Earth’s chang­ing tides

Su­per­con­ti­nent for­ma­tion may be linked to a cy­cle of su­per­tides that have long-term ef­fects on the whole planet, sug­gests Dr Mat­tias Green of Ban­gor Univer­sity

Western Mail - - AGENDA -

EARTH’S crust is made up of frac­tured slabs of rock, like a bro­ken shell on an egg. These plates move around at speeds of about 5cm per year – and even­tu­ally this move­ment brings all the con­ti­nents to­gether and forms what is known as a su­per­con­ti­nent.

The last su­per­con­ti­nent on Earth was Pan­gaea, which ex­isted be­tween 300-180 mil­lion years ago.

This col­lec­tion and dis­per­sion of the con­ti­nents is known as a su­per­con­ti­nent cy­cle, and the world is now 180m years into the cur­rent cy­cle.

It is pre­dicted that the next su­per­con­ti­nent will form in about 250m years, when the At­lantic and Pa­cific oceans both close and a new ocean forms where the large Asian plate splits.

Be­cause the plates move around, ocean basins change their shape and size. For ex­am­ple, the At­lantic is cur­rently ex­pand­ing at about the rate your fin­ger­nails grow (a cou­ple of cen­time­tres per year), whereas the Pa­cific is slowly clos­ing.

These changes in the ocean basins can have a very large im­pact on the tides over mil­lions of years.

This is be­cause the tide moves around the oceans like a very long wave, with more than 1,000km be­tween two peaks. The way this wave moves is largely con­trolled by the shape of the ocean basin and its depth, and if the ocean has the right size – if the length of a basin is half that of the wave, or “res­o­nant” – the tides can be­come very large.

Res­o­nance can hap­pen in any sys­tem that swings or os­cil­lates if you force it at its nat­u­ral pe­riod. For ex­am­ple, if you give a child on a swing a small push at the right time, they will swing higher and higher, be­cause you are forc­ing them at the nat­u­ral pe­riod of the swing.

The pe­riod of the tide is set by the mo­tions of the Earth, moon and sun – and the nat­u­ral pe­riod of an ocean basin is set by its ge­om­e­try.

For ex­am­ple, to­day, the north At­lantic is very near res­o­nance be­cause these two pe­ri­ods are al­most the same. This is why the tides in the At­lantic are much larger than those in the Pa­cific or In­dian Oceans.

But this has not al­ways been the case.

From ex­per­i­ments with com­puter models which can sim­u­late the tides with great ac­cu­racy, we know that the tides were weak for long pe­ri­ods of the cur­rent su­per­con­ti­nent cy­cle, be­cause the shape and size of the basins couldn’t sup­port large tides.

In fact, of the past 250 mil­lion years, it is only the last two mil­lion years or so that have seen large tides on Earth. Since we are ap­proach­ing the half­way point of the su­per­con­ti­nent cy­cle, we asked our­selves a ques­tion: what will hap­pen to the tides as the next su­per­con­ti­nent as­sem­bles in 250m years or so? Is it pos­si­ble that there is a su­per­tidal cy­cle linked to the su­per­con­ti­nent cy­cle?

Us­ing the com­puter model, we have now found that there is in­deed a su­per­tidal cy­cle linked to the su­per­con­ti­nent cy­cle. In fact, there are two: we are cur­rently at the start of one “tidal max­i­mum”, a pe­riod of time when the tides are very large. They will then weaken sig­nif­i­cantly, be­fore briefly be­com­ing large again in around 150 mil­lion years from now.

Af­ter that, the tides will again drop down to less than half of the en­ergy lev­els they have at present as the next su­per­con­ti­nent forms.

This will hap­pen be­cause the basins go in and out of res­o­nance as their shape changes.

The tidal max­ima are brief in ge­o­log­i­cal terms and only last 20 mil­lion years or so.

For most of the time, the tides are less en­er­getic than they are to­day and, over the 400-600 mil­lion years be­tween the for­ma­tions of the two su­per­con­ti­nents, the tides are only large for 50 mil­lion years in to­tal.

Tides are a ma­jor en­ergy source for the ocean: the en­ergy pumped into the tide by the sun and the moon is lost, or dis­si­pates, within the ocean.

This en­ergy helps stir the ocean – much like a spoon stirs a cup of cof­fee.

In the same way as the spoon moves sugar and milk around in the cup, the tide can drive move­ments of nu­tri­ents, heat and salt be­tween the deep ocean and the sur­face.

Fluxes of heat and salt are key to the large-scale cli­mate con­trol­ling ocean cir­cu­la­tion and fluxes of nu­tri­ents help sus­tain bi­o­log­i­cal pro­duc­tion, es­pe­cially in shal­low seas.

Changes in tides on any timescale can have large ef­fects on the whole Earth sys­tem.

While the changes de­scribed here may not have im­pact on us in the im­me­di­ate fu­ture, it adds to our un­der­stand­ing of how the tides in­ter­act within var­i­ous dis­ci­plines – in­clud­ing plate tec­ton­ics, the cli­mate sys­tem, nu­tri­ent re­cy­cling and, even­tu­ally, the ocean’s abil­ity to evolve and host life.

■ Dr Mat­tias Green is a reader in phys­i­cal oceanog­ra­phy at Ban­gor Univer­sity’s School of Ocean Sciences.

■ This ar­ti­cle first ap­peared on www.the­con­ver­sa­tion.com

Changes in tides on any timescale can have large ef­fects on the whole Earth sys­tem

> The ocean’s tides change as the Earth’s crust al­ters. Pic­tured is Bali Is­land, In­done­sia

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