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Palaeogeography
Palaeogeography is the geography of the Earth in ancient times, so tries to establish the arrangement of the continents back through time.
There had been a time when the continents were different, Rincewind understood, and then they'd sort of shuffled more comfortably together, like puppies in a basket"
("Sourcery", Terry Pratchett)
Palaeogeography is a science that becomes harder the further back we go, as evidence is lost 'in the mists of time', and into the interior of the Earth.
However; its study has produced many lines of evidence which have led to the construction of a sequence of theoretical maps, showing how the Earth looked in prehistoric times.
Whilst different map sequences show differences caused by uncertainties in many areas, we have high confidence in the overall picture presented.
This section covers a brief overview of the 'dance of the continents' in Mesozoic to Cenozoic times, and is illustrated with plate tectonic maps by C. R. Scotese, from the PALEOMAP Project (www.scotese.com)
The following discussion covers the World-view Palaeography since the start of the Mesozoic
World-view Palaeography
Triassic Overview
At the dawn of the Mesozoic, all of the Earth's continental masses were in the form of one single 'supercontinent', now called Pangea. Prior to that time, there had been many separate continents, but by then, they had all collided and formed one vast expanse, surrounded by the Panthallasic Ocean.
This continent of Pangea was approximately wedge-shaped, and open to the east, with an ocean intruding into the continent. This ocean is now known as the Tethys, and its remnants in today's world geography is the Mediterranean Sea - which is now closed to the east, but open to the west.
As the Mesozoic progressed, the super-continent of Pangea began to break-up and form the continents that we recognise today.
The British Isles in the Triassic
During the Triassic, the 'British Isles' were not islands, but had an intra-continental position within Pangea. The area that now constitutes our islands was drifting northwards as Pangea rotated, to a latitude of ~10o - 20o N, equivalent to the latitude of the present day Saharan desert.
Erosion of the then-recently uplifted landmass formed aeolian "New Red Sandstone". It is an iron-rich silica sandstone that oxidised to give it its red colouration. (Oxidisation of iron is rusting, and is what gives Mars its red colour - Mars is rusty). British deposits of the era consist of these red beds, together with alluvial, fluvial and lake deposits, with some shallow-water marine and evaporite deposits. Permo-Triassic outcrops can be seen on either side of the Pennines.
Jurassic Overview
In the early Jurassic, South America and Africa had not yet started to separate. However, latitudinal rifting of Northern America had began, creating a roughly-equatorial sea-way between the North and South.
By the Late Jurassic, possibly earlier, this rift joined up with the Tethys Ocean to its east, and formed a continuous but narrow seaway between the northern and southern continents.
These are known as Laurasia and Gondwana; Laurasia in the North consisting of what is now North America and most of Europe and Asia, and Gondwana in the south, consisting of what is now South America, Africa, Arabia, India, Madagascar, Antarctica and Australia.
The British Isles in the Jurassic
The British Isles continued its northwards drift. As the global sea-level began to rise in the Early Jurassic, the consequent marine transgression replaced the desert land and occasional land-locked seas by more permanent, warm, shallow shelf-seas. The sea-level rise over the British Isles from the Mid-Triassic to the Mid-Jurassic was partially due to a global sea-level rise, but also to the "doming up" of the floor of the area that is now the North Sea, which further increased the flooding of the land.
Most of the south-east of Britain was submerged throughout the Jurassic, although Scotland, Wales and the South-West were emergent. An isolated island, the Anglo-Berant landmass, sat on the area around London.
Cretaceous World View
At the start of the Cretaceous, the continental arrangement on Earth was beginning to resemble that of today; but still had distinct differences.
The continuous equatorial seaway between Laurasia and Gondwana that first developed in the Late Jurassic is a significant feature of Cretaceous. The oceanic flow between them (despite obstructions and restrictions in places because the seaway was shallow and narrow) had great significance on the climate of the time.
Early in the Cretaceous, or possibly already in the the Late Jurassic, South America began to rift away from Africa, forming the South Atlantic Ocean.
Next, the Central Atlantic (between North America and Africa) opened. However, the rifting between North America and Western Europe that produced the North Atlantic did not begin until the Late Cretaceous.
Today, about 140 million years later, the South Atlantic is around 8 000 km wide, and continues to widen today by about 2cm per year.
On the sea floor, the rift between the continents is marked by a continuous submarine, volcanically-active mountain range, known as the Mid-Atlantic Rift.
Iceland in the North Atlantic, and the Ascension Islands and St. Helena in the South Atlantic, are points along it that have breached the surface. Iceland sits above an active mantle plume, which has been implicated in the initial formation of the rift.
The Island of Surtsey off the south coast of Iceland only appeared in very recent times - 1963.
There, you may not only be 'older than the hills', but many of us are older than the land itself.
By the Late Cretaceous, a continuous but locally-restricted Atlantic Ocean, joining the oceans in the south and north, had formed. The Atlantic continued to widen throughout the Late Cretaceous, and continued into the following Cenozoic era, to today.
In the south in Gondwana, other break-ups were occurring. However, there were still no deep-water connections between the oceans. This limited the poleward transport of heat, and consequently affected the global climate.
The British Isles in the Cretaceous
Britain continued to drift north, to around 58oN, a higher latitude than it is at today.
In the Early Cretaceous, much of Britain was above sea-level.
In southern England, Late Jurassic and the earliest Cretaceous strata were deposited under continental lagoonal, lake, and fluvial conditions, whilst further north, the Lower Cretaceous is entirely marine in character.
However, only a few millions of years into the Cretaceous, the sea rose over most of Britain, thereby flooding most of the early Cretaceous landmass.
Deep water conditions then prevailed until the Tertiary.
The Cretaceous ended with a very definite change to the climate and biota, when a meteorite smashed into Chicxulub, Mexico. Its location is marked on the map above, approximately between North and South America. The impact probably caused world-wide fires, and threw ash into the atmosphere. This ash would have risen to the higher levels of the atmosphere, the stratosphere where it would have circulated the Earth for many years, blocking out sunlight and cooling the climate by many degrees.
More about the Chicxulub impact and its effect on the world's climate can be read in the Cretaceous Boundary page.
Cenozoic World View
The Northern continents continued to revolve and twist, a dance leading to continental collisions which lead to the formation of the mountain ranges we see today, the Appalachians of the US, The Himalayas of Asia, and the more modest Alps and Pyrennes of Europe.
India and Madagascar had by then moved away from their original position nestling between the single landmass of Africa and Antarctica, and were heading north.
Later, Madagascar rifted away, and remains as an island off the east coast of Africa. India, however, continued to move northwards until it collided with the Asian plate to form the Himalayas and Tibetan Plateau.
The Himalayas
The Himalayas are the youngest, and highest, of the world's mountains. There formation began when India collided with the Asian landmass about 55 Million years ago, since when they have been continually pushed up. With time, mountains erode away, but this range is still growing.
The movement of India into Asia has not yet stopped. India is still pushing north, at about 1.8m per century, which is about the same rate as the Atlantic is widening. This results in the mountains being 'pushed up' at a rate of about 10cm per 100 years - 'a metre a millenium'. Consequently, huge pressure is also being built up, which is released in major earthquakes.
The last such was in 1505, and has been estimated to be of the order of magnitude 8.2. The 500 years since then have 'absorbed' pent-up motion of around 9 metres, so another massive earthquake may be due.
Despite the continued push of India into Asia, the overall height of the mountains is not increasing. The mountains are being eroded at about the same rate as they are being pushed up, with the vast majority of the world's sediment being carried from the Tibetan Plateau by its major rivers. The many rivers, such as India's Ganges and China's Yangtze, that flow from the Tibetan Plateau carry away vast amounts of eroded rock, which amounts to more than one fifth of the world's river-borne sediment. This is a hugely disproportionate amount, given the area from which they are scoured is only about 5% of the Earth's continental surface.
The Mountains of Europe
The Alpine orogeny was far more complex, and included the northward movement of the African continent. Africa and Europe never had an exact border; and no date can be put upon the 'collision of Africa into Europe'.
Instead, a number of microcontinents and islands that lay in between in the Tethyan Ocean were crushed, a process which continues today.
The orogeny consisted of several phases, from the Middle Cretaceous through to the Neogene.
It was an extremely complex phase of tectonic movement, in which the northward movement of Africa into European plate and microcontinents resulted in much mountain-building, as Africa partially subducted under Europe, whilst also shearing along it.
- The Pyrenean range, formed from collision of the Iberic plate with Europe
- The Franco-Italian Alps, formed from collision of an 'Italian' (Apulian) plate with Europe,
- Further south, accretions of small terranes against Africa, i.e. in Kabyles mountains.
In Western Europe, the Pyreneans are mainly Eocene, while the main
tectonic events in the W. Alps are rather Oligocene to Miocene - but with older deformation occurring as early as the Middle Cretaceous.
During the Cretaceous (at least, for the Western Alps), the mountain-building process was one of subduction and building of an accretionnary wedge, rather than collision 'stricto sensu'.
The collision is still continuing and won't stop within the next few tens of millions of years. Eventually, the Atlas Ranges will be situated near Paris and Berlin.
The Tethys and the Mediterranean
Remnants of the Tethyan ocean floor can be seen above ground and even walked over, in some places such as Cyprus today. Although a small amount of the current Mediterranean Sea floor is Tethyan Sea floor, most is new ocean. The Tethys closed up a number of times and evaporated away, leaving thick deposits of gypsum and halite. Its first closure was probably about 5.5 - 6.5 million years ago, and [xx Ma ago ] it was breached to the west. For xx years, a torrent of water poured into the enclosed basin at the Straits of Gibraltar to the west, but this is an intermittent opening. During its development from the Tethys into the Mediterranean, the basin has been open to the east, sometimes closed entirely, and is now open to the west.
The Future
No one knows what will happen in the future; but it is clear that Britain within the Eurasian plate is still moving northwards, at about 0.8 cm per year.
Palaeographers are sure that this movement will continue for at least the next 50 million years, moving north and east towards the North Pole.
Prof. Christopher Scotese has gone further, and predicts that in 250 million years Britain will lie where Siberia is now. He sees the continents floating "like a scum" on the molten rocks that form the interior of the Earth. "They come together and break up in a kind of cycle". We now have indications to suggest that at least three supercontinents like Pangea have existed at different times in the Earth's distant past, each of which broke up and rejoined. Whatever we do to our atmosphere and climate, even Man is unable to affect this 'dance of the continents', and can only to speculate - although we will not be around long enough to witness the accuracy of any of the predictions.
You may now either return Home or go to the Site Map.
Alternatively, read about Cretaceous Period, during which time the Chalk of Portsdown was deposited.
For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk