Alfred Wegener first proposed the theory of continental drift in the early 20th century, after he noticed the remarkably close fit between the continental shelves of South America and Africa. Based on fossil records, he extended the connections between continents to postulate a super-continent, Pangaea. Wegener theorized that Pangaea began to break apart, and eventually its parts became the continents we know today. However, Wegener’s theory had a major flaw: he was unable to explain the process which could cause the solid masses of the continents to move in the way he proposed. What forces could possibly be strong enough? Because he could not answer this question, his theory was rejected by the scientific community (Kious & Tilling, 2012). But when further evidence was gathered in the latter part of the 20th century, a new theory, plate tectonics, was created to explain Wegener’s ideas, and today this theory is accepted.
The path that Wegener’s theory took towards acceptance was an excellent example of the scientific method. Ideally, all scientific discoveries and theories should follow the scientific method. This process has several steps: 1) making observations, 2) creating a hypothesis to explain what was observed, 3) using the hypothesis to predict other observations, preferably in the context of controlled experiments, and 4) completing experiments or other observations to determine how well facts fit the hypothesis (Wolf, 2013). At the end of #4, the method often starts again at #1, because it is rare that a hypothesis will be 100% correct the first time it is tested! By using the scientific method, one can expand knowledge of the natural world — not just knowledge of facts, but also knowledge of processes and mechanisms. Observations can be qualitative or quantitative. Qualitative information asks questions of what, where, when, and how. Quantitative information asks how many or how much. Both types of information are essential to the scientific method.
In the case of continental drift, Wegener observed that the continents fit together, and that there were similarities in the southern continents with regard to rocks, fossils, and glacial deposits (qualitative observations). His hypothesis was the concept of continental drift. Unfortunately, Wegener died before evidence was found that would further explain his observations (Kious & Tilling, 2014).
One of the keys to the puzzle was the depth and contours of the ocean floor. New methods of measurement showed that it was not flat and featureless; in some areas the ocean floor was much deeper than other areas, and there were trenches and mountains under the water. Analysis of the sediment on the ocean floor also suggested it was much younger than previously thought. Another piece of information was the discovery of magnetic striping — strips of land that were magnetized in different directions. It was already believed that magnetic poles had shifted over the history of earth, so this striping must mean that the rocks involved were formed during times of shifting polarities. This evidence supported Wegener’s theories (Kious & Tilling, 2012).
The 21st century theory of plate tectonics states that the continents and oceans are made up of solid masses of rock (crust) that rest on hot, liquid rock called magma. The movement of plates became widespread over a billion year period between 3 and 4 billion years ago (Bercovici & Ricard, 2014). Boundaries between plates are categorized as convergent (moving together destroys crust), divergent (moving apart creates crust), transform (horizontal sliding), and plate boundary zones (which do not fit the other categories) (Kious & Tilling, 2012).
Plate tectonics can explain the formation of geographic features such as the Himalayas and the Pacific Ring of Fire. The Himalayas developed from the collision of the Eurasian Plate with the Indian Plate. Instead of one plate going under the other (subduction), the two plates crumpled up, forming the highest mountain range on Earth. The Pacific Ring of Fire refers to the volcanoes and earthquakes that occur around the edges of the Pacific Ocean, which may be caused by convergent, divergent, or transform boundaries. First, the sea floor is spreading in several locations; that is, magma is rising to the level of the crust and forming new crust when it is cooled by ocean water (Bercovici & Ricard, 2014). This is an example of divergent movement, and it causes nearby plates to move outward, as when the Nazca plate converges with the South American plate, moving under it via subduction. Transform boundaries occur when plates slide horizontally past each other; this often causes earthquakes, such as those that result from shifts in the San Andreas fault line. A transform fault is also known as a fracture zone (Kious & Tilling, 2012).
- Bercovici, D., Ricard, Y. (2014). Plate tectonics, damage, and inheritance. Nature 508:513-516. http://dx.doi.org/10.1038/nature13072
- Kious, W.J., Tilling, R.I. (2012). This Dynamic Earth — The Story of Plate Tectonics. Washington, DC: U.S. Geological Survey.
- Wolf, S. (2013). Introduction to the Scientific Method. Retrieved from http://teacher.nsrl.rochester.edu/phy_labs/appendixe/appendixe.html