Earth is over four billion years old but all of the rocks underlying the world’s ocean basins are much younger and formed less than 200 million years ago. Earth constantly recycles its ocean basins and the first part of this process is the formation of new oceanic lithosphere at divergent plate boundaries. In this video we will discuss the geological features and processes that characterize divergent plate boundaries and talk about how ocean basins are created.
We’ll start by reviewing some observations about divergent plate boundaries. Plates move away from each other along divergent boundaries as shown by the red lines on this map. We can find examples of these boundaries in every major ocean basin.
In contrast, this map shows the locations of hundreds of moderate to large earthquakes that occurred in a single year. Note that there are discontinuous lines of shallow earthquakes associated with divergent boundaries in the Pacific, Indian, and Atlantic oceans. Geoscientists use the term – bathymetry – to refer to the depth and topography of features on the ocean floor.
Divergent plate boundaries are characterized by the presence a continuous oceanic ridge that forms a submarine mountain range that can be traced through the major ocean basins. The ridge is typically more than a thousand meters higher than the surrounding sea floor. Observations of the ridge system reveal that it is a source of volcanic activity.
Finally, divergent plate boundaries are where we find the youngest oceanic lithosphere. These rocks are forming today as volcanic and plutonic igneous rocks. The age of the ocean floor gets progressively older moving away from the divergent boundary.
We can compare the bathymetry and age of the ocean floor on these spinning globes. It is relatively easy to identify the divergent boundaries toward the centers of the ocean basins using the features we have just described. These patterns are even more apparent when we superimpose both maps.
So, our observations are that plates move apart at divergent plate boundaries and these boundaries are typically home to oceanic ridges, young ocean floor and earthquake activity. Next we will use these and other observations to interpret the evolution of divergent plate boundaries and describe how oceanic basins are formed. If we are to form an ocean, we have to start without one.
So we begin by examining a location where a continent is being split apart. Then we will look closely at the features of a narrow, young ocean, before turning our attention to the familiar wide ocean basins of the Atlantic and Pacific. The Atlantic Ocean formed after the break-up of the supercontinent Pangaea.
Similarly today, plate tectonic forces are acting to break apart the African continent. The first stage of this process is marked by an initial stretching and thinning of the continental crust as the mantle rises to form a regional uplift. With time, the crust thins and breaks apart on normal faults to form a feature known as a rift valley.
The rift valley may contain volcanoes or lava flows fed by magma rising from the mantle. This process is currently happening in Africa as plate tectonics is gradually slicing off a sliver of the eastern part of the continent. Note the concentration of volcanoes marking the presence of the rift valley extending southward into the continent from the Red Sea region.
Tectonic forces are pulling the continent apart. We can see landforms known as “fault scarps” that form steps in the land surface where faults cut across the landscape. The fault scarps are aligned perpendicular to the stretching of the continental crust.
With continued stretching, the elevation of the rift valley floor will drop below sea level and it will be flooded to form a narrow ocean. The rift that holds the Red Sea started to open approximately 30 million years ago as the African and Arabian plates began to separate but it didn’t begin filling with seawater until about five million years ago. We see evidence of volcanism in the Red Sea in newly formed volcanic islands in the southern segment of the basin, just north of the east African rift.
There is actual oceanic crust between the two neighboring plates so we can consider this a legitimate plate boundary. Consequently the Red Sea represents a very youthful ocean basin. With sufficient time, the ocean basin expands in width from a few hundred kilometers across to several thousand.
Just how wide the ocean becomes will depend on the rate of plate motions and the length of the process. Much of the Atlantic and Pacific oceans began opening at about the same time and contain rocks of similar ages. However, the Pacific is much wider because the plates underlying the ocean basin have been separating faster than the North American and African plates.
So, to summarize . . .
Plates separate along rift zones associated with oceanic ridges. This process is driven by mantle convection. Faulting in the rift zone generates shallow earthquakes along the ridge.
As the plates move apart, a gap opens up that is filled by magma rising from the asthenosphere below. Magma forms new, young oceanic crust. Magma that reaches the surface forms lava with a basaltic composition or it may solidify at depth to form the plutonic igneous rock like gabbro.
The high heat flow results in lower density along the ridge and produces higher elevations, explaining the presence of the ridge system. The earliest or oldest oceanic crust forms adjacent to the continents and the latest or youngest crust is along the ridge, toward the center of the ocean basin. You can test your knowledge of the things discussed in the video by drawing a labeled sketch of the features and processes of divergent plate boundaries on a blank piece of paper.
Go on, give it a go. So, we had two learning objectives for this lesson. How well do you think you can respond to each?
