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CONTINENTAL DRIFT AND PLATE TECTONICS
Since it was created, the earth has undergone many changes, including landform creation
and destruction and changes in the relative position of the continental landmasses.
Geologists have put forward many theories to explain earth surface changes but none have
succeeded in providing universal explanation. The theory of plate tectonics which builds
upon the concept of continental drift, however is a unifying theory that explains many of
Earth's major geologic features.
(1) CONTINENTAL DRIFT
Alfred Wegener proposed the theory of continental drift back in 1912. The theory suggests
that there has been large-scale movement of continents across the globe and that during the
Permian period, 225 million years ago, all the continents were joined as one super continent
Pangaea. Around 200 million years ago, Pangaea split into Laurasia and Gondwanaland.
The continents have continued to move and today's configuration of continents represents
the most recent stage in their movement.
Continental Drift theory is based on the following evidence
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THE JIGSAW FIT OF THE CONTINENTS - there is a noticeable jigsaw fit between
many of our continents - for example, between the East Coast of South America and
the West Coast of Africa, which suggests that at some point in time the continents
were once assembled together.
PLANT / ANIMAL FOSSILS - A number of identical fossils have been found
distributed across the southern continents, again suggesting that they were once
joined. Fossils of the Mesosauras dating back 280 million years ago have been found
in South America and Africa (and nowhere else) - it is known from the fossil that this
animal could not swim. Plant Fossils, such as Glossopteris (a tree) have been found
in South America, Africa, India and Australia.
GEOLOGICAL SEQUENCES - A number of continents show evidence of matching
geological sequences with rocks of simillar age, type, formation and structure occuring
in different countries - e.g. the Appalachians (E USA) show a geological match with
mountains in NW Europe and if they were fitted together would form a single
continuous mountain belt.
CLIMATOLOGICAL ANOMALIES - A number of climatic anomalies have been
discovered which suggest that continents must once have been in a different position
and therefore have experienced a different climate. Coal which only forms under wet /
warm conditions have been found beneath the Antarctica ice cap and there is
evidence of glaciation in Brazil (which now has a equatorial climate)
Whilst continental drift theory is very important, at the time it was not widely accepted due
to the lack of explanation of how / what forces would be capable of resulting in moving the
huge rock masses.
(2) THE THEORY OF PLATE TECTONICS
It was following developments in the exploration of the ocean floor in the 1950s that new
evidence was found to revive continental drift theory and led to the development of a theory
to explain the movement of the continents - plate tectonic theory.
The theory of Plate Tectonics is based around the idea that the crust is broken up into a
series of large crustal plates which "float" on the asthensophere below. Motions in the
asthenosphere, called convection currents, cause plates to move away from each other at
the rising limb of a convection current, forming a constructive plate boundary where new
oceanic crust is formed. As plates continue to move outwards, eventually the oceanic plate
may be subducted at a destructive plate boundary.
Supporting evidence for Plate Tectonics Theory: (You must learn this..)
1. Discovery of the Mid-Atlantic Ridge (Ewing) - Ocean floor mapping led to the
dsicovery of a global mid-oceanic ridge mountain chain zig-zagging around the continents.
2. Magnetic Variations on the Ocean Floor (Palaeomagnetism) - during cooling,
minerals in the Basaltic rock, align themselves along the Earth's magnetic filed - forming a
permanent record of magnetic field in the rocks. Periodic variations in the earth's magnetic
field, have produced almost symmetrical magnetic patterns in the rocks either side of the
Mid-Atlantic ridge (althernating stripes of magnetically different rocks).
3. Theory of Sea-Floor Spreading (Hess) (Good overview of Sea Floor Spreading from
ThinkQuest) - Hess, put forward an idea that mid-ocean ridges are a structurally weak point
where magma is able to rise to the surface and where due to the upwelling and eruption of
this material, new crust is created. This helps, to support the continental drift theory as it
helps to explain how the continents may be moving, as they are carried on the 'spreading'
ocean floor. Hess's theory was supported by the fact that the youngest rocks are nearest to
the ridge (showing the present day magnetic polarity in their mineral alignment) and the
oldest rocks (showing reversed polarity) are further away from the ridge.
The problem with Hess's Theory was that "sea floor spreading" and the associated
development of new oceanic crust, suggests that the earht is increasing in size, although we
know this is not the case. Hess therefore realised that whilst in some areas, new crust was
being formed, elsewhere old crust must be being destroyed. Examples of where this occurs
were found around the Pacific Ocean with crust being subducted and destroyed at deep-sea
ocean trenches.
Therefore the theory suggests that newly formed oceanic crust moves away from a
ridge rather like a conveyor belt and descends (millions of years later) into a
deep-oceanic trench.