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EARTH’S MATERIALROCKS FORMING PROCESSES

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ROCKS• Defined• A solid naturally occurring mass of

consolidated mineral matter.• This is because rocks are made up of granules

of different minerals that form bigger and hardmasses.

• Geologically, it is a combination of mineralparticles in a solid state irrespective ofhardness which constitutes an integral part ofthe lithosphere

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Characteristics of minerals

• Lustre: surface appearance of a mineral as itreflects light. (metallic or non metallic) non-metallic lustre shines like metal. E.g. Galena,Gold and Ilmenite. Non metallic e.g. Cinnabarappear dull and clay like.

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• Cleavage: differences on splitting of mineralse.g. Mica splits in one direction and forms thinsheets, halite has 3 cleavage directions anddiamond 4.

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• Hardness: Mineral s differ in hardness. Can betested by scratching one mineral with another

• Colour: colour of minerals depends on thesubstances that make up the crystals.

• Moh’s hardness scale can be used indetermining hardness of minerals

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Moh’s Hardness Scale

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• Appearance: Gold found of nuggets anddiamond as crystals halite as grains or clumps.

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• Streak: The rubbing of a mineral across aslightly rough, white plate.

• The rubbing grinds some of the mineral topowder and leaves a coloured streak on theplate

• E.g. Haematite varies from reddish to brown toblack, but leaves a red steak. Chalcopyrite (abrassy yellow) leaves a green to black streak

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Haematite streak

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CLASSIFICATION OF ROCKS

Mode of formationStructureGeological age

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Mode of formation

• Rocks which form in the same way areclassified together

• 3 types of rocks can be found- Igneous- Sedimentary- metamorphic

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Igneous Rocks

• Rocks of volcanic origin• Also, called fire formed rocks• Form after the cooling and solidifying of semi-

molten rocks (magma)• Classified into- Extrusive and intrusive igneous rocks

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Chemical composition of Igneous rocks

• Acidic/Felsic Rocks:- Have a high silica content of about 65% wit

relatively high potassium and sodium• Basic/Mafic Rocks:- Have a silica content between 45%-52%- They are considered to be silica deficient- They are composed of mostly oxides of iron,

aluminum and magnesium

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• Intermediate Rocks:- Have silica content 45%-66%- Form as a basic magma moves through the

continental crust to absorb more silica- May also form due to mixing of felsic and mafic

magma (e.g. Diorite, andesite and dolerite.• Ultra-basic/Ultra-mafic rocks- Silica content below 45%, contains little quartz

or feldspar- They have mostly ferromagnesian minerals,

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Bowen's Reaction Series Model

• In the early 1900's, N. L. Bowen and others atthe Geophysical Laboratories in WashingtonD.C. began experimental studies into the orderof crystallization of the common silicateminerals from a magma.

• The idealized progression which theydetermined is still accepted as the generalmodel for the evolution of magmas during thecooling process.

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Principles of Bowen’s Reaction series

a) As a melt cools minerals crystallize that are inthermodynamic equilibrium with the melt(dissolution equals crystallization; if noequilibrium either crystallization willdominate [supersaturation], or dissolution[undersaturated]).

b) As the melt keeps cooling and minerals keepcrystallizing, the melt will change itscomposition.

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c) The earlier formed crystals will not be inequilibrium with this melt any more and will bedissolved again to form new minerals. In otherwords: these crystals react with the melt to formnew crystals, therefore the name reaction series.

d) The common minerals of igneous rocks can bearranged into two series, a continuous reactionseries of the feldspars, and a discontinuous reactionseries of the ferromagnesian minerals (olivine,pyroxene, hornblende, biotite)

e) This reaction series implies that from a single"parental magma" all the various kinds of igneousrocks can be derived by Magmatic Differentiation

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Summary on Bowen’s Reaction Series• Bowen determined that specific minerals form at

specific temperatures as a magma cools.• At the higher temperatures associated with mafic

and intermediate magmas, the generalprogression can be separated into two branches.

• The continuous branch describes the evolution ofthe plagioclase feldspars as they evolve frombeing calcium-rich to more sodium rich. Thediscontinuous branch describes the formation ofthe mafic minerals olivine, pyroxene, amphibole,and biotite mica.

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• At a certain temperature a magma mightproduce olivine, but if that same magma wasallowed to cool further, the olivine would"react" with the residual magma, and change tothe next mineral on the series (in this casepyroxene).

• Continue cooling and the pyroxene wouldconvert to amphibole, and then to biotite. Thereason for this "stepped" evolution of mineralsis that with dropping temperature we havedecreasing thermal vibration of molecules, andthat allows silica to form more complexstructures.

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• Thus, olivine with its isolated silicatetrahedrons forms at the highesttemperatures, and as temperatures drop silicatetrahedrons first manage to join together inchains (pyroxenes), then in ribbons(amphiboles), and then sheets (micas).

• Finally, at the lowest temperatures the twobranches merge and we get the minerals thatare common to felsic rocks - muscovite mica,orthoclase feldspar,and quartz

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SEDIMENTARY ROCKS

• formed on the earth’s surface under normalsurface temperature and pressures.

• Result from the accumulation of the productsof weathering of other rocks and organicmaterials.

• Products of weathering are either transportedor may accumulate where they are formed.

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• The processes of transforming loosefragmented rocks into a compact solid cohesivemass is called lithification.

• This process is also known as consolidation,and the resultant rock is said to beconsolidated.

• Sandstone is a consolidated rock, while sand isan example of an unconsolidated rock.

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METHODS OF SEDIMENT EROSIONAND TRANSPORT

• There are five main agents of sedimenterosion and transport. Rivers – (or fluviatile effects) Sea – (marine effects) Glaciers – (glacial effects)Wind – (Aeolian effects) Landslides

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SEDIMENT GRAINSIZE, SORTING ANDROUNDING

a) Grain size of sediment – this is the diameter inmm of the particles i.e. the grains that makeup the rock. This can vary from less than 1 mmto 100 mm or more.

b) Sorting of the rock – in sedimentary rocks allthe grains of the rock may not be of the samesize. The more or nearly they are of the samesize, the better is the sorting.

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Degree of sorting in a sediment

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c) Rounding: - as the rock fragments are beingtransported, they collide with other fragmentsand become less angular i.e. more rounded.

Fig belows hows the progressive processes ofrounding of fragments.

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DIAGENESIS AND LITHIFICATION• Diagenesis processes are those changes of

various kinds occurring in sediments betweenthe time of deposition and the time at whichcomplete lithification (consolidation) takesplace.

• The changes may be due to bacterial action,digestive processes of organisms, to solution andre-deposition by permeating water, or tochemical replacement. On the other hand,lithification is the process of converting looseunconsolidated sediment into a cohesive rock.

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• On the other hand, lithification is the processof converting loose unconsolidated sedimentinto a cohesive rock.

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DIAGENESIS

• Sediments are derived by weathering anderosion of the surface rocks of the crust.Sediments are usually transported to a placewhere they accumulate or deposited tobecome a sedimentary rock

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DIAGENESIS PROCESSES

• 1. The sediments may not undergo transportbut may be deposited at its point ofweathering. This is a sedentary rock.

• 2. If this undergoes transport, it becomes atransported sediment

• 3. When it accumulates it is usually a loosemass e.g. sand and pebbles. These are theones referred to as unconsolidated rocks.

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• 4. After accumulation the diagenesis andlithification process convert the unconsolidatedsediment into an indurated or consolidatedsedimentary rock which is hard, compact andcoherent.

• 5. Diagenesis process describes all theprocesses that occur between deposition/accumulation and lithification. Diagenesis occurin relatively low temperature and pressureenvironments near the surface of the earth.

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LITHIFICATION

• This is the final induration of the sedimentswhere chemical and physical reactions convertit from an unconsolidated rock into aconsolidated rock.

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LITHIFICATION PROCESSES

A. Compaction - as more sediment is beingdeposited, there is an increase of weight orpressure that usually expels much of theconnate water and forces the rock grains tocome much closer together. As the grains areforced against each other, their outersurfaces usually dissolve and re-crystallizethus welding the grains together.

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B. Re-crystallization – this includes pressuresolution as described in (a) above, Percolatingwater can also dissolve material from one areaand re-deposit it elsewhere or alternativelywater can introduce substance into thesediments which then crystallize.

C. Cementation – Deposition of substances fromaqueous solutions usually occurs in the voidsor other spaces between the grains. Whenthese solutions crystallize they bind thesediments and in the process they convertedthe loose aggregate into a solid coherent rock

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CLASSIFICATION OF SEDIMENTARYROCKS

1) Clastic rocks – consists of grains that ofmechanical products of weathering

2) Chemical sediments – formed dominantly bychemical processes and more so from directprecipitation of compounds from solutions

3) Organic sediments – formed from organicdebris such as mollusks, shells, plant debrisetc.

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CLASTIC ROCKS

• Clastic sedimentary rocks are formed from theproducts of the mechanical breakup of otherrocks.

• Also named as Physically formed sedimentaryrocks

• The clastic rocks are most often named andclassified on the basis of the average grain sizeof the particles that form the rock. (tablebelow)

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Classification of Clastic rocks

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CHEMICAL SEDIMENTS

• Chemical sediments are basically formed bychemical processes usually from directprecipitation of solutions. There are two maindivisions of chemical sediments:

• Precipitates, and Evaporites

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PRECIPITATES

• (a). Crystalline limestone: Crystallinelimestone is precipitated according to thefollowing reaction:

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• The loss of CO2 from the solution may becaused by the rise in temperature. An exampleof the occurrence of the above chemicalprocess is the carbonate deposition along theBahama island banks in the Pacific ocean.

• The carbonate deposition is caused byagitation as a result of water hitting the cliffs.

• Other types of carbonate deposition occurs onthe lips of waterfalls due to the lowering ofpressure; as well as in caves due to evaporationto form structures such as stalactites andstalagmites

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(b). Siliceous Deposits

• Siliceous deposits consists dominantly of silica(SiO2). Silica in solution is usually incorporatedinto the skeletons of marine organisms such asradiolaria and sponges.

• Siliceous deposits can also be deposited fromhigh temperature volcanic springs or sometimesprecipitated directly from sea water like chert andflint (crypto-crystalline silica) i.e. the crystals areonly visible by ray methods.

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• Chert in particular is a dense, microcrystallinehard rock, that fractures with splinterlyconchoidal fracture.

• Flint is similar to chert only that it has darkcolour and a smoother fracture surface. Thecolored form of chert is named Jasper. Chertand flint are often found in nodules inlimestone beds with the original layers orbanding passing right through them

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• Silica frequently replaces other minerals, cell bycell, grain by grain, through the action ofground water.

• A mineral which has been silicified will oftenretain the appearance of the original, combinedwith the hardness of silica.

• Such a mineral is also described as an agate.Colorful agatized minerals are often made intojewelry, for they will take a high polish.

• Wood and other organic matter are oftenreplaced by silica.

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(c ). Ferruginous Deposits

• The ferruginous deposits are usually rich iniron and usually deposited in marshes, lakes,and lagoons as well as in the sea.

• The more common examples are the iron richoolites as well as the impregnation of otherrocks with haematite and limonite nodules.We have also rounded masses of pyrite andmarcasite and siderite (FeCO3).

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EVAPORATES

• When a sea has no outlet, the water willgradually evaporate if the climate is dry. As itdoes so, the dissolved salts become more andmore concentrated; familiar examples of thisare the Dead Sea (Israel) and Lake Magadi(Kenya). Each of these has a higher saltcontent than the ocean. Eventually these maydry up completely, leaving beds of salts whichare collectively termed evaporates.

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• Sea water, in particular, usually containsnumerous dissolved salts e.g. NaCl, MgCl2,CaSO4, Potassium chloride, (KCl), andmagnesium bromide. If a body of sea water isextensively evaporated, these salts willcrystalline out progressively from the least tothe most soluble.

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Examples of Evaporate Rocks• Many of these are actually minerals but since they have

extensive impurities they are referred to as rocks.Evaporite rocks show ancient conditions of extensiveevaporation.

- (a) Gypsum deposits (CaSO4 2H2O) – when it is massiveand translucent, it is referred to as Alabaster. If itoccurs in fibrous veins, it is called satinspar. If it occursas clear crystals, it is called selenite. If it occurs inearthy opaque form it is referred to as gypsum.

- (b) Anhydrite (CaSO4) – it is usually precipitated above25˚C

- (c) Rock Salt (NaCl)

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ORGANIC SEDIMENTS

• These include mainly organic limestone e.g.coral, peat and coal. These are classified asorganic products or precipitates that arecaused by organic processes.

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STRUCTURES IN SEDIMENTARY ROCKS

• Bedding- Bedding is the most distinctive feature of

sediment sequence and this consists of beds orstrata separated from each other by beddingplanes.

- The bed thickness may vary from mm range toseveral meters and the most thinner ones arereferred to as laminae. Fine laminae arecharacteristic of suspension deposits. Ifsedimentary rocks show beds then they arebedded or stratified rocks

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Bedding structure in sedimentary rocks.

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Graded Bedding

• Graded bedding usually occurs within a singlebed where there is variation in grain size fromlarge/coarse near the bottom to fine/smallnear the top

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Cross bedding

• Cross bedding occurs as a result of the slopingof the sub parallel planes within and at anangle to the main bedding plane. They areproduced by deposition of sediments belowmoving water and strong winds and hence notall bedding structure is originally horizontal

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Ripple Marks

• The ripple marks occur on top of beds and aredue to water movements over the bed duringits formation e.g. on modern beaches. Theirpresence indicates higher water speedsinvolving the movement of coarse particles inthe bed-load.

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Mud Cracks

• Mud cracks also occur on top of beds wheredrying out of sediments causes them tocontract forming polygonal cracks e.g. those indried up lake beds and in tidal mud flats.

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METAMORPHIC ROCKS

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