PX266 Geophysics (2010/11)
Lecture 13 Handout – Plate Tectonics
Dr. Gavin Bell
The basic concept of plate tectonics is that the Earth’s lithosphere is divided into a
small number of nearly rigid plates (there are seven major plates and several more
minor ones). Most of the deformation – stretching, folding and shearing – in the
lithosphere takes place near the boundaries of the plates. Deformation well inside the
plate boundaries is much smaller than deformations occurring at or near boundaries.
A seismicity map neatly outlines many of the plate boundaries since most volcanism
and earthquake activity occurs in regions of strong lithospheric deformation. The
main points are:
The generation of new plate material occurs by seafloor spreading, where new
oceanic lithosphere is generated along active mid-ocean ridges.
Once generated, the new ocean lithosphere forms part of a rigid plate which
may or may not include continental material.
The surface area of the Earth remains constant. Consequently, seafloor
spreading must be balanced by the consumption of plate material elsewhere.
The rigidity of the lithospheric plates means that they can transmit stress over
great distances without buckling.
The relative motion between plates is taken up almost entirely along plate
boundaries.
The continents can be considered as “rafts” of less dense material which remain on
the surface, while the denser oceanic lithosphere is subducted beneath other oceanic
or continental lithosphere. A single tectonic plate can comprise both continental and
oceanic lithosphere, but the latter is created or destroyed far more easily.
