Rodinia and its reconstruction

Over the 420 million years or so between 1.2 billion and 780 million years ago, a global jigsaw puzzle of continent-sized pieces moved across the surface of the Earth to form a supercontinental agglomeration known as Rodinia. It is only over the last decade or so that enough information has piled up from different sources to convince geologists that such an important event in Earth Time did actually happen. There are still plenty of problems and arguments over the configurations of the continents and many gaps in the story, but the assembly of Rodinia is the earliest such event that is more or less generally accepted by the geological community.

Over the last 50 years, reconstruction of the past distribution of the Earth's tectonic plates has become increasingly sophisticated and more secure in its configurations. Most is known about recent events such as the opening of the North Atlantic from around 55 million years ago. And, as we work back in time, the basic story is pretty clear back as far as the assembly of the Pangean supercontinent in late Palaeozoic times and the even earlier formation of Gondwanaland in early Palaeozoic times. But even then, the data is becoming less secure and the constraints on plate configurations grows increasingly problematic and unreliable.

Fundamental to our understanding here is the study of the Earth's magnetic field in the past and the way in which some measures of that past magnetism are recorded in certain rocks.

Without the palaeomagnetic record it would not have been possible to attempt any reconstruction of Rodinia, but the data are still scarce and open to various radically different interpretations. The main contender for the Rodinian reconstruction at the moment has Laurentia straddling latitude 30 degrees south and rotated clockwise from its present position, so that the east coast is facing the South Pole across the Grenville orogenic belt to the continental mass of Amazonia that extends south across the Antarctic Circle. The north coast of Laurentia (today's west coast) faces north and the continental mass of East

Palaeomagnetism

Recovery and interpretation of what is known as the palaeomagnetic record are indispensable for the reconstruction of Earth Time. The Earth's magnetic field might seem pretty weak to us humans, but it is remarkably powerful and persistent. It has probably been in existence since the initial layering of the Earth's interior into a thin outer cool and brittle rocky crust, below which is the thick and hot rocky mantle and even hotter innermost metallic core with its solid centre and liquid outer layer.

The Earth's magnetic field is strong enough not only to orient compass needles but also any magnetically susceptible iron-rich particles that are moved around in a fluid medium such as a hot lava, air or water. Once deposited and solidified in a rock, the particles' record of the magnetic field is locked in for posterity as a kind of 'fossil compass'. Surprisingly perhaps, they can survive being folded and faulted, shuffled about the Earth's surface and buried several kilometres deep, all provided that the parent rock is not heated too much.

Palaeomagnetism records the orientation of the contemporary magnetic north and the declination of the field. This latter is very important because it varies from being near to vertical at the poles to horizontal at the equator. Subsequent recovery of this data informs us of the rock's latitude at the time of formation. However, it unfortunately does not inform us about the longitude. That has to be recovered from other criteria and introduces an often highly debatable and contentious element into past reconstructions.

When palaeomagnetism was first discovered and measured, scientists were puzzled to find that in some instances polarity was swapped over, with north becoming south, and that the magnetic pole seemed to 'wander' well away from its present location. For some time it was known that there is a constant slight but discernible shift in the magnetic field and that allowances have to be made on a year-to-year basis for accuracy of navigation and so on. But even so, some of these palaeopoles were way off course.

When the location of the palaeopoles was calculated through a succession of rocks and over a significant period of geological time, they followed well-defined but meandering paths. These often diverged well away from the known variation. Furthermore, measurement of polar wandering for successions of strata in different parts of the world showed that each region has its own path. When paths converge or coincide, it shows that those regions were in close proximity. For instance, polar wandering curves from northern Scottish rocks coincide closely with those of northern Laurentia (today's eastern Greenland) over a long period. This extends from around a billion years ago until 55 million years ago when the North Atlantic first began to open, as rifting and volcanism generated new ocean floor rocks between eastern Greenland and Ireland along with Britain. Consequently, Scotland was split away from its ancient Laurentian allegiance and joined the rest of Britain and Ireland to form the northwestern continental margin of Europe.

Antarctica, which extends north across the Equator to where India lay, 'docked' alongside Antarctica's northern coast. By comparison with modern geography, everything is 'topsy-turvy'. An alternative arrangement has Australia abutting against today's west coast of Laurentia and the Grenville orogenic belt extending into East Antarctica. Much more research will be needed before the picture becomes in any way clearer.

Comparison of polar wander curves for Laurentia, East Antarctica, India and Australia suggests that following some 300 million years of existence as part of the supercontinent of Rodinia, Laurentia broke away from East Antarctica and so on by 755 million years ago. It began to 'drift' away following a path that seems to have taken it right over the South Pole and eventually up into higher latitudes by Cambrian times. Anatarctica, India and Australia remained in low southern-hemisphere latitudes for the rest of Proterozoic times until the beginning of the Palaeozoic. Again, there are still big questions over the timing of the break-up of Rodinia. Some experts claim that, following the initial breakup, there was a brief re-assembly of another supercontinent that has been called Pannotia, but the waters of this issue are still very muddy. Either way, the overall result was a major reconfiguration of continents and oceans.

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