Nodules form on top of the sea floor because they are denser than the surrounding sediment. This density difference causes the nodules to sink until they reach a layer of sediment that is more compact and less dense. The nodules then rest on top of this layer, preventing them from sinking any further.

In addition to their density, the shape of nodules also helps them to stay on top of the sea floor. Nodules are typically round or oval in shape, which reduces the amount of surface area that is in contact with the sediment. This reduces the friction between the nodules and the sediment, making it less likely that the nodules will sink.

Finally, the surface of nodules is often covered in a layer of mucus. This mucus helps to lubricate the nodules and further reduces the friction between the nodules and the sediment.

All of these factors combine to help nodules stay on top of the sea floor. This allows them to be easily collected by mining operations.