Tectonic Plates and Earthquakes
It is important to understand the world that surrounds us as knowledge allows us to progress. Whether we recognise it or not, the science of the Earth is highly significant as it allows us to understand the ground we stand on and its impacts on us. The theory of plate tectonics paints a very clear picture illustrating the Earth beneath our feet.
It is important to understand the world that surrounds us as knowledge allows us to progress. Whether we recognise it or not, the science of the Earth is highly significant as it allows us to understand the ground we stand on and its impacts on us. The theory of plate tectonics paints a very clear picture illustrating the Earth beneath our feet.
Tectonic Plates?
Prior to the discovery of plate tectonics, the world was ignorant to why the Earth was so unsettled. Why did the ground violently shake, what caused mountains to grow of heights such as Everest, why did volcanoes expel lava? Over centuries, these puzzles baffled scholars, who until the eighteenth century pointed to a Biblical Flood shaping the surface of the Earth. Catastrophism was the name assigned to the belief that geological events were a result of abrupt series of catastrophic phenomenon. The nineteenth century led to the belief of uniformitarianism, which suggested that these geological events were simply phenomenon which had always existed. It was not until the turn of the twentieth century when a valid explanation was proposed. |
Alfred Wegener and Continental Drift
In 1912, a German meteorologist Alfred Wegener had assembled a promising proposition. Upon noticing how the Earth’s continents seemed to piece together like a puzzle, Wegener suggested that the continents had previously been compressed together into one large supercontinent called Pangea and since had drifted apart, forming the world we know today. Subsequent to the development of his theory, Wegener came across fossils of the same plants and animals on different continents. With these plants and animals being unable to travel across the oceans, the widely accepted explanation at the time was that land bridges, which later collapsed, allowed these animals to cross continents. Wegener’s theory of ‘continental drift’, however, offered a different proposition, stating that these fossilised organisms lived in the period when the continents were conjoined and therefore were able to cross continents. Wegener’s theory also provided a sufficient explanation to the discovery of animal and plant fossils of differing climate to the location of their finding, such as tropical plants fossils found in Antarctica. Wegener then discovered that South America and Africa shared the same Glacial striae patterns and the Scottish highlands shared identical geology to the American Appalachians, further backing his theory. Although being able to provide sufficient evidence to his theory of continental drift, Wegener was unable to explain the mechanism which drove continents to drift apart as he believe they simply ploughed through the seafloor. Due to this Wegener's theory was disregarded by the scientific community.
In 1912, a German meteorologist Alfred Wegener had assembled a promising proposition. Upon noticing how the Earth’s continents seemed to piece together like a puzzle, Wegener suggested that the continents had previously been compressed together into one large supercontinent called Pangea and since had drifted apart, forming the world we know today. Subsequent to the development of his theory, Wegener came across fossils of the same plants and animals on different continents. With these plants and animals being unable to travel across the oceans, the widely accepted explanation at the time was that land bridges, which later collapsed, allowed these animals to cross continents. Wegener’s theory of ‘continental drift’, however, offered a different proposition, stating that these fossilised organisms lived in the period when the continents were conjoined and therefore were able to cross continents. Wegener’s theory also provided a sufficient explanation to the discovery of animal and plant fossils of differing climate to the location of their finding, such as tropical plants fossils found in Antarctica. Wegener then discovered that South America and Africa shared the same Glacial striae patterns and the Scottish highlands shared identical geology to the American Appalachians, further backing his theory. Although being able to provide sufficient evidence to his theory of continental drift, Wegener was unable to explain the mechanism which drove continents to drift apart as he believe they simply ploughed through the seafloor. Due to this Wegener's theory was disregarded by the scientific community.
Arthur Holmes’s Convection Currents
In 1929, British geologist Arthur Holmes stepped into the picture, introducing a hypothesis that elaborated on Wegener’s rejected theory of continental drift. Holmes proposed a mechanism that justified continental drift: convection. Holmes suggested that the rock deep within the Earth would buoyantly rise due to the heat radiated from the radioactive decay of elements in the Earth’s core. This current of heat thermal expansion causes rock to spread when it reaches the crust, the rock then loses heat and sinks back to the core. These convection currents are responsible for influencing the movement of continents. Holmes’s deduction had laid out the proof for the mechanism of continental drift that Wegener lacked. Despite this, there was no solid evidence to support Holmes’s theory prior to World War II, when radar technology was able to map out the seafloor.
In 1929, British geologist Arthur Holmes stepped into the picture, introducing a hypothesis that elaborated on Wegener’s rejected theory of continental drift. Holmes proposed a mechanism that justified continental drift: convection. Holmes suggested that the rock deep within the Earth would buoyantly rise due to the heat radiated from the radioactive decay of elements in the Earth’s core. This current of heat thermal expansion causes rock to spread when it reaches the crust, the rock then loses heat and sinks back to the core. These convection currents are responsible for influencing the movement of continents. Holmes’s deduction had laid out the proof for the mechanism of continental drift that Wegener lacked. Despite this, there was no solid evidence to support Holmes’s theory prior to World War II, when radar technology was able to map out the seafloor.
The Ocean Floor and Harry Hess
The advancement in radar technology during World War II allowed the ocean floor to be viewed and understood. The general perception ocean floor was that it was rather flat, however radar mapped out a very rough and uneven surface with large mountain ranges and rifts. There was also only a thin layer of sediments on the ocean floor. This was another shocking finding as the ocean floor was believed to be close 4 billion years old and a thick layer of sediments would have accumulated over time. To add to this confusion was magnetic striping. Rocks contain a mineral called magnetite, which has poles identical to a compass needle. Magnetite particles line up with the Earth’s magnetic field and all point to the same direction once molten rock is solidified. As the magnetic field changes between time, the magnetic field’s direction from different times are cast in stone. Symmetrical bands of rock with alternating weak and strong magnetism were discovered on the seafloor. These symmetrical bands are known as magnetic striping. Enter American geology professor Harry Hess. In 1962, Hess published his theory of seafloor spreading. Hess’s theory stated that the rifts in the Earth’s crust allowed magma, driven by convection currents, to flow to the surface. The magma would then cool to form new seafloor which, directed by convection currents, would spread on each side of the crack, giving room for magma to rise and form new seafloor. As the convection currents cool, the older seafloor would sink deep into the Earth, recycling the crust. Magnetic striping is attributed to seafloor spreading as the differently magnetised rocks would be directed symmetrically outwards from rifts. This also explained why the layer of sediments on the ocean floor was so thin as the sediments would subside along with the seafloor. Further evidence for seafloor spreading arose when the age of the seafloor was discovered to have varied, with the oldest rocks being only 200 million years old. This was evidence of the recycling Earth's crust as stated in Hess's theory. With these solid pieces of evidence, seafloor spreading had not only been accepted into Earth science, it had also provided solid evidence for convection currents and continental drift.
The advancement in radar technology during World War II allowed the ocean floor to be viewed and understood. The general perception ocean floor was that it was rather flat, however radar mapped out a very rough and uneven surface with large mountain ranges and rifts. There was also only a thin layer of sediments on the ocean floor. This was another shocking finding as the ocean floor was believed to be close 4 billion years old and a thick layer of sediments would have accumulated over time. To add to this confusion was magnetic striping. Rocks contain a mineral called magnetite, which has poles identical to a compass needle. Magnetite particles line up with the Earth’s magnetic field and all point to the same direction once molten rock is solidified. As the magnetic field changes between time, the magnetic field’s direction from different times are cast in stone. Symmetrical bands of rock with alternating weak and strong magnetism were discovered on the seafloor. These symmetrical bands are known as magnetic striping. Enter American geology professor Harry Hess. In 1962, Hess published his theory of seafloor spreading. Hess’s theory stated that the rifts in the Earth’s crust allowed magma, driven by convection currents, to flow to the surface. The magma would then cool to form new seafloor which, directed by convection currents, would spread on each side of the crack, giving room for magma to rise and form new seafloor. As the convection currents cool, the older seafloor would sink deep into the Earth, recycling the crust. Magnetic striping is attributed to seafloor spreading as the differently magnetised rocks would be directed symmetrically outwards from rifts. This also explained why the layer of sediments on the ocean floor was so thin as the sediments would subside along with the seafloor. Further evidence for seafloor spreading arose when the age of the seafloor was discovered to have varied, with the oldest rocks being only 200 million years old. This was evidence of the recycling Earth's crust as stated in Hess's theory. With these solid pieces of evidence, seafloor spreading had not only been accepted into Earth science, it had also provided solid evidence for convection currents and continental drift.
Plates Tectonics
Following up the theories of Wegener, Holmes and Hess as well as the discovery of rifts and mountain ranges, scientist were finally able to paint a picture of the Earth’s outer structure. Scientist concluded that the Earth’s crust is divided into separate plate and that mountain ranges and rifts were the boundaries of these plates. These boundaries was where plates interacted with each other, due to convection currents. Diverging plate boundaries are where plates separate from each other. Magma rises through rifts in the crust forming a new crust. Converging boundaries are where plates collide, creating subduction zones where one plate slides under another, submerging into the Earth. Transform boundaries are where plates slide against each other. These plates floated upon the asthenosphere, the uppermost section of the mantle. New oceans are made when the crust cracks and weakens, allowing water in from the ocean. An additional theory suggests that gravity also plays a role in tectonic movement in one of two measures: slab pull and ridge push. Slab pull would be the gravitational pull on denser plates in subduction zones, which pulls the plate back into the Earth. The differing theory is ridge push, which suggests that gravity pulls the new crust downwards due to new ocean crust forming at ridges above the crust. Older crust would then be pushed back into the Earth. With this, the theory of plate tectonics established and finally accepted into the scientific community. Plate tectonics have since then proven to be as crucial to Earth sciences as the atom to Chemistry and Physics.
Following up the theories of Wegener, Holmes and Hess as well as the discovery of rifts and mountain ranges, scientist were finally able to paint a picture of the Earth’s outer structure. Scientist concluded that the Earth’s crust is divided into separate plate and that mountain ranges and rifts were the boundaries of these plates. These boundaries was where plates interacted with each other, due to convection currents. Diverging plate boundaries are where plates separate from each other. Magma rises through rifts in the crust forming a new crust. Converging boundaries are where plates collide, creating subduction zones where one plate slides under another, submerging into the Earth. Transform boundaries are where plates slide against each other. These plates floated upon the asthenosphere, the uppermost section of the mantle. New oceans are made when the crust cracks and weakens, allowing water in from the ocean. An additional theory suggests that gravity also plays a role in tectonic movement in one of two measures: slab pull and ridge push. Slab pull would be the gravitational pull on denser plates in subduction zones, which pulls the plate back into the Earth. The differing theory is ridge push, which suggests that gravity pulls the new crust downwards due to new ocean crust forming at ridges above the crust. Older crust would then be pushed back into the Earth. With this, the theory of plate tectonics established and finally accepted into the scientific community. Plate tectonics have since then proven to be as crucial to Earth sciences as the atom to Chemistry and Physics.
References
- Rickard, G. (2011) Pearson science 9 student book. Frenchs Forest, NSW, Australia: Pearson Education Australia.
- Historical perspective [This dynamic earth, USGS] (no date) Available at: http://pubs.usgs.gov/gip/dynamic/historical.html (Accessed: 12 August 2016).
- Simison, B.W. (no date) Plate tectonics: History of an idea. Available at: http://www.ucmp.berkeley.edu/geology/techist.html (Accessed: 12 August 2016).
- Developing the theory [This dynamic earth, USGS] (no date) Available at: http://pubs.usgs.gov/gip/dynamic/developing.html (Accessed: 12 August 2016)
- FAQ on earth sciences - Earth Observatory of Sinagapore (no date). Available at: http://www.earthobservatory.sg/faq-on-earth-sciences/why-seafloor-so-recent-and-continental-crust-so-old (Accessed: 12 August 2016).
- SciShow (2012) Alfred Wegener: Great minds. Available at: https://www.youtube.com/watch?v=nbU809Cyrao (Accessed: 12 August 2016).
- Geo Dharma (2010) Plate tectonics. Available at: https://www.youtube.com/watch?v=ryrXAGY1dmE (Accessed: 12 August 2016).
- Arthur Holmes: Harnessing the mechanics of mantle convection to the theory of continental drift(2005) Available at: http://www.amnh.org/explore/resource-collections/earth-inside-and-out/arthur-holmes-harnessing-the-mechanics-of-mantle-convection-to-the-theory-of-continental-drift/ (Accessed: 12 August 2016).
- WGBH (1998) A science odyssey: People and discoveries: Arthur Holmes. Available at: http://www.pbs.org/wgbh/aso/databank/entries/boholm.html (Accessed: 12 August 2016).
- The geological society - pioneers of plate tectonics (no date) Available at: https://www.geolsoc.org.uk/Plate-Tectonics/Chap1-Pioneers-of-Plate-Tectonics/Harry-Hess (Accessed: 12 August 2016).
- FAQ on earth sciences - Earth Observatory of Singapore (no date) Available at: http://www.earthobservatory.sg/faq-on-earth-sciences/brief-history-plate-tectonics-theory (Accessed: 13 August 2016).
- Rickard, G. (2011) Pearson science 9 student book. Frenchs Forest, NSW, Australia: Pearson Education Australia.
- Historical perspective [This dynamic earth, USGS] (no date) Available at: http://pubs.usgs.gov/gip/dynamic/historical.html (Accessed: 12 August 2016).
- Simison, B.W. (no date) Plate tectonics: History of an idea. Available at: http://www.ucmp.berkeley.edu/geology/techist.html (Accessed: 12 August 2016).
- Developing the theory [This dynamic earth, USGS] (no date) Available at: http://pubs.usgs.gov/gip/dynamic/developing.html (Accessed: 12 August 2016)
- FAQ on earth sciences - Earth Observatory of Sinagapore (no date). Available at: http://www.earthobservatory.sg/faq-on-earth-sciences/why-seafloor-so-recent-and-continental-crust-so-old (Accessed: 12 August 2016).
- SciShow (2012) Alfred Wegener: Great minds. Available at: https://www.youtube.com/watch?v=nbU809Cyrao (Accessed: 12 August 2016).
- Geo Dharma (2010) Plate tectonics. Available at: https://www.youtube.com/watch?v=ryrXAGY1dmE (Accessed: 12 August 2016).
- Arthur Holmes: Harnessing the mechanics of mantle convection to the theory of continental drift(2005) Available at: http://www.amnh.org/explore/resource-collections/earth-inside-and-out/arthur-holmes-harnessing-the-mechanics-of-mantle-convection-to-the-theory-of-continental-drift/ (Accessed: 12 August 2016).
- WGBH (1998) A science odyssey: People and discoveries: Arthur Holmes. Available at: http://www.pbs.org/wgbh/aso/databank/entries/boholm.html (Accessed: 12 August 2016).
- The geological society - pioneers of plate tectonics (no date) Available at: https://www.geolsoc.org.uk/Plate-Tectonics/Chap1-Pioneers-of-Plate-Tectonics/Harry-Hess (Accessed: 12 August 2016).
- FAQ on earth sciences - Earth Observatory of Singapore (no date) Available at: http://www.earthobservatory.sg/faq-on-earth-sciences/brief-history-plate-tectonics-theory (Accessed: 13 August 2016).