Sherlock Holmes rushes into the living room where Dr. Watson is ready for action in front of the fireplace.
Holmes: I have to leave early today so you better keep it short, Watson. I am off to the yearly convention in the Northumbrian Angler’s Federation, the dry fly section, where I am giving a lecture on Cock-Y-Bondhu.
– Oh – I clearly see the utmost importance of that endeavor; I will start immediately, Dr. Watson responds.
Part 1: Sherlock Holmes and the Hippopotamus in the Basin
Part 2: Sherlock Holmes applies “meteorology” to geology
Part 3: Sherlock Holmes gets a big surprise from Dr. Watson
– I will give you my impression on the importance of including the Coriolis Effect when trying to understand the movement of continental plates on Earth, he continues.
– As usual, let`s start with a continental plate situated just north of the Equator. Let`s further assume that it is being pulled northwards by the subduction of an oceanic crust attached to it. Because the whole plate moves from a region with relatively high rotational velocity into a region with lower rotational velocity, the whole plate will deviate eastwards. However, there is more. We have established earlier that the rotational velocity is proportional to the cosine of the latitudinal angle. The rate of change in rotational velocity when moving northwards, the velocity gradient, is proportional to the sine of the latitudinal angle in question.
– Because of this, the northern and southern edges of the moving plate will experience a different change in the eastwards rotational velocity of the Earth. The sine function goes from zero to one between the Equator and the pole; hence, the northernmost section of the plate will experience a greater change in rotational velocity than the southern part. If the plate is strong and rigid, it will therefore rotate clockwise. Of course, being attached to the subducting oceanic plate and being surrounded by rigid oceanic crust elsewhere, there will be great resistance, however, if anything – this is what should be observable, Dr. Watson concludes.
Holmes: That sounds reasonable my friend, what more is to be said about this? My train leaves in 45 minutes.
– Our favorite places to reflect on concerning the Coriolis Effect are the Equator and the poles. Let`s consider the Equator first, Dr. Watson continues.
– Consider a continental plate centered on the Equator and half of it to the north and south from the equatorial line. The entire plate is moving northwards for some reason. Based on the arguments presented a minute ago, the northern part would tend to deviate eastwards while rotating clockwise. The southern part would also try to rotate clockwise because the southern edge of it is experiencing a greater change in rotational velocity than the part along the Equator. In addition, the southern half will deviate westwards as it is approaching regions with higher rotational velocities. The northern half will experience more of a Coriolis Effect since it is moving into regions with higher sine values. The southern part is moving more on top of the Equator, where sine-values are closer to Zero.
– As you may have guessed Holmes, this will put some strain on the plate and it might even split into two parts right along the Equator. It might be interesting to consider what happens then. In fact, I have made a figure this morning to illustrate this.
Holmes injects: I clearly see what you are talking about Watson, I am not stupid but still in a hurry. Keep it short.
– Well, anyway here is my drawing. As you may see, a divergent shear zone will appear between the two halves. Now my friend Holmes, as you hinted a day ago, we might look at Africa and South America. Maybe their shape and the way they separated isn`t coincidental.
– In fact, if we study the drifting of continents over the eons, while always considering things relative to the location of the Equator, we should find that the following applies:
- Northwards drifting continents on the northern hemisphere rotate clockwise while deviating eastwards.
- Northwards drifting continents on the southern hemisphere rotate clockwise while deviating westwards.
- Southwards drifting continents on the northern hemisphere rotate counter-clockwise while deviating westwards.
- Southwards drifting continents on the southern hemisphere rotate counter-clockwise while deviating eastwards.
- The location of the axis of rotation will move within a continent as it drifts, due to the differences in change of velocity experienced by the edges of the continent (The sine-function).
Holmes: What happens with continents approaching the poles?
– That might require a fair amount of explaining however, we might look at Antarctica. It is located on the South Pole, Dr. Watson says.
– It must have ended up there after coming from the north since every direction to the South Pole is from the north. This implies that as the continent approached the pole, it experienced a Coriolis Effect because every part of it had an eastwards rotational velocity higher than the region nearer to the pole. Initially, this would make the southern part deviate eastwards a bit more than the northern part due to the aforementioned sine function. In sum, this would make Antarctica rotate counter-clockwise while deviating eastwards relative to the original path. As the continent continued moving its northern parts further south, the former southern parts would start moving away from the pole, i.e. northwards. This would lead to westward deviation and clockwise rotation of the part having crossed the pole.
– To make things a bit more understandable, I have made a sketch.
Dr. Watson explains his drawing:
– The South Pole is colored red in all four stages. Straight arrows indicate the general direction of the continental mass and curved arrows indicate rotation. My drawing includes four stages in the development.
– In A), Antarctica is starting to move towards the South Pole. In B), Antarctica is approaching the pole and influenced by the Coriolis Effect. In C), part of the continent has traversed the pole, and this part is now moving northwards. This is when opposing Coriolis Effects emerge on the former northern- and southern edges of the plate. The continent is from this point in time put under considerable strain. During stages A) to D) Antarctica, would revolve around a constantly moving axis (pole of rotation). In the event of rifting of the continent, rotation might occur around two rotational axes, also on the move as the two parts move. In D) a convergent strike-slip zone is emerging on the left-hand side of Antarctica and in continuation of this on the right-hand side, Antarctica is rifting. The rifting is caused by the opposite rotation of the two parts of the continent similar to the “co-operating gears” of the Equatorial Mid-Atlantic Ridge. In the Antarctic situation, there is also some left lateral slip caused by linear deviation in addition to the convergence and divergence caused by rotation.
– By the way, Holmes, have you reflected on the similarities between the mountains in the Antarctic and the Lomonosov ridge next to the North Pole?
Holmes: Can`t say that I have.
Dr. Watson has now stopped talking, nearly out of breath. Holmes gets up and prepares to leave.
– If I might add one more thing, Holmes, Dr. Watson adds hastily.
– India is an interesting case. I believe including the Coriolis Effect might resolve its detailed movements before and during the collision with Asia. If you wait a second Holmes, I might give you a crash course on India before you run.
Holmes: Make it extremely concise, Watson.
– Well – ca. 75 million years ago India was located south of the Equator and was pulled northwards by a subducting plate across the Equator before it started to collide with Asia ca. 48 million years ago. The collision is still ongoing. Because India started moving from a position centered at ca. 30 degrees south, it should initially deviate westwards from its direction of travel, while rotating clockwise. After crossing the Equator, it should start to deviate eastwards while rotating clockwise. During the drift phase before colliding with Asia, India would have drifted northwards along a S-curve while rotating clockwise.
– If I am correct, Dr. Watson concludes, the evidence for this should be set in stone, or the Himalayas, as is the name of this particular stone.
Holmes: I`m leaving. Thank you for your definite proof of intellect. Next time I will lecture you – on subduction.
And in an instant, Holmes has left the building.
HANS K JOHNSEN
Inspired by Arthur Conan Doyle