In this tutorial, we’ll learn about the three types of plate boundaries: convergent, divergent, and transform. These boundaries occur across the two types of plates: oceanic plates, which have a thinner and denser crust made primarily of basalt, and continental plates, which have a thicker and less dense crust primarily composed of granite. But first, a little bit about plates in general.
Tectonic plates are sections of the Earth’s lithosphere (the solid, rigid upper part of the mantle and the crust) that glide over the asthenosphere, a solid but much more ductile layer below the lithosphere. The movement of the plates is determined by convection currents in the Earth’s mantle, with rising currents creating new lithosphere at divergent plate boundaries and sinking currents pulling down old lithosphere back into the deeper parts of the mantle.
The image below shows a cross-section of the Earth with the lithosphere, asthosphere, remaining mantle (sometimes called the mesosphere), outer core, and inner core. Note how much thinner the lithosphere and thicker the astheosphere are for oceanic plates compared to continental plates.
Convergent Plate Boundaries
There are three types of convergent plate boundaries: oceanic/oceanic, oceanic/continental, and continental/continental. Depending on the specific type of boundary, convergent plate boundaries can host the world’s largest volcanic chains, highest mountains, or deepest trenches. Powerful earthquakes are commonly found at these boundaries, and the “megathrust” earthquakes along the subduction zones of oceanic plates are the largest ones in the world.
Oceanic/Oceanic Plate Boundary
When two oceanic plates converge, the denser of these plates is subducted and uplifts the other, creating a trench and associated island ark. The Aleutian Island chain is a great example of a convergent oceanic/oceanic plate boundary, with the older, denser Pacific Plate subducting under the North American Plate.
Oceanic/oceanic convergent plate boundary.
Retrieved from USGS
Continental/Oceanic Convergent Plate Boundary
Much of the Eastern Pacific coast lies along a continental/oceanic convergent plate boundary. The West Coast of the United States have the Cascades and Sierra Nevada, and the West Coast of South America has the Andes. The Andes in particular are a textbook example of this type of plate boundary, as they have an extremely deep trench and very high (over 20,000 feet!) mountains in close proximity to each other. They have the strongest earthquakes in the world; the 1960 Nazca Earthquake was a 9.5 magnitude earthquake, which is approximately 800 times more powerful than our relatively puny Nisqually earthquake in 2001 (remember, earthquakes are measured on a logarithmic scale). These cataclysmic earthquakes are not just limited to continental-oceanic plate boundaries though; they occur at oceanic-oceanic convergent plate boundaries too. A 7.9 earthquake actually just struck the Aleutian Islands less than a year ago.
Continental-Continental Convergent Plat Boundary
Continental-continental convergent boundaries are best known for creating the world’s highest mountain ranges, like the Himalayas in Nepal/Tibet and the Karakorum in Pakistan. Because continental plates are thick and of similar density, they have much more trouble subducting under one another and instead tend to push up directly against each other, creating extremely high mountain peaks and massive plateaus (like the Tibetean Plateau). These boundaries do not have volcanoes because the lithosphere is too thick to allow magma to rise to the Earth’s surface, and they do not have “megathrust” earthquakes because they do not have subduction zones. They can still generate extremely powerful quakes, such as the Nepal Earthquake in 2015.
Divergent plate boundaries arise when hot masses of the asthenosphere rise and burst through the lithosphere. This mass then cools, creating a new basalt crust and lithosphere. This formation of new lithosphere pushes each plate away from each other, causing them to diverge; hence the name divergent plate boundary.
Ironically enough, divergent boundaries can create mountain ranges as well due to the magma rising through the lithosphere and cooling, forming a range. However, most of these ranges are underwater, because when plates diverge, seawater inevitably flows in after the rift between them has gotten large enough that seawater can flow through. Ever notice how the east coast of South America and west coast of Africa look like they fit with each other? Well, it is widely accepted that they were once connected in Pangea, which was a “supercontinent” composed of all the current continents, and that they began to break up 180 million years ago. In the middle of the Atlantic between these continents is the Mid-Atlantic Ridge. The Mid-Atlantic ridge is only one of many; the ocean is filled with these ridges.
Not all divergent plate boundaries occur underwater though. The East African Rift, a divergent plate boundary stretching from Ethiopia to Malawi, is a prime example. All of Africa’s 17 14,000+ foot volcanoes are located in the East African Rift.
Transform boundaries neither converge nor diverge; they slide right past each other. They differ from convergent and divergent boundaries in they are conservative plate boundaries, meaning lithosphere is neither created or destroyed. When plates are subducted or created, they are generally not done so in a uniform fashion, and rather have many individual transform faults connecting the overall subducting or diverging structure of the plate boundary itself. While they are most common around mid-ocean ridges, the most destructive ones occur near subduction zones on land. The San Andreas Fault in California is one of the most famous transform faults in the world, and is responsible for delivering many catastrophic earthquakes to the sunshine state.
Southern California and Baja California are currently sliding northwards, and in approximately 50 million years, they will become subducted under the Aleutian Trench. So in the very very slight possibility that Los Angeles is still a bustling city 50 million years from now, citizens there will have to move somewhere else.
Now that you’re an expert on plate tectonics, click here to learn about the different types of faults that cause earthquakes.
Written by Charlie Phillips – charlie.weathertogether.net. Last updated 12/1/2017