Sherlock Holmes and Dr. Watson are once again resting in their favorite location, in front of the fireplace. Holmes keeps staring at the mantelpiece, in silence. No drinks, no pipe.
– In our last after-dinner discussion, you brought up the word “subduction” – something that was unknown to me. Do you have more to say about this phenomenon, Dr. Watson asks.
Part 1: Sherlock Holmes and the Hippopotamus in the Basin
Holmes replies: Well – there is probably a lot to be said about this phenomenon. However, I will only touch on it briefly because today I have much bigger fish to fry. Maybe we might call what I will tell you, “The Dinosaur in the Mantle”.
– Steady now Holmes, you might sound a bit more serious if you did not invent all these, well frankly, a bit silly expressions, Dr. Watson says.
Holmes, with a determined expression on his face: I am sorry to say, Watson, your lack of knowledge and imagination prevents you from seeing the necessity of such grand words. However, you will get it.
Holmes continues: Let’s finish off the subduction story and get on with the big stuff. Last time we spoke about basin formation due to uplift, erosion, and faulting caused by mantle upwelling. I also told you that the reason for all of this is “subduction”.
– Exactly, Dr. Watson interrupts, and right now I was hoping for the continuation of your story.
Holmes explains: Well Watson, subduction is the process where old and dense oceanic crust, earlier formed in the deep oceans by upwelling magma, sinks in because of its density. You see, contrary to ice that floats on water, the cold and dense oceanic crust does not “float” on the hot mantle below. Eventually, it will sink in. Because the oldest oceanic crust is found where the new oceanic crust was first created – often near the coast of continents where rifting occurred – that is where it starts to sink in first. This is also the region where most of the sediments are brought in by rivers and glaciers over the millennia. This aids in pushing the oceanic crust further down.
Holmes continues: But the thing is, since both the oceanic crust and the sediments were “born” in the sea, they contain a lot of seawater that subducts with it. This water, often bound to certain minerals – that we will not take time to discuss – and its dissolved salts, are later liberated as the crust reaches certain depths. The fluids invade the mantle, make it less dense and less viscous, and produces the mantle upwelling we spoke about and also produces volcanism. We will leave it there because as I said – we have bigger fish to fry.
– Ok then, I guess we might return to the subject on a later occasion if I can think of an intelligent question to ask, Dr Watson replies.
Holmes replies: Indeed, Watson. Have you ever heard about the French scientist Gaspard-Gustave Coriolis?
– No, Holmes, why should I?
Holmes: That is a question you will have the answer to in a minute.
He continues with the following story:
“A person on a moving train is pointing a rifle straight out the window. The trigger is pulled at the exact moment when the rifle points directly at a non-moving target situated by the railway. When standing next to the target and seeing the bullet arrive – in slow motion of course, you might see the bullet deviating sideways and miss the target. If the difference in velocity between the target and train/rifle is larger, or the bullet moves more slowly, the deviation will be greater.”
Holmes adds: This is an example of what is called the Coriolis Effect – it is not a force in itself, but forces may arise due to the relative movement of different objects contacting each other.
The face of Dr.Watson looks like a big question mark.
– What on earth is the connection between this rifle-man on a train, and geology, he exclaims.
Holmes replies: Well, Watson, good question, I am certain that many geologists would ask the same question. Many years ago, meteorologists observed that winds blowing towards a low-pressure system in the atmosphere in the northern hemisphere made the air circulate counter-clockwise.
Holmes continues: And on the southern hemisphere, the air in a low-pressure system rotates clockwise. The reason is this: The Earth and the atmosphere may be regarded as a rigid body, rotating from the west to the east. One revolution takes 24 hours, regardless of your location. Hence, the rotational speed is greatest along the Equator. Away from the Equator, the rotational speed is proportional to the cosine of the latitudinal angle above/below the Equator, because this defines the radial distance between the axis of rotation and the surface of the Earth.
Holmes: Winds blowing northwards from the Equator are moving faster eastwards than the location of a low-pressure system further north. Therefore, just like the bullet on the train, the wind deviates to the east of the low-pressure system. Winds blowing southwards towards the low-pressure start their journey moving at a slower pace eastward and deviating to the west of the low-pressure system. If we regard the low-pressure system as some sort of wheel, the winds will combine to push it around in a counter-clockwise direction.
– I will have to make a drawing of that to understand what you are saying, Dr. Watson laments.
He makes the following drawing:
Holmes adds: Anyway, let it be no surprise that the air escaping from high-pressure systems, produces a rotation opposite to that of low-pressure systems. He continues: do you know what causes low pressures and high pressures in the atmosphere, Watson?
– I am afraid not, Holmes, Dr.Watson replies.
Holmes explains: Low pressure systems are created by ascending light air that requires new air to flow in and replace it. High-pressure systems are created by descending air. Usually, these vertical velocity components are not given much attention in weather forecasts. This might be because humans normally spend their time at the surface of the Earth. What goes on at five km elevation in the atmosphere is not that important to common people.
Holmes continues: Anyway, in geology, vertical motion should not be overlooked. Rotational velocities are also a function of the radial distance from the Earth’s axis of rotation. In other words, the Coriolis Effect is relevant for masses moving towards, or away from, the axis of rotation. This is true whether the masses are moving along the surface or moving up or down below the surface. Masses moving away from the axis of rotation will deviate westwards because they are initially rotating slower towards the east than the regions further away from the axis. Masses moving towards the axis of rotation will deviate eastwards relative to the region they are approaching. This is independent of what hemisphere we are observing.
Holmes: However, Watson, we are not finished yet. Do you remember the pressure systems in the air where rotation is created in the air flowing near the surface?
– Yes, Holmes, – If you don’t mind, I have a slightly better memory than a goldfish, Watson replies.
Holmes: Ok then Watson, let’s finish off these very rudimentary elaborations on the Coriolis Effect in the mantle.
He continues: As mantle masses approach a location where they are allowed to ascend, due to weakening of the mantle/crust above, they may be allowed to flow in from many directions just like the air in the low-pressure system. Hence, in this situation, they will also rotate counter-clockwise on the northern hemisphere and clockwise on the southern hemisphere. On the Equator itself, rotation is cancelled out but the westwards deviation will still be present. Now my friend, are you perhaps seeing some light at the end of the tunnel?
Holmes: This is the Dinosaur in the Mantle. Geologists seem to have overlooked the fact that subducting slabs, upwelling mantle, and continental drift are all subjected to the Coriolis Effect, sometimes in three dimensions. They probably think that the effect is not visible since rocks are much more rigid than air. However, a fundamental principle in physics is not turned off just because someone thinks it is. Fortunately, rocks tend to preserve signs that reveal their previous experiences. For this to occur, the forces arising from relative movements have to exceed the strength of the rocks in question and produce long-lasting deformations. In other words, very old events may still be detectable in the structure of rocks if they were exposed to substantial Coriolis Effects.
– I am impressed, Dr. Watson states, have you checked if your theories actually might be valid?
Holmes: Of course, I have! Do you think that I am just rambling along to keep you amused? Where is that drink I ordered, by the way?
Holmes continues: Anyway, because of the Coriolis Effect and the slightly different behaviour on the northern and the southern hemispheres, the rocks will tell a story of a “Tectonic Equator”. Rigid rocks will develop what is called strike-slip faults if they are subjected to large enough twisting forces. This will lead to so-called right-slip and left-slip faulting according to where the twisting occurred, relative to the Equator. Mid-ocean, volcanic islands will show signs of rotation, just like any other low-pressure system, because they are located directly on the oceanic crust with the deformable mantle below. Have a look at Fiji, for instance.
Holmes asks: Do you remember that I told you about the hiatus that will always be present in the vicinity, or within the big sedimentary basins?
– Yes, of course, is there more, Dr. Watson asks in anticipation.
Holmes says: Indeed, at the very bottom of intracratonic basins, where complete rifting has not destroyed the “evidence”, you may still observe a strike-slip fault in the basement to further confirm the upwelling and twisting mantle millions of years ago. And, in addition, the type of strike-slip fault will tell you if the event occurred to the north or to the south of the Equator.
Holmes: There is so much more, and so many implications of what I have been telling you however, it is once again getting late. Now, do you believe that this is a Dinosaur in the Mantle?
Watson nods in silence.
– And you have just touched on “upwards” movements, without considering the velocity vectors of different kinds of movements on, or within the Earth. There is surely more to this story, he adds slowly.
Holmes exclaims: You impress me, Watson!
Holmes, getting slightly impatient and raising his voice: Where is that drink I ordered?
– Make it two, Watson shouts.
The butler seems to be in serious trouble now.
HANS K JOHNSEN
Inspired by Arthur Conan Doyle