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Gill Sans Bold Earth and Environmental Science Preliminary Course Stage 6 Dynamic Earth Part 3: Plate movement 0 20 I er b to T S c O EN g in D M t a r EN o p or AM c n 2 Number: 43180 Title: Dynamic Earth This publication is copyright New South Wales Department of Education and Training (DET), however it may contain material from other sources which is not owned by DET. We would like to acknowledge the following people and organisations whose material has been used: Photographs courtesy of Upgrade Business Systems and Ric Morante Part 2 p 14, Part 4 p 8 Photograph of tillite courtesy of Barbara Gurney Part 2 p 14 Photograph courtesy of Tim Reid Part 4 p 7 Photographs of Japanese mountains courtesy of Richard Alliband Part 4 pp 16, 17 Diagram from Veevers, JJ et al (1991) Australian Journal of Earth Sciences 38 p 384, courtesy of Geological Society of Australia Part 6 p 25 COMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING This material has been reproduced and communicated to you on behalf of the New South Wales Department of Education and Training (Centre for Learning Innovation) pursuant to Part VB of the Copyright Act 1968 (the Act). The material in this communication may be subject to copyright under the Act. Any further reproduction or communication of this material by you may be the subject of copyright protection under the Act. All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith. Published by Centre for Learning Innovation (CLI) 51 Wentworth Rd Strathfield NSW 2135 _______________________________________________________________________________________________ _ Copyright of this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or transmittal in whole, or in part, other than in accordance with provisions of the Copyright Act, is prohibited without the written authority of the Centre for Learning Innovation (CLI). © State of New South Wales, Department of Education and Training 2008. Gill Sans Bold Contents Introduction ............................................................................... 2 Moving continents ..................................................................... 3 Different names for plate margins .......................................................3 Divergent plate boundaries..................................................................5 Conservative plate boundaries ............................................................6 Destructive plate margins ..................................................................10 Mechanism for plate movement .............................................. 15 Suggested answers................................................................. 19 Exercises – Part 3 ................................................................... 21 Appendix ................................................................................. 25 Part 3: Plate movement 1 Introduction In the previous part you analysed a number of different types of evidence used to support the Plate tectonic theory. In this part you will gain further insights into the causes of plate movement and the mechanisms that are thought to be responsible for these movements. Within this part you will be required to model interactions between plates as well as model mechanisms for plate movement, by completing a series of activities. In doing so you will develop a greater understanding of the interaction between different plate boundaries and the processes that are occurring at each of the three boundaries. This part will provide a good grounding for the work you will undertake in the first Higher School Certificate module Tectonic impacts. In these parts you will be given the opportunities to learn to: • describe the processes that may occur when two plates collide • define the term ‘subduction zone’ and identify the geological features that are characteristic of a subduction zone • describe the plate tectonic model and use it to explain the distribution and age of continents and oceans. Extract from Earth and Environmental Science Stage 6 Syllabus © Board of Studies NSW, amended October 2002. The most up-to-date version can be found on the Board’s website at http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html 2 Dynamic Earth Gill Sans Bold Moving continents You have already been introduced to the concept that Earth’s lithosphere is made up of a series of plates. In the section, Lithospheric plates you labeled each of the major plates. Look closely at the map once again. What are the two types of plate boundaries shown in this diagram? _________________________________________________________ _________________________________________________________ Check your answer. You should have noticed a series of shaded triangles representing one type of plate boundary known as a destructive plate margin. The other type of plate boundary is shown as a series of parallel lines. This represents a constructive plate margin. If you look at the shape of the destructive margin you will notice it is more continuous and can curve. On the other hand, constructive margins are shown as distinct straight sections. The single line joining these straight constructive plate margins go to make up the edges of the plates and represent the third type of plate boundary. These boundaries are known as passive plate margins or transform faults. Transform faults allow the constructive margins to move around in a curve-like shape by sliding along these faults. Different names for plate margins Each plate margin is known by a different descriptive names. These names are derived from the processes or structures present at these sites. For instance the three plate margins are referred to above as being constructive, destructive and passive, depending on whether they create, destroy or maintain the amount of existing crustal material. Part 3: Plate movement 3 However plate margins can also be named according to their own type of geological structure. For example: • constructive boundaries are also referred to as mid oceanic ridges, • destructive boundaries are also be referred to as subduction zones. They are associated with trenches • conservative boundaries are referred to as transform faults or passive plate margins. Names have also been given to plate boundaries according to their relative movement. On either side of the mid oceanic ridge the plates move away from each other and are therefore also referred to as divergent plate boundaries. On the other hand at destructive plate boundaries plates converge on each other and collide. These are often referred to as convergent plate boundaries. Conservative plate boundaries can also be referred to as strike-slip faults because of the movement of the plates along the transform faults. As you can appreciate there are quite a few different names for the same type of plate boundary – all are correct for different reasons. Complete the following self-correcting question to help summarise these different names. Fill in the missing names for the following plate boundaries. Plate boundary 1 Plate boundary 2 Plate boundary 3 subduction zone (or trench) divergent conservative (or passive) Check your answers. 4 Dynamic Earth Gill Sans Bold Divergent plate boundaries The term divergent means to move away, and that is what is happening at these boundaries. The two adjacent plates are forced in opposite directions as new oceanic crustal material is being inserted and added to the inside edge of each of the plates, forcing them to move away from each other. mid-ocean ridge rift ocean crust continental crust Movement at divergent plate boundaries. Mid oceanic ridge producing ocean floor material between continental land masses. Because new material is being added to the plates through the creation of new oceanic floor, these boundaries are sometimes referred to as constructive plate boundaries. The mountain range that is produced, as a result of the magma upwelling from the upper mantle, is known as the mid oceanic ridge. These ridges make up the largest mountain ranges in the world, not only in length but also in average height above the level of the surrounding solid surface. It is not uncommon for these mountain ranges to rise some two to three kilometres above the level of the ocean floor. Ridges can range in width from one thousand to four thousand kilometres. Sea floor spreading not only gives a mechanism for the movement of plates, but it also gives an explanation as to why ocean floor crust is much younger than any of the continental crustal material. Mechanisms for plate movement will be dealt with in greater detail later in this module. Look again at the first section in Part 2 under the heading of Lithospheric plates. Look at the map showing the location of Earth’s major plates and observe the position of the constructive plate margins. What comment can you make about the location of these margins? _________________________________________________________ _________________________________________________________ Check your answer. Part 3: Plate movement 5 Conservative plate boundaries These plate boundaries are called conservative due to the fact that they neither create or destroy crustal material. Conservative plate boundaries form the edges of plates and occur in association with mid oceanic ridges. These conservative boundaries occur along transform faults where one plate will slide past another plate, separating sections of mid oceanic ridge. Transform faults at passive plate margins allow sections of lithosphere to slide past each other in opposite directions. Although these boundaries can also be referred to as passive plate margins, again because crustal material is not created or destroyed, they are in reality far from passive. What do you think will occur along the fault plane between the two opposing slabs of lithosphere? _________________________________________________________ _________________________________________________________ Check your answer. These regions produce some of the world’s strongest earthquakes. ocean ridge-rift Plate A transform fault Plate B lithosphere asthenosphere va rising la Transform faults separating a mid oceanic ridge into sections. Earthquakes occur along the transform fault between the mid oceanic ridge. 6 Dynamic Earth Gill Sans Bold The following activity will help demonstrate how plate movement at the mid oceanic ridge and transform faults can lead to the production of large earthquakes. A mid oceanic ridge and transform fault model What you will need: • scissors • sticky tape • coloured pencils • paper sheet printed with a model from the Appendix • a frame sheet from the Appendix. Creating the model: 1 Use your coloured pencils to shade the numbered segments on the sheet in the Appendix. Try to use a different colour for each number. For example if you select red for the number 1 segments then you must colour in the six segments numbered 1 in red. Use a different colour for all the segments numbered 2 and so on. 2 Use your scissors to cut along the two transform fault lines. Make sure you do not cut through either end of the sheet. Only cut between the two scissors symbols. ➩ Part 3: Plate movement ➩ 7 Use sticky tape to reinforce the handles at either end of the sheet. This will ensure that the sheet does not tear at the ends of the transform faults. 4 Get the sheet of cardboard from the Appendix. 5 Cut along the three lines to create three slits in the cardboard. These three slits represent the mid oceanic ridge. 6 Return to the sheet that you had coloured in and fold along the three dotted lines. Make sure you have the folds pointing down and away from you and not sitting up. 7 Place these folds through the top of the slits in the cardboard and then pull them through from behind until colour number 6 is just beneath the cardboard. ➩ 3 ➩ 8 ➩ Dynamic Earth Gill Sans Bold Using the model: Feature of model What the model feature represents three slits in the cardboard mid oceanic ridge two slits on the sheet of paper transform fault segments on sheet segments of ocean floor crust colour each colour represents a period in time, therefore all segments that are the same colour are also the same age region to the left of the three slits plate A region to the right of the three slits plate B 8 Hold on to the edges of the sheet and slowly pull the sheet away from the centre allowing the folded paper to come up through the three slits. ➩ 9 ➩ Take note of the colours that emerge from the three slits. Remember that these three slits represent three sections of the same mid oceanic ridge. 10 Take note of the direction that each section of the ocean floor is moving in relation to the other sections. Part 3: Plate movement 9 Analysing the results: 11 Draw arrows either side of the transform fault (on the ocean floor segments) indicating the direction the plate is travelling. 12 Mark in large letters on the ocean floor segments A for one plate and B for the other plate. Earthquakes mostly occur along the section of transform fault where plates are moving in opposite directions. 13 Use your model to find out where these sections are and then mark these sections with a red dot near the particular section of transform fault. Check your answers. Destructive plate margins If material is being added to a rigid plate at one end, then it must either be crumpled or destroyed at the other end. The term destructive plate margin is used because crustal material is being destroyed (destructive) over a wide area (margin rather than boundary). Colliding plates Crustal material is destroyed when two plates collide. The denser crust will be subducted down deep trenches underneath the less dense crust to be melted into the upper mantle. This is why these types of plate boundaries are also given the name of subduction zones. A convergent plate boundary. Describe the direction of the forces for each of the three plate boundaries. • divergent ____________________________________________ _____________________________________________________ • convergent ___________________________________________ _____________________________________________________ • transform or conservative _______________________________ _____________________________________________________ Check your answers. 10 Dynamic Earth Gill Sans Bold When plates collide they can generally fall into one of three categories: • continental – ocean collision • continental – continental collision • ocean – ocean collision. Continental – ocean collision This is where an oceanic plate collides with a continental plate. In these situations the denser oceanic crust is subducted underneath the more buoyant continental crust. The subducted oceanic crust is melted and the resulting magma rises back up through the overlying continental crust to form volcanoes. oceanic crust oceanic trench subducting oce ani c lit 100 km hos 200 km asthenosphere p he volcanic arc re continental crust asthenosphere melting continental lithosphere An ocean – continent collision One example where this is happening is along the west coast of South America. Here the Nazca plate is being driven towards and subducted beneath the South American plate. The force of these plates colliding and the volcanism has resulted in the production of the Andes Mountain range. Continental – continental collision In this category are collisions between two continental plates. An example where this type of collision is occurring is in northern India. Here the Indo-Australian plate has collided with the Eurasian plate. As a result India has subducted underneath the Eurasian plate. The crustal material making up both plates have a similar density. Therefore instead of India being subducted deep into the upper mantle it has continued to follow the underneath side of the continental crust on the Eurasian plate. Part 3: Plate movement 11 100 km asthenosphere 200 km Continental lithosphere Continental lithosphere A continental – continental collision This gives rise to continental crust reaching a thickness of about 70 km where as the average thickness of continental crust is about 40 km. The mechanism for why this type of subduction occurs is still being debated. One theory suggests the attachment of the oceanic plates either side of the Indian plate is strong so that the subduction of the oceanic crust is literally dragging the Indian subcontinent plate forward beneath Eurasia. Compare the diagram showing an oceanic – continental boundary with the diagram showing a continental – continental boundary. List two differences between these two boundaries shown by the diagrams. _________________________________________________________ _________________________________________________________ _________________________________________________________ Check your answers. Ocean – ocean collision This type of plate boundary occurs when two oceanic plates collide. oceanic crust 100 km 200 km oceanic trench subducting oce ani c lit hos asthenosphere p he island arc re melting continental crust asthenosphere continental lithosphere Ocean – ocean collision 12 Dynamic Earth Gill Sans Bold The Tonga-Kermadec Trench is a subduction zone that has been created from the collision of oceanic crustal material from different plates. The Tonga-Kermadec Trench occurs to the north east of New Zealand. Features of subduction zones You’ve seen that collisions between lithospheric plates can produce subduction zones where one plate moves under the other. What are the typical features of a subduction zone? The diagram following shows some typical features of an ocean – continent collision. accretionary wedge at trench oceanic crust (MORB) ✳ ✳ ✳ lithosphere continental crust ✳ ✳ ✳ ✳ lithosphere ✳ ✳ ✳ ✳ asthenosphere ✳✳ ✳ ✳ ✳ ✳ ✳ asthenosphere earthquakes (in ‘Benioff Zone’) rising magmatic plumes metamorphic rocks (folded) Features of a typical ocean – continent collision. Part 3: Plate movement 13 The features shown on the diagram are described below. Trench This is where the oceanic plate is being subducted. These trenches can be up to 11 km deep. Accretionary wedge This is the wedge of oceanic sediments that have been scraped off the subducting oceanic plate. This wedge of material accumulates in the trench between the subducting oceanic plate and the overlying continental plate. Wadati-Benioff zone The Wadati-Benioff zone (often abbreviated to Benioff zone) is a region of earthquakes generated along the leading edge of a subducted plate. The sites where earthquakes begin increase with depth and outline the angle and depth to which the plate is being subducted. Rising magmatic plumes When one plate is subducted beneath another the heat and pressure increases as the depth of subduction increases. Eventually the subducted plate melts and forms pockets of magma known as magmatic plumes. Because these plumes are much hotter than the surrounding country rock they rise and force there way up through the overlying plate. Metamorphic rocks The tremendous amount of heat and pressure generated from the contact of the colliding plates causes rocks to deform. During this deformation the rocks are folded and the minerals recrystallise to form metamorphic rocks. These minerals assemblages include the so called blue-schist facies or collection of minerals. These minerals are found only where high pressures are found with low temperatures. 14 Dynamic Earth Gill Sans Bold Mechanism for plate movement Although Alfred Wegener had clarified the theory of continental drift and was certain that continents that were once joined had since separated, he was at a loss to explain the driving mechanism for his continental drift theory. As a result Wegener received much criticism and his theory was left to stagnate until the late 1950s when Professor Harry Hess from Princeton University proposed his theory of sea floor spreading. Sea floor spreading provided a mechanism for continental drift. Hess proposed that mantle material had risen and erupted at mid oceanic ridges as a result of upward moving convection currents, and was deposited as new crustal material on either side of the ridge. The addition of new oceanic crust was said to have forced the lithospheric plates away from each other, thereby separating and moving the continents. continental crust oceanic crust rift valley (divergent convergent boundary) boundary divergent boundary oceanic crust convergent boundary transform fault convection cells plate movement Hess’ model of a mechanism for plate movement. As the oceanic crust moves away from the mid oceanic ridge the crustal material is allowed to cool increasing its density. At the same time oceanic sediments accumulate on top of the crust. Eventually this dense crust subducts into the partially molten upper mantle known as the asthenosphere in much the same way as an over loaded boat sinks into water. Part 3: Plate movement 15 This cool and dense subducted crust sets up a downward moving convection current, which then acts to drag the dense oceanic crust down further into the mantle where it is remelted. These convection currents set up convection cells within the upper mantle that contribute to driving the plates over the asthenosphere, similar to a conveyor belt system. Although this model has been widely accepted there is still much debate about where convection cells are set up, how large they are and sometimes even if they are set up at all. Some scientists argue that convection cells, if they are formed, would not be able to apply enough force to move continental plates laterally across the globe. One theory that has grown in popularity since the mid 1990s is that it is subduction of the dense slab that drives plate tectonics. This is a cold down driven convection rather than a warm up driven convection current. The primary heat source in Earth and therefore for the warm convection currents is thought to be from heat being generated by decaying radioactive elements within Earth. This heat is thought to assist the process of continental drift by heating up the mantle and allowing the mantle to penetrate areas of geological weakness in continental crust. As this molten magma from the mantle finally penetrates the crust along fissures in the ocean floor it solidifies as it cools. Oceanic crust is added as the continents drift away from each other. This process will be dealt with in greater detail in the module Tectonic impacts. The role of gravity A lot of emphasis in the past has been placed on the role of warm driven convection currents as the main driving force for plate movement. Much discussion and debate has taken place over the degree to which convection currents operating at the MOR push the plates apart. Another view is that the differences in crustal density result in the dragging of oceanic crust down into the upper mantle at subduction zones. If a continental plate is firmly connected to the other end of the subducting plate it is dragged along too! Oceanic crust is composed almost entirely of basalt. This basalt has a greater density than most continental crustal material. Basalt produced at mid oceanic ridges has a density of approximately 3.0 gcm–3 whereas continental crustal material can vary considerably, but would perhaps average somewhere near 2.7 gcm–3. 16 Dynamic Earth Gill Sans Bold When a lithospheric plate composed of oceanic crust confronts another lithospheric plate composed of continental crust, gravity acting on the denser oceanic plate would pull this plate beneath the less dense continental plate. closing ocean crust (density 3.0 g/cm3) continental crust (2.7 g/cm3) trench lithosphere asthenosphere Density of oceanic and lithospheric plates. It has been argued that this dragging force, and not convection currents, is largely responsible for the movement of plates. In reality it is probably a combination of both, but to what degree is a topic of further geological debate. A quick review 1 Look back through the parts of the module that you have completed. In your own words and using dot points, summarise the main features of the plate tectonic model. _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ Part 3: Plate movement 17 2 Do you think that crustal push at mid oceanic ridges is mostly responsible for plate movement? Or do you think that crustal drag at subduction zones is the main mechanism for plate motion? Give evidence to support your opinion. ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________ Turn to the end of this part and complete Exercise 3.1. Congratulations! You have now come to the end of this Part. 18 Dynamic Earth Gill Sans Bold Suggested answers Moving continents The two types of plate boundaries shown are constructive plate boundaries and destructive plate boundaries. Different names for plate margins Plate boundary 1 Plate boundary 2 Plate boundary 3 transform fault subduction zone (or trench) mid oceanic ridge strike-slip convergent divergent conservative (or passive) destructive constructive Divergent plate boundaries These margins are located within the oceans, usually away from continental margins. Conservative plate boundaries Friction between the two opposing slabs of lithosphere will produce earthquakes. Part 3: Plate movement 19 A mid oceanic ridge and transform fault model Your completed model should be similar to the diagram below. MID OCEANIC RIDGE ➩ PLATE A ➩ PLATE B ✳✳✳✳✳ TRANSFORM FAULT ➩ PLATE A ✳✳✳✳✳ PLATE A ➩ ➩ ➩ PLATE B TRANSFORM FAULT PLATE B ✳ Earthquake foci Colliding plates • At divergent plate margins the forces are tensional. They are pulling away from each other. • At convergent plate margins the forces are compressional. They are pushing towards each other. • At conservative plate margins the forces are moving side by side in opposite directions to each other. Continental – continental collision 20 • The thickness of the lithosphere at the point of subduction is much greater at continental – continental collisions than at oceanic – continental collisions. • The subducting oceanic crust in an ocean – continental collision is denser than the continental crust. Dynamic Earth Gill Sans Bold Exercises – Part 3 Exercise 3.1 Name: _________________________________ Exercise 3.1 1 2 The following terms refer to the three types of plate boundaries. Link names that refer to the same type of plate boundary with an arrow. constructive transform fault mid oceanic ridge passive diverging oceanic trench destructive converging When an oceanic plate converges up against a continental plate, the oceanic plate will always subduct beneath the continental crust. a) Define the term subduction zone. _________________________________________________ _________________________________________________ b) Explain why continental crust doesn’t subduct beneath oceanic crust. _________________________________________________ _________________________________________________ c) Outline how this provides an alternative to convection currents as a mechanism for continental drift. _________________________________________________ _________________________________________________ d) Outline why oceanic crust is much younger than continental crust. _________________________________________________ _________________________________________________ Part 3: Plate movement 21 3 The diagram below shows a cross-section through a number of plates. Look at this diagram carefully and answer the questions that follow. plate X plate Y plate Z fold mountain West A D sea level D East B E D C D C a) Name the features at: A _______________________________________________ B _______________________________________________ C _______________________________________________ b) Identify the type of crust shown at E. __________________________________________________ c) State the direction of movement of the following plates: X _______________________________________________ Y _______________________________________________ Z _______________________________________________ d) What name is given to the zone beneath the lithosphere labelled as D? __________________________________________________ e) Explain how zone D is said to aid movement of the plates. __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ 22 Dynamic Earth Gill Sans Bold 4 In the following box, draw a labelled diagram showing convection currents operating underneath a mid oceanic ridge. Ensure that you label the following: • mid oceanic ridge • oceanic crust • continental crust • direction of convection currents • plate A • plate B • oceanic sediment (near the ridge and away from the ridge) • direction of plate movement • asthenosphere. Diagram of convection currents underneath a mid oceanic ridge Part 3: Plate movement 23 24 Dynamic Earth Gill Sans Bold Appendix Materials for the ‘A mid oceanic ridge and transform fault model’ activity are on the following pages. These materials are: • a printed sheet to colour, cut and fold • a frame sheet for your model. Part 3: Plate movement 25 ✂ ✂ 8 13 10 7 12 9 6 11 8 5 10 7 4 9 6 3 8 5 2 7 4 1 6 3 5 2 1 4 1 2 3 3 2 4 1 FOLD 1 FOLD 3 1 2 3 FOLD 2 5 4 1 6 2 7 6 3 8 7 4 9 8 5 10 11 9 6 TRANSFORM FAULT 10 ✂ ✂ 26 5 Dynamic Earth Gill Sans Bold MID OCEANIC RIDGE ✂ CUT OUT SLOT 3 ✂ MID OCEANIC RIDGE CUT OUT SLOT2 TRANSFORM FAULT ✂ CUT OUT SLOT 1 TRANSFORM FAULT MID OCEANIC RIDGE Part 3: Plate movement 27