Re: Will the Earth ever be 100% land or water?

Dear Chaz,

The short answer to your question is no, the Earth’s surface will probably 
never be covered completely by water or by land.  To understand why we can 
briefly review the Earth’s history back almost to the beginning some 4.5 
billion years ago.

There have been times in the geological past when far more of the Earth’s 
surface was covered by water than today.  Oceans are known to have been 
present even before 4 billion years ago, so they must have formed very 
early in the Earth’s history.  Indeed, during the first 2 billion years or 
so the Earth’s surface may have been almost entirely covered by water.  
The reason is that plate tectonic processes, which are the means by which 
continental crust is created, took a long time to create continental crust 
of a scale that we would now recognize as continents.  Although it is 
unlikely that there was ever a time with no land at all, the early oceans 
are now thought to have been dotted by volcanic island chains, which were 
the precursors of continents.  So imagine a planet covered by a global 
ocean from which isolated volcanic island chains, perhaps like the 
Hawaiian Islands today, protruded.  The movement of tectonic plates 
resulted over time in collisions between island chains, causing them to 
accrete into larger blocks of continental crust.  By the Proterozoic eon, 
which began about 2 billion years ago, continent-sized blocks of crust had 
formed.  We know that some of this crust was above sea level because there 
is evidence of an ice age, with ice sheets on land, in the early 
Proterozoic.

Ever since the Proterozoic, over the last 2 billion years of Earth 
history, global sea level has risen and fallen in long-term cycles, 
lasting 10s to 100s of million years.  Thus there were times, as in the 
early Palaeozoic era and in the Cretaceous period, when global sea level 
was much higher than it is today.  During these times much of the area of 
the continents was flooded by shallow seas in which much of the limestone 
we find commonly around us today was formed.  But never was all the land 
submerged.  For example, in the Cretaceous period sea levels are estimated 
to have stood some 200 meters higher than today, and the Chalk in southern 
England and many other parts of the world was deposited in shallow seas 
covering continental crust.  There was still plenty of land, of course, on 
which plants and animals (notably the dinosaurs) flourished.  Today the 
Earth is in a glacial state, and sea levels are standing low because so 
much water is locked up in glaciers and ice sheets on land.  Long-lasting 
ice ages were also a feature of the early Permian period and the late 
Proterozoic, and sea levels stood relatively low at these times too.

The processes of plate tectonics have created continental crust throughout 
the Earth’s history, and these processes continue today.  New land is 
being formed over hot spots such as Hawaii and Iceland and over subduction 
zones, such as Japan, where volcanic islands rise above sea level.  This 
new land will be accreted onto the existing continents in time.  Because 
the creation of new land continues it is unlikely that the oceans can ever 
completely cover the Earth – one might say that they had their best chance 
on the early Earth, and ever since the balance between oceans and 
continents has slowly been shifting in favour of continents.

It is also important to understand that plate tectonics creates not only 
continental crust but also oceanic crust.  Oceanic crust is created at mid-
ocean ridges, and destroyed at subduction zones, at a far more rapid rate 
than continental crust is created.  Oceanic crust is denser than 
continental crust, and because of this it rides lower on the underlying 
mantle than continental crust, forming the great basins that hold the 
oceans.  As long as plate tectonic processes continue, both types of crust 
will be created and we can expect both ocean basins and continental land 
above sea level to co-exist.

A final point to note is that it has taken 4.5 billion years for 
continents to grow to cover 30% of the Earth’s surface.  Throughout that 
time the pace of plate tectonics has been slowing down as the radioactive 
elements in the Earth’s mantle and crust that create the heat that drives 
plate tectonics have been progressively depleted.  At that rate plate 
tectonics will cease long before the Earth’s surface can become completely 
paved over by continental crust.

I hope I have answered your question.

Kind regards,

David Scarboro
Associate Lecturer
The Open University