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Plate Tectonics Plate Tectonics factsheet pdf www.bgs.ac.uk/ask www.bgs.ac.uk The outer crust of the Earth is divided into rigid plates. They vary in shape and size and move relative to one another over the globe, at an average speed similar to the growth rate of your fingernail. There are nine major plates: the Eurasian, African, South American, North American, Nazca, Antarctic, Pacific, Juan De Fuca and Indian-Australian. Eurasian Plate 9cm 1cm 1cm North American Plate Eurasian Plate Juan De Fuca Plate African Plate Pacific Plate 9cm Indian-Australian Plate South American Plate 6cm Nazca Plate Arabian 1cm 4cm 4cm Somali sub-Plate 7cm Antarctic Plate Most of the edges of these plates are geologically active. There are three types of boundaries between plates: 1 Divergent boundaries occur where plates move away from each other and fresh magma wells up to fill the gap creating new crust as it cools and solidifies. Divergent boundary Oceanic plate (5-7km) Convection drives the motion of the lithospheric plate 2 1 Lava cascades, Hawaii Volcanoes National Park, 1969. US Geological Survey, Department of the Interior/USGS Upwelling magma 2 San Andreas Fault from Palo Alto to Saratoga, USA, 1980. US Geological Survey, Department of the Interior/USGS photo by R E Wallace 1 Unless otherwise stated, British Geological Survey ©NERC 2007. All rights reserved. www.bgs.ac.uk Plate Tectonics factsheet pdf www.bgs.ac.uk/ask Convergent boundaries occur where plates collide and one plate is pushed underneath the other (subducted). Crust is returned to the interior of the Earth and as the old plate sinks the rock melts and erupts as volcanoes at the surface. Volcanoes Continental crust Oceanic crust Plate is subducted beneath continental plate as it is denser Molten rock wells up to erupt from volcanoes Transform fault boundaries, where plates slide past one another. Movement is not smooth but more like a stick-slip process, where sudden slips can cause damaging earthquakes e.g. the San Andreas Fault. 1 Div erg ent bou t faul ra ryT nda rm nsfo Dive rgen t bou nda ry ing ell a w Up agm m 2 1 San Andreas fault close to Olema, San Francisco, USA, 1906. US Geological Survey, Department of the Interior/USGS photo by G K Gilbert 2 San Andreas Fault, California. A view to the east showing the fault line bisecting the Highway 14 road. US Geological Survey, Department of the Interior/USGS photo by R E Wallace For further information: www.bgs.ac.uk/education/earthquakes.html www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html 2 Unless otherwise stated, British Geological Survey ©NERC 2007. All rights reserved.
