Page 46 - (DK) Ocean - The Definitive Visual Guide

44 OCEAN GEOLOGY

The Evolution of the Oceans

EVER SINCE THE ATLANTIC COASTS OF SOUTH AMERICA and Africa were accurately charted,
spreading continent
ridge carried it has been apparent that they match like the pieces of a jigsaw puzzle. We now know that
on plates
the continents move, that they were once joined together, and that today’s oceans arose
when the landmasses split apart. The evolving oceans have modified the global climate,
and sea level has fluctuated in response to climate change and geological factors.

Plate Tectonics 1. CAMBRIAN (500 MYA)
The remains of the first supercontinent, Rodinia,
The numerous convection cells (see p.41) that gave rise to the were scattered, with the largest piece, Gondwana,
subduction zone lying in the south. The Iapetus Ocean separated
first fragments of continental crust gradually gave way to fewer,
convection larger-scale convection cells as the mantle cooled. The continental Laurentia (North America) from Baltica (northern
Europe). The Panthalassic Ocean occupied
cell drives
plate motion fragments became consolidated into larger areas, and rifts most of the Northern
formed at the thinnest parts of the ocean crust, Hemisphere.
splitting it into large plates. When the density
of the oceanic and continental plates became
sufficiently different, the oceanic crust PANTHALASSIC OCEAN
PLATE MOVEMENT sank where it met the more buoyant
Crustal plates move
around under the continental crust, creating subduction
influence of convection zones. Since then, the evolution of
cells, which probably the oceans and continents has been LAURENTIA
reach deep down dominated by plate tectonics (see SIBERIA
to the boundary
between the outer pp.48–49). As the plates move, they IAPETUS
core and the mantle. carry the continents with them, with OCEAN
oceans opening and closing in between.
BALTICA GONDWANA

“ancestral” North Atlantic
SIBERIA
lies between North
America and Europe
scattered
PANTHALASSIC remnants
OCEAN 2. DEVONIAN (400 MYA) of Rodinia
The Rheic Ocean opened when a
AUSTRALIA string of islands, which were to
EURAMERICA become western and southern
Europe, broke away from
Gondwana and moved toward
Euramerica, closing the Iapetus
RHEIC OCEAN shallow continental
GONDWANA Ocean in the process. -shelf seas
Ural Mountains

southern Europe
joins Euramerica
(Laurentia and first plants on land
Baltica) as Iapetus form vegetated
Ocean closes areas PANTHALASSIC SIBERIA
OCEAN PALEO-
Through the Ages TETHYS SEA

As Earth’s plates have moved around, largely driven by the PANGEA
spreading ridges and subduction zones of the rapidly recycling
oceanic crust (see p.48), continents have come together and moved SOUTH AFRICA AUSTRALIA
apart—periodically grouping together to form “supercontinents.” AMERICA
The German scientist Alfred Wegener proposed that 250 million extensive GONDWANA southern ice cap covers
INTRODUCTION was another grouping about 1 billion years ago called Rodinia, and 3. CARBONIFEROUS (300 MYA) KEY subduction zone
years ago (mya) there was a supercontinent called Pangaea, centered
on the equator and surrounded by one great ocean. It seems there
deserts
most of South America,
Africa, and Australia
perhaps an earlier grouping before that. Each time the continental
landmasses have come together, they have eventually been broken
As the supercontinent Pangaea came together,
continental masses stretched from pole to pole,
apart as deep rifts opened up in their interiors, as is happening today
almost encircling the Paleo-Tethys Sea to the
in the Red Sea and the East African Rift. Computer models of the
east. Today’s coal seams were laid down in
crustal fragments and the locations of spreading and subduction have
spreading ridge
swampy forests along the shores of equatorial
allowed fairly reliable reconstructions of the geography of earlier
shelf seas. An extensive ice cap built up as
outline of modern landmass
times back to 500 million years ago.
Gondwana moved over the South Pole.