Why is burial usually necessary to turn a sediment into a sedimentary rock?

Answer 1

The key to converting sediment into sedimentary rock is often though of as being pressure. And it is only by burial that enough pressure can be generated on a layer of sediment to "press" it into sedimentary rock. With pressure and time comes what is called compaction and the expulsion of (most of) the space between the particles of sediment that were deposited. Let's look at sediments being laid down and turned into sedimentary rock.

When sediments are created, moved and deposited, they can consolidate and form a layer or layers. These strata will be composed of particles of organic and inorganic material. Included will be the intersticial spaces between the sediments. Any water present in the pore spaces (and there usually is) will bring with it dissolved minerals. These can precipitate out in the spaces, and will cause cementation, which is the binding of these minerals to the particles of sediment and the "sticking together" of the sediments. Following the deposition of more material, pressure (lithostatic pressure) will build on the underlying strata. The compaction forces the layer to become more dense and to "turn to stone" over time. And sedimentary rock is the result of this process.