Later in 1912, a German meteorologist named Alfred Wegener published a theory that all the continents as known today were once joined into one super continent named Pangaea which split by unknown means and form 2 more continents named Laurasia and Gondwanaland which then split again to form today’s continents. Wegener collated evidence from several different sciences. He first said that the remains of a small reptile which lived in Permian times had been found in South Africa and South America, also a plant which existed when coal was being formed has also been located in India and Antarctica, also rocks of similar type, age, formation and structure occur in South Africa and South America, coal has also been found in Antarctica yet coal can only be formed in warm wet conditions, evidence of glaciation in Brazil and India and also rocks which could not have been formed under current conditions have been found in Britain. All these pieces of evidence have pointed towards the continents once being joined or by having some sort of bridge between them.
Wegener’s theory of continental drift was dismissed by several scientists as he was not considered a specialist in any of the sciences he had used to develop his theory and he could not explain how solid land masses had managed to change their positions, he was unable to suggest the mechanism for the drift.
Wegener’s theory of Continental drift.
Since Wegener put forward his theory of continental drift there have been several pieces of new evidence that support his unproven theory. The discovery of the Mid-Atlantic ridge, which is a range of mountains that run the length of the Atlantic Ocean, found by Ewing in 1948 during an investigation of islands in the Atlantic has shown that the mountain range consists of fairly new volcanic rock – not ancient rock as previously presumed. This evidence proves that there is still physical activity of the earth’s crust still occurring today.
Studies of palaeomagnetism in the 1950’s showed that the rocks that were either side of the Mid-Atlantic ridge had small deposits of iron that were aligned along the earth’s magnetic field. It was known before the 150’s that the earth changed its magnetic field around every few 100,000 years, These deposits of iron have been shown to almost be symmetrical either side of the ridge in their alignment along the earth’s magnetic field. This shows that there have been new rocks generated along the ridge during the history of the earth leading many scientists to believe that the plates of the earth were increasing in size.
In 1962, Hess studied the age of the rocks from the middle of the Atlantic outwards to the shores of North America. He confirmed that the newest rocks were in the centre of the ocean and the oldest near the coast of the U.S.A. Hess also suggested that the sea floor of the Atlantic was widening by 5cm a year.
Sea-floor spreading and Magnetic reversal pattern
The evidence provided resulted in one major difficulty concerning the suggestion of sea-floor spreading. If the sea floor was increasing in size therefore it must mean that the earth was also increasing in size yet there was no evidence to suggest this. Evidence was needed to show that some parts of the crust were being destroyed, such areas of destruction were found to correspond to the fringes of the Pacific Ocean.
These discoveries have led to the construction of the theory of plate tectonics. This theory suggests that the lithosphere is split into 7 large and several smaller plates, which are rigid and float atop the mantle, and are moved by convection currents. These plates are categorised into 2 different sorts, continental and oceanic, the oceanic crust is more dense than continental and consists of silica and magnesium and is also made up of younger rock. The continental plates are older, less dense and consist of silica and aluminium.
As a result of the convection currents these plates are moved about either towards, away or sideways to other plates resulting in Fold Mountains, volcanoes, earthquakes and other major landforms.
Due to the lower density of the continental crust when the continental and oceanic crusts collide the oceanic crust is forced downwards into a subduction zone where the rock is heated and turned back into a semi-molten state, which rejoins the mantle or increases the pressure in the mantle and causes eruptions. This has occurred in the Pacific Ocean around Asia. When plates move apart to form constructive plate boundaries magma from the mantle surfaces and cools to form new rock between the plates forming mid-ocean ridges and volcanoes.
Plate tectonics
Using all the evidence that has been collated over the years has let us contrive a reliable theory which looks at all possible ideas that may have been overlooked it has lead us to complete a detailed model of the earth’s surface and the explanations for landforms and major natural hazards i.e. Earthquakes and volcanoes.
