Which term best describes rock formations that intrude into preexisting rock layers, forming features like sills and laccoliths?

Which term best describes rock formations that intrude into preexisting rock layers, forming features like sills and laccoliths?

• A. Intrusive igneous
• B. Extrusive igneous
• C. Sedimentary
• D. Metamorphic

Answer: (A)

Intrusive igneous rocks form when magma pushes into preexisting rock layers and cools underground. This slow cooling produces larger mineral crystals and creates bodies like sills, which run parallel to the layering, and laccoliths, which are dome-shaped intrusions that push the overlying rocks upward. Because these formations originate from magma cooling below the surface, they’re classified as intrusive igneous. In contrast, extrusive igneous rocks come from lava erupting onto the surface and cooling quickly to form fine-grained or glassy textures, while sedimentary rocks form from the deposition and lithification of sediments, and metamorphic rocks arise from heat and pressure altering existing rocks.

Intrusive igneous rocks form when magma pushes into preexisting rock layers and cools underground. This slow cooling produces larger mineral crystals and creates bodies like sills, which run parallel to the layering, and laccoliths, which are dome-shaped intrusions that push the overlying rocks upward. Because these formations originate from magma cooling below the surface, they’re classified as intrusive igneous. In contrast, extrusive igneous rocks come from lava erupting onto the surface and cooling quickly to form fine-grained or glassy textures, while sedimentary rocks form from the deposition and lithification of sediments, and metamorphic rocks arise from heat and pressure altering existing rocks.