Why is sea level where it is? Because the volume of water in the oceans fills all of the valleys that make up the oceans to a certain depth, and that depth is sea level.
When water is poured into a container, the surface of the water will always be flat, no matter how flat the bottom of the container is. There are two reasons for this. The first is the physical structure of water and the second is the force of gravity. There are four states of matter: solid, liquid, gas, and plasma. Not every form of matter can become a plasma. The atoms in each state have a different amount of energy and can move more or less depending on that amount of energy. Water is obviously a liquid. The solid state is ice, the gas state is steam, and the plasma state is impossible because the oxygen and hydrogen molecules wouldn’t be able to stay together with that much energy.
The molecules in liquid water and solid water are held together by a molecular bond called a hydrogen bond. This happens when the positively charged hydrogen atom is connected to a negatively charged molecule, in this case oxygen. When water is a solid, this molecular bond is rigid and it keeps the molecules very close together. They vibrate, but they are not able to move freely. When water becomes a liquid, the molecules are still connected with the same hydrogen bonds, but they are able to stretch a lot more. This means that the volume changes because of the extra space and the atoms are able to move more freely. This produces the properties of a liquid and explains why it will fit any container that is any shape. However, without the force of gravity, water wouldn’t perfectly fit the container. If you put water in a container in space, the water will form a sphere because all of the atoms attract each other equally. If you put water in a container of any shape on Earth, gravity pulls all of the atoms down. The atoms are pushed up by the atoms below them and gravity can only pull the atoms down to the point where the pressure pushing up is too strong, and this is the surface of the water and why the surface is flat.
So, why is sea level where it is? All of the world’s oceans are connected and it is one giant body of water. There is an estimated 1.3 billion cubic km of water in the seas. That is a staggering amount, but all of that water still follows the principles that make water fit a tank and end up with a flat surface. Gravity pulls down on the oceans until it equals the pressure pushing up from below and that is sea level.
However, sea level isn’t equal all over the world. There are several things that can have an effect on sea level. The first is temperature. When things warm up, the molecules have more energy and they spread out, increasing in volume. When the sea warms up, it expands and this can raise sea level. This is one other problem caused by global warming. Sea levels don’t only rise because of melting ice. The warmer temperatures make the sea expand.
The second thing is the amount of salt in the ocean. Salinity affects density. When there is more salt dissolved in areas of the ocean, it makes the sea denser, which means its volume decreases, lowering the sea level. The third thing is the atmospheric pressure. If there is high atmospheric pressure, it can push down on the sea, lowering sea level. The fourth thing is ocean currents. Large ocean currents move water from one area to another and they can cause water to pile up, or be reduced in certain places, which raises or lowers the sea level. The fifth thing is gravity. We tend to think of gravity as a constant, but it varies by slight amounts over the planet. In areas with greater gravity, the sea will be pulled down slightly more and sea level will be ever so slightly lower.
We have taken sea level to be zero and we measure all altitude from there, but as the ice caps melt, sea level will change. We will have to reevaluate our zero and perhaps recalculate the heights of things, such as Mt. Everest. And this is what I learned today.
Photo by Kellie Churchman: https://www.pexels.com/photo/landscape-photograph-of-body-of-water-1001682/
Sources
https://www.worldofmolecules.com/3D/why-do-solids-liquids-and-gases-behave-differently.html
https://www.visionlearning.com/en/library/Chemistry/1/Properties-of-Liquids/222
https://education.nationalgeographic.org/resource/sea-level/
https://en.wikipedia.org/wiki/Sea_level
https://sealevel.nasa.gov/understanding-sea-level/overview
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