Plate tectonics and the creation of continents

All the continental areas have cores of ancient rocks. These rocks range in age from 2.5 Ga (= 2500 million years old) to 3.75 Ga. These areas of ancient rocks are termed cratons or continental shields. They were formed during the very early stages of the Earth’s evolution, during which buoyant silica-rich material segregated from the mantle and became concentrated into continental rafts above descending mantle convection currents. Subsequent growth of these continents took place by the addtion (accretion) of new continental material to the edges of these core regions.

Continental drift

One implication of plate tectonics is that continental land masses have not remained fixed in one place over geological time, but have continuously moved in relation to each other. This diagram (right) shows a proposed sequence of such continental drift through the Phanerozoic (click on image to enlarge).

Continental drift: the evidence

1. Topographic fit
Inspection of any global map will show that the edges of continents such as South America and Africa appear to match. This apparent jigsaw-like fit is no coincidence - the two continents split apart from one landmass during the Cretaceous period, and oceanic crust developed in between them.
(Click image to enlarge)

2. Tectonic fit
When the continents are fitted together according to topographic fit, the positions and ages of tectonic features which occur on the various continental landmasses seem to join up. This is best seen in the cratons of Africa and South America, and in the mountain belts of northern Africa and southern Europe. Another line of evidence is the alignment of glacial striations (scratches caused by ice) in rocks caused by the movement of ice sheets over the southern continents during the Permian and Carboniferous periods.

3. Stratigraphic correlation
Continents which were once part of the same landmass will show similar rock sequences along their margins up until the time when they split apart. After the continental divide, each landmass will show a different stratigraphic sequence (sequence of rocks). In the African and South American plates, similar freshwater rocks found along the continental margins can be correlated up until the end of the Lower Cretaceous, when continental rifting split the continents apart. The onset of rifting is marked by a sequence of evaporites on both landmasses, but after this the two continents show different marine stratigraphic sequences, indicating that they had drifted away from each other and became separated by ocean.

4. Palaeontological correlation
In much the same way as above, fossils contained within stratigraphic sequences which are now located on different continents can indicate that the two landmasses were once joined. The best example of this are the mesosaurus fossils found in early Permian freshwater shales in both Brazil and southern Africa - the presence of this fossil on both continents suggests that the two areas were connected during the lifetime of the Mesosaurs.
(Click image to enlarge)

5. Apparent polar wandering curves
Newly-formed oceanic crust generated at mid-ocean spreading ridges preserves a record of the orientation of the Earth’s magnetic field at the time of rock crystallisation. As well as preserving the orientation of the magnetic field (either normal or reverse polarity), these oceanic crustal rocks also tell us the location of the magnetic poles at the time of crystallisation. In addition, the information recorded in oceanic basalts enables us to calculate how far away the magnetic pole was when the igneous rock crystallised.
Evidence collected from various magnetised igneous rocks showed that the magnetic poles appeared to wander around the surface of the earth with time. Lines connecting positions of where the poles were thought to lie at certain points in geological history are known as apparent polar wandering curves. However, polar wandering curves from different continents show different paths, so it was soon realised that it was unlikely that the magnetic poles moved about the surface of the earth so much, and the obvious explanation was that it was the continents that had been moving, not the poles.
Polar wandering curves from two present day continents will match together at certain times in history if those two landmasses were once joined. The point of divergence between the two polar wandering curves marks the point of continental rifting.

6. Ocean floor magnetism
One of the key lines of evidence in the development of the sea floor spreading theory was the discovery of symmetrical magnetic stripes on either side of the mid-ocean ridges. Once it was discovered that the stripes on one side of the spreading ridge could be correlated with a almost identical stripe on the other side of the ridge, sea floor spreading theory became more widely accepted.

7. Palaeoclimatic evidence
Evidence of past climates preserved in the British rock record shows that Britain has gradually drifted northwards across the globe over the past 600 million years.

Find out more about plate tectonics:

Plate tectonics make the world go round: introduction
Constructive margins: Continental (rift valleys)
Oceanic (mid-ocean ridges)
Destructive margins: Continental collision
Ocean-continent destructive margins
Ocean-ocean destructive margins (island arcs)
Conservative margins
Continental drift
Plume and hotspots

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February 2007