Earth’s lithosphere (surface) is broken into massive, irregularly-shaped pieces, called tectonic plates. Some of the plates are so large that they span entire continents. Others are smaller. Tectonic plates sit atop a denser, soft part of the mantle called the asthenosphere. If you need a refresher, view my Layers of the Earth post.
The theory of plate tectonics is a scientific theory central to the field of geology. This theory explains that the earth’s surface is broken into large plates that are slowly moving. These plates slide past and over one another. Their motion gradually changes the appearance of Earth’s landmasses and oceans over time. Plate movement also causes earthquakes and volcanic eruptions along plate boundaries.
Where two tectonics plates meet is called a boundary. There are three main tectonic plate boundary types. Without further ado, let’s take a look at each of them.
1. Convergent Plate Boundary
At convergent boundaries, two plates move towards one another. There are two convergent boundary subtypes:
Convergent with Subduction
When one or both plates are oceanic plates, the denser plate is subducted, or drawn underneath the less dense plate. This creates volcanic activity, severe earthquakes, and tall coastal mountains or islands.
Convergent without Subduction
Once an oceanic plate fully subducts, two continental plates may converge. Continental plates cannot be subducted. This creates huge inland mountain ranges, like the Himalayas.
2. Divergent Plate Boundary
At divergent boundaries, plates move apart. This allows new crust to form between the plates. Divergent boundaries can happen in the ocean or on continents. You will find volcanic activity and mild earthquakes at divergent boundaries. If the divergent boundary is in an ocean, you may see mid-ocean ridges due to volcanic activity. The volcanic activity can also create islands along this boundary type. Iceland is an example of an island on a divergent boundary.
3. Transform Plate Boundary
At transform boundaries, plates slide past one another. This can happen in oceans or on continents. The sliding motion produces faults with mild to severe earthquakes, such as in the San Andreas region of California. Landscape features, such as mountain ranges or riverbeds, may appear broken and offset due to the sliding of the plates.
My Plate Tectonics Lesson Plan:
If you enjoyed the illustrations and information from this post, I know you will love my complete Plate Tectonics Unit. It is a set of classroom materials that I designed using my artwork to create a fun learning experience!
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References and Further Reading
- C.C. Plummer, D.H. Carlson, and L. Hammersley (2019). Physical Geology. McGraw-Hill Education. (Chapter 1)
- National Oceanic and Atmospheric Administration (n.d.). What are the different types of plate tectonic boundaries? Ocean Exploration. Available: https://oceanexplorer.noaa.gov/facts/plate-boundaries.html
- The National Park Service (2020). Types of Plate Boundaries. Available: https://www.nps.gov/subjects/geology/plate-tectonics-types-of-plate-boundaries.htm
- U.S. Geological Survey (1996). Tectonic plates (public domain image). Available: https://commons.wikimedia.org/wiki/File:Plates_tect2_en.svg
