Tectonics : the Earth quakes [2]

Tectonique : la Terre tremble [2]

The Earth : Drift away, plates - The drift [Genista]

By Nicolle Mathé, Genista Informations No 294, June 2003 (Plate tectonics)

Since Alfred Wegener and his "Theory on continental drift", for which, in 1911, he suffered a large number of detractors, the well-informed person is aware of the existence of plates and the turbulences generated by their movements.

Despite this, there is a very large number of people who live dangerously for having settled on their outskirts.

the movements of tectonic plates (in centimetres per year) [les mouvements des plaques tectoniques (en centimètres par an)]

Currents to be supposed

Some plates drift apart ³/₈ to 8 inches (1 to 20 cm) per year while others get closer to each other in the same limits.

They move, like a boat on water, and it is been imagined that the viscous magma underneath must be drifting.

The supposition was that, in the same way as hot air creates ascending currents and cold air, being heavier, creates descending currents which birds can use, very hot magma (1700°C) at the basis of the asthenosphere is going up under the lithosphere and once there, being colder (1300°C), it goes down, thus making circular or convection currents.

Plates go away from each other

Diagram I
effects of ascending convection currents [effets des courants convectifs ascendants]

Convection currents close to each other and going in opposite directions may push masses of magma upwards.

They exert a high pressure on the crust of the Earth (arrow 1) which, when it is thin, swells and finally breaks letting magma out.

The rest of the magma continues its movement under the crust in two opposite directions, a traction is exerted laterally on the lithosphere, on each side of the newly-made cleft (arrows 2) which opens ceaselessly.

This takes place permanently at the bottom of oceans, and lava, while getting cold, becomes basalt which constitutes the ocean floor, without stopping, from the cleft.

Thus, the bottom of oceans has a bulge 1·25 mile high (2000 m), 620 to 1850 miles wide (1000 to 3000 km) and 37,000 miles (60,000 km) long : it is the oceanic ridge. Its central zone, or rift, is full of cracks and always active.

Plates which go well together

Diagram II
effetcs of descending convection currents [effets des courants convectifs descendants] /

When convection currents close to each other going in opposite directions they may push masses of magma towards the base of the asthenosphere (arrow 1), and this creates a suction why may make a thin plate to plunge under a thicker one.

This is subduction (arrow 2). It takes place between some oceanic plates and continental masses.

Deep oceanic troughs, 6·2 miles deep (10,000 m) are formed along their edges. The plates which goes underneath becomes molten and gives magma again while the continental mass becomes folded to make a subduction range. Two continental masses may also collide with each other and make collision ranges.

Synthesis

Diagram III
Section from the Pacific plate to the Indian plate.

section from the Pacific plate to the Indian plate [coupe de la plaque pacifique à la plaque indienne] /

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Part 1 : The Earth - Its quakes • [Première : La Terre - Ses frémissements]

Cette Page en français • [This Page in French]

Part 3 : It folds or it breaks - an active planet • [Troisième Partie : Ça plisse ou ça casse - Planète active]

Contents : Nature & Environment • [Sommaire Nature & Environnement]