Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 55

former continental edges themselves may show up as ancient, eroded
mountain belts—crumple zones that mark where continents collided
to destroy the ocean between them. Here and there, slivers of oceanic
crust may have been rescued from subduction, caught up in the
crumple zone and scraped on to the continental edge—where they
may lie still, to be interrogated by geologists today.
For instance, one can follow the line of a 400 million-year-old belt
of eroded mountains from Scandinavia, across Britain and Ireland,
into Newfoundland (that used to be just next door to Britain before
the Atlantic Ocean opened), then down the Appalachians. These, the
Caledonian Mountains, represent the giant crustal scar tissue where
there was a former ocean—the Iapetus Ocean—that is in many
respects the forerunner of the Atlantic Ocean. Once several thousand
kilometres across, it was swallowed up into the depths of the Earth:
that is, the crust of the sea floor was swallowed up, while the water
itself simply spilled across into other ocean basins.
One can occasionally still find slivers of this ocean floor—for
instance at the small Ayrshire village of Ballantrae on the west coast
of Scotland. 45 Here there are telltale signs of ancient ocean floor, some
480 million years old, which include basalt lavas with 'pillow' struc-
tures (the pillows formed by water chilling as the lava is extruded) and
the fossilized ocean floor oozes that are associated with them.
Ballantrae is a tiny sliver, a few kilometres long, of that ancient ocean
that once separated Scotland from England and Wales. A little dis-
tance away, in southern Scotland, there are more oceanic slivers—but
these just represent the ocean floor sediments (now hardened into
mudstones) that were scraped off the basaltic ocean floor and stacked
vertically—with uncanny gentleness, as though by a giant and expert
plasterer—against what was then the edge of a Scottish/North Ameri-
can continent. This crustal scraping and plastering took place over
a period of about 30 million years, as the Iapetus Ocean was