There seems to be a region where the ocean surface is lower than elsewhere: There is a ‘gravity hole’ in the Indian Ocean, and scientists now think they know why | CNN and it seems to coincide with a place where local gravity is also lower, probably something to do with the arrangement of less-sloshy stuff under the ocean. My head gets kind of spinny trying to think out why the water surface should be lower in a spot with lower gravity. (Or maybe the "lower gravity" in question is what's measured at the surface, and has something to do with the reduced mass of water there.) Is there a less-dizzying way for a person to think about this?
CNN — There is a “gravity hole” in the Indian Ocean — a spot where Earth’s gravitational pull is weaker, its mass is lower than normal, and the sea level dips by over 328 feet (100 meters). This sentence confused me (easy enough to do. ) There must be a higher gravitational pull (an increase in mass or density below that spot) at that location to cause a depression in the ocean's surface.
The CNN article phrasing was implying that there was a direct connection between the low gravity and low sea level, which doesn't jive. The connection is indirect. The low gravity lets the higher gravity areas nearby to pull more water to them. Likely abetted by the constant sloshing going on. "As a result of the low pull of gravity there, combined with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average" https://www.scientificamerican.com/article/giant-gravity-hole-in-the-ocean-may-be-the-ghost-of-an-ancient-sea1/
Speculation, gravity is a field, not a single vector. So the surrounding water is ‘pulled away’ from the depression. Bob Wilson
Sounds like what the SciAm piece was getting at. I'm in the habit of picturing a field where all the vectors point radially inward, but when you're not simplifying Earth to a point mass and there are higher densities around you, the vectors near you must be pointing downward-and-a-little-away-from-your-feet.
A simple experiment that demonstrates the effect: Suspend a large cooking pot from a rope at three points around the rim Twist the pan so when released, the twisted rope will spin the cooking pot. Half fill cooking post with water. Release the cooking pot so the rope makes it spin as it descends. Watch the water level develop a depression in the center like the ocean dip By having the pan of water spin, centrifugal force will move the water from the center up against the sides. This has the same effect as the decreased gravity in the depression that allows stronger gravity around the depression to pull the water away and make a dip. There is a simpler, less 'splashy' version. Half fill the pot with water while it sits in sink. Using a large cutlery, stir the water so it circulates. Take the cutlery out and the water will soon form the depression in the center as long as it spins in the pot. A third approach suitable for the summer heat, half fill a circular, inflatable child's pool with water. Then use a strong stream from the water hose to impart a circular motion. The center will be lower than the edges but you may need a spirit level to see the dip. Less precise, insert small children. Bob Wilson
Another way to say it: when the pan is still, the only force pulling on the water is straight down, but when it spins, there's an additional component (the centrifugal part) pulling water horizontally, away from the center, toward the sides. Enough to raise it up around the sides (even though there's still a downward force opposing that). If the earth were some weird empty sphere with all its mass at one central point, then all of the gravitational force vectors would point straight at the center all the time. But its mass is really distributed through the whole sphere. If there's a little bit less straight below you, then there is enough surrounding the place where you're standing so the force vectors point a little bit out to the sides of you, instead of straight down. That vector can be resolved into a straight-down part and an out-to-the-side part, and that's what carries some water out to the sides, lowering it where you are and piling it up a bit higher around you.
This Indian Ocean "hole" is not a concave depression as in the above examples. It is still convex like the surface of a sphere, but with its surface very slightly depressed compared to a "reference Earth" sphere or ellipsoid. The horizontal dimensions of this "hole" are so large that its depth is a tiny fraction of the amount of Earth's curvature across it. At 3 million km^2, it is almost 2,000 km across, or 18 degrees of longitude (if at the equator). If we 'flat-spotted' a patch of Earth this size, its center would be about 78,000 meters below the reference sphere. To be concave, it would have to be even deeper still. But this "hole" is only 106 meters deep, so it is still very much convex, with a curvature indistinguishable to the human eye from the rest of the Earth. It takes very careful measurements to detect it at all.
Think about the inverse. Over the decades I've seen numerous articles mentioning how mass concentrations pull the local ocean surface up. These concentrations include large mountains, large thick ice caps (e.g. Greenland), and even underwater seamounts and ridges. Some of the later have even been coarsely mapped by satellites using radar to measure ocean surface level down at the centimeter scale. If extra mass can raise the local ocean level, it just makes sense that 'missing' mass, e.g. lower density rock or crust or magma below, would do the opposite.
Ah.....the good old days. I remember LOTS of fun with gradiometers back when we only had bear skins and stone knives. Even with today's fancy-dancy geo-stationary gear it's fairly to have a good model when you're worried about the OTHER 2-3 dimensions. Flat-earthers are wrong but a lot of folks don't recon that sometimes the reference ellipsoid.....doesn't. It's NOT a ball. Oh...and sometimes the 'big 4' GPS/GLONASS/Beidou/Galileo are subjected to ah.....'local service interruptions.' If you don't have a bathroom dressing room full of documents or a home-brew server you might be out of luck. Did I forget to mention that the rock that we live on ISN'T a Rock? It's not standing still either. If you're doing nap-o'-the earth stuff you might be in for surprise if you don't consider the lumps and bumps even if you're NOT using a rock knife of a hairy coat to aim your spear.
