Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 54

time backwards, close up the segments that have recently opened,
magnetic stripe by magnetic stripe and time-slice by time-slice. Run-
ning time backwards this way, the Atlantic Ocean progressively nar-
rows and disappears: by some 200 million years ago, in the early
Jurassic, it was entirely closed.
At that time, oceans that have now vanished, or are just remnants,
become enormous. The Mediterranean Sea is expanded from its
present shrunken state into the mighty Tethys Ocean of ancient times,
as Africa pulls back from Asia and India tracks back south of the
equator. The Alpine and Himalayan mountains, meanwhile, subside
back into the original, uncrumpled shallow seas and coastal plains
that they sprang from.
But to go yet farther back? After all, close up the present-day ocean
and we are back just a couple of hundred million years, which is less
than 5 per cent of the age of the Earth. How does one reach yet further
back, to divine the shapes of yet more ancient oceans?
The detective work here is a little less straightforward, and there-
fore more fascinating. With virtually all of that ancient oceanic crust
destroyed we need to make enquiries among their more durable
neighbours, the continents, by tracking their movements. Once we
know where the continents were, then the oceans must have been
everywhere else.
Ancient continental positions can be tracked by measuring the ori-
entation of preserved magnetic particles in the strata that still point
towards where the North Pole used to be when those strata formed.
This kind of information is patchy (the magnetic data can be over-
written if the rocks are heated too strongly, for instance), and it pro-
vides information only on the ancient latitude, not the longitude, of
strata. Nevertheless, the magnetic information can be combined with
other approaches, such as making a 'best fit' of the edges of ancient
continents in what is essentially an Earth-sized jigsaw puzzle. These