Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 62

more) are of this type, fed by the magma rising from the 'wedge' of
mantle that lies above downgoing crustal plates.
Yet more fluid travels farther down with the slab into the mantle, to
be released, many millions of years later, as the subducted material is
remelted to produce basaltic magmas at mid-ocean ridges. Hence,
there are complex but substantial pathways of water between the sur-
face and the depths of the Earth.
There is a large question here. After the initial influx and (pre-
sumed) 'folding in' of water, what has been the overall trend of water
between the Earth's surface and its interior? Has water, on balance,
been taken out of the mantle, causing the oceans to grow in volume?
Or has water been continuously folded into the mantle, causing the
surface ocean waters to further diminish? Simply by considering what
we know of the inputs and outputs of water (the error bars are large),
there seems to be more going into the Earth's interior from the oceans
than is coming out. If so, that has consequences for the far future (see
Chapter 8). From the geological evidence, though—that is, from clues
in rocks and strata—it is hard to tell.
Using the proportions of land to sea (which we explored earlier in
this chapter) is a crude measure, because there is the changing depth
of oceans to consider too. We simply have to use the evidence of the
continents as best we can, exploiting these permanent, buoyant struc-
tures as a kind of dipstick to see how shallowly or deeply they have
been submerged in the past. In effect, one has to track past shorelines
as they advanced inland on to higher ground, or receded, leaving
former shorelines high and dry.
That is easier said than done. One problem is that sea level has bobbed
up and down in the past for all kinds of reasons—as land ice has grown
and melted, say. Twenty thousand years ago, when the Ice Age was at its
height, sea level was some 120 metres lower than today (while melting all
the ice that we have today would raise sea level by some 70 metres).