Deformation and Metamorphism - GeoScience - and 4 Deformation and... · Deformation and...
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Deformation and Metamorphism
Overturned microfolding of marble (pale layers) and phyllite (dark layers) produced by low-temperature regional metamorphism of Cambrian strata, Second Valley, Fleurieu Peninsular (Photo: Bernd Michaelsen). The geometry of these folds is related to the major fold in the area the Normanville anticline that outcrops along the coast.
Regional Processes Deformation
Key Ideas Intended Student Learning Deformation
Compressional and tensional forces acting on rocks cause joints, faults, and folds.
Contrast the conditions under which rocks are likely to break or fold. Explain the difference between a joint and a fault. Explain the difference between normal, reverse, and lateral faults. Describe forces that cause each type of faulting to occur. Explain the difference between an anticline and a syncline. Recognise, in the field, at least one of the deformation structures listed above.
The sections of the Intended Student Learning that are italicised must form part of the fieldwork or practical materials submitted for moderation. They will not be examined in the public examination
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Regional Processes Metamorphism
Rocks change in the solid state to become metamorphic rocks
Define the term metamorphism. Explain how heat, pressure, fluids, and time contribute to metamorphism.
Thermal metamorphism is caused by heat from an igneous intrusion.
Explain the formation of a metamorphic aureole surrounding an igneous intrusion. Identify the following thermal metamorphic rocks, name their parent sedimentary rocks, and describe the textural and mineralogical changes that have occurred: Hornfels Quartzite. Marble
Regional metamorphism is due to directed pressure and heat.
Explain the difference between load pressure and directed pressure. Explain, with the aid of diagrams, the development of foliation by directed pressure. Explain the difference between cleavage and bedding in rocks that have been subjected to regional metamorphism. Identify the following regional metamorphic rocks: Slate Schist. Gneiss Describe the textures and mineralogies of the rocks listed above. Describe the progressive formation of these three regional metamorphic rocks from their parent sedimentary rock, shale. Explain why the changes: sandstone quartzite limestone marble may occur in both thermal and regional metamorphism.
Topics 1.3.3 & 1.3.4 Deformation & Metamorphism Page 3 of 33
1.3 Regional Processes 1.3.3 Deformation BENDING OR BREAKING
Forces (or pressure) acting on rocks may cause deformation, which is likely to take one of two forms bending or breaking.
In response to the forces acting on it, the undeformed strata shown in the adjacent diagram may either:
bend to form folds,
or break to form joints.
FACTORS AFFECTING NATURE OF DEFORMATION
The pressure acting on a rock mass is likely to be a combination of load pressure, due to the weight of overlying rocks, and directed pressure caused by forces within Earth's crust. Load pressure acts equally in all directions, merely compressing the rock. By contrast, directed pressure squeezes the rock in only one direction.
Deformation of rocks is caused by directed pressure but the load pressure affects the nature of the deformation. If the load pressure is low, directed pressure is likely to cause the rocks to fracture. If the load pressure is high, the rocks are more likely to bend under the influence of directed pressure. Rocks deep within the crust are therefore likely to fold, while those closer to the surface are more likely to break.
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Changes in temperature affect the way in which rocks behave when subjected to stress. At higher temperatures, folding is more likely to occur than breaking. This is another reason why folding usually occurs deep below Earth's surface, whereas breaking occurs closer to the surface. 3. Fluids
Fluids which are present in the pore spaces of rocks and joints, act as lubricants, making is easier for rock layers to slide over each other. Thus the presence of fluids makes it more likely that the rocks will bend rather than break. 4. Time
Solid, brittle materials normally bend rather than break if they are deformed very slowly. Small forces applied over very long periods of time may cause rocks to deform by a process called creep. This can be observed in old buildings, where floor boards and marble mantelpieces may sag. JOINTS & FAULTS
Joints and faults are formed at or near Earth's surface when sudden forces act (e.g. earthquakes).
When rock fractures a joint is formed.
A fault is created if the rocks on one side of the joint move relative to rocks on the other side
The terms associated with faults are shown in the adjacent diagram. Learn all these terms and their meanings (i.e. footwall, fault scarp, hanging wall)
Topics 1.3.3 & 1.3.4 Deformation & Metamorphism Page 5 of 33
Extensional forces (i.e. forces pulling the rocks apart) produce normal faults. Here, the hanging wall block moves down relative to the footwall block. Reverse Faults
Compressional forces cause reverse faults. The hanging wall moves up relative to the footwall.
Lateral (Strike-slip) Faults
Lateral faults are formed when two blocks slide horizontally past each other. The San Andreas Fault (discussed in more detail later in the course) is an example of a lateral fault.
Folds develop deep in Earth's crust over very long periods of time, usually in response to horizontal compressional forces. The world's great mountain ranges were formed due to large-scale folding of Earths crust.
The diagram below shows some of the terms used to describe different types of folds, and parts of folds learn all these terms.
Visit the excellent web site on structural geology at:
and link to the sections on folding and faulting.
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(Tightly) folded rock formation at Moruya, NSW (photo D Vernon 2006; http://en.wikipedia.org/wiki/Image:Folded_Roc
k.jpg The process of forming dragfolds (fault-propagation folds) like these are associated with faulting and can be can be viewed in cartoon movie at http://www.uib.no/people/nglhe/StructModules
Folded Neoproterozoic (Ediacaran) sedimentary layers, Finnmark, northern Norway. (http://www.geo.uib.no/struct/Figures.html)
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The diagram above illustrates an orogeny (caused by compressional tectonics) in eastern North America between 543 and 440 Ma. Folding and faulting are associated with regional metamorphism (Source of image: http://upload .wikimedia.org/wikipedia/en/2/20/Taconic_orogeny.png). The diagram shows how orogenic processes caused the North American continent to grow eastwards. The Australian continent has similarly grown eastward since 543 Ma (i.e. the beginning of the Phanerozoic eon).
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1.3 Regional Processes 1.3.4 Metamorphism DEFINITION
The term metamorphism encompasses processes by which rocks within Earth's crust are changed over a long period of time by heat, pressure and fluids.
NB The processes of weathering and sedimentary rock formation are excluded from the definition of metamorphism, since these processes occur at or near Earths surface, rather than within the crust.
Processes in which the rocks actually melt are defined as igneous and are not defined as metamorphic.
Metamorphic changes take place in the solid state. FACTORS AFFECTING METAMORPHISM
1. Temperature (below the melting point of rock)
Minerals in sedimentary rocks (usually quartz, calcite and clays) are stable at low temperatures. Increasing temperature causes these minerals to change to different minerals (which are stable at these higher temperatures).
Increasing temperatures also cause a change (usually a decrease) in the water content of the rock. 2. Pressure
The pressure acting on a rock mass is likely to be a combination of load pressure, due to the weight of overlying rocks, and directed pressure caused by forces within Earth's crust, such as those which cause folds and faults to form. Both types of pressure compress minerals, squeezing their atoms together to form denser minerals that are stable under higher pressures.
Pressure can also alter the texture of a rock, resulting in an increase in grain size. Directed pressure results in the formation and alignment of flat (platey) minerals such as micas (e.g. biotite, muscovite). Micas are therefore characteristic of metamorphic rocks which have been affected by directed pressure. This texture is known as foliation. Fossils, or the pebbles in a conglomerate, can also become elongated by directed pressure.
Topics 1.3.3 & 1.3.4 Deformation & Metamorphism Page 9 of 33
Fluids from magma bod