Tectonic Plates and Faults - Earthquake Engineering for Structural Design - page 143

Chapter 5
Tectonic Plates and Faults
5.1 PLATE TECTONICS
5.1.1 Major Plates
Today it is accepted that the Earth is covered by some rigid plates which move
across its surface, over and on a partially molten internal layer. Using geological
terms, the plates form the lithosphere , which is the Earth's solid rock. The rigid
lithosphere can be considered to float on the ductile asthenosphere , which flows.
So, the lithosphere (surface of the Earth) is broken up into what are called tectonic
plates ; Plate tectonics (from the Greek tecton , meaning one who constructs and
destroys) being the theory of geology developed to explain the phenomenon of
continental drift. This theory defines the tectonic plates and their boundaries. In
function of their surface, the tectonic plates are divided into major and minor
plates. Tectonic plates can include continental crust, oceanic crust or both. The
distinction between continental crust and oceanic crust is based on the density of
constituent materials. The continental crust is composed primarily of granite, so it
is relatively light. An average thickness may by around 30-40 km, while the
thickest part is about 70 km in the zone of the Himalayas and Tibet. The oceanic
crust is denser than the continental one, being composed of basalt. It is relatively
thin, about 7 km thick. As a result, the oceanic crust generally lies below the sea
level, while the continental crust is situated above the sea level. There are 52
important tectonic plates (USGS, nd, Wikipedia, nd). The 14 major tectonic plates
are shown in Figure 5.1. Among them, the seven most important plates are:
- African plate covering Africa (continental plate).
- Antarctic plate covering Antarctica (continental plate).
- Australian plate (known also as Indo-Australian plate) covering Australia
(continental plate).
- Eurasian plate covering Europe and Asia (continental plate).
- North American plate, covering North America and North-East Siberia
(continental plate).
- South American plate, covering South America (continental plate).
- Pacific plate (the biggest), covering the Pacific Ocean (oceanic plate).
In addition to the above great tectonic plates, the following can be also
considered as major plates: Arabian plate (continental plate), Caribbean plate,
Cocos plate, Juan de Fuca plate, Nazca plate and Philippine plate (all being oceanic
plates).
It is very important to underline that the tectonic plates do not coincide with the
continent forms.