I think it makes something more memorable when you know where something came from, so I thought I would look up the history of plate tectonics – which I find has been around for a surprisingly short time!!!
I read a few writings on tectonic history at http://www.ucmp.berkeley.edu/history/wegener.html from the University of California Museum of Paleontology, http://pangaea.org/wegener.htm SciLinks from NSTA and http://scign.jpl.nasa.gov/learn/plate2.htm, the Southern California Integrated GPS education module. The whole idea of plate tectonics had its origins in 1912 with a German scientist, Alfred Wegener, who noticed that if all the pieces of the earth were jigsaw pieces they would kind of fit in together perfectly. He actually wasn’t the first to notice this – some explorers had noticed the Africa looked as if it could fit into South America and there had been a paper written some 300+ years ago that suggested it – but Wegener was the first to propose they had been stuck together, were still moving apart and postulated a mechanism as to why they were moving apart.
Everyone at the time thought Wegener was pretty silly but he did have some evidence to back up his claims. Palaeontologists, digging up fossils, had found similar remains on the coasts of two continents that Wegener had predicted were once joined together, but were now separated by many thousands of kilometers (previously paleontologists had said the continents must have been linked by bridges). Wegener went further by developing his “Continental Drift” theory.
Wegener’s Continental Drift theory, although inaccurate, paved the way for the modern theory of plate tectonics. Wegener suggested that the continents had, in the past, all been joined together into a giant super-continent – which was later labelled as Pangea (Ancient Greek for Entire Earth) and that continents were moving away from each other toward the equator due to centafugal force from the earths rotation. He also hypothesized that as the continents move they encounter resistance from the crust in-front of them. The resistance causes areas of the continent to compress and fold upwards forming mountains near the continents edges. Wegener also suggested that India had drifted north into Asia forming the Himalayas and Mount Everest.
Wegener’s theory that continental drift caused mountains was reasonable to many followers – the alternative theory for why there were mountains was called called the “Contraction” theory. The Contraction Theory was that the planet was once a molten ball and, in the process of cooling, the surface contracted folded back over itself to form mountains. The big hole with this theory is that if it were right then you would expect all mountains to be the same age – and this was known not to be true. The Continental Drift theory was not without it’s problems – it was known the centrifugal force exerted on the continents would be far too low to cause them to move, but Wegener and some of his close supporters continued plugging away at the theory of continental drift and eventually alternative mechanisms were proposed (Weneger’s involvement eventually came to a tragic end in 1930 when he froze to death in Greenland). Two of the most notable advocates of Wegener’s theory of continental drift were an English Geologist, Arthur Holmes, who came up with the idea of convection cells, and Harry Hess, a US geologist, who developed the sea-floor spreading concept. Both of these geologists died in the 60′s – at the beginning of the wider spread acceptance of tectonic plate theory. Up until the mid 60s most scientists poopood the whole continent moving thing…
Eventually ridges under the sea were mapped and were found to be connected to sea floor spreading – and were also found to be connected to mountain patterns on land. The connection of the ocean ridges to mountains led to the idea that it was plates that were moving rather than continents and that plates were made up of the earths crust (containing both land and sea), and were slotted together at the ridges to totally cover the planet. The 1950s and 60s also saw the collection of evidence for Holmes’ convection cell sea floor spreading concept (still haven’t found out what that is yet…) through the development and installation of sonar equipment set up for war purposes. At this time there was also a comprehensive collection of seismology data – from readers set up to measure nuclear testing!!! and this data that made the significant link between these “ridges” , earthquakes and volcanic activity. Gosh could you just imagine the look on the geologists face when they stumbled across that!!! that would most certainly be an OMG moment…
The actual “tectonic plate theory” that is understood today, was written in full in papers submitted 1965-1967. It is now largely accepted in its current form by most geologists.
Further evidence to support tectonics is being gathered all the time. One of the significant technological advances which has allowed accurate mapping of tectonic movement is the development of GPS technology… the info from our little devices in the car actually comes from a satellite. NASA GPS monitoring satellites give indications of how much continents move during earthquakes (eg – they say Japan moved 2 meters!!!)
hmm so this is all really interesting… but is it a good idea to talk about what the past theories actually were – I think not… just incase students remember “centrifugal force and earths rotation” rather than the fact it was not an accurate mechanism…
Perhaps summarizing this to mention Wegener was the first to develop a theory of why continents were drifting apart, and even though his mechanism was not accurate, he did pave the way for other geologists to come up with a viable mechanism. Actually – this could be done as an example of scientific method at work. The other thing to note – because it is an essential learning from memory (shall look it up) – is that advances in technology have been part of the reason for the development of the theory of plate tectonics – seismic readers, marine geology equipment, satellite imaging, GPS… This info would best be suited to an engage phase in the first lesson.
So before delving into what tectonic plate theory is… I thought I would look up to see where Australia fits into this continental drift thing… I can see us squished into the right hand side of the Pangea map above…
The current proposed time line is that 200 million years ago the continents were joined together as a giant landmass – Pangaea – surrounded by a large Ocean – Panthalassa. (before this, apparently the continents were separated and moved together into the Pangaea formation.
150 – 200 million years ago, Pangaea broke into two super continents – Laurasia (forming a northern hemisphere continent) and Gondwanaland (forming a southern hemisphere continent).
These two supercontinents have broken into smaller continents which have drifted to their current positions – the Laurasian continent breaking into North America, Europe and Asia and Gondwanaland breaking into South America, Africa, India, Antarctica, Australia and New Zealand.
About 65 million years ago, Australia and Antarctica were still narrowly connected and shared the same cool temperate rainforest. About 50 million years ago, Australia separated from Antarctica and headed north at a rate of around 8 cm per year. Along with heading north came increased temperature, mountain building and isolation. We have been our own continent for 50 million years and this has led to the evolution of unique native flora and fauna which is not seen anywhere else in the world. The continents of the world continue drifting at an average rate of around 2.5-5cm per year – we can see this now more clearly through accurate measurements from GPS satellites – Australia is drifting 35degrees east of North at a rate of around 6.7cm per year.
This comes from Geoscience Australia’s website http://www.ga.gov.au/image_cache/GA10096.pdf (or more accurately, one of their teaching resource posters)
It would also be interesting for students to see this in the engage phase… Perhaps the poster link could be printed for students too – since it shows the different ages on a time scale
Next step… I think I need a refresher on the earth before tackling tectonic plate theory…