The world map shows that our planet is mostly covered by the waters of the oceans. But the familiar present-day continental outlines have not always been in the same place. Over geological time the continents have moved over great distances, typically at speeds of about 50 km per million years (km/Myr) or 50 mm/yr. At the start of Phanerozoic times, about 540 million years ago (540 Ma), the present-day southern continents – Africa, Antarctica, South America, India and Australia – had emerged from Precambrian time as an amalgamation of old continental crust (present-day pieces of which are shown in yellow in the animation) as a single huge continent known as Gondwana [https://en.wikipedia.org/wiki/Gondwana]. Gondwana remained essentially intact for the next 350 million years. But then, early in the Jurassic period, about 185 Ma, this huge continent started to fragment. The animation shows how this happened, leading to the familiar present-day configuration of the southern continents, separated by the enormous areas of new, ocean-covered crust created in the process.
The configuration of reassembled Gondwana has been speculated for more than 100 years, initially in the works of Wegener (1927) and du Toit (1937). The first computer-aided fit was presented by Smith and Hallam (1970) and used in the first geological map of reassembled Gondwana (De Wit et al, 1988). A geologically plausible and tight Early Jurassic reassembly of the southern continents is a more recent development that is now accepted generally (IGCP-628) [https://gondwana.geologia.ufrj.br/o-projeto/].
The Gondwana continent underwent several episodes of rifting and many of the rift zones evolved, in turn, into continental margins separated by mid-ocean ridges. At these ridges or ‘spreading axes’ the space created by the separating continental plates was filled with magma arising from below the earth’s crust. In this way new oceanic crust is created and carried away from the active ridge. The oldest oceanic crust, therefore, is usually found near the continental margins and the youngest near the central ridge. A record of the intervening tectonic process is preserved in the topography of the sea floor. But who has actually seen the ocean floor?
Detailed worldwide observations of the ocean floor only became available about 1997 (Smith and Sandwell, 1997) by way of satellite altimetry (Sandwell et al, 2014). Historically, the study of our planet – geology – has of necessity been largely focused on the study of what may be seen on the continents. An important approach to understanding the evolution of the much larger oceanic area – more than two-thirds of the earth’s surface – lies in careful interpretation of the topography of the ocean floor.
Since rock is much denser than sea water, a mountain on the ocean floor is more massive than the volume of sea-water it displaces. This excess mass gives rise to a slight but permanent distortion of the sea surface above it (typically no more than about 10 centimetres) that may be measured from an earth-orbiting satellite. All the world’s oceans have been mapped in this way on a grid of resolution better than 10 km. This is sufficient to reveal and define a plethora of hitherto unseen features interpretable as the tectonic history of the oceans. Watching the animation above is an easy way to assimilate the results of our study and see the order in which different parts of the southern oceans were created.
Understanding the behaviour and evolution of mid-ocean ridges (Figure 1) is central to the story. The two most important elements of a mid-ocean ridge are (1) the ridge sections or rifts (shown in red in the animation) and (2) the offsets in the ridge known as transforms (white). On each ridge section, the conjugate tectonic plates move apart in a direction that is at right angles to the ridge. The transforms follow closely this same direction such that a mid-ocean ridge is typically made up of a series of right-angle bends making a stepped pattern extending over thousands of kilometres below the oceans. This geometry is remarkably stable and self-replicating over long periods of time. This results in the transform-offsets in the ridge creating long ‘fracture zones’, often thousands of kilometres in length, sometimes traceable across an entire ocean. In contrast to the active ridges and transform offsets, these features do not produce earthquakes so are said to be ‘aseismic’. Note that the direction of continental separation varies over time, leading to the gently curved nature of the resulting fracture zones.
Our model features the positioning of the mid-ocean ridges over time. In principle, active ridges are located mid-way between the conjugate continental fragments. However, modelling them with such simple precision reveals that the tectonic record preserved in the ocean floor departs from the simple plate model for many ridge sections. For example, ridges occasionally ‘jump’ to a new location when fracturing the pre-existing oceanic crust in a new way offers an easier path for inter-plate movements in the whole global plate system. Some jumps of several thousand kilometres are recorded, such as between India and Madagascar.
Clearly the long fracture zones have always been connected to a transform at the mid-ocean ridge and this constraint may be applied to reconstructions of continental plates at times in the past (Reeves and de Wit, 2000). The animation illustrates this principle for the fracture zones interpreted from the satellite altimetry. It is straightforward to achieve the telescoping for one pair of continents but to achieve it across all the continent pairs in Gondwana dispersal with a legitimate timescale has required patient adjustments over many years.
Periodic reversals in the polarity of the earth’s magnetic field over geological time (Ogg, J.G., 2020) have been recorded in the magnetisation of the solidifying magma as it progressively makes up the new oceanic crust [https://glossary.slb.com/zh-cn/terms/g/geomagnetic_polarity_time_scale]. In the interval from the present-day back until 83.64 Ma there were 68 such reversals and the time scale is well established from many magnetometer traverses across most of the oceans by marine research vessels.
Before 83.64 Ma, however, there was almost 40 million years of steady geomagnetic field during which the ocean crust created is devoid of these distinctive reversal-related ‘stripes’. By the time we trace back to 121.4 Ma (the youngest of an earlier series of geomagnetic reversals) the separation of the Gondwana continents is quite small and magnetometer traverse mapping is far from complete. The picture is confused by the proximity of the coastline, irregular early rifting patterns and thick sedimentary deposits in the near-offshore. Unfortunately, this is the most interesting time interval for understanding the stratigraphy of the continental margins, i.e. the time interval after initial rifting and before the margins became stable or ‘passive’.
The work has mostly accepted published interpretations of plate movements for more recent movements, back to 83.64 Ma, except where the result is clearly at variance with the new ocean-floor topographic data. Our own interpretations of the new data have been focused mainly on earlier events and building a consistent model of continental movements across all Gondwana from 83.64 Ma back to an intact Gondwana at 184 Ma. We have worked from first principles and honoured published M-series marine magnetic anomaly positions and the 2020 Geological Time Scale [Ogg, 2020].
Static continental reconstructions may be made easily. Showing any model in animation, however, usually reveals errors and inconsistencies that require adjustments to be made. Logic constrains movements to be smooth and gradual in animation unless there is credible evidence to the contrary. Certainly ocean growth never goes into reverse anywhere across the model, except where subduction is known. Achieving this criterion of credibility is an iterative process that has extended over many years of refinement. Geological evidence from exploration work in developing rifts and margins has been honoured wherever available (e.g. https://www.africageologicalatlas.com/).
A key element of new constraint to the Gondwana dispersal system was provided by the marine magnetometer work of Mueller and Jokat (2018) off the conjugate margins of Mozambique and Antarctica. Without this the early positions of the two largest fragments of Gondwana – Africa and Antarctica - were only vaguely determined. In general, the more closely the data is honoured, the fewer the options for a model that works satisfactorily across all fragments. With care, a seemingly intractable problem with an infinite number of possible solutions converges on something approaching a single unique one. One important discovery is the clear change in the direction of Antarctica’s path away from Africa 130-110 Ma, an interval when Gondwana’s dispersal appears to have been accelerating generally.
At a small number of locations worldwide, three mid-ocean ridges meet at a common point called a triple junction. Central to our story of Gondwana dispersal is the triple junction off southern Africa known as the Bouvet triple junction after the small, desolate volcanic island found close to its present-day location in the southernmost Atlantic Ocean. It has been associated with a particularly active supply of magma since early in Gondwana disruption when a huge igneous event affected much of southern Africa and parts of Antarctica at about 183 Ma. The area covered by such an event is known as a large igneous province (LIP) [https://en.wikipedia.org/wiki/Large_igneous_province] and each LIP is thought to be associated with a ‘mantle plume’ arising from thousands of kilometres within the earth. Mantle plumes are shown as large (1000 km diameter) pale yellow stars in the animation.
Initially, the movements of the other continents were constructed relative to a fixed Africa, the largest fragment. But Africa has no special status; it too has moved with respect to the axis of the earth’s rotation. We have adjusted Africa’s movement such that the LIPs recorded in present-day geology across all the oceans (mostly defined by large accumulations of magma on the ocean floor and in some cases only scantily recorded on the continents) coincide as closely as possible with the locations of the mantle plumes at the time of their primary eruption. Neither the LIPs nor the exact centre of the mantle plume can be defined more precisely than about 1000 km and active eruption sites must, in part, be pre-determined by weaknesses in the earth’s crust at the time.
Uncertainties notwithstanding, the perhaps-surprising conclusion is reached that the system of mid-ocean ridges separating the Gondwana continents has been approximately fixed with respect to this array of mantle plumes and, hence, the earth’s axis of rotation. Stated briefly, the continents of Gondwana have drifted while the mid-ocean ridge system separating them has remained more-or-less fixed in space. The Bouvet plume head (Bouvet or B in the animation) has been active or re-activated throughout Gondwana dispersal and probably played a key role in its initiation. Note in the animation how all the southern continents find ways of moving away from Bouvet from the onset of dispersal to the present day.
A major meteorite impact crater has been discovered in southern Africa at Morokweng (M in the animation) [https://en.wikipedia.org/wiki/Morokweng_impact_structure]. A large asteroid impacted at about 146 Ma making a crater in the earth’s crust that is still at least 75 km in diameter - but presently hidden below Kalahari sand. The timing is close to the as-yet-ill-defined time boundary between the Jurassic and Cretaceous periods and could well have been a defining event in earth history, similar to the Chicxulub impact in Mexico’s Yucatan [https://www.wikidata.org/wiki/Q55816]. Whether Morokweng had any significant effect on continental dispersal patterns is unknown. But it is clear from the modelling that rates of plate movement accelerated markedly once the Cretaceous period got under way. Antarctica-and-Australia first rifted off India and, at about the same time, South America started rifting off Africa. Direct evidence of the impact has yet to be identified in the stratigraphic column.
The four phases or ‘regimes’ of Gondwana dispersal proposed by Reeves and de Wit (2000) may now be defined more closely:
There were two basic aims to this work. The first was to use ocean-floor topographic data, new in 1997, to understand exactly how Gondwana fragmentation evolved in order to gain new insight into the development of continental margins and rifts hosting immense natural resources. The second was to present results based on the new data in a format that could be quickly assimilated by those seeking a graphic introduction to a fundamental aspect of earth science in the southern hemisphere. The first edition, published almost 25 years ago, has proved attractive to many who are simply curious about the workings of the natural world.
The work started about 1997 as an internally funded research project of ITC designed to demonstrate the power of geographic information systems (GIS) in geological research. The original version of this website originates from that time.
From 2004 to 2009 the work continued within the School of Geosciences at the University of the Witwatersrand, South Africa, while also attracting the support of several organisations involved in resource exploration in the southern hemisphere. Throughout the work, the ‘Atlas’ plate-reconstruction software developed at the University of Cambridge has been employed to facilitate adjustments to model geometry and visualise the results of new models quickly in animation. This software was developed by Lawrence Rush and Alan Smith over more than 40 years and their personal support and encouragement was invaluable for much of that time. Suzanna Reeves undertook a great deal of basic cartographic data-capture in the summer of 2010. From 2010 the work has formed part of IGCP-628, the Geological Map of Gondwana, [https://gondwana.geologia.ufrj.br/o-projeto/] based at the Federal University of Rio de Janeiro, Brazil under the supervision of Renata Schmitt. The project has produced a new geological map of reassembled Gondwana, superseding that of de Wit et al (1988).
Since I retired from independent project work in 2018 I have been able to amuse myself with identifying and solving ever-finer inconsistencies in the model, an opportunity that few others have enjoyed. Presenting results at the ‘Africa’ meetings of the Petroleum Exploration Society of Great Britain/Geological Society of Houston and various other international meetings has been a regular encouragement and a unique opportunity to learn from others about the petroleum geology of Africa for which this work can be seen as the global context. The valuable input of Duncan Macgregor (https://www.africageologicalatlas.com/), Jon Teasdale and many other earth science professionals over many years is gratefully acknowledged.
This website was originally instigated and has been maintained by Barend Köbben at the ITC–University of Twente. A much fuller explanation of the work, supported by numerous animations, may be found on my own website https://www.reeves.nl/gondwana. Enquiries are always welcomed.