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Plate Tectonics 10.2 Objectives Summarize the theory of plate tectonics. Identify and describe the three types of plate boundaries. Identify the causes of plate movement. Theory of Plate Tectonics • By end of 1960s evidence supporting continental drift and sea-floor spreading led to the development of a theory called plate tectonics. • Plate tectonics: Theory that Earth's outer shell is divided into several plates that are in motion (glide) over the mantle • Helps explains why and how continents move and is the study of the formation of features in Earth’s crust PLATE TECTONICS the surface of the Earth is made of rigid plates Size and position of plates can change over time PLATE BOUNDARIES Edges of plates, where two plates “float” side-by-side, is where geologic activity takes place Plates movement is from convection currents deep in the mantle World Plates The Crust • This is where we live! • The Earth’s crust is made of: Continental Crust - thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old -Rich in silica Oceanic Crust -thin (~7 km) - dense (sinks under continental crust) - young -Rich in Fe & Mg • 5 structural layers – – – – – Lithosphere Asthenosphere Mesosphere Outer core Inner core • 3 Compositional Layers: – Core – Mantle – Crust Structure of the Earth • Geophysical surveys: seismic, gravity, magnetices, electrical, – Acquisition: land, air, sea and satellite – Geological surveys: fieldwork, boreholes, mines How do we know what the Earth is made of? What are tectonic plates made of? • Plates are made of rigid lithosphere. – Consists of the crust and upper mantel What lies beneath the tectonic plates? • Below the lithosphere (which makes up the tectonic plates) is the asthenosphere. – Lower part of mantel – Mantel has molten rock (magma) How do we know where plate boundaries are? • Scientists identify plate boundaries by studying data from earthquakes. – When plates move, shifts occur along boundaries – Sudden movements are called earthquakes • Frequent earthquakes in given location are evidence of a plate boundary • The locations of volcanoes can also help identify the locations of plate boundaries. Earthquakes There are thousands of small earthquakes every day “Strong” earthquakes (~M7) occur once a month. >M8 occur about once/year. Where are the deepest earthquakes? For earthquakes of the past 2 weeks, go to http://www.iris.edu/seismon/ Earthquakes & Plate Boundaries Notice that the earthquakes coincide with plate boundaries, and the deepest quakes (blue) are in subduction zones. Question: Where would you expect to see volcanoes? Create your own maps at http://www.iris.edu/quakes/maps.htm Modified from USGS Graphics Volcanoes & Plate Boundaries This map shows that locations of volcanoes (ones above sea level) also tend to occur along the plate boundaries Modified from USGS Graphics Tectonic Plates How fast are the plates moving? Plates move 1-10 centimeters per year (≈ rate of fingernail growth). Fingernail growth plotted: http://jclahr.com/science/earth_science/thumbnail/index.html Modified from USGS Graphics