Why does the ocean get deeper?
This is a map of the seafloor elevation or bathymetry in the Indian Ocean. The red and yellow areas represent shallow parts of the ocean; the blue and violet areas are deeper. Some obvious features, like Indonesia and the Ninety-East ridge, stand out obviously. If you look at the overall seafloor there is also another clear pattern. The Indian Ocean is shallow near the mid-ocean ridge, and the ocean gets deeper farther away from the ridge.
This property of the ocean floor is almost entirely due to heat. At mid-ocean ridges, the hot mantle comes up nearly to the depth of the seafloor. That mantle can be about 1200°C. Once that mantle melts to make ocean crust and the ocean crust solidifies and begins cooling, everything starts to contract. Over a period of millions of years, heat diffuses out of the ocean crust and even the upper mantle, cooling off the upper hundred kilometers of the planet. As the rocks cool, they contract and their density goes up, gradually dragging the ocean floor downwards under its own weight.
Some of the deepest places on Earth, including areas like the Mariana Trench, are found where there is exceptionally old oceanic crust. The older the oceanic crust, the colder it is and the denser it is, and the more it wants to sink into the planet.
This feature is observed on land as well. Where the land surface was recently rifted or active, it can be warmer than an ancient, cold continent. As this land surface cools, it will settle and subside, creating space for sediment or water to flow in and be trapped.
This simple story still leaves room for some complexities – for example, right next to the Sunda trench in this photo, the ocean floor actually rises because it’s being bent just before it goes down the subduction zone. Other areas can be warmed up by volcanic activity or by extra heat coming up from the mantle.
-JBB
Image credit: https://go.nasa.gov/2o8a1NX