www.bgs.ac.ukPlate TectonicsThe 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.

Most of the edges of these plates are geologically active.There are three types of boundaries between plates:

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.

1

Somalisub-Plate

PacificPlate

Indian-AustralianPlate

Eurasian Plate Eurasian PlateNorthAmerican

Plate

NazcaPlate

Antarctic Plate

SouthAmerican

Plate

AfricanPlate

Arabian

Juan De FucaPlate

9cm

7cm

9cm4cm

4cm

1cm1cm

1cm

6cm

Divergent boundary

Oceanic plate (5-7km)

Convection drives the motionof the lithospheric plate

Upwelling magma

1

2

1 Lava cascades, Hawaii Volcanoes National Park, 1969.US Geological Survey, Department of the Interior/USGS

2 San Andreas Fault from Palo Alto to Saratoga, USA, 1980.US Geological Survey, Department of the Interior/USGS photo by R E Wallace

Plate Tectonics factsheet pdfwww.bgs.ac.uk/ask

Unless otherwise stated, British Geological Survey©NERC 2007. All rights reserved.

www.bgs.ac.uk

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.

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 damagingearthquakes e.g. the San Andreas Fault.

For further information:www.bgs.ac.uk/education/earthquakes.htmlwww.cotf.edu/ete/modules/msese/earthsysflr/plates1.html

2

Transform faultDivergent boundary

Divergent boundary

Upwelling

magma

Volcanoes

Oceanic crustContinental crust

Plate is subductedbeneath continental plate

as it is denserMolten rock wells up

to erupt fromvolcanoes

Plate Tectonics factsheet pdfwww.bgs.ac.uk/ask

Unless otherwise stated, British Geological Survey©NERC 2007. All rights reserved.

1

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