Revision - Tectonics

Apparently our exams are coming up? I don't really know, I think Millie said something about doing 7 hours of revision a week. That sounds a bit extensive to me, I'll give it a few more weeks until I start...






So I am going to start delving back into my favourite topic and prove my nerdiness to the world as I get excited by the Laki eruption of 1783 (and the synoptic links it offers), the Wilson cycle, the possibility of predicting earthquakes etc. I'm like a child before christmas. 


The first topic shall be... (drum roll)


THE EVIDENCE FOR CONTINENTAL DRIFT AND IT'S RELATIVE USEFULNESS 


So alrighty then, lets get this show on the road. Firstly what is continental drift? Continental drift is the movement of the Earth's tectonic plates in relation to each other, this can be moving towards each other, away from each other or laterally against each. 

Geologists (and aristocrats with nothing else to do) often identified the simplest evidence for continental drift. This is the fact that certain coastlines match with each other, for example in 1620 Sir Francis Bacon recognised that the coastlines of East South America and West Africa were closely related (though he failed to note that they can also form the tectonic rex...). This represents the first piece of evidence that can be used to prove continental drift, matching coastlines to each other. This however is very subjective which makes it unreliable and can match coastlines that are completely unrelated for example the UK looks as if it could slot quite snugly into the Gulf of Carpentaria (other people may disagree, illustrating the subjectiveness). What this evidence does allow is for further investigation, it's a cheap method of identifying areas which may be worth looking into. 


Biology and Climate have also been used to identify and help prove continental drift. By finding common fossils in vastly distant locations such as the Mesosaurus (which has been found in both Brazil and South Africa) we are able to distinguish that these locations were once connected. We can interpret these locations as being connected due to the traits of evolution, the chances of the same species evolving simultaneously in separate locations is almost impossible. A prime example of why we interpret evolution this way is by examining Madagascar. Madagascar split from Africa 160 million years ago meaning it's flora and fauna evolved independently of the rest of the world's. This has resulted in species similar to those found elsewhere but actually not related, for example the Tenrec which is often mistaken as a common hedgehog. This is an example of species filling the same niches in different areas but not evolving commonly, crucial to establishing the continental drift theory. 

The diagram illustrates sea floor spreading and how it can be
measured using palaeomagnetism

The discovery of the Mid-Atlantic Ridge in 1948 helped provide evidence for continental drift. Along the ridge there is a line of volcanoes which is where new oceanic crust is created, thus if oceanic crust is being created then the Earth is getting bigger but it's not. Therefore it must be being destroyed in a separate location. This, when coupled with the discovery of sea floor spreading in the 1960s provides (arguably) the strongest evidence for continental drift. Investigations into sea floor spreading were made possible with the development of Radio Carbon 14 dating (RC14) which has proven that as you move away from the mid-ocean ridges the age of the rocks increases. The final piece of 'modern' evidence for continental drift is Palaeomagnetism. This was discovered in the 1940s due to use of military SONAR equipment and then then theorised into what we understand today by Fred Vine and Drummond Matthews. Palaeomagnetism explores the concept that when iron bearing magma cool below the Curie point the iron within the rocks acts a "fossil compass" pointing towards the prevailing global magnetic field (the Earth's magnetic field switches between North and South and vice versa, every 400ka) . If the sea floor is spreading then the oceanic crust should show symmetrical magnetic stripes as you move away from the mid-ocean ridge. 


I reckon that's enough to be getting on with at the moment, if anyone has any questions feel free to ask away, we're here to help. I'm off to procrastinate instead of doing revision.