Hypabyssal rocks are igneous rocks that form at medium depths, typically within the Earth’s crust, between volcanic (extrusive) and plutonic (intrusive) environments. These rocks are often fine-grained due to their intermediate cooling rates and are important in studying the processes of magma intrusion and solidification within the crust.
Reference: Johannsen, A. (1931). A Descriptive Petrography of the Igneous Rocks. University of Chicago Press.