Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 41

sea level, as an array of more or less flat plains (that, very slowly, slope
upwards to long, low ridges that snake across this overall level). These
areas are, geologically, the ocean basins and mid-ocean ridges. The
ocean waters are held within these basins, but also overspill them a
little on to the higher of the two levels.
This second level is about a kilometre above sea level. It is our
familiar terrestrial landscape, but also extends beyond it, across the
shoreline, on to the shallow sea floors that surround us. Together,
the land and shallow sea make up the geological continents, which
occupy a little less than a third of the Earth's surface. Between these
two levels, of continental and ocean floor surfaces, there is a distinct
step called the continental slope. It is not quite a cliff, but rather
forms a marked slope that descends more or less steeply from the
higher level to the lower one. We cannot see it directly, because it is
entirely under water. It is, though, the most important boundary in
the world. It separates two fundamentally different types of the
Earth's crust.
Two further elements can be added. On the continents there are the
raised, rugged masses of the mountain chains. These may run along
the edges of the present-day continents, like the Andes, or stretch
across them, like the Himalayas or the Urals. To mirror these, in the
oceans there are long narrow trenches (rather narrower than are the
mountain belts). These are the ocean trenches, which descend in
places to quite monstrous depths, most famously in the bottom of the
Marianas Trench in the western Pacific Ocean, 11 kilometres below
the sea surface.
Finally, let us take just two more elements. One is the lid that keeps
the water in, high in the sky. It is a cold trap at the base of the strato-
sphere, where temperatures fall so low that water vapour, so much a
part of the underlying troposphere, freezes into ice crystals within
clouds, that then fall back to lower levels. The stratosphere, hence, is