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Slab Pull? Density of Plate Slab The plate tectonic story www.earthscienceeducation.com Geography Workshop GA Conference 2021 Duncan Hawley in association with the Earth Science Education Unit and EarthLearningIdea www.earthlearningidea.com Image: Free download https://www.kissclipart.com/visual-tectonic-plates-clipart-crust-earth-plate-t-nnw9dx/ The plate tectonic story www.earthscienceeducation.com Aims of this session The workshop and its activities aims to: • provide an integrated overview of the concepts involved in teaching the processes of plate tectonics at KS3, KS4 and A level • survey of some of the recent evidence and key ideas in understanding how plate tectonics works • offer improved explanations for the distribution and characteristics of volcanoes, earthquakes, and some surface landforms • suggest approaches to teaching the abstract concepts of plate tectonics • help teachers decide if and how they should adjust what they presently teach to reflect the current understanding about the way plate tectonics operates • help teachers develop students’ critical sense of ‘the plate tectonic story’ encountered in textbooks, on diagrams, on the news, via the internet and in other media. The plate tectonic story www.earthscienceeducation.com Where on Earth are earthquakes and volcanoes? - geobattleships www.earthlearningidea.com/PDF/79_Geobattleships.pdf Battleship grid for Geobattleships © Dave Turner Galunggung eruption by USGS, public domain ‘North All Trucks’ © USGS The plate tectonic story www.earthscienceeducation.com Where on Earth are earthquakes and volcanoes? - geobattleships Earthquakes Volcanoes Battleship grid for Geobattleships © Dave Turner adapted by Duncan Hawley The plate tectonic story www.earthscienceeducation.com Distribution of earthquakes – what does the distribution show? Distribution of earthquakes – source unknown, redrawn by ESEU The plate tectonic story www.earthscienceeducation.com Distribution of earthquakes – what does the distribution show? Shallow to deep focus earthquakes = subduction zones The plate tectonic story www.earthscienceeducation.com Global Heat Flow Map showing areas of partial melting under oceanic areas (derived from mixed data sets in Hamza, Cardoso & Alexandrino, 2010) What is the heat source? The plate tectonic story www.earthscienceeducation.com Why are the Earth’s tectonic plates called plates? Picture of a Plate Image source. www.latzio.com/ The plate tectonic story www.earthscienceeducation.com Interior detection by a seismic scan Graphic credit: Richard Callender. Source: IRIS www.iris.edu/gallery3/general/posters/exploring_earth/WavePropagation The plate tectonic story www.earthscienceeducation.com Interior detection by seismic waves Wave type Primary wave Secondary wave Name meaning fastest wave, so slower wave, arrives arrives first, called second, called primary secondary Other names longitudinal – transverse – travels travels by vibration by lateral movement along the material push/pull wave; shake wave; shear comPressional wave; sideways wave wave; slow wave Transmission through solids through solids only and fluids (liquids and gases) Earthquake damage is caused mainly by seismic Surface waves, and not by Primary or Secondary waves The plate tectonic story www.earthscienceeducation.com Velocities of P and S waves as they travel into the Earth Graph of 'Velocities of P and S waves as they travel into the Earth © ESTA, redrawn by ESEU The plate tectonic story www.earthscienceeducation.com The seismic evidence for the structure of the Earth The Structure of the Earth – from the seismic evidence – reproduced with kind permission of USGS, redrawn by ESEU 13 The plate tectonic story www.earthscienceeducation.com Structure and Composition of Tectonic Plates Earth’s Outermost Layers Chemical composition • Continental crust (30-70 km) Granites av density 2.7 g/cm3 • Ocean crust (5-10 km) Basalts av density 3.0 g/cm3 • Mantle (Fe, Mg) to 2890 km >3.6 g/cm3 Physical behaviour • Lithosphere (100 km) - hard, rigid unit that forms the tectonic plates (they break!) • Asthenosphere (100-250 km) - softer unit capable of flowing Image source: Institute of Hydrological sciences National Taiwan University The plate tectonic story www.earthscienceeducation.com Lithosphere & Asthenosphere Mars™ Bar – analogy Faults in a MarsTM Bar (A rift valley) © Peter Kennett The plate tectonic story www.earthscienceeducation.com Modelling the mantle - the mantle can deform, can flow and can fracture Potty Putty™ Photographs of potty putty™ © Peter Kennett The plate tectonic story www.earthscienceeducation.com QUESTION What happens to the mantle when something sinks into it? The mantle is an elastic solid ( like Potty Putty ™) • It flows • Warmer mantle material flows (slowly) • Colder material holds its form The plate tectonic story www.earthscienceeducation.com Seismic ‘Scans’ Tomograms or Tomographic Sections c.700 km Red = high velocity c.2900 km = relatively warm Blue = low velocity = relatively cold c.700 km c.2900 km Source: based on the work of Van der Hist et al (1997) and Fukayo et al (2001) The plate tectonic story www.earthscienceeducation.com Source: www.nbvm.no/en/platetect.htm The plate tectonic story www.earthscienceeducation.com Source : What drives tectonic plates? Science Advances 2019 Oct; 5(10) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6821462/bin/aax4295_Movie_S3.avi The plate tectonic story www.earthscienceeducation.com The original idea explaining how plates move Arthur Holmes 1944 Mantle Convection cells Source: Holmes, A. 1944 Principles of Physical Geology. Thomas Nelson and Sons Ltd Chapter XXI The plate tectonic story www.earthscienceeducation.com So…what does drive the plates? Minute Earth Plate Tectonics Explained https://www.youtube.com/watch?v=kwfNGatxUJI The plate tectonic story www.earthscienceeducation.com QUESTION What happens to the mantle when something sinks into it? The plate tectonic story www.earthscienceeducation.com Where do plates ‘spread’ fastest? Image Source: Adapted from Figure 3. Zhu Ze et al. 2012 Global crustal movement and tectonic plate boundary deformation constrained by the ITRF2008 CC BY-NC-ND 4.0 The plate tectonic story www.earthscienceeducation.com The plate tectonic story www.earthscienceeducation.com What drives the plates? (Part of the map published by Eric Gaba (Wikimedia Commons user: Sting) under the Creative Commons Attribution-Share Alike 2.5 Generic license) The plate tectonic story www.earthscienceeducation.com What drives plate motion? Ridge Push? Mantle Drag? Slab Pull? density of plate slab Source: British Geological Survey www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/whatDrivesTectonicPlates.html The plate tectonic story www.earthscienceeducation.com Modelling the model - What drives the plates? Demonstrating how injection of magma at an oceanic ridge is unlikely to cause plate movement. Source: Earth Science Education Unit (ESEU) Slab pull © David Bailey The plate tectonic story www.earthscienceeducation.com Modelling the model - What drives the plates? Demonstrating how subduction of a cold dense plate exerting a pull on the whole plate is more likely to lead to horizontal movement of the plate and ‘splitting’ at the oceanic ridge Source: Earth Science Education Unit (ESEU) Slab pull © David Bailey The plate tectonic story www.earthscienceeducation.com Why do plates sink into the mantle? The density issue Source EarthLearningIdea: What drives the plates? In slab pull what pulls the slabs? The plate tectonic story www.earthscienceeducation.com What drives the plates? On a plate map: • Identify the Pacific plate, the Nazca plate and the South American plate. • For each if these plates measure (approximately) the total length of the plate margin - all parts of the margin, including the ridges/rifts, transform faults, and subduction zones. • Measure the length of that plate margin which is subducting (in the direction of the triangular ‘teeth’ shown on most maps (Part of the map published by Eric Gaba (Wikimedia Commons user: Sting) under the Creative Commons Attribution-Share Alike 2.5 Generic license) The plate tectonic story www.earthscienceeducation.com Global Heat Flow Map showing areas of partial melting under oceanic areas (derived from mixed data sets in Hamza, Cardoso & Alexandrino, 2010) What is the heat source? The plate tectonic story www.earthscienceeducation.com Why is there volcanic activity at plate margins? Increasing temperature (creates geotherm) but also Increasing pressure If the mantle is solid what causes melting ….anywhere? The plate tectonic story www.earthscienceeducation.com Lithosphere & Asthenosphere Source: Reproduced with permission of the Earth Science Education Unit (ESEU) The mantle is solid – but close to its melting point at 1300°C The plate tectonic story www.earthscienceeducation.com Global Heat Flow Map showing areas of partial melting under oceanic areas (derived from mixed data sets in Hamza, Cardoso & Alexandrino, 2010) What is the heat source? The plate tectonic story www.earthscienceeducation.com Why on Earth do rocks melt? Geotherm shows increasing temperature with depth – this is not linear as pressure (at depth) prevents melting due to molecules (in peridotie) not being able to move freely. The plate tectonic story www.earthscienceeducation.com Why on Earth do rocks melt? Partial melting can be achieved when the temperature of the mantle (indicated by the local geotherm) exceeds the melting point of peridotite. This can occur in a number of ways and in different geological environments as
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