True Polar Wander As a spinning top slows down, it will start to wobble to the side and eventually t

True Polar WanderAs a spinning top slows down, it will start to wobble to the side and eventually tip over all the way. The top is, well, “top-heavy”. If it’s spinning rapidly enough, it stays balanced with the weight at its farthest edge, but as it slows down the weight wants to shift position. Planets can do something kind of like this; the Earth can actually tip over like a slowing top. This process is called True Polar Wander.The Earth’s outer core is made of liquid iron. Outside of it is a solid mantle; the mantle is literally a solid floating on top of a liquid. The planet is actually broken into these layers and so the mantle can actually change its position and rotate relative to the core.The Earth also spins, like a top, around its north-south axis, which we call the poles. The stable position for the Earth is one where any extra mass is also at the equator. For a metaphor, imagine that you’re swinging a weight around your body on a string. The weight will pull your arms out as you swing it around. If you try to hold your arms straight up, as the weight moves around it will tug your arms to the side.This weight is like any weight on the Earth’s surface; as long as it’s at the equator, the weight is happy. But, if there’s extra weight at the poles, the planet becomes unbalanced. Just like the swinging weight would pull your arms farther out, weight near the poles will pull itself towards the equator.Extra weight near the poles will therefore be able to actually cause the Earth to roll, taking mass near the poles and moving it to the equator. This process is called “True polar wander”, because the spot that was once the North/South pole actually shifts to a different point, and another spot rolls into its place. This name is in contrast with “apparent polar wander”, a seeming motion of the poles relative to the continents that is caused by plate tectonics. Apparent polar wander happens even if the poles don’t move; true polar wander is the poles actually shifting (https://tmblr.co/Zyv2Js1LJUEUa).Geologic evidence suggests that true polar wander has happened several times in Earth’s history, most recently a pair of shifts of roughly 9°, starting in the Cretaceous when the planet rolled to the side and then shifting back the other way during the Paleocene-Eocene when the modern day pole rolled back. There are arguments for far larger shifts in the Earth’s pole position going farther back in time.What types of mass shifts might cause one of these rollover events? Mostly it is thought to be big things in the mantle. A plume of hot material rising through the mantle will be low density because hot things expand and rise. That low-density material might want to be closer to the poles. However, if the hot mantle reaches the surface and triggers massive volcanic eruptions, that pile of igneous rock on the Earth’s surface could be a high-density feature that would want to roll towards the equator.This image shows a pair of mantle plumes rising up through the planet, depositing large amounts of lava and mass on the surface, and triggering true polar wander. There was a mantle plume near the equator about 70 million years ago when the planet started rolling – the end result is the Deccan traps, a large lava field in India that erupted 65 million years ago. That plume is right around the time where it could have triggered the rolling in the geologic record.A recent study simulated what would happen on an Earth-sized planet closer to a star, as has been seen in many Exoplanets. The simulations showed that the greater the gravitational force is at the equator, the more easily planets would roll over. True polar wander, therefore, could be a regular feature of planets orbiting stars other than our own.-JBBImage credit and Exoplanet Paper:https://www.nature.com/articles/s41561-018-0071-2References:https://www.nature.com/articles/s41561-018-0079-7http://onlinelibrary.wiley.com/doi/10.1029/2011JB009072/fullhttps://www.nature.com/articles/nature10174 -- source link

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