Chapter 4. Plate Tectonics

Figure 4.1 Iceland is known for its volcanoes, which are present because Iceland is located on the Mid-Atlantic Ridge, where the Atlantic Ocean is spreading apart and new crust is forming. In fact, Iceland exists because that volcanic activity has built up the island from the ocean floor. Iceland is cut by rift zones (white lines on the map at left) where the island is splitting apart along with the rest of the Atlantic Ocean. Rift zones are marked by belts of young volcanic rocks (dark green). You can stand on a rift zone if you visit Thingvellir National Park (right). Rifting has produced a valley where the crust has settled downward. The margins of the North American and Eurasian tectonic plates are visible as ridges on either side of the valley. The photographer was standing on a ridge on the North American side. Source: Karla Panchuk (2018), CC BY-SA 4.0. Photo: Ruth Hartnup (2005), CC BY 2.0. Image source. Click for more attributions.

Chapter Goals

Complete this chapter so you can:

  • Discuss the early evidence for continental drift, and Alfred Wegener’s role in promoting this theory.
  • Describe other models that were used early in the 20th century to understand global geological features.
  • Summarize the geological advances that provided the basis for understanding the mechanisms of plate tectonics, and the evidence that plates and are constantly being created and destroyed.
  • Describe the seven major plates, including their size, motion, and the types of boundaries between them.
  • Describe the geological processes that take place at divergent and convergent plate boundaries, and explain why transform faults exist
  • Explain how supercontinents form and how they break apart.
  • Explain why tectonic plates move.


Plate tectonics is the model or theory that we use to understand how our planet works: it explains the origins of continents and oceans, the origins of folded rocks and mountain ranges, the presence of different kinds of rocks, the causes and locations of earthquakes and volcanoes, and changes in the positions of continents over time. So… everything!

The theory of plate tectonics was proposed to the geological community more than 100 years ago, so it may come as a surprise that an idea underpinning the study of Earth today did not become an accepted part of geology until the 1960s. It took many decades for this theory to become accepted for two main reasons. First, it was a radically different perspective on how Earth worked, and geologists were reluctant to entertain an idea that seemed preposterous in the context of the science of the day. The evidence and understanding of Earth that would have supported plate tectonic theory simply didn’t exist until the mid-twentieth century. Second, their opinion was affected by their view of the main proponent, Alfred Wegener. Wegener was not trained as a geologist, so he lacked credibility in the eyes of the geological community. Alfred Wegener was also German, whereas the geological establishment was centred in Britain and the United States- and Britain and the United States were at war with Germany in the first part of the 20th century. In summary, plate tectonics was an idea too far ahead of its time, and delivered by the wrong messenger.


Thordarson, T., and Larsen, G. (2007) Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history. Journal of Geodynamics, 43, 118–152.


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