According to modern geologic understanding, Earth once had all its land
area contained within a single supercontinent called Pangaea that sat
one side of the globe surrounded by a single ocean. Pangaea
then broke into many pieces that drifted across the Earth’s surface and
into the positions in which we see them now.
Pangaea ~200 million years ago showing today’s continents overlayed.
This is known as the plate tectonic theory. It states that
continents and surface land sit on ‘tectonic plates’ that drift over
ocean areas. As they do so, the plates push the ocean crust
in front of them down into the molten area underneath in a process
called subduction, and the seafloor expands behind them in a process
called seafloor spreading.
There’s also a lesser known yet very interesting idea that holds the
Earth was originally somewhat smaller and expanded to what it is
now. The basis for saying this is the observation that if one
takes the dry landmasses (continental regions) and rearranges them onto
a smaller sphere whose area is equal to the sum-area of those
landmasses, the land appears to fit together like pieces of a jigsaw
puzzle.
A good summary is given by this video:
A few liberties appear to be taken with morphing/distorting the land
shapes but otherwise the fit is quite good. It is also
important that the landmasses are joined with adjacent landmasses that
they could have drifted away from.
Below is my own arrangement attempt that doesn’t use
morphing. Click on the image to start the
interactive presentation.
The concept of an expanding Earth has been around since the late 19th
century but was officially abandoned in favour of plate tectonics in
the 1960s. Since then a few authors have continued to pursue
the idea; each proposing differing mechanisms for how the expansion
process might have occurred, or leaving left that aspect
unanswered. It will be object of this article to propose a
different mechanism than theirs and add further thoughts.
Comparison of Theories
The plate-tectonics/Pangaea theory has its benefits. It
explains the observed current motion of plates and why the continents
appear to join together across the Atlantic. But it also has
its problems. For example, it doesn’t explain why all the
land would be situated on one side of the globe, why it would break
apart, or how the subducted land can be pushed into a liquid magma
of higher density.
The Expanding Earth theory has similar benefits, with the upside that
it doesn’t require the land to start on one side of the globe or for
subduction to occur. But has the obvious downside in that it
requires the Earth to grow in size – which on the face of it makes no
sense.
So it could be said both theories have their share of benefits and
problems. Overall however, the expanding theory appears to
have more benefits and might be a better idea – providing, of course,
that there was a way for the expansion to occur.
Continental vs. Oceanic crusts
So is it possible that the Earth could have greatly expanded, and if
so, how?
An important clue lies in crust thicknesses. The Earth’s
crust varies in thickness but is typically 30-50km over land areas and
5-10km over ocean areas. There is also a lack of a smooth
transition between the two as the thickness changes suddenly at the
edge of continental shelves.
Why the distinct difference? If Earth were originally a ball
of molten magma, as is our modern understanding, and its surface
cooled, wouldn’t the crust be the same thickness all over?
A response to this might be that ocean crust is thinner because of the
water. But this makes no sense. Firstly because you
can’t have water sitting on hot magma. If Earth had water at
the time, it would be in the form of steam in a very thick and tall
atmosphere. Secondly, water would make the crust underneath
cool faster, since it removes heat via convection, making the ocean
crusts thicker.
So here is a possible timeline of events that accounts for the
different thicknesses:
1. The Earth started as a lump of molten magma, pulled into a
sphere by gravity (where the magma came from is unimportant here).
2. The surface gradually cooled, losing heat into space, and
slowly hardened. After many millions (or billions) of years,
the surface hardened into a thin crust. Let’s say it was 20km
thick at that point.
3. Something happened that caused the interior of Earth to
expand.
4. The crust cracked into many pieces like fragments of an
egg shell. Those pieces then floated on top of a giant ‘lava
lake’.
5. After a while the expansion process stopped. The
pieces continued floating about, sometimes colliding with other pieces.
6. The exposed magma began to cool, forming a thin
crust. The land pieces continued to cool and thicken.
7. The thin crust gradually thickened to around 10km thick
and the land pieces thickened to 30km, and mostly stopped drifting.
8. As temperatures cooled, the steam in the atmosphere
condensed and fell, eventually settling into the thinner crust areas
and forming the oceans.
“Something happened”?
The above timeline approximately matches the state of Earth as we see
it now. To make it more accurate, ocean crusts would need to
be continually forming as the expansion occurred. This would
cause sections of the crusts to be of different ages; becoming younger
farther away from continents.
The part we are missing is point 3 where “something happened” to cause
the interior of Earth to expand. What could possibly do that?
The answer to this is complicated and needs to be broken into several
parts. We’ll begin by looking at this density chart:
This shows the density of Earth as a function of radius. Note
that the interior (the core) has the highest density and the exterior
(the crust) is the least dense. There are said to be two
reasons for this. One is that the heavier elements have sunk
downward, and the other is that the pressure is higher farther down,
leading to greater compression.
The core has a density of 13.1 g/cm3 and is said to be made primarily
of iron. But iron’s density is only 7.8 g/cm3. So
this tells us the iron has been compressed somewhat.
Meanwhile the lower mantle (halfway down) has a density around 5.5
g/cm3 and is said to be mostly silicates. But the density of
silicates (and rock in general) is around 2.5 g/cm3. So this
too has been compressed.
It may seem odd to think of rocks being compressed but it needs to be
pointed out that pressures down deep are much higher than what would be
normally experienced.
Non-linear compression
The next thing to consider is that compression doesn’t occur in direct
proportion to force. For example, say we have a block of
rubber that is 10cm tall. We place a 1kg mass on it and
notice it compresses by 1cm and is now 9cm tall. We then
put another 1kg on it (making the total 2kg) and notice it
compresses another 1cm, making the height 8cm tall.
At this point we might conclude that each additional kg will reduce the
height by a further cm. But that couldn’t be true because
otherwise 10kg would reduce the height to zero and 11kg would somehow
make it negative.
Instead the amount of compression needs to become less and
less. E.g. 3kg might compress by 2.9cm (height=7.1), 5kg
might compress by 4.5cm (height=5.5), 7kg by 5.8cm (height=4.2), etc.
So say we have 5kg sitting on top and the block’s height is
5.5cm. If we add a further 2kg the height will reduce by
1.3cm (making the height 4.2cm). But if we instead subtract 2kg,
the height will increase by 1.6cm (making height 7.1cm). So
adding to an already compressed block yields a small amount of extra
compression. Whereas subtracting the same amount of mass
yields a larger amount of expansion. This is going to be
important when looking at Earth.
As additional weight is placed on the rubber, further compression
becomes less and less,
making this compression vs. force chart non-linear.
Pressure within a sphere
Next we’ll consider the pressures inside a sphere that results from
gravity. Suppose we have a sphere of uniform
density. It has a mass of one unit (M=1) and a radius of one
unit (R=1) The density works out to
ρ=M/volume=3/(4*pi)=0.24, as shown:
What will the internal gravity function look like? If we set
Newton’s gravitational constant G=1 we get:
So gravity starts at zero at the core and goes to 1 at the
surface. What will the internal pressure function
be? Pressure is the sum of gravity times density for all the
mass above a location. It looks like this:
It’s an inverted parabola with a pressure of 0.24 at the core and zero
at the surface.
Let’s suppose that this sphere is made of compressible materials and
the radius it has is due to gravitational compression. That is to say,
its radius would be somewhat larger if there was no gravity.
We are going to redistribute the material inside the sphere.
Suppose this sphere was made up of materials of different atomic
weight. We will bring heavier elements to the central region,
and lighter elements to the outer, while keeping the radius the
same. Suppose that after doing so, the central region has
double the original density and the outer has half the original
density. Since the total mass remains unchanged, the
‘boundary’ works out at to be at cube-root of 1/3= 0.69.
Comparing this to the previous situation, the gravity function (blue
line) looks like this:
The red line is the previous gravity function and the density is shown
in brown. We can see gravity becomes double at the boundary
and identical at the surface. Now how does the pressure
function look?
The pressure (blue) at the core has increased slightly compared to the
previous (red) situation, while pressure in the outer part decreases
largely. This is most important!
Now what do you think will happen? Refer back to the rubber
block example. Most likely the inner region will compress by
a small amount and the outer part will expand by a larger
amount. This sets off a chain reaction...
1. As the outer region expands, this reduces both gravity and density
in the outer region, leading to the outer region expanding further.
2. As a result of the outer region expanding, pressure above the ‘core’
decreases. This leads to the core region expanding slightly.
3. As a result of the core expanding, the total radius increases, which
decreases gravity farther out.
4. This further decreases pressure in the outer region, allowing it to
expand some more.
And so the process repeats itself over and over until the sphere has
expanded to a somewhat larger size.
Comparisons to Earth
Does the above sound like Earth? The inner core of Earth is
said to be heavier metals, mostly iron, and the outer regions mostly
silicates. We can be reasonably sure it wasn’t that way
originally. Instead the heavier elements would have been
uniformly distributed throughout the magma (assuming Earth was
originally a molten magma ball). Those elements would have sunk
slowly inward. As they did so, the whole pressure dynamic
would change and the upper portions would begin expanding, followed by
the inner portions, etc.
In short, the sinking of heavier elements to lower depths would cause
Earth to expand. These diagrams give an overview of the
process:
Original Earth has heavy and light elements evenly mixed.
Its size is small due to very high pressure. Outer surface
cools to a solid crust.
Heavier elements, like iron, sink toward the centre. This
reduces pressure in the outer regions, causing them to expand, which
then allows inner regions to expand. Surface crusts are
cracked and pieces float on newly-exposed magma.
Exposed magma cools yielding a thinner oceanic crust.
Edges remain hot and volcanic as expansion continues.
Earth ends up with a much larger diameter. Atmospheric
water condenses and flows into the oceanic areas.
Conclusion
It is possible that Earth once had a much smaller size due to
gravitational compression of the elements that composed it.
As heaver elements sunk inward, this changed the interior pressure
dynamics and caused the Earth to decompress/expand to its current size.
