Sedimentary rocks are formed through lithification of pre-existing rocks or pieces of once-living organisms. Which process is lithification?

• A. Erosion
• B. Compaction
• C. Weathering
• D. Deposition

Answer: (B)

Lithification is the process that turns loose sediment into solid rock, mainly by grains being pressed more tightly together and by minerals cementing them. The part of this that fits best among the options is compaction—the weight of overlying layers squeezes the sediment, reduces its porosity, and helps bind the grains together as a solid. Erosion and weathering describe how rocks are broken down and transported, not how loose material becomes rock. Deposition is the settling of sediments from water or air, which is a precursor to lithification but not the lithification step itself. So, compaction is the lithification process in this context.