Divergent Boundary: Definition & Examples
In this lesson, you will learn the meaning of the term ‘divergent boundary’ in the context of plate tectonics. A number of features and examples of divergent boundaries will be introduced. A short quiz will follow.
What is a Divergent Boundary?
You’ve just boiled an egg and are removing the shell. Suddenly; it slips from your hand and hits the floor with a crack. You pick it up to find the whole surface littered with lines and breaks. While nobody has ever dropped the Earth (as far as we know!), the cracked surface of the egg is just like the Earth’s crust. While it might seem solid enough, it is actually broken into sections called tectonic plates. These plates float on a sea of hot magma. This magma is solid but becomes a liquid when it reaches the surface through volcanoes.
A divergent boundary, or constructive boundary, is where two sections of the eggshell – two tectonic plates – are moving away from each other. When this happens, they leave a gap in-between, and this gap can be filled by the magma from below. This creates a series of volcanoes all the way along the boundary.
Features of Divergent Boundaries
Exactly what happens when two plates diverge depends on the two types of plates involved: oceanic plates and continental plates. Oceanic plates are, unsurprisingly, plates below sea level and under the oceans. But the ‘oceans’ part isn’t actually what makes it an oceanic plate because there are some exceptions to this rule. Instead, it’s more about the plate’s composition. Oceanic plates are composed of mafic or basaltic rock. Continental plates are indeed above sea level, but again, what makes a plate officially continental is the presence of felsic or granitic rock.
Most divergent boundaries in the world today are between two oceanic plates. When these plates separate, the magma rushes up to fill the gap and creates underwater volcanoes. This is the creation of brand new crust! This crust can remain deep under the oceans, creating underwater ridges, or it can rise to the surface over time to form islands.
When two continental plates separate, it is theoretically possible for a similar thing to happen; volcanoes could form to create new crust. However, continental plates are far thicker than oceanic ones. Because of this, what usually happens instead is that a continent will gradually break apart, as water from the sea rushes in to fill the gap. By the time magma has a gap to fill, that gap is already deep underwater in a brand new ocean. This starts out as an enormous rift valley across the land, until that valley fills with water.
When one continental and one oceanic plate diverge, there can be features of both. It is more a matter of whether the boundary itself lies over an ocean or land than whether the plates are continental or oceanic.
Examples of Divergent Boundaries
The mid-Atlantic ridge is an example of a divergent boundary, where the Eurasian Plate that covers all of Europe separates from the North American Plate. This underwater mountain range is constantly growing as new crust is formed. Further up that same boundary, it passes through Iceland. There it forms characteristics of a continental divergence, and a rift is forming on the island. Eventually, Iceland will break into two separate islands, with sea in between.
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