I want to tell you a story.  Its about the earth, the way it evolved and the way it is now.

I shall start just after the sun was shining brightly.

The sun shone brightly and spun around on it axis  Circling around it was lots of gases, dust and rock…  They circled around in all sorts of different orbits not necessarily aligned with the suns rotational axis.  The sun shone on these bits and made them hotter.  The bits aligned themselves into a disk and they all rotated around the sun’s equatorial plane.  Gradually the bits got together under the effects of gravity. So that as one lump formed it clearer the zone near it of smaller bits. 

As they fell together they all got hotter. To the extent that much of the rock was molten.  One such lump was a binary system.  A big lump and a small lump.  They rotated around their common center of mass.  The earth lump spun on is axis once every 20 hours.  The moon lump went around the earth every 20 days.  Its orbital period dictated that it orbit more closely. Both had molten surfaces.  Both raised tides of molten rock on each other. The tides were bigger because they were closer.  The tides caused friction in the molten rock and slowed down the rate of rotation. 

When the dust and rock was spread out the sun was able to heat it and increase its temperature.  As the bits came together in a disk the heating effect was diminished and then further diminished as they formed small round planets.  The planets began to cool.  They continued to be warmed by the sun.  They had their own residual heat.  They had warmth created by atomic reactions from the interior.  They had the warmth of friction caused by the tides. They were warmed by the occasional arrival of a further lump of rock and dust.

The moon being smaller cooled more rapidly.  The surface froze. 

In the molten earth the various components sorted themselves by density.  The denser bits sinking to the center and lighter bits floating to the surface. So the nickel and iron being denser collected at the center while the lighter rocks floated to the surface. Whatever air or water was about was heated and flung off away from the earth.

The earth cooled more slowly.  The earth’s rotational axis was not aligned with the axis of it rotation around the sun.  So for half the time during the orbit around the sun a polar region was tilted away from the sun and cooled more quickly.  The surface of the molten rock froze.  A good example of this would be the TV pictures of an eruption in Hawaii.  Molten rock arrives at the surface and cools due to radiation and conduction into the air.  The frozen rock forms a crust on the outside of the molten rock.  Inside this crust the molten rock can still flow.  So each winter a frozen rock island formed in a sea of molten rock. Each summer the heating effect of the sun melted the frozen rock and the bits floated away toward the equator.  Just like we can see TV pictures of icebergs breaking away from the icecap each spring and drifting toward the equator.

Time progressed and the frozen rock covered nearly the whole surface.  The equator continued to be molten but with an increasing coverage of “iceberg” lumps.  At some stage all the “iceberg” lumps all joined up.  But there was a problem.  The moon was still raising tides on the molten rock.  So once a day the molten rock heaved up and then down, cracking the nearly solid surface.  Into every exposed crack molten lava welled up and froze. So the cracks spread apart and pushed it neighbours harder against the next one and in some cases up and over its neighbour and this formed a more rigid bond between the pushed up mass. 

Where the original crack had occurred it was more susceptible to further cracking and next day it cracked again and up came the molten lava and as the tide subsided so it pushed it neighbours away again.  So the whole plate moved away and on the other side up and over the adjoining plate.  So once established the cracks were self perpetuating.  And once a few bits had been crushed together and ridden up and over and under each other these too were self sustaining because they were less susceptible to cracking. 

As the lumps became bigger they sank deeper into the molten rock and since the rock was hotter toward the centre the bottom of the lump was melted.   That was the bit of the plate that had ridden under its neighbour.  Again, once this process became established the part which rode over was protected from melting and became more established while the bit that went down had the end melted off – making room for more for it. 

The moon raised the tide, and at any one pass it lifted up a body of molten rock all along a line of longitude.  I. e. a bulge that extended from north to south with maximum bulge near the equator and nearest to the moon.  So the cracks that occurred tended to be north and south orientated.  But this organised system did not last.  It became fragmented and buckled.  Some crack would expand more rapidly than others and other bits that were sliding underneath would get caught on other bits and not progress evenly.  The process of expanding the cracks continued but became disorganised so that the neatly organised north-south cracks swung around and caused additional cracks along established bits of crust and the whole process would get going again.  But higgdely piggdely with no organised pattern.

So after a short while the whole thing was composed of lots of separate plates, of various shapes a various sizes.  When you watch ships in a harbour with a strong wind and a lot of swell – they each roll and pitch at a rate defined by their shape and mass. If they were not all held separate by mooring lines they would scrape up and down against each other.  And so it was with the plates – they each had their own harmonic motion.  So that the driving frequency of the tide was adopted by motions that the plates could use. So some which were near a harmonic of the driving frequency would leap and thrash about while others would slump docilely.  Such enormous bodies of rock treated leaping and thrashing as tiny movements but still sufficient to crack the the crust. 

You can see the effect of driven harmonic motion in the Lake Lundy in Canada where the body water has a frequency near to the di-urnal cycle.  The tides (of water) are the highest in the world.

The plates were floating on viscous treacle.  Well not really treacle but viscous molten rock so any violent motion was damped down.  But the parts that tended to crack were those places where a node occurred – that’s a point where the oscillations are at a maximum. And cracking did not occur at anti-nodes.  In special cases there could be parts where two plates move up and down in unison and that would tend to limit cracking.  So cracking did not occur all along a boundary but at particular places where differential movements occurred.  And so the molten lava would spring into the crack and expand the crack  – but not evenly along the whole length of a crack and so the plates became even more disorganised. 

And then the surface cooled even more. And then it rained!  Where the water came from I cannot say.  It rained on the tops of the heaped up bits. And trickled downhill to the valley.  The lowest point was the edge of the plate and the water dribbled down the crack at the edge of the plate and cooled the upwelling lava even more.  The process of cracking and spreading continued but even more slowly.  It rained some more and the water accumulated in seas and hid the edges of the plates.

The seas were affected by the moon and formed tides.  And the plates continued to be affected by the moon.

And that is the end of my story.  But they did not live happily ever after, because they squabbled about the reason for the spreading plates.