1 Basic Geology

7/28/2019 1 Basic Geology

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PgDip/MSc The Energy Programme/Subsurface Basic Geology

Basic Geology

Review

In this topic the student is introduced to the fundamentals of the Earthsstructure, plate tectonics and rock types.

Content

Earth Structure

Figure 1 illustrates the structure of the Earth. There is a central solid iron core,surrounded by a liquid iron core, the lower mantle and the upper mantle. The uppermantle consists of a weak, partially molten asthenosphere and a strong lithosphere witha surficial crust of light rock. About 90% of the earths crust is made up of the fourelements: iron, oxygen, silicon and magnesium, which are the fundamental buildingblocks of most minerals. Iron, being heavy, sinks to the core, and lighter elements suchas silicon, aluminium, calcium, potassium and sodium have risen to the crust.

Figure 1. The Earths Structure. (From THE DYNAMIC EARTH by B.J . Skinner and S.C. Porter,copyright 2000 J ohn Wiley and Sons. This material is used by permission of J ohn Wiley and Sons, Inc.)

The Robert Gordon University 2006 1


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Plate Tectonics

Plate Tectonics was first proposed in the 1960s. The central idea is the division of thelithosphere into 12 rigid plates (6 major ones), which each move as distinct units (Figure2). The plates consist of rigid lithosphere (with either thin, dense oceanic crust or thick,less dense continental crust), which floats on the partially molten asthenosphere(Figure 3). Convection currents within the asthenosphere are thought to be the drivingforce behind the plate movement. Where hot matter rises under the ocean it flows apartand carries the plates along with it (Figure 4). When this hot matter cools and sinks theplates also begin to sink. The plates are constantly moving, which explains why theAtlantic Ocean did not exist 150 Ma (million years ago). At this time it has beenestablished that Eurasia, Africa and the Americas were all one continent called Pangea.It is possible to trace the effects of tectonics back approximately 4.6 billion years,although the rock record and hence history becomes hazy after about 1 billion years.

The margins between the 12 plates are Divergent (spreading apart), Convergent(colliding together) or Transform (sliding past each other). Plates are constantlyproduced and consumed. Volcanic and seismic activity along plate margins variesdepending on type. Trailing edges tend not to be particularly active (most of Europe)

wheras leading edges tend to be very active.

Figure 2. Tectonic Plates Today (Peter J Sloss, NOAA-NESDIS-NGDC).



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Figure 3. Close-Up of Crust and Asthenosphere. (From UNDERSTANDING EARTH by FrankPress and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)

Figure 4. Convection Currents and Plate Movement Theories. (From UNDERSTANDINGEARTH by Frank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used withpermission.)



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Divergent Margins

Figure 5A illustrates a divergent plate boundary. Related features include linear MidOcean Ridges (the Mid Atlantic Ridge) where the lithosphere breaks and a rift develops.As the lithosphere breaks hot lava rises from the asthenosphere. The rift continues toopen thus separating the two plates. This occurred between America and Africa andlead to the formation of the Atlantic Ocean basin. The Mid Ocean Ridge (MOR) ischaracterised by earthquakes and volcanism. Different lavas have differentviscosities.This leads to a variation of divergent speeds, and in turn to offsets in theplate margin. The mid atlantic ridge shows an average speed of 2.5 cm/year whereas 18cm/year can be found in the South Pacific.

Convergent Margins

When two plates are being pushed together the denser one will ride below the lighterone, creating a subduction zone. Less buoyant oceanic crust usually sinks below thethicker, lighter continental crust. Features associated with this subduction includemountain building, trench formation, earthquakes and volcanism. The contact of theNazca plate and the South American plate led to the formation of the Andes mountainrange and the Chilean deep-sea trench (Figure 5B). The Nazca plate (plate 1) bucklesdownwards and the overriding South American plate (plate 2) is crumpled and uplifted.As the subducted plate sinks it will melt, generating a source of hot molten rock thatrises into the overlying crust, inducing volcanism.

Where two plates converge at thick continental crust edges, subduction is low and anever growing mountain range is formed, termed a collision boundary (Figure 5C). TheHimalayas are formed due to collision of the Asian and Indian plates for example.

Transform Faults

Transform faults occur where two plates slide past each other (Figure 5D). Themovement is generally not regular and uniform but occurs abruptly as a series of suddenslip faults. The San Andreas Fault in America where the Pacific plate slides past theNorth American plate is an example. The sudden slip movements produce a series ofdamaging earthquakes along the fault.

In summary, divergent zones are sources of new lithosphere and subduction zones aresinks. Material is created and consumed in equal amounts. If this were not true, theEarth would change in size.

Figure 5. Types of Plate Margin. (From THE DYNAMIC EARTH by B.J . Skinner and S.C. Porter,copyright 2000 J ohn Wiley and Sons. This material is used by permission of J ohn Wiley and Sons, Inc.)



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Magnetism

Motions in the fluid iron core of the Earth set up a dynamo action thus generating theEarths magnetic field (Figure 6). Rocks are magnetised in the direction of the magneticfield at the time of their formation. The rocks can be dated radiometrically and thus thehistory of the magnetic field recorded. Such studies have shown that the field reversesdirection (the reason for which is unexplained) with such reversals evident on theseafloor. Figure 7 illustrates the symmetrical pattern of magnetised rocks either side of aMOR.

Figure 6. Magnetic Field Lines. (From THE DYNAMIC EARTH by B.J . Skinner and S.C. Porter,copyright 2000 J ohn Wiley and Sons. This material is used by permission of J ohn Wiley and Sons, Inc.)

Figure 7. Magnetised Rocks Either Side of a MOR. (From UNDERSTANDING EARTH byFrank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)



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Minerals and Crystals

A mineral is defined as any naturally formed, solid, chemical substance having a specificcompostion and characteristic crystal strucure. Diamond is a mineral as it has a definedcomposition (pure carbon) and crystal structure (the atoms are packed in a threedimensional array). Graphite is also a mineral of pure carbon, but with a sheet likecrystallographic strucure. Coal is not a mineral as it is composed of many differentcompounds (although mainly carbon), the proportion of which varies from one place toanother, and has no defined structure. Coal is a rock, which is an aggregate of minerals.Most minerals are made up of several elements.

Table 1 shows the percentage of different elements in the Earths continental crust.These elements combine to form molecules, which in turn combine to form minerals.Silicates form the majority of the Earths minerals. Figure 8 shows the evolution of rock.Crystals take on seven basic shapes or structures (Figure 9). Some elements andcompounds are polymorphic, ie, they can take on more than one crystal strucure(carbon forms both diamond and graphite). Examination of the crystallographic strucureof a particular rock mineral can tell us a lot about its history and formation. If a crystal isallowed to grow unhindered space wise, it will take on a perfect shape (Figure 10). Saltfor example forms cubic crystals. Commonly however in rock formation, crystal growth ishalted by growth of neighbouring crystals, or the crystals are abraded and fractured.Although there are many hundreds of minerals, there are 20-30 major rock formingminerals.

Table 1. Most Abundant Elements in the Earths Crust.

Element % by weight Element % by weight

O 45.2 Na 2.32

Si 27.2 K 1.68

Al 8 Ti 0.86

Fe 5.8 H 0.14

Ca 5.06 Mn 0.1

Mg 2.77 P 0.1

All Other 0.77

Figure 8. Evolution of Rock.



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Figure 9. Basic Crystal Shapes.

Figure 10. Example of Quartz Crystal in Rock Matrix Pore Space (approximately10m across).

Mineral Properties

Each mineral has properties dependant on composition and structure. Once we knowwhich properties are characteristic of which minerals it may not be necessary to carryout a chemical analysis. Various tests can be used to identify the type of structure, andto indicate the mineral present. Properties such as crystal shape, colour & streak, luster,hardness (Mohs scale), cleavage, specific gravity and optical characteristics can beused for identification (Figure 11).



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Figure 11. Examples of Common Mineral Properties. (Photos top to bottom: Breck P Kent, EdDeggenger & Bruce Coleman, Chip Clark, Chip Clark)

Mohs Scale of HardnessMineral Scale

NumberCommonObject

Talc 1Gypsum 2 FingernailCalcite 3 Copper coinFluorite 4Apatite 5 Knife bladeOrthoclase 6 Window glassQuartz 7 Steel file

Topaz 8Corundum 9Diamond 10Mineral Lustre

Metallic Strong reflections produced byopaque substances

Vitreous Bright, as in glassResinous Characteristic of resins, such

as amberGreasy The appearance of being

coated with an oily substancePearly The whitish iridescence of

materials such as pearlSilky The sheen of fibrous materials

such as silkAdamantine The brilliant lustre of diamond

andsimilarminerals

Some Chemical Classes of MineralsClass Defining Atoms Example

Native elements None: no charged atoms Copper (Cu)Oxides &hydroxides

Oxygen ion (O2-)

Hydroxyl ion (OH-)

Hematite (Fe2O3)

Halides Chloride (Cl-), fluoride (F

-),

bromide (Br-), iodide (I

-)

Brucite (Mg[OH]2)Halite (NaCl)

Carbonates Carbonate ion (CO32-) Calcite (CaCO3)

Sulphates Sulphate ion (SO42-) Anhydrite (CaSO4)

Silicates Silicate ion (SiO44-) Olivine (Mg2SiO4)

Physical Properties of MineralsProperty Relation to Composition & Crystal StructureHardness Strong chemical bonds give high hardness. Covalently bonded

minerals are generally harder than ionically bonded minerals.

Cleavage Cleavage is poor if bond strength in crystal is high and is good ifbond strength is low. Covalent bonds generally give poor or nocleavage; ionic bonds are weak and so give excellent cleavage.

Fracture Type is related to distribution of bond strengths across irregularsurfaces other than cleavage planes.

Lustre Tends to be glassy for ionic bonds, more variable covalent bonds.

Colour Determined by kinds of atoms and trace impurities. Many ioniccrystals are colourless. Iron tends to colour strongly.

Streak Colour of fine powder is more characteristic than that of massivemineral because of uniformly small grain size.

Density Depends on atomic weight of atoms and their closeness ofpacking in crystal. Iron minerals and metals have high density.Covalent minerals have more open packing, hence lowerdensities.



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Table 2 shows commonly occurring minerals in different rock types.

Table 2. Common Minerals in Rock.

Igneous Sedimentary Metamorphic

Quartz * Quartz * Quartz *

Feldspar * Clay minerals * Feldspar *Mica * Feldspar * Mica *

Pyroxene * Calcite Garnet *

Amphibole * Dolomite Pyroxene *

Olivine * Gypsum Staurolite *

Halite Kyanite*

* Indicates mineral is a silicate.

Basic Rock Types (Rock Clans)

The rock cycle (Figure 12) illustrates the relationship between the three main rock typesor clans: Igneous, Metamorphic and Sedimentary.

Figure 12. The Rock Cycle. (From UNDERSTANDING EARTH by Frank Press and Raymond Siever,1998, 1994 W.H. Freeman and Company. Used with permission.)

Igneous

The cooling and solidification of hot molten magma from the mantle forms igneous rock.Igneous rock can be classified as intrusive (intrinsic, plutonic) or extrusive (extrinsic,



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volcanic). Intrusive igneous rocks form as magma pushes its way up through cracks andfissures into surrounding rocks. Intrusives cool relatively slowly and crystals thereforehave time to develop. They are characterised by large crystal growth. Extrusive igneousrocks form when magma reaches the Earths surface, for example as lava flows fromvolcanic eruptions. These rocks are cooled rapidly and are characterised by fine crystalsthat have not had time to develop (Figure 13). If the lava is cooled extremely rapidly, theatoms have no time to rearrange into crystalline structures, and glass type structures orminaraloids are formed, obsidian for example.

Figure 13. Intrusive and Extrusi ve Igneous Rock Sources and Terms. (From THEDYNAMIC EARTH by B.J . Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. This material isused by permission of J ohn Wiley and Sons, Inc.)

Igneous rocks are the most abundant type of rock found in the Earth today, about 70%.Minerals such as quartz, feldspar, mica and olivine are important building blocks ofigneous rocks (Figure 14). Characteristically, the mineral crystals in igneous rocks havebeen restricted in growth by surrounding crystals, so their edges are amorphous inappearance (Figure 15). Igneous rocks of the same composition can be classified asdifferent rocks depending on cooling rate and resultant texture. For example, granite(intrusive) is coarse grained, but when the same compositional lava is cooled rapidly it

forms fine grained rhyolite (extrusive).



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Lavas vary from extremely fluid basalts to viscous and explosively eruptive rhyolites,depending on composition. Basalts are the most common fortunately as all majorvolcanic disasters around the World have been related to rhyolitic eruptions.

Figure 14. Minerals in Common Igneous Rocks . (From UNDERSTANDING EARTH by FrankPress and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)

Figure 15. Igneous Rock and Crystal Structure.

Sedimentary

Sedimentary rocks form when igneous, metamorphic or pre-existing sedimentary rocksare subjected to erosive forces (glaciation, wind, rain, and snow) (Figure 16). The rocksare broken down, and the individual grains and rock particles (detrital or clasticsediment) are transported away from the source area and redeposited in low-lyingareas. It is within such low lying basin areas that the majority of petroleum is found.Stratification of sedimentary rocks results from the arrangement of sedimentary particles

in distinct layers known as beds.



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The conversion of unconsolidated sediment to rock is termed lithification. Diagenesisis a term used to describe all the chemical, biological and physical processes involved ina rocks formation during and after lithification.

Clastic particles can be defined by size (Table 3) which in turn form different types ofrock (Figure 17). Crystals within sedimentary rocks that have been formed bymechanical erosion of source rocks tend to be rounded in appearance due to abrasion.

Figure 16. Erosion and Sources of Sedimentation. (From UNDERSTANDING EARTH byFrank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)

Table 3. Clastic Particle Definitions.

Name ofParticle

Range Limits ofDiameter (mm)

Name of LooseSediment

Name of ConsolidatedRock

Boulder >256 Boulder gravel Boulder conglomerate (b)

Cobble 64 - 256 Cobble gravel Cobble conglomerate (b)

Pebble 2 - 64 Pebble gravel Pebble conglomerate (b)

Sand 1/16 2 Sand Sandstone

Silt 1/256 1/16 Silt Siltstone

Clay (a)


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Figure 17. Rocks from Sedimentary Particle Types. (From THE DYNAMIC EARTH by B.J .Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. This material is used by permission ofJ ohn Wiley and Sons, Inc.)

Sedimentary rocks are the primary rocks involved in oil and gas formation and will becovered in greater detail in Topic 2.

Metamorphic

Metamorphic rocks form when igneous, sedimentary or pre-existing metamorphic rocksare altered by heat and pressure due to their deep burial in the Earth or due to a hotmolten rock intrusion. For example, in the subduction zone the pressure, temperatureand deformation which rocks are subjected to will lead to the formation of new mineralgrains, textural changes and thus new metamorphic rocks.

Metamorhpic rocks can be characterised by both grade and type of metamorphism.Figure 18 illustrates the grades of metamorphism depending on pressure andtemperature. The end result is controlled by factors such as chemical reactivity of inter-granular fluids, pressure, temperature, differential stress across the zone ofmetamorphosis and of course the time span involved.

Figure 18. Metamorphic Grades. (From THE DYNAMIC EARTH by B.J . Skinner and S.C. Porter,copyright 2000 J ohn Wiley and Sons. This material is used by permission of J ohn Wiley and Sons, Inc.)

The types of metamorphosis are defined relative to the physical conditions that arepresent during metamorphosis.

Regional is most common in the continental crust and may occur over tens ofthousands of square kilometres. Regional metamorphism involves high differential



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stress levels and a considerable amount of mechanical deformation, along with chemicalrecrystallisation. Low grade, regional metamorphosis of shale or mudstone forms slate.

The slaty cleavage planes are formed perpendicular to the direction of maximum stressduring metamorphosis. Regional metamorphism is a consequence of plate tectonics.

Contact metamorphism occurs more locally adjacent to bodies or intrusions of magma,due mainly to chemical recrysatallisation. The zone affected is termed an aureole.

Mechanical deformation tends to be minor due to generally homogenous stressesaround the magma intrusion.

Cataclastic, or dynamic, metamorphism may be found along faults where tectonicmovement leads to high differential stresses, and rock deformation. The rocks may befractured and ground almost to a paste resulting in a pulverised texture. Cataclasticrocks are often found alongside regionally metmorphosed rocks in narrow zones alongfault perimeters. These rocks often act as a major fluid barrier between rocks.

Burial metamorphism genarally occurs in deeply buried sedimentary basin rockswhere temperatures may be as high as 300 Celsius. The presence of water within thesedimentary rock speeds up chemical recrysatallisation processes. As with contactmetamorphism, there is little mechanical deformation. The resultant rock may appearphysically very similar to the original sedimentary rock, but will differ in its mineral

content.Hydrothermal metamorphism occurs due to chemical reactions between fluids and

heated rocks, and is often associated with mid ocean ridges.

Figure 19 shows examples of the rock types formed during metamorphosis dependanton pressure and temperature zones, termed facies.

Figure 19. Metamorphic Facies with Common Tectonic Settings Superimposed. (FromUNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H. Freeman andCompany. Used with permission.)

Figure 20 illustrates the minerals present during metamorphosis of shales. Quartz seenall way through, but changes in character. Plagioclase is only found in metamorphicrocks. Muscovite is an index for low and intermediate grade metamorphosis, Biotite forintermediate and Garnet for high grade metamorphosis.

Figure 21 illustrates areas of metamorphosis related to plate tectonics.



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Figure 20. Metamorphosis of Shales. (From THE DYNAMIC EARTH by B.J . Skinner and S.C.Porter, copyright 2000 John Wiley and Sons. This material is used by permission of John Wiley and Sons,Inc.)

Figure 21. Plate Tectonics and Metamorphosis Examples. (From UNDERSTANDINGEARTH by Frank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used withpermission.)



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Summary

Figure 22 Pictorially summarises rock types and Earth processes involved in theirdevelopment.

Figure 22. Interaction o f the Water, Rock and Tectonic Cycles. (From THE DYNAMIC

EARTH by B.J . Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of J ohn Wiley and Sons, Inc.)

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1 Basic Geology