Which are the three main rock types?

• A. Granite, Sandstone, Limestone
• B. Obsidian, Coal, Shale
• C. Igneous, Sedimentary, Metamorphic
• D. Marble, Gypsum, Slate

Answer: (C)

Rocks are grouped by how they form into three main types: igneous, sedimentary, and metamorphic. Igneous rocks come from molten rock that cools and hardens. If it cools underground, crystals grow larger (like granite); if it erupts onto the surface, it can be glassy or have tiny crystals (like obsidian). Sedimentary rocks form from bits of other rocks or minerals that settle from water or wind, stack up, and become solid through pressure and cementing; they often show layers and may contain fossils (like sandstone or limestone). Metamorphic rocks arise when existing rocks are changed by heat and pressure deep underground, altering minerals and texture without melting (like marble from limestone or slate from shale).

The other options just name specific rocks, which belong to these categories but don’t identify the groups themselves. So the three main rock types are igneous, sedimentary, and metamorphic.