How are sedimentary rocks primarily formed?

Sedimentary rocks are primarily formed through the process of lithification of sediments. This involves the accumulation and compaction of mineral and organic particles that have settled over time, usually in bodies of water. These sediments can include fragments of other rocks, minerals, and biological materials like shells and plant debris.

During lithification, these deposits undergo cementation as minerals precipitate from groundwater and bind the particles together, transforming the loose sediments into solid rock. This process is crucial because it not only helps in the formation of sedimentary rocks but also provides insight into Earth's history, as these rocks often contain fossils and records of past environments.

In contrast, other processes mentioned involve the formation of igneous and metamorphic rocks. For instance, the solidification of magma leads to igneous rock formation, while crystallization under pressure is characteristic of metamorphic processes. Cooling lava at the surface also results in igneous rocks rather than sedimentary ones. Thus, the key mechanism for sedimentary rock formation is indeed lithification of sediments.