What defines a dyke in geological terms?

A dyke in geological terms is specifically defined as an intrusion of igneous rock that cuts through existing layers of rock. This geological feature forms when magma from beneath the Earth's surface is injected into pre-existing strata and solidifies as it cools. Dykes typically appear as vertical or near-vertical structures, and their presence provides valuable information about the geological history and the processes that shaped the region.

The other options refer to different geological concepts. A narrow valley formed by erosion would describe a canyon or ravine, which emerges through the action of water or wind wear over time. A raised area resulting from volcanic activity could be referring to a volcano or a lava dome, rather than a dyke specifically. Fossilized remains of a large organism are known as fossils and serve to highlight past life forms and ecosystems rather than geological formations like dykes. Thus, the definition aligns perfectly with the correct choice, emphasizing the significance of dykes in understanding igneous rock processes.