During lithification of clastic sediments, which processes reduce pore spaces and cement grains together?

• A. Compaction and cementation
• B. Sedimentation and dissolution
• C. Weathering and metamorphism
• D. Crystallization and growth

Answer: (A)

The idea being tested is how loose clastic sediments turn into solid rock through diagenetic changes that reduce pore spaces and hold grains together. As overlying layers accumulate, the weight compresses the grains, squeezing out fluids and air and reducing porosity—this is compaction. Then minerals dissolved in groundwater precipitate to form cements that bind the grains, consolidating the sediment into rock; this is cementation. The other options don’t describe this pair of steps: deposition (sedimentation) adds material, not lithifies it; dissolution would remove material rather than glue grains; weathering and metamorphism involve surface breakdown or high-temperature/pressure transformation, not the typical diagenetic cementing and squeezing; crystallization and growth refer to crystal formation in magma or solution, not binding loose sediment.