Solid Earth

3.1 From Rifting to Spreading

Video: From Rifting to Spreading

(To play the video, please click on the image above)

Photo: Graben structure at Kverkfjölljökull, Iceland, 2006

Chapter 3.1

From Rifting to Spreading

From Rifting to Spreading

A rift graben is formed when a plate is stretched and the lithosphere breaks apart, as shown in the two images above. The breakup of a continent and the development of such a graben can be the beginning of a cycle that we call the Wilson cycle. It is named after one of the founders of plate tectonic theory, John Tuzo Wilson. A Wilson cycle involves the formation, opening and closing of an ocean. The individual stations of a Wilson cycle will be explained in more detail in a later video.

A continental rift can lead to the formation of an ocean along this graben (Fig. 3.1.1). However, an ocean does not necessarily have to open up, as can be seen in the example of the Upper Rhine Graben. The tearing of Europe had already begun there, but a change in the direction of plate movement deactivated the Upper Rhine Graben as a rift, i.e. as an extensional structure.

Rift graben

A graben can open up in a symmetrical or asymmetrical structure (Fig. 3.1.2). When rising asthenosphere softens the lithosphere from below, a zone of weakness is created where a plate can break apart. The two sides are then developed approximately symmetrically and a symmetrical graben is formed, in which most of the volcanoes are found in the axis of the graben.

If, on the other hand, a large oblique fault zone develops along which the continental lithosphere can slide apart, an asymmetrical graben is created in which the volcanoes are not found in the axis of the graben, but often far away from it.

An asthenospheric bulge has developed under the East African rift, which has been causing the graben to form at this point for over 10 million years (Fig. 3.1.3).

Most volcanoes, such as Ol Doinyo Lengai in Tanzania or Nyiragongo in Congo, lie in the graben axis, others such as Kilimanjaro are only slightly off. Accordingly, it is a predominantly symmetrical rift system (Fig. 3.1.4a).

Some geologists suppose that the African Plate has already completely torn through at this graben and therefore consider the eastern side to be a separate lithospheric plate, which is called the Somali Plate. However, the southern continuation of the plate boundary has not yet been clarified.

If you imagine that a plate will soon break away along the graben axis, i.e. in a few million years, and a new ocean will form between them, it could perhaps look like in Fig. 3.1.4b. In this illustration, the Somali Plate has separated from the African Plate along the trench axis of the western branch of the graben. The eastern branch would also be possible, but at the moment it is not clear where the crack will ultimately run.

Development of a spreading zone

The Red Sea is the best example of a young, recently opened ocean (Fig. 3.1.5). In the middle of the Red Sea there is a fissure in which real oceanic crust is already being formed and the two plates, the African and Arabian plates, are drifting away from each other. In the Wilson cycle, this stage is referred to as the Red Sea stage.

From this stage, it is only a matter of time before a broad ocean forms. The Atlantic, which presents itself to us today at several thousand kilometers, also began in the same way when Africa and South America still formed a common plate.

A spreading zone is forming in the middle of the ocean, which after some time will be characterized by a mid-oceanic ridge. This ridge has a rift graben at its center on the ridge axis (Fig. 3.1.6). This is a graben structure very similar to a continental rift graben with a median valley, graben shoulders and graben margin faults.

The spreading zone is a divergent plate boundary, i.e. two lithospheric plates move away from each other as the white arrows in the figure show. The constantly re-opening crack is closed with a magmatic melt that penetrates from below from the asthenosphere into the oceanic crust and builds it up.