Geopix UK - Tectonics

Plate Tectonics

A number of 19th and 20th century scientists, notably Alfred Wegener (in 1912), commented on the similarity between the coastlines of South and North America on the west coast of the Atlantic and the coastline of Africa and Europe on the east side of the Atlantic and suggested that at one time these continents fitted together. Wegener called this supercontinent Pangaea (meaning "all the world"). At some time in the geological past, the supercontinent split apart, and the pieces drifted away from each other.

Do continents move? If so, how?

The other Study Topics in this group describe the evidence that is used to show that continents have been moving slowly across the surface for the Earth for hundreds of million of years. The present system of movements has been in operation for about 200 million years. Before that, different systems were active with the continents appearing to move in other patterns.
The evidence comes from

rocks deposited in particular environments, such as coral limestones and coal;
remanent magnetism showing palaeolatitudes.

Not only do the continents move, but the ocean floors spread as they are created at the mid-ocean ridges.
The evidence comes from

magnetic anomalies (magnetic stripes) which show patterns of reversals in the polarity of the Earth's magnetic field;
the symmetry shown by the stripes on either side of the mid-ocean ridges;
a study of the age of the ocean floor, which increases away from the mid-ocean ridges.

The uppermost zone of the Earth, the lithosphere, is divided into a number of sections or lithospheric plates of varying shapes and sizes. The lithosphere is supported by the underlying, more ductile asthenosphere. Instead of the continents drifting by themselves, floating on a "sea" of mantle material, it is the plates which move, carrying with them a crust of continental landmass, or oceanic crust, or a bit of both.

The main lithospheric plates

We now know a lot about the inside of the Earth, enough to suggest that there are convection currents within the mantle (or within the asthenosphere itself) which move very slowly, at a rate of a few centimetres a year. Where a convection current rises towards the surface and spreads sideways, plates are split apart and carried sideways. Where two convection currents meet, one plate is carried down beneath the other, into the mantle.

This suggests that the boundaries between the lithospheric plates are particularly active, with zones of volcanoes and earthquakes. Perhaps we should look at it the other way round - we can identify where the boundaries are by following the zones of volcanic and earthquake activity.

Japan is a group of volcanic islands on the tectonically active rim of the Pacific Ocean. Sakurajima is one of the country's most active volcanoes. The nearby town of Kagoshima has shelters from earthquakes and volcanic bombs, and well-rehearsed evacuation procedures.

Three main types of plate boundary can be identified:

constructive margins where the plates move apart and new ocean crust is formed as molten magma comes to the surface;
destructive margins where plates "collide" and one of them is destroyed as it is assimilated into the mantle; the immense forces compress the crust into fold mountains;
conservative margins where two plates move past each other, with little to show - except for the frequent earthquakes.

The fault (in Iceland) which is covered by this path is claimed (in some guide books) to be the plate boundary between America and Europe. That is not quite accurate, but it is the closest that most of us will get to a plate boundary. In fact, this fault is one of many which make up one side the the axial rift which forms the boundary.

In summary, new plates are created from mantle material at the constructive margins and assimilated back into the mantle at the destructive margins - re-cycling on a grand scale.

Fortunately, not everything is taken down into the mantle. The lighter continental crust stays 'floating' on the surface, giving us somewhere to live. But it is as well to remember that, if there were no Plate Tectonics, there would be no earthquakes, no volcanoes, no mountain building - and no land for humans to live on.

Geopix Study Topic "Plate Tectonics"

describes the evidence for continental drift and sea-floor spreading
fit-similarity
remanent magnetism
polar wandering
magnetic anomalies
age of the ocean floors
gravity anomalies and heat flow

describes the major features found at plate boundaries - such as the Benioff zone, accretionary prisms;
describes the major types of plate boundary, including plate movement, volcanoes, earthquakes: