What is the term for a slab of volcanic rock formed when magma squeezes between layers of rock?

The term for a slab of volcanic rock that forms when magma squeezes between layers of rock is a "sill." Sills are horizontal sheets of igneous rock that intrude between existing layers of sedimentary or metamorphic rock, creating a layered appearance. This occurs when magma rises from deeper parts of the Earth and pushes its way laterally between the layers, solidifying as it cools.

Understanding the concept of a sill is important in the study of geology, particularly in igneous processes. Sills are typically formed at lower pressures than dykes, which are vertical intrusions, and they can contribute to the formation of unique geological structures. Recognizing the distinction between different types of intrusive rock formations helps clarify how magma behaves under the Earth's surface and leads to various rock formations.