A Cretaceous Intrusion
The dramatic, rocky peaks of Songnisan National Park are the exposed tops of a massive body of granite known as a pluton. This rock formed deep within the Earth's crust during the Cretaceous Period, approximately 100 million years ago, from a huge volume of cooling magma. This event was part of a widespread period of igneous activity across the Korean Peninsula related to the subduction of an oceanic plate beneath the Eurasian continent. This process, known as the Daebo Orogeny, folded and faulted older rocks and injected magma that would cool to become large granitic batholiths.
The Songni Mountain pluton is primarily composed of biotite granite. This type of granite is identifiable by the presence of the dark, flaky mineral biotite, alongside the more common quartz and feldspar. The slow cooling process deep underground allowed large mineral crystals to grow, giving the rock its coarse-grained texture. These granites are part of a larger belt of Mesozoic-era plutonic rocks that run through the region, forming the geologic core of many of the peninsula's mountain ranges. The specific granites of this era in Korea are often referred to as "Younger Granites" to distinguish them from earlier Jurassic intrusions. The magma that formed them originated from the partial melting of the lower continental crust, a process triggered by the intense heat and pressure associated with the active continental margin.