From what are carbonaceous types of sedimentary rocks made of?

Step 1: Analyze the question The question asks what carbonaceous sedimentary rocks are made from. The key word is "carbonaceous", meaning carbon-rich.

Step 2: Explain the concept Sedimentary rocks are grouped by how they form:

• Mechanically formed (clastic): sandstone, conglomerate, shale, made from rock fragments.
• Chemically formed: rock salt, gypsum, some limestone.
• Organically formed: made from the remains of living things. These come in two types: • Calcareous, rich in calcium carbonate, such as limestone and chalk, from shells and coral. • Carbonaceous, rich in carbon, such as coal, peat and lignite, from plant material.

Step 3: Work through the options

• A. Tropical heat: this is a climate condition, not a material, so it is wrong.
• B. Molten magma: this forms igneous rocks, not sedimentary rocks, so it is wrong.
• C. Vegetable matter: plants buried in swamps and forests are compressed over millions of years. They turn first into peat, then lignite, then bituminous coal and finally anthracite. This is the carbonaceous process, so it is correct.
• D. Coral remains: these form calcareous rocks such as coral limestone, not carbonaceous ones, so it is wrong.

Step 4: Answer Carbonaceous sedimentary rocks, such as coal, are made from vegetable matter.

Exam tip: Link the words to their sources:

• Carbonaceous means carbon, which comes from plants (coal).
• Calcareous means calcium, which comes from shells and coral (limestone).