Plates- The Basics

The basics

Plates map

  • Lithospheric plates are made up of crust and the upper mantle. There are no gaps between plates. There are two types of plate – oceanic and continental.
  • Oceanic crust is more dense than continental crust. The denser oceanic lithospheric plate sinks beneath the continental lithospheric plate at subduction zones. For example the Nazca Plate descends (subducts) beneath the South American Plate near Chile.

Cross section plates

  • Oceanic crust is made largely of basalt, dolerite and gabbro rocks. These rocks are usually found in distinct layers with basalt at the surface and then dolerite followed by gabbro underneath the basalt.

Oceanic Crust

  • The crystal sizes of basalt are very small as the rock cools very quickly, the dolerite has slightly larger crystals than basalt as it cools less quickly and gabbro at the bottom cools even slower so has still larger crystals.
  • Basalt, dolerite and gabbro all contain crystals of augite, olivine and plagioclase feldspar.
  • Beneath the gabbro there is a rock called peridotite. This is largely green in colour and a mantle rock. When peridotite reacts with water it can form serpentinite.
  • Serpentinite is a very soft rock and can lubricate faults. This may be the cause of lubrication for the detachment fault we are looking at.
  • The asthenosphere is a partially molten layer (1 -5% liquid) within the otherwise solid mantle.


  • The solid mantle is not static but geologists do not generally believe that mantle convection provides the force to drive plate movement.
  • What drives the plates is disputed.
  • Many scientists think that the old cold dense oceanic crust descending beneath continental crust pulls the slab of crust causing movement at the ridges.
  • Other scientists think that the young, hot, less dense crust at Mid Oceanic Ridges pushes the plates outwards.

There is much research into how and why plates move and scientists are learning all the time as more data is collected.

Any questions please just ask via comments on the blog.

Hope this helps with basic understanding.
For more info please see: http://www.geolsoc.org.uk How do plates work?


Angela 🙂


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