What is a laccolith?

A laccolith is indeed defined as a mass of rock in the Earth's crust that takes on a dome-like shape. This geological structure forms when magma intrudes between layers of sedimentary rock but does not reach the surface, causing the overlying rock to bulge upward. This results in the characteristic dome shape that can be observed in various locations where laccoliths have formed.

The process of laccolith formation involves the magma's pressure forcing its way into pre-existing rock layers, and over time, the surrounding materials can cool around the intruding magma, solidifying into igneous rock. Consequently, this leads to the development of a dome structure, which is a significant feature in the landscape and provides crucial insights into the geological history of the area.

The other options, though related to geology, describe different geological features: a below-ground reservoir of magma refers to a magma chamber, an underground volcanic chamber is typically associated with different volcanic structures, and a layered sedimentary rock formation describes stratification that is not specific to the characteristics of a laccolith. Understanding these distinctions helps clarify why the definition of a laccolith specifically aligns with a dome-shaped rock mass in the crust.