What does the term "intrusion" refer to in geology?

The term "intrusion" in geology specifically refers to the process by which magma from beneath the Earth's surface penetrates into pre-existing rock formations and solidifies into igneous rock. This process creates sizable bodies of rock known as intrusive igneous rocks or plutons, which form at variable depths beneath the surface.

Intrusive rocks are characterized by their coarse-grained texture, which forms as the magma cools slowly over time, allowing larger crystals to develop. This contrasts with extrusive igneous rocks, which form from lava that exits the Earth and cools more rapidly at the surface. The concept of intrusion is fundamental in understanding the geological history of an area, as it reveals information about past magmatic activity and the conditions prevalent deep within the Earth's crust.

The other choices refer to different geological concepts that do not align with the essence of 'intrusion.' Surface level rock formations, for example, are not formed through the process of magma solidification but rather through erosion and sedimentation. Small volcanic rocks would relate more to ejecta resulting from volcanic activity, while rocks formed from sedimentation pertain to a completely different process of rock formation involving compaction and cementation of sediments.