Supercontinent: Ten Billion Years in the Life of Our Planet (23 page)

Supercontinents, like people, get married and they get divorced. They may also repeat this process more than once. When continents rift, however, they leave traces in the geological record, even after the oceans that the divorcing continents leave in their wake have long been destroyed and forgotten.

Some pieces of evidence, such as the clustered radiometric dates of mountain-building episodes, show us when continents joined together. Different types of evidence have to be used if we are to date the moment when supercontinents broke up. For example, when rocks undergo tension they crack along lines at right angles to the pulling force. Molten magma may rise up and fill these cracks, creating what geologists colourfully term ‘dyke swarms’ (a dyke being a flat,
sheetlike
, cross-cutting body of once-molten rock). The dykes’ orientation betrays the direction of the tension.

Another consequence of tension is rifting, just as we saw
happening
in the North Sea Basin at the end of the Permian, when the Atlantic Ocean was beginning to open. In rifting, rock in the valley floor drops down like a keystone in an opening arch, and sediment rushes in to fill the space. Looking at the distribution of rift valley deposits and dyke swarms can help geologists work out how a
supercontinent
broke apart.

The leading configuration for the supercontinent Rodinia was
published
by Paul Hoffman in 1991 in the American journal
Science
, and it is based on the assumption that the Grenville-age mountain belts of the world were all created by the elimination of oceans. Hoffman’s configuration placed the west coast of North America against East Antarctica, a solution known as SWEAT (South West north America-East AnTarctica), and which was originally proposed by the Scottish-American geologist Ian Dalziel. However, such is the
uncertainty
in these reconstructions that there is as yet no general
agreement among scientists even about this basic configuration. Palaeomagnetic evidence may be geophysics, but it doesn’t tell you everything; the results leave a lot of room for interpretation, at least as regards longitude. Also, very ancient rocks lack the fossil controls that can be used to help determine the relative positions of Cambrian and younger continental fragments at given times; and the
geological
controls (matching mountain belts, dyke swarms, rift systems and the like) – as Wegener himself found – need not of themselves compel particular solutions to the jigsaw.

For example, the chief rival configuration to SWEAT puts Australia alongside North America (and is known as AUSWUS, for
Australia
-Western United States). Other possible configurations are also occasionally dropped into this alphabet soup at international meetings, all serving to demonstrate the extreme difficulty of solving the Rodinia jigsaw puzzle from the scanty evidence of the ultimate palimpsest.

For supercontinents even older than Rodinia the situation is
predictably
even worse, though just to show that controversy does not necessarily increase proportionately with age, many geologists believe (with Ian Dalziel) that in between Rodinia and Pangaea another supercontinent, Pannotia, was created. In this vision of events
present
-day Australia, East Antarctica and India rifted off
en masse
from Rodinia about 760 million years ago and became reattached to the eastern side of Africa and Arabia. However, whether Pannotia
qualifies
as a true supercontinent depends on whether this event did any more than build the megacontinent Gondwanaland. Opinion on this remains resolutely divided. One recent textbook on the subject, for example, makes no mention at all of Pannotia among the panoply of pre-Pangaean supercontinents.

We have seen how supercontinents may form by two processes, for which geologists have borrowed the psychological terms ‘
introversion
’ and ‘extroversion’. Introversion is another name for the Wilson
Cycle, sometimes also called ‘accordion tectonics’, whereby a
continent
rifts apart, forms an ocean within itself and then closes again along the same line, destroying the interior ocean and forming a new range of mountains more or less where an older range once stood. Extroversion simply envisages this rifting continuing, so that the original supercontinent is turned inside out and all its fragments meet one another along their leading edges somewhere else on the planet.

The solution to the question of continental drift did indeed lie at the bottom of the ocean, as many geologists suspected. The problem that geologists have in putting pre-Pangaea supercontinents like Rodinia back together (in other words, in distinguishing between such possible solutions as SWEAT and AUSWUS) is the lack of ocean floor from that time, because it has all long since been destroyed. However, it would be an immense help if, even without that ‘road map’, they could somehow tell whether a given supercontinent (whose existence and approximate date of fusion we should be able to tell from such evidence as clustered radiometric ages) formed by introversion or extroversion. The question is, how?

Consider this. In the case of an interior ocean (like the modern Atlantic, opening between fragments of a disintegrating Pangaea) all the ocean floor that has formed is obviously younger than the
break-up
of Pangaea. If the two sides of the Atlantic should decide to close again and form Chris Scotese’s Pangea Ultima, the ages of all
ocean-floor
rock that will have to be consumed will fall (at their oldest) between the dates of Pangaea’s initial break-up and (at their youngest) its eventual reunification.

But on the other hand if Roy Livermore’s vision comes true and
250 million years from now his Novopangaea forms by the opposite process of extroversion, the ocean floor that is consumed in forming the new supercontinent (mainly the Pacific Ocean) will all have lain outside Pangaea at its state of maximum packing: the floor of an ‘exterior’ ocean called Panthalassa. Nearly all of this ocean floor therefore formed before Pangaea was fully assembled; and therefore nearly every piece of it, especially the very first pieces to be destroyed, would yield ages
older
than the break-up of that supercontinent.

In other words, in introversion, all ocean floor consumed in making a supercontinent will be
younger
than the break-up of the previous supercontinent, while in extroversion it will be
older
.

This presents a potential method of telling which of the two
mechanisms
broke apart and then formed supercontinents older than Pangaea. But how useful could it be? It relies, after all, on dating ocean floor that is consumed in the creation of a new supercontinent. But surely, you may ask, subduction
consumes
all ocean floor, so it is no longer available for sampling. If there is indeed none left, how can this idea move us forward?

The answer lies in remembering the difference between perfect models and imperfect reality. It may be true in textbook diagrams that subduction destroys all ocean floor, but it is not so in real life. Real subduction is not the clear-cut business represented in these tectonic cartoons; and sometimes, instead of diving down into the Earth like they should, something goes wrong and pieces of ocean crust become scraped off on to the continents to become parts of mountains: true ‘topographic oceans’.

Geologists have long recognized these distinctive rocks. They
consist
of three basic elements: basalts, erupted underwater and thus forming characteristic pillow shapes; the vertical dykes that fed these submarine eruptions with lava; and the glassy mineral chert (silica, or silicon dioxide) sitting between the pillows. The pillows typically
have chilled margins (small crystals, or even glass) where the hot magma met the ocean and cooled very quickly. Below, the dykes that fed their eruption formed as tension at the mid-ocean spreading ridge opened up long, parallel cracks at right angles to the direction of tension. The cherty sediments between the pillows, rich in silica, partly precipitated from solution and partly derived from the
skeletons
of such creatures as sponges and the microscopic diatom. (There are few other microfossils because, in the low temperatures of the deep sea, those with skeletons made of calcium carbonate dissolve away.)

 

Cartoon representing plate tectonics. Ocean floor, produced at the mid-ocean ridge, is pulled back down into the crust at subduction zones. However, the process is not this neat in nature, and bits of the ocean floor get scraped off to survive on top of the continental plate, as ‘ophiolite’.

Pillow lavas, dykes and cherts form a classic threesome first noted in 1905 by Gustav Steinmann (1856–1929) and grandly named the ‘Steinmann Trinity’ in his honour by the eccentric Scots geologist Sir Edward Battersby Bailey.

But the Steinmann Trinity is only part of what geologists now call an ‘ophiolite suite’, an ocean-floor remnant that may run to
thicknesses
of three to five kilometres. Deeper still within the sequence, below the sheeted dykes, come massive, coarsely crystalline rocks called gabbros. These rocks are the solidified remains of the magma chambers that fed the dykes and that cooled more slowly because of their greater volume and depth, and thus formed rock with the same chemistry but larger crystals. Lying below the gabbros are the
deepest
rocks of all, including peridotite (which has sometimes been altered by seawater to form a well-known rock called serpentine, often used for ornaments, ashtrays and cheeseboards). These dark-green rocks are slices of the Earth’s mantle.

Only with the coming of plate tectonics were these distinctive rock sequences recognized for what they are: the last remnants of long-vanished ocean crust, scraped off on to the continent but destroyed everywhere else by a subduction process that once drew two continental crust blocks together in a mountain-building episode. If enough ophiolite formed during the accretion of a
particular
supercontinent could be dated, the spread of results should enable us to tell if that supercontinent formed by introversion or extroversion.

However, there is a big problem with this idea, and it has to do with resetting of radiometric clocks. Ophiolites commonly have three important ‘ages’. There is the date they were created, their ‘magmatic age’. Their second radiometric age is an overprint that dates from the point at which they began to be involved in subduction processes, as the increase in heat and pressure began to alter their constituent
minerals
. Then there is their third age, which is when they were scraped off on to the oncoming continental crust. For this method to work, geologists need to know the rock’s first, true age: the date when it was first born from the mantle.

In 1991 Paul Hoffman wrote a paper with the title ‘Did the breakup of Rodinia turn Gondwanaland inside out?’ According to this model of how Rodinia fragmented, about 760 million years ago a
megacontinental
landmass made up of the continents we now know as Australia and Antarctica rifted off from Rodinia along a line that now defines the western edge of North America. (This is not the
present-day
coast of North America, because since Pangaea split up, the USA and Canada have ridden over much of the ancient Pacific Ocean floor, colliding with many small landmasses on the way. These have accreted as what geologists call
terranes
to the west coast, and built up the mountainous western seaboard of North America in a process akin to the way that litter collects against the top of an ‘up’ escalator, when the steps are finally subducted into the bowels of the machine.)

In making this move, the Australia-Antarctica continent opened up an interior ocean that became the ancestral Pacific. Ancient ocean floor encircling the fragmenting Rodinia was subducted, and the process
continued
until Australia-Antarctica had swung round and collided with another continent consisting of South America-Africa, which at that time were still joined along the line that would one day open to form the present South Atlantic. Australia-Antarctica thus became fused with South America-Africa, creating a megacontinent we have seen before: Suess’s Gondwanaland. Many geologists also believe (with Ian Dalziel) that at this time other pieces of continental crust were also very close together, possibly fused, and have given this supercontinent assemblage the name ‘Pannotia’ (‘all southern continents’).

Because the ocean that was consumed in this process was all ‘exterior’ to Rodinia, it provides a known example of extroversion; and a possible test for the dating method, because the maximum ages obtained from any ophiolite remnants of the consumed ocean floor should pre-date the break-up of Rodinia.

When Pannotia subsequently split, about 550 million years ago, the interior oceans created by this event, such as Iapetus, Tuzo Wilson’s so-called ‘proto-Atlantic’ separating present-day North America from Western Europe, were subsequently destroyed as the next
supercontinent
(Pangaea) was created. They were, we know, destroyed by the accordion tectonics of the Wilson Cycle process – that is, by
introversion
. Dating fragments of
those
vanished ocean floors should therefore yield ages
younger
than Pannotia’s break-up.