"Solid Earth Tides"
- What Useful Things They Are!

 

Welcome!  My name is James D. Oglethorpe and I'm a retired Industrial Engineer living in Sydney, Australia. 

Since you've found this page, you'll be interested in the Geo-Sciences.  I hope you enjoy my brief points below - the story is quite interesting...

Oceanic Tides  ■  Continental Drift  ■  Geomagnetic Field  ■  Sunspot Cycle

I often go walking on the shores of Rushcutters Bay with my wife and the tidal movement of Sydney Harbour can easily be observed there.  Some years ago, I was explaining the conventional theory of oceanic tides (ie. water attracted by the Moon, etc.) and my wife asked the excellent question - "So if the New Moon is directly above us right now, why is it low tide here?"

I did some reading to find the answer, and to my surprise it wasn't just due to the shallowness of Sydney Harbour, or some such. 

Deep-water places such as Hawaii also have the same "lag".  I read that the average “tidal lag” around the world is six hours.  However, six hours is half the wavelength of the tidal cycle! - In other words the observable tides are completely opposite what one would expect if the Moon was attracting the water.

I did some calculations to work out the gravitational pull of the Moon on a litre of water off Sydney heads.  It is indeed infinitesimally tiny, and is not going to accelerate a kilogram of water anywhere in only six hours.   ...So what makes the stuff leap through the Heads at five knots?

Fortunately I could see one answer where the calculated forces do add up: the influence of Solid Earth Tides.

Now, while ocean tides have been known to Man since antiquity, Solid Earth Tides have only been measured since the 1960s.  (Although we can’t discern it, our houses are actually going up and down by a large fraction of a metre twice a day!)  Solid Earth Tides are easy to calculate; the column of rock and iron below our feet expands as the gravitational pull of the Moon overhead very slightly reduces (by one quarter millionth!) the normal downwards pull of the Earth itself.

Solid Earth Tides act exactly as one would expect: they are "high" when the Moon is above; there is no phase lag and no complexity.  (In essence, they are boring and unchallenging, and so are usually relegated to a footnote in Tidal textbooks.)  My contention however was that the oceanic tides are not caused by the Moon "attracting water".  Instead, it seems clear that the primary effect of the Moon's and Sun's gravity is to deform the whole Earth.  Each section of the crust is thus slightly tilted backwards and forwards twice every day, as the tidal bulge passes.  So the seawater surges sideways through Sydney Heads because it is chasing a temporarily-revised definition of "level".

This paradigm-shift is easily visualised below, in the NZ Govt's animation of their daily tides.  If one looks only at the water, then the setup appears very complex.  The tide is always "high" on one side of the islands when it is "low" on the other.

- Yet those two "opposite" tides are often physically quite close to each other, not separated by 90 degrees of Longitude, as first principles would suggest if the Moon was attracting the water.  So what's going on?

If we simply shift the paradigm to focus on the land, we can see that it is actually being "rocked around" by the passage of the Solid Earth Tide.  The water runs up the beach ("high tide") on the side of the island that has been tipped down.

Learning: Oceanic tides are actually a secondary reaction driven by Solid Earth Tides.  The water is just sloshing around to find a level.

(There is a significant parallel here with the realisation that the Earth rotates, which caused such a scientific shock 450 years ago – and incidentally coined the political meaning of the word “Revolution”.)

And in case you're wondering how a Solid Earth Tide of a fraction of a metre can raise oceanic tides sometimes much more than a metre, the answer mainly lies in the changing cross-section of the Earth's crust at the coast, where the Continents, tens of kilometres thick, are connected to the oceanic crust only 3km thick. The flexibility of the rock that makes up our Continents is quite different from that of the sea-floors and these two elements do not bend uniformly when the Solid Earth Tide passes by.  The resulting "kink" produces higher water levels on many parts of the coast.  Note that oceanic tides are highest right on the coastal boundary and then lose height rapidly as one goes out into the ocean.

(Continental “Granite-like” rocks “float” on the Earth's surface after they are formed, because they are 20% less dense than the “Peridotite” mantle below.  Meanwhile, the sea floor crust is made of much younger Basalt rocks, having a density between that of the Continents and the Mantle.)

Considering evidence such as the NZ animation above led me to another important Engineering insight – that the rocking and rolling of the land can provide a clear explanation for “Continental Drift” that has none of the logical/observational problems of the conventional 1960s “magma plumes/ridge push” model.

In my calculation of the height of the Solid Earth Tide, I looked up the “bulk modulus” of Granite, Basalt and other materials.  Amazingly, Granite is 30% more springy than Basalt.  This means that where the two are in contact, at the “subduction zones” at the edges of the some of the continents, the passing of the Solid Earth Tide twice a day can create a downward ratcheting effect, where the Granite first expands and then contracts against the basalt.  Although the deflections involved (plastic creep) are only a fraction of a millimetre per day across tens of kilometres of contact zone, this is in fact the speed of seafloor subduction (a few centimetres per year).  This mechanism can improve the “slab pull” view of Continental Drift currently held by a minority of scientists.

This “tidal-ratchet” mechanism fits the observation that the biggest mountain ranges, such as the Andes, pull in the seafloors at the highest rate.

Another direct observation supporting tidal driving of Continental Drift (rather than magma plumes) is to compare the planet Venus to Earth.  Both are very similar in size and composition, yet the surface of Venus shows none of the Continental Drift seen on Earth.  Significantly, Venus has 99% less tidal activity on its surface than Earth does.  (Due to its slow rate of rotation and lack of any moons.)

Learning: Continental Drift on Earth is driven by Solid Earth Tides, through “slab pull” of Continental Granite against seafloor Basalt.

Moving from the horizontal to the vertical effects of Solid Earth Tides, a very straightforward mechanism immediately suggests itself for generating the Geomagnetic Field.  When the Earth’s surface rises by 600mm with the Solid Earth Tide, a proportional rise occurs all the way to the centre of the Earth.  The top of the planet’s Iron core is about halfway down, so we are talking about up to 300mm physical displacement of the conducting material.  Simple Electrical Engineering says that the forced movement of the conductor in an existing magnetic field will produce an electrical current flow.  In the case of the “rising” Solid Earth Tide, the current flows east.  This will in turn induce a local magnetic field around the current, pointing “North”.

However, going right around the circumference of the Core, there will always be two “rising” and two “falling” tides.  This leads to a model of the core with four induced magnetic zones, like the segments of an orange, two “North Polarity” segments interleaved with two “South Polarity” segments.  (The reason that we have any observable magnetic field at all on the outside of the planet, despite these apparently conflicting components, is that the tidal influences of the Moon and Sun are misaligned to the Earth’s Equator by up to 23 degrees, thereby inducing one polarity to dominate slightly.)

However, the Geomagnetic field tends to “flip” each million years or so.  The tidal induction model handles this very elegantly, as the “South” segments simply need to strengthen slightly in excess of the “North” segments.  This can occur as the Earth’s alignment with the Sun and Moon wobbles over time.  (The last magnetic flip occurred near a time of a maximum wobble In the Milankovitch Cycle.)

The tidal-induction model explains several features of the Geomagnetic Field in a much better way than the current mainstream theory (which imagines physical convection plumes of billions of tons of hot iron):

- Why the magnetic field is not aligned with the Earth’s spin axis and why it does not pass exactly through the centre of the Core (electromagnetic induction angle).
- Why the field only has two flavours, North and South (rather than coming out, let’s say, at Cuba);
- Why the historical magnetic flips recorded in lava flows happen very quickly (a few days for full transition from North to South).
- Why the historic magnetic field strength recorded in rocks shows a long-term variation reminiscent of Milankovitch Cycles.
- Why the core of the Earth stays hot (it’s made of iron-nickel alloy similar to a toaster element, and it has a strong electrical current flowing through it!).

Also I tried to make an observational prediction, to convince myself that this model had legs.  If there are really four magnetic segments to the Core, then this should show up as a “square” symmetry within the Aurora. Amazingly, the very first picture that I found of the Aurora Australis shows exactly that:

The eddy-currents between the conflicting interior segments may also explain why the Auruoa (when observed from the ground in Antarctica) often transitions from long straight east-west curtains to turbulent swirls.

(You may wonder where the massive energy for this would come from? - Very interestingly, Tidal driving of the Geomagnetic Field and Continental Drift can actually help to account for the “missing” power being subtracted from the spinning Earth’s angular momentum each day.  (ie. Our length of day gets longer by about one "leap-second" each year.)  When one calculates the power needed to push the observable acceleration of the Moon and compares it with the huge braking force that is gradually slowing the Earth's spin, we are still left with over 2 Terawatts of constant power-load unaccounted for.  This amount of power is of the right scale to shift continents and point compasses.)

Learning: The Geomagnetic Field is induced by the action of Solid Earth Tides on the conducting Core. 
Long-term polarity flips are induced by wobbles in the alignment of the Moon and Sun with the Earth’s spin axis.

Scaling up the tidal magnetic induction model to other bodies in the Solar System also seemed to offer many corollaries:

- The planet Venus has a similar core to Earth, however it has no moon and rotates much more slowly, so the “Solid Venus Tide” is less than 1% of our Solid Earth Tide.  Spacecraft observations show that Venus has a miniscule magnetic field, far less than 1% of the strength of that on Earth.

- Large moons (such as Io and Enceladus) of the massive outer planets are well-known to undergo “tidal heating” which produces continuous eruption of volcanoes and geysers.  While the conventional thinking is that these moons are being physically "flexed" and warm up through friction (ah, but what is rubbing against what, exactly?) it is much simpler to envisage large electric currents passing through resistant materials in these moons, due to the movement of their solid-tides and also the moon’s orbits cutting through the strong magnetic fields of their mother planets.  (An electric induction heater!)

- Another significant benefit is a better explanation of the sunspot cycle.  As with the tides on Earth, where the amplitude of the tide is modulated very significantly by the relative angle of the Moon and the Sun (maximum when they are aligned, minimum when they are at 90 degrees to each other) the sunspot cycle depends on the alignment of the planet Jupiter catching up against the moving background of the outer planets.  (This is why the regular sunspot cycle is 10% less than Jupiter’s orbital period.)  NASA’s sunspot graph shows the regular 11-year cycle is also modulated by the angles between the other major planets, predominantly Saturn (29 year orbit) Uranus (84 year orbit) and Neptune (164 year orbit).

Learning: Tidal induction of magnetic fields can be scaled up to the level of Stars.

 

If strong tidal forces can produce strong polar magnetic fields, scaling this up once more could also provide an explanation of the “polar jetting” of Black Holes and Active Galaxies.

(A short stroll from Rushcutters Bay to the edge of the Universe! )