Conjugate continental margins have a finite length. Explain, then, how mid-ocean ridges increase in length over time. (An old Tripos exam question, paraphrased). The above animation of the Southwest Indian Ridge (SWIR) gives a graphic example of how this happens. From a length of 1500 km at 130 Ma, the SWIR has reached a length of 6500 km at the present day.

In principle, a mid-ocean ridge may be modelled as moving at half the angular velocity of its conjugate continental margins. The result may be checked against the tectonic record in the topography of the ocean floor. In many cases this corroborates surprisingly well. The main exceptions are (a) when a mid-ocean ridge ‘jumps’ to a new location and (b) in the complexity of small, often short-lived, minor plates in the vicinity of triple junctions for example, the Bouvet triple junction.

A ridge jump usually involves the demise of an active mid-ocean ridge and the onset of a new and active one elsewhere, most often in pre-existing oceanic crust. Rifting in pre-existing, stable oceanic crust may be a class of tectonic feature new to many familiar only with continental geology. Examples abound in the oceans. For example, watch the separation of Broken Ridge from the Kerguelen Plateau across the Southeast Indian Ridge. Many others are shown in green in the animation above.

The SW terminus of the SWIR is the Bouvet triple junction. The NE terminus is the Rodrigues triple junction that lies centrally in the present-day Indian Ocean. Unlike the Bouvet triple junction, the Rodrigues cannot be related to the activity of a single mantle plume. Both the Marion and the Reunion plumes played roles in different ways and at different times. Watch the animation again closely. (An aside: Does perhaps the longevity of a single, stable and central ridge within the Africa-Antarctica Corridor (AAC) owe something to its being midway between the Bouvet and Marion plumes throughout the animation?)

While East Gondwana was still intact (i.e. until about the start of Cretaceous time, 142.3 Ma in our model) there was no Rodrigues triple junction and the eastern margin of the AAC was the long offset to the north called the Davie Fracture Zone. The first Africa-Madagascar-Antarctica triple junction came into existence as India started to separate from Antarctica-Australia. The westward-propagating ridge, initially between India and Sri Lanka, joined that in the AAC by means of India moving eastwards away from the AAC, creating space that was filled by the Madagascar Rise (and possibly also the Gunnerus Ridge) by about 120 Ma. This was followed by the rapid separation of Sri Lanka from Antarctica in the following 10 Myr and Sri Lanka reaching its present position with respect to India in the same interval.

Note in the animation the various positions of the mid-ocean ridge immediately east of the AAC. Once India’s rapid NE journey starts at about 89 Ma, the active mid-ocean ridge is south of a small fragment that now lies north of the Crozet Islands and a small ocean is created atop the Marion plume within which the submarine plateau west of the Crozets is now central. From about 58 Ma, this ridge was abandoned in favour of a more northerly location as the SWIR propagated NE between the Madagascar Rise and the Antarctic plate. Note also that the Madagascar Rise itself moves south from Madagascar in the interval 89-60 Ma. The (Greater) Vishnu fracture zone to its NE records the movement of the Madagascar Rise with respect to India (not that India started well south of Madagascar as advocated in Reeves and de Wit (2000)). Note the role of the Vishnu FZ as a ‘leaky transform’ lying between the Reunion and Marion plume heads in the interval 70-50 Ma, contemporaneous with the change in the spreading direction in the AAC and the subsequent propagation of the SWIR to the NE from about 60 Ma.

The Crozet Islands themselves are made up of relatively recent volcanism (c6 Ma), perhaps indicating a tendency of the present mid-ocean ridge to reactivate an old line of weakness into the failed ocean lying from the Prince Edward Islands east to the Crozets.

Now give yourself 40 minutes to answer the question in the first paragraph without using animation… How many discrete ridge locations have existed between Madagascar and the Antarctic plate? To what extent might the model over-simplify the real situation? The tectonics of the oceans is not as simple as the Atlantic Ocean (and the text-books) might lead you to believe.