Which sedimentary rock category forms from lithification of organic debris?

• A. Clastic
• B. Organic
• C. Chemical
• D. Metamorphic

Answer: (B)

Sedimentary rocks are grouped by how they form. When organic debris—like plant material or the remains of organisms—accumulates, becomes buried, and undergoes compaction and cementation, it turns into an organic sedimentary rock. Coal is the classic example, formed from compressed plant matter, with other rocks in this category arising from accumulated organic material as well. In contrast, clastic rocks come from fragments of preexisting rocks, chemical rocks form from minerals precipitated from solution, and metamorphic rocks are rocks that have been altered by heat and pressure. So lithification of organic debris specifically yields organic sedimentary rocks.