of the Earth’s crust. The most common metamorphic sequences in relatively young rocks (e.g. younger than 450 Ma (million years old)) occur in fold mountain belts which are produced by tectonic processes associated with the development of these belts. We call such regions metamorphic belts. Within these sequences, the higher grade regional metamorphic rocks generally occur within the lower crust regions and the lower grade ones occur in the upper crust. Older cratonic regions (also termed shields) also contain numerous regional metamorphic sequences. Both older and younger regions contain abundant exposures of granitic rocks whose formation is strongly linked to the metamorphism.

The geology of eastern Australia is dominated by a number of these fold mountain belts. The largest are the Lachlan and New England fold belts. Both of these fold belts contain relatively low-grade regional metamorphic rocks, along with numerous granitic intrusions. The New England Fold Belt contains small amounts of blueschists and eclogites that formed in the collision zones of subducting slabs. These fold belts have formed over hundreds of millions of years by plate tectonic processes.

The Al2SiO5 polymorphs

These are the minerals andalusite, kyanite and sillimanite which occur in metamorphosed pelitic rocks but have different stability fields with respect to temperature and pressure. Because of this, these minerals are used to determine the grade and pressure of the metamorphic rocks that they occur in.

Identification of metamorphic rocks

The first thing that we look at in identifying metamorphic rocks is their texture. A first subdivision can be made between:
  • non-foliated (or massive) rocks. These are termed:
    • hornfels
    • marble
    • quartzite
    • skarn
    • granulite
  • foliated (a planar penetrative feature). These are termed:
    • slate (those with a cleavage)
    • phyllite (those with a cleavage and micaceous sheen)
    • schist (parallel arrangement of platy minerals)
    • knotted schist (similar to schist but with distinctive large non-platy minerals surrounded by the platy minerals)
    • gneiss (alternating layers of different composition usually on the scale of a few mm to a cm)

The second thing that we look at to identify a metamorphic rock is its mineralogy. After this has been determined, to name the rock we simply list the mineralogy (in decreasing order of abundance) and then give the rock a general textural suffix. For example:

  • A rock composed of 60% muscovite and 40% biotite, both occurring as interlocking platy grains (i.e. a schistose texture) is called a muscovite-biotite schist.
  • A massive rock composed of 50% grossular garnet, 40% epidote and 10% wollastonite is called a grossular-epidote-wollastonite skarn.

The Rock Cycle PDF (30 kb) GIF (31 kb)

source:http://www.austmus.gov.au

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Sedimentary Processes and Rocks

Posted on April 25, 2008 by Rashid Faridi

Sedimentary rocks form by the breakdown (both physical and chemical) of pre-existing rocks (these may be of igneous, metamorphic or sedimentary origin). The main factors that control this breakdown are:

  • climate
  • topography
  • vegetation
  • properties (physical and chemical) of the rock.

The particles that are broken down are called sediments. Sediments are classified according to their size, ranging from silts and muds up to gravels and boulders. The main factors that control the transportation of sediments are:

  • water
  • wind (particularly in arid regions)
  • gravity (with all sediments flowing downhill regardless of the slope)

Stratification

Horizontal layering in sedimentary rocks is called bedding or stratification. It forms by the settling of particles from either water or air (the word sediment comes from the Latin sedimentum, meaning settled). Layer boundaries are natural planes of weakness along which the rocks can break and fluids can flow. As long as the sequence of layers has not been deformed or overturned, the youngest layers are at the top and the oldest are at the bottom. This sequence of stratification is the basis for the stratigraphic time scale. These observations were first made by a Danish physician, Nicolaus Steno, who in 1669 formulated the principles of horizontality, superposition (younger layers on top of older ones) and original continuity (sedimentary layers represent former continuous sheets).

Sedimentary geologists (known as sedimentologists) tend to study both present-day sediments and older sedimentary rock sequences.

  • Laminations are thin discrete layers of rock.
  • Formations are groups of sedimentary rocks which have formed at the same time and contain similar sedimentary rocks. They are mappable units that formed under distinctive environmental conditions.
  • Unconformities are major time-gaps between layers.

Principle of uniformitarianism

The principle of uniformitarianism is that processes which operate on the Earth’s surface today are similar to those that operated in the past. It is a fundamental principle in sedimentary geology and was first proposed by Charles Lyell in 1830. Using this principle when studying present-day sedimentary environments (e.g. coral reef systems, delta systems, river systems), we can determine fundamental principles such as:

  • rates of sedimentation
  • geometry of sediment sequences
  • rates of compaction
  • amount of water present in the sediments

which can then be applied to much older sedimentary rock sequences.

Weathering

  • Introduction
  • Weathering and feldspars
  • Weathering and carbonates
  • Resistance of minerals to weathering
  • Weathering in different rock types
  • Shapes of weathered fragments
  • Rates of erosion

Introduction

Weathering is the process where rocks break down under the effects of water and air. It consists of two processes which always act together:

  • fragmentation (known as mechanical or physical weathering)
  • decay (known as chemical weathering)

Erosion is the process of the movement of weathering products, by water and air.

The smaller the pieces (or fragments), the greater the surface area available for chemical attack and the faster the pieces decay. Important agents in weathering are rainfall, wind, ice, snow, rivers, seawater, vegetation and living organisms.

Soil is both a factor in weathering and the result of it. Once soil starts to form, rock weathers more rapidly and more soil is formed. Rainwater, which is mainly H2O, also contains small amounts of dissolved CO2 and H2S which react strongly with many rock materials.

Weathering and Feldspars

Feldspar is the most common mineral in many igneous and metamorphic rocks, particularly in granites. In temperate climates, granites are quite resistant to erosion whereas in humid to tropical regions, granites decay easily. This is because in areas of greater rainfall, the feldspars decay into clay minerals that are then easily released from the rock. Feldspars generally only remain untouched in very arid climates.

The most common clay mineral formed is kaolinite. In the weathering of feldspars to kaolinite, potassium, sodium, calcium and silicon are released from the feldspar structure and go into solution. As the speed of chemical reactions increases with increasing temperature, weathering occurs at a much faster rate in tropical areas than in more temperate ones. Under extreme weathering conditions in tropical climates, the clays themselves decay further into the mineral gibbsite (Al (OH)3) the main ore of aluminium. The resulting rock is a bauxite or laterite.

Weathering and carbonates

Whereas feldspar minerals decay into clays, carbonate minerals can completely dissolve. Caves are a characteristic product of the weathering of limestone (rocks made up predominantly of the carbonate minerals calcite and dolomite) in humid climates.

The main agent of weathering in this case is groundwater containing dissolved CO2. This dissolution of limestone results in the enrichment of the groundwater in Ca ions, leading to what is known as ‘hard water’. CO2 is extracted from the atmosphere and from organic material in the soil and dissolves in the groundwaters. The greater the weathering of limestone, the more CO2 is removed from the atmosphere. As limestone dissolves faster than silicate rocks, the chemical weathering of limestone accounts for more of the total chemical erosion of the land surface than any other rocks, even though much larger areas of the Earth consist of silicate rocks.

Resistance of minerals to weathering

Measurements of weathering in the field can be combined with experiments in the laboratory to determine the relative resistance of minerals to weathering. The products of weathering are generally more resistant to further weathering than other minerals, particularly iron oxide and clay minerals. As quartz is generally insoluble and chemically stable, it tends not to weather readily under most conditions. The order of mineral stability under weathering conditions is related to the stabilities of chemical bonds and crystal structures under different temperatures and pressures.

Weathering in Different Rock Types

Layered rocks break into slabs or plates along bedding planes whereas massive rocks break along regularly spaced planar cracks, called joints. In some igneous rocks, the joints take the form of sheets – sets of parallel closely-spaced planar surfaces. In areas of large daily temperature gradients, thermal expansion often accompanies frost action and chemical weathering.

Exfoliation is the peeling-off of large curved sheets or slabs of rock from the weathering surface of an outcrop.

Spheroidal weathering is a similar phenomenon in which rounded boulders split off layers or shells from the weathered surface.

Shapes of weathered fragments

The shapes of fragments caused by weathering and erosion are largely inherited from the patterns of joints, bedding and other structures in the parent rock rather than being produced by transport. The size of fragments is a good clue to the intensity of mechanical erosion. In general, the higher or steeper the topography, the larger the fragments. Once fragments have been moved from their source rock, they enter new environments of weathering and erosion and undergo further breakdown. Once boulders fall into streams they break and abrade quickly, and the size of pebbles downstream decreases rapidly with distance from the source. The different sizes and shapes of eroded particles (from huge boulders to clay-sized particles) are attributable to the characteristics of their source rocks and their distance from the source.

Rates of Erosion

Rates of erosion can be averaged over regions and are called rates of denudation (measured in mm per thousand years). These rates are greatest in valley glaciers and badlands (deeply eroded areas) and lowest in areas of low relief and temperate and rainforest regions. The rate of denudation is primarily controlled by topography and climate. Human influences accelerate the rate of denudation by three to ten times, with the highest figures being recorded in areas of intensive land use.

One of the highest rates of denudation measured is in the Tamur Basin of the Himalayas. The combination of steep slopes, unconsolidated material (sediment), a glaciated terrain and human modification has resulted in a rate of 4700 mm per 1000 years.

Soils

What is soil?

Soil is a thin layer of loose sediments (usually very fine-grained) and organic matter that lies above a layer of partly-weathered broken rock (called subsoil). Subsoil lies above the bedrock. The term regolith describes all of the loose, incoherent sequence of rock fragments, sand, alluvium, and soil that rests upon solid rock (also known as bedrock).

Soil layers

Soils are made up of three layers and are thickest where they are older and in warm and wet environments. The layers are called horizons. These are known as:

  • A-horizon: The top layer of soil. This is usually the darkest as it contains the greatest concentration of organic material. This layer mainly consists of clay minerals and quartz with soluble minerals being absent.
  • B-horizon: The second soil layer, which contains little organic matter but abundant iron oxide and soluble minerals.
  • C-horizon: The bottom soil layer. This is really just slightly altered bedrock (i.e. subsoil) and contains broken fragments of the bedrock, along with clay minerals.

Formation of soils

It is important to conserve the soil horizon as it takes many thousands of years to develop a mature soil. The A-horizon can take up to 10 000 years to fully develop and B-horizon may take as long as 100 000 years. A mature soil is one that is in chemical equilibrium. As the soil is slowly eroded from the top by natural processes, it is gradually deepened by chemical reaction. However, if the erosion is rapid, the rate of chemical decay is not fast enough and the soil is drastically thinned. Soil formation involves the response of mineral decay and mechanical erosion to variabilities in time, temperature, rainfall and biological activity. Palaeosols are ancient soils recognised in the sedimentary rock record.

Soil types

Three major soil types can be classified based on the minerals present in their A and B horizons:

  • Pedalfers: (Ped is Greek for soil, al for aluminium and fer for iron). In areas of high rainfall, the A and B horizons of the soil are leached, resulting in the soil being rich in quartz, clay minerals and iron oxide minerals. These soils are rich in aluminium and iron.
  • Pedocals: In warm and dry climates, the soils are enriched in soluble minerals, particularly calcium carbonate.
  • Laterites: In tropical regions, the soils are so highly leached that all silicate minerals are completely altered leaving the soil enriched in aluminium and iron oxides. This type of soil is usually deep red in colour.

Sedimentary processes and structures

  • Introduction
  • Sedimentary transport

Most sedimentary sequences that are preserved in the rock record are formed from catastrophic deposition such as floods, mud flows, rock slides and melting of glaciers. For a sediment sequence to be preserved and lithified (turned into rock), it must be covered over by younger sediments soon after it is deposited and water within the sequence must be expelled (this usually achieved through compaction by the weight of overlying layers).

Sedimentary transport

Sediments are transported downhill by either water or air until they meet a depression in the surface of the Earth. Most sediments are eventually deposited in large depressions which we call sedimentary basins. As more sediment is deposited within a sedimentary basin, subsidence occurs (brought about by the weight of the sediments) and the basin becomes deeper, thus allowing even more sediment to be deposited. Sedimentary basins can form in a number of different environments, both on land (e.g. lakes) or sea (e.g. ocean basins).

Sediments can be transported in three ways:

  • Suspension load is when sediments are carried in suspension (usually fine-grained sediments that can be carried along easily by the flow)
  • Bed load is when the forward force of the moving current acts more directly on the larger particles at the bottom as it pushes, rolls, and slides them along
  • Saltation is more complex and usually affects sand-sized particles. Here, the particles are sucked up by eddies into the flow, travel with the flow for a while, and then fall back to the bottom

Water and sedimentary processes

Water plays a vital role in most sedimentary processes. Most of the water on Earth occurs in the large ocean basins. However, water is also produced by volcanic activity on the surface of the Earth and some of it actually disappears into space each year. As well as carrying sediments in suspension, water also carries large quantities of sediments in solution (e.g. dissolved components) and this sometimes exceeds the total amount of sediments carried by other processes.

Pure H2O itself has little effect on rocks. It is the dissolved gases within water – particularly CO2 (carbon dioxide) – which cause the chemical decay of minerals and mineral dissolution.

The water table

The groundwater or water table is the surface between two zones of rock or soil below which the pores are completely filled with water. This is also called the saturated or phreatic zone.

Above the water table is the unsaturated or vadose zone. This zone is also called the zone of aeration because the pore spaces are filled partly by air and water.

The water table in any area is rarely stable, even for a few months. In periods of low rainfall, the water table falls, while in periods of heavy rainfall, it rises, eventually leading to flooding when all the pore spaces are filled with water.

Aquifers are permeable beds or rock masses (e.g. vesicular basalt) whereas aquicludes are relatively impermeable beds or rock masses (e.g. quartzite). In some areas, the water in the aquifer is very old (e.g. in the Great Artesian Basin of Australia, much of the water is believed to be older than 50 000 years) and the aquifer recharge may not balance the removal of water from the aquifer. This will eventually lead to the aquifer becoming depleted of water.

Organic matter and sedimentary processes

The biosphere (all biological activity such as plants, animals, and their remains) also plays a vital role in sedimentary processes. All organic matter eventually decomposes, releasing vital nutrients (such as N, Ca, C) into the soil and sea. Both coal and oil are formed by the interaction of buried organic matter with sedimentary processes.

Diagenesis

Diagenesis is the alteration of the mineralogy and/or texture of sediments at low temperatures and pressures. It affects sediments close to the Earth’s surface. There are two main processes operating,

  • compaction by overlying sediments (involving the close-packing of the individual grains by eliminating the pore space and expulsion of entrapped water)
  • cementation – development of secondary material in the former pore spaces which then binds the sedimentary particles together. This material may be introduced from the passage of groundwater or derived from solution.

Sediment characteristics

Characteristics of sediments include:

  • Porosity
  • Permeability
  • Roundness and Sphericity
  • Sorting
  • Matrix

Porosity is the volume of voids within a rock which can contain liquids. The porosity of a sedimentary rock made up of perfectly closely-packed spherical particles is 27%, with open-packing this rises to 47%. Because sedimentary rocks are never perfectly packed and also contain cement, the porosity generally varies from 1 to 50%. In sandstones, porosity varies from 5 to 15%, while in loose sands and gravel it may reach 45%. Clays are exceedingly porous, up to 50%. The degree of porosity largely depends upon the geometrical arrangement of particles in the sediment.

Permeability is the ability of water or other liquids (e.g. oil) to pass freely through a rock. It is important to note that some rocks (e.g. sandstones) may have a high porosity but may still be impermeable. A rock is pervious if it is permeable by mechanical discontinuities such as joints and bedding planes.

Roundness refers to the roughness of the surface of the sedimentary grain. In general, grains become more rounded the further they are from their source rock. This is an important tool in mineral exploration, particularly for diamonds and other gemstones. Sphericity refers to the shape of the grain and is largely inherited from the host rock.

Sorting refers to the range of particle sizes in a sediment or sedimentary rock. In general, sediments which have travelled relatively long distances from their source are well sorted while those that haven’t travelled far are poorly sorted.

Matrix is the fine-grained material (usually clays or silt) that is deposited originally with the coarser-grained material (e.g. sands and gravels) in a sediment. It is also derived from the weathering and erosion of the source rocks.

Sedimentary structures

Sedimentary structures are very important as they provide us with information on the palaogeography and palaeoclimate of the areas in which they occur. They can also indicate the direction of palaeocurrents of rivers and seas. Sedimentary structures can be of either physical (e.g. wave action) or biological (e.g. disruption of sediments by animals) origin.

Physical structures: The movement of sand grains in a current creates ripples and dunes on the stream bed. These are known as bedforms. Ripples are the low narrow ridges that are separated by wider troughs. Physical structures form on sand dunes, on underwater sandbars in rivers and streams, and under the waves at beaches. They come in a wide variety of shapes and sizes which are characteristic of the currents that form them:

  • Asymmetrical ripples are ripples that have a gentle slope upstream and a steep slope downstream.
  • Cross-bedding is inclined bedding and commonly forms in alluvial environments.
  • Potholes are rounded depressions caused by swirling currents and eddies.
  • Mud cracks are formed by evaporation on mudflats or in shallow lakes.

Biological structures: include worm burrows (usually in soft-sediment, particularly at high tide levels along beaches), crab burrows (particularly those of hermit crabs), tubeworm colonies and animal tracks on soft sediment.

Sedimentary environments

Sedimentary environments can be classified according to the climate in which they occur and/or the geometrical arrangement of the sediments. In general, we classify ancient sedimentary rocks according to their similarity to current sedimentary environments. When describing sedimentary sequences, we usually do so in terms of facies (the sum total of all sedimentary features) columns in which the sequence is graphically illustrated from the base to the top. Facies columns include all details on sedimentary structures, variations in grainsize, contacts between the individual beds and fossils present.

Rivers

In geology, river environments are known as fluvial environments. Within fluvial environments deposition occurs with normal settling of relatively coarse to medium-grained sediments on the river bottom and along bends in the river, and also during times of flooding when finer-grained sediments are deposited along the river banks.

Lake Yenga, near Balranald, New South Wales. Photo: SG Lane Collection © Australian Museum.

Lakes

When rivers flow into lakes, the coarser-grained sediments are usually deposited at the toe of the deltas while the finer-grained sediments are carried in suspension to be deposited in the centre of the lakes. In many cases, the river breaks up into tributaries when it enters into the lake, and the banks between these tributaries are mainly composed of sand-sized sediments. As it contains a relatively high porosity, detrital organic material (particularly plant material) is deposited into the centre of the lakes along with the clay and silt-sized sediments.

Hawkesbury river mouth (Lion Island). Photo: SG Lane Collection © Australian Museum.

River mouths

All major rivers (along with many minor ones) eventually reach the sea. As they approach the sea, they generally divide into many channels before eventually reaching it. The banks of these channels are generally composed of sand deposits along with minor mud which is deposited in times of flooding. Between the channels, shallow bays are filled by fine-grained sediments such as sands and muds. When the river eventually reaches the sea, the coarser sand-sized particles are deposited first (sometimes forming bars between the river mouth and sea) and the finer-grained silt and clay particles are deposited in deeper water out to sea.

Coral Reefs

Coral reefs only occur in warm shallow water environments. They are formed by the interaction of biological (i.e. corals) and sedimentary (i.e. silts and muds) processes. Many coral reefs are formed on pre-existing drowned volcanic necks (i.e. many of the islands in the Pacific have formed in this way) while others form on pre-existing masses of rock (e.g. the Great Barrier Reef). Coral reefs grow far more rapidly on the windward side as the currents driven by the wind bring in abundant nutrients.

Coral reefs are very fragile environments. As the coral itself is the shell of a living organism, it needs abundant sunlight (for photosynthesis) and nutrients. If the coral is blanketed in sediments (such as silts produced by rapid erosion caused by intensive irrigation on land), sunlight will not penetrate and the coral (and reef community) will die out. Also, if there is too much nutrient in the water (sometimes produced on land by farmers using fertilisers, e.g. super-phosphate, to increase their crop yield), other predators (such as the crown of thorns starfish) will move in, eat the coral and eventually sterilise the reef.

Glaciers

In polar and cold climates, glaciers are produced. As glaciers move downhill, they grind down any underlying rocks into fine powder and carry along with them any pebbles or boulders that have fallen into the glacier from avalanches and rockfalls. When the glacier starts to melt, these sediments are deposited at the foot of the glacier (particularly the coarser-grained sediments) and are called moraine deposits. After glaciation, suspended valleys known as hanging valleys are formed, along with steep waterfalls and alluvial fan deposits.

Arid environments

In arid environments, there is little available water or vegetation to act on the rocks and sediments present. Most weathering and erosion is caused by wind action and frost. Because of the lack of water and vegetation, sedimentary structures produced in these environments last for a relatively long time period. The Namib Desert of Namibia contains the world’s largest sand dunes which are believed to be about 30 million years old. Most sediment deposits in arid regions are wind-blown in origin, and hence tend to be very well sorted. Other characteristic sedimentary deposits in arid environments are evaporite deposits that form in shallow lakes. Because of low rainfall the water table in these areas is very low, and soluble minerals occur near the surface and accumulate over time.

Tropical Environments

In areas of very high rainfall (i.e. tropical areas) the rainfall is so high and the vegetation is so dense that there are very few visible rock outcrops. The soil profile is very deep because the interaction of rain with the vegetation forms humic acids that strongly leach and rapidly break down any rocks present. Sedimentary deposits in these environments are largely formed by mudslides, particularly if the topography is steep.

Classification of sedimentary rocks

Clastic sedimentary rocks (those composed of rock fragments) are classified according to the predominant grainsize present. Using this scheme, they can be classified as:

Prefixes can also be added to indicate the dominant mineralogy. For example, a quartz-rich sandstone is a quartzose sandstone, a feldspar-rich sandstone is an arkose, a mica-rich sandstone is a micaceous sandstone, and a lithic-rich sandstone or shale is a greywacke.

Conglomerate
Conglomerate (7.5 cm x 4 cm). Newcastle District, New South Wales. Photo: S Humphreys © Australian Museum.

Conglomerates

Conglomerates can be further subdivided according to their texture (grain-supported versus matrix-supported), composition (pebbles of one rock type or several different rock types) and source rocks (pebbles derived from within the depositional environment or from outside).

Limestones

Limestones are a different case as they can either be of clastic or chemical origin. The clastic limestones can contain either:

  • rock fragments
  • fossils (e.g. fossiliferous limestone)
  • ooliths
  • pellets

In contrast, the chemically-precipitated limestones consist solely of crystalline carbonates (e.g. microcrystalline limestone). The clastic limestones can be further subdivided according to their grainsize, much like other clastic sedimentary rocks.

Non-fossiliferous limestone
Non-fossiliferous limestone (8 cm x 5 cm) Jenolan Caves, New South Wales. Photo: S Humphreys © Australian Museum.
Fossiliferous limestone
Fossiliferous limestone (8 cm x 8 cm) Rockley, New South Wales. Photo: S Humphreys © Australian Museum.
Other chemical sedimentary rocks include:
  • banded-iron formations
  • ironstones
  • cherts
  • evaporites
  • silcretes
  • calcretes

Zebra rock or Zebra stone is a distinctive reddish-brown and white banded claystone from the Ord River area of Western Australia where it forms discontinuous lenses within a Late Proterozoic shale sequence. Most occurrences are now submerged beneath the dam waters of Lake Argyle. Since its discovery in 1924, it has been used widely as an ornamental stone. It is composed of small particles of quartz, white mica, clay minerals, hematite and alunite. The striking colour banding was probably formed after the original sediments were deposited by the rhythmic precipitation of hematite-rich bands during alteration of the rock in a highly oxidising environment.

Some examples of sedimentary rocks:

Chalk is a fine-grained limestone, usually formed by compacted microscopic animals called foraminifera.

Chert is a fine-grained hard sedimentary rock composed of microscopic silica grains, and has a flat fracture.

Flint is a variety of chert (mostly of upper Cretaceous age) that has a conchoidal fracture.

Marl is a calcareous mudstone.

Micrite is a finely-crystalline calcite.

Mudstone is a fine-grained sedimentary rock that lacks a well-developed bedding plane.

Shale is a fine-grained sedimentary rock with a well-developed bedding plane.

Siltstone is similar to mudstones but consists predominantly of silt particles.

Sparite is a coarsely-crystalline calcite.

source:http://www.austmus.gov.au

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Classification of Igneous Rocks

Posted on April 25, 2008 by Rashid Faridi

Igneous rocks are classified by using grainsize, silica content, and/or silica saturation. Other methods of classification include the International Union of Geological Sciences (IUGS) scheme and the Total Alkalies vs Silica scheme.

Grain Size

Based on the predominant grainsize that reflects the depth at which molten rocks form within the Earth. Three types of rock can be identified:

Volcanic rocks solidify close to the Earth’s surface. Because they cool quickly they have a finer-grained matrix (called ground mass). They may contain some larger crystals that formed earlier further down (called phenocrysts).
  • Plutonic rocks form deeper within the Earth and the slower cooling allows them to crystallise as coarse-grained rocks.
  • Hypabyssal or subvolcanic rocks form at intermediate depths (generally as dykes and sills) and so tend to be medium-grained.

 

Silica content

Silica content (SiO2) which also controls the minerals that crystallise is used to further classify igneous rocks as follows:

  • Acid: usually above 63% silica (mostly feldspar minerals and quartz), e.g. granite.
  • Basic: 45 to 55% silica (mostly dark minerals plus plagioclase feldspar and/or feldspathoid minerals), e.g. basalt.
  • Ultrabasic: usually less than 45% silica (mostly dark minerals such as olivine and pyroxene), e.g. peridotite.

Silica saturation

This is similar to modal composition, but allows distinction between feldspathoid-bearing and feldspathoid-free rocks.

  • Rocks that are undersaturated in silica will crystallise silica-poor minerals like feldspathoids or olivine.
  • Those that are just saturated in silica will lack feldspathoids or quartz.
  • Those that are oversaturated in silica will crystallise quartz among the minerals.

International classification

The International Union of Geological Sciences (IUGS) classification scheme is the standard scheme for igneous rocks and uses the minerals present. It is in the form of triangular diagrams where each apex of the triangle represents 100% of a mineral. It splits into feldspathoid-bearing (i.e. nepheline, sodalite) and feldspathoid-free rocks. Then, it takes the relative percentages of quartz (Q), plagioclase (P) and alkali feldspars (A). The value of Q, P, and A determines a given field for each rocktype. Similar schemes also classify ultramafic rocks (but using olivine, orthopyroxene and clinopyroxene) and gabbroic rocks.

TAS classification

The TAS (Total Alkalies vs Silica) classification scheme uses chemistry to classify volcanic rocks. The analysis is recalculated to 100% after removing H2O and CO2. It is a simple X-Y graph with the X-axis being SiO2 Wt% and the Y-axis being Na2O+K2O Wt%.

Rock types

  • Common plutonic rocks
  • Common subvolcanic rocks
  • Common volcanic rocks
  • Other volcanic rock types
  • Other types
Common plutonic rocks
Acid
  • Granite: the most common igneous plutonic rock. Contains essential quartz, plagioclase and alkali feldspar, usually with hornblende and/or biotite and/or muscovite.
  • Granodiorite: a plutonic rock with essential quartz and plagioclase, with lesser amounts of alkali feldspar and small amounts of hornblende and biotite.
  • Tonalite: a plutonic rock with essential quartz and sodic plagioclase, usually with lesser biotite and amphibole.
  • Aplite: fine-grained, composed mostly of quartz and feldspars.
  • Pegmatite: a very coarse-grained, usually plutonic granitic rock.

Basic

  • Diorite: a plutonic rock composed of sodic plagioclase, commonly hornblende, and/or biotite or augite.
  • Gabbro: a coarse-grained plutonic rock with essential calcic plagioclase, pyroxene and opaque minerals, and/or hornblende or olivine. The plutonic equivalent of basalt.
  • Monzonite: a plutonic rock with equal amounts of plagioclase and alkali feldspar, along with lesser amphibole and/or pyroxene.
  • Syenite: a plutonic rock composed mainly of alkali feldspar, with subordinate sodic plagioclase, biotite, pyroxene, and amphibole.

 

Ultrabasic
  • Anorthosite: a plutonic rock with mostly calcic plagioclase and small amounts of pyroxene.
  • Dunite: a plutonic rock composed mostly of olivine.
  • Clinopyroxenite: a plutonic rock composed mostly of clinopyroxene.
  • Harzburgite: a plutonic rock composed of essential olivine and orthopyroxene.
  • Hornblendite: a plutonic rock composed mostly of hornblende.
  • Lherzolite: a plutonic rock composed largely of olivine, along with lesser clinopyroxene and orthopyroxene, and sometimes spinel.
  • Orthopyroxenite: a plutonic rock composed mostly of orthopyroxene.
  • Peridotite: a plutonic rock composed of olivine with lesser pyroxene and/or amphibole.
  • Pyroxenite: a plutonic rock composed largely of pyroxene.

 

Common subvolcanic rocks

  • Dolerite: a subvolcanic rock with essential plagioclase, pyroxene and opaque minerals. The subvolcanic equivalent of basalt.
  • Porphyry: a general term for igneous rocks that contain phenocrysts in a finer-grained groundmass.

 

Common volcanic rocks

Acid
  • Rhyolite: silicic volcanic rocks composed of quartz and alkali feldspar, with minor plagioclase and/or biotite. The volcanic equivalent of granite.
  • Comendite: a porphyritic alkali-rich rhyolite with phenocrysts of quartz, alkali feldspar and sodic pyroxenes and/or amphiboles.
  • Dacite: a common volcanic rock composed of quartz and sodic plagioclase, along with small amounts of biotite and/or hornblende, and/or pyroxene.

 

Basic
  • Basalt: the most common volcanic rock on Earth. Composed of essential calcic plagioclase and pyroxene, sometimes with olivine, feldspathoids or interstitial quartz.
  • Andesite: an intermediate, commonly porphyritic volcanic rock composed of plagioclase, pyroxene, hornblende and/or biotite.
  • Latite: an intermediate volcanic rock with equal amounts of sodic plagioclase and alkali feldspar.
  • Trachyte: a volcanic rock composed largely of alkali feldspar.

 

Ultrabasic
  • Komatiite: these rocks are high in magnesium and crystallised at high temperatures. They commonly display a spinifex texture consisting of intergrown skeletal and bladed olivine and pyroxene crystals in a glassy groundmass. Most are Archean in age.

 

Other volcanic rock types

  • Agglomerate: a rock of coarse pyroclastic material consisting dominantly of rounded fragments.
  • Amygdaloidal: a volcanic rock containing mineral-filled cavities (or vesicles).
  • Bomb: a volcanic projectile with an average diameter greater than 64 mm and whose shape or surface indicates that it was partially or completely molten during its formation and transport through the air.
  • Ignimbrite: a pyroclastic tuff consisting of crystal and rock fragments in a matrix of glass shards that are usually welded together.
  • Obsidian: a volcanic glass that is commonly dark in colour and glassy, with a water content less than 1%.
  • Perlite: volcanic glass that exhibits numerous concentric cracks. Some are high in water and expand when heated.
  • Pumice: a pyroclastic glassy volcanic rock containing abundant empty cavities (or vesicles), usually of rhyolitic composition.
  • Scoria: a pyroclastic volcanic rock containing abundant empty cavities (or vesicles), usually of basaltic composition.

 

Other types

  • Carbonatite: volcanic or subvolcanic rocks composed mostly of primary carbonate minerals, such as calcite, dolomite or Na-carbonates.
  • Kimberlite: a volcanic or subvolcanic rock composed largely of serpentinised olivine, with variable amounts of phlogopite mica, orthopyroxene, clinopyroxene, carbonate and chromite. Characteristic accessory minerals include pyrope garnet, rutile, monticellite and perovskite, and in some cases diamonds.
  • Lamproite: a volcanic or subvolcanic rocks that are rich in potassium and magnesium, composed of unusual rare minerals such as K-Ti-richterite, priderite, wadeite, jeppeite, Fe-orthoclase and leucite. The hostrock for diamonds in the AK1 pipe of the Argyle diamond mine, Western Australia.
  • Lamprophyre: a name for a group of subvolcanic rocks that are strongly porphyritic in mafic minerals such as biotite, amphiboles and pyroxenes in a groundmass of feldspar. They commonly occur as dykes.

Reference

Le Maitre, R.W., 1989. A classification of igneous rocks and glossary of terms. Blackwell Scientific Publications, 193p.

Terms

  • Acid – usually above 63% silica (mostly feldspar minerals and quartz), e.g. granite.
  • Basic – 45 to 55% silica (mostly mafic minerals plus plagioclase feldspar and/or feldspathoid minerals), e.g. basalt.
  • Ultrabasic – usually less than 45% silica (mostly mafic minerals such as olivine and pyroxene), e.g. peridotite.
  • Felsic – mostly feldspar minerals and quartz, e.g. granite.
  • Mafic – mostly pyroxenes and olivine plus plagioclase feldspar and/or feldspathoid minerals, eg. basalt.

source:http://www.austmus.gov.au

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Limestone Topography

Posted on April 25, 2008 by Rashid Faridi
Limestone caves

Caves form in limestone (calcium carbonate), and occasionally in dolomite (calcium magnesium carbonate), when water containing dissolved carbon dioxide (carbonic acid) seeps into rock crevices and joints. The carbon dioxide comes from decaying organic matter in soil, and also directly from the atmosphere. This slightly acidic water dissolves the rock, forming cavities which can enlarge and join up to make larger cave systems of interconnected chambers. An underground water flow can develop when many rain-fed subsurface drainages join up, or a river can be captured by an open cavity collapse structure (sink hole or doline) and flow underground. This leads to further enlarging and sculpturing of the caves by chemical and physical weathering. Gravel, sand, silt and clay can be deposited in the caves from outside, or fine sediments from internal springs may build up. This dissected and dissolved landform with a complex groundwater system is called karst.

Cave decorations

When underground rivers work deeper, or water-filled cavities are drained, the resulting free spaces (often the higher or older chambers) can be decorated with various types of crystalline calcium carbonate formations.

After passing through limestone, the acidic water contains calcium carbonate in soluble form, now present in solution as calcium bicarbonate. When the solution loses carbon dioxide, it deposits solid calcium carbonate again. The resulting mineral is usually calcite, but occasionally a calcium carbonate with different crystal form can grow (aragonite). The solution can lose carbon dioxide when degassing through direct exposure to air, especially when it splashes, or flows over impediments. The resulting cave formations have a waxy, glistening appearance. Loss of carbon dioxide through evaporation plays a much lesser role, usually in areas of high airflow near cave entrances, but the resulting cave formation usually look dull and chalky. Bacterial and algal colonies can also play a role in deposition of calcium carbonate. Cave deposits or decorations are called speleothems.

How a cave is decorated depends on whether the water drips, seeps, flows or sits in pools.

Dripstones

  • Straws grow from hanging drops of water. The crystals form on the outer rim of each successive drop, building up a hollow tube with internal diameter of a few millimetres. Straws sometimes reach lengths of over 6 metres.
  • Stalactites grow from water dripping off a cave roof. A long, thin hollow tube, a straw, forms first, but its internal channel becomes blocked, forcing water to flow over its outer surface, depositing successive layers of calcite, resulting in a stalactite with typical pointed carrot shape.
  • Stalagmites grow from splashing drips beneath the stalactites. Stalagmite shapes tend to be ‘stumpy’ or rounded, compared to the more pointed shape of stalactites.
  • Columns result when a stalactite and its corresponding stalagmite meet.
  • Shawls grow from water trickling down sloping surfaces. Layers of calcite build up into fluted curtains.

Seepstones

Helictites seem to defy gravity. They grow when water slowly seeps through porous roof or wall surfaces, producing tiny droplets that precipitate calcite in the form of fine tubes with minute internal channels. The water flows through the tubes by capillary action, and changes in the direction of growth may be due to blockages in the internal channels. As the calcite grows from the tip at various angles, interesting twisted shapes result.

Flowstones

Flowstone forms from layers of calcite deposited when thin films of water flow down walls and across cave floors. Some resemble ‘frozen waterfalls’.

Flowstone often forms over gravel and silt left behind by underground rivers. When this material is washed away to leave an undercut, canopies of flowstone are left unsupported.

Rimstone dams build up on slopes as slow-moving water flows over them. Bumps on the sloping surface promote turbulence and assist carbon dioxide loss. The resulting low walls can hold back water to form a pond.

Poolstones

Rafts are formed from thin skins of crystalline calcite which float on the surface of a pool.

Calcite crystals grow as ‘dog-tooth spar’ when caves or smaller cavities are filled with still water. These solutions are saturated with dissolved calcium carbonate and can precipitate crystals with perfect geometrical shapes. They stop growing when the water drains out.

Calcite can crystallise as a very thin and fragile shell around air bubbles, which will eventually sink to the bottom of the pool.

Cave pearls are formed when calcite grows around small grains in a pond, adding layer after layer to form small spheres.

Source:http://www.austmus.gov.au
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