An Ancient Magma Chamber Exposed
The dramatic rock formations at Concón are composed of granodiorite, an intrusive igneous rock that forms from silica-rich magma cooling slowly deep beneath the Earth's surface. This specific outcrop is part of the immense Coastal Batholith of central Chile, a vast chain of cooled magma chambers, or plutons, that runs along the coast. Geologists have dated the granitoids in this batholith to the late Paleozoic Era, specifically the Carboniferous to Permian periods, making the rock under your feet approximately 300 to 320 million years old.
The rock is phaneritic, meaning its interlocking crystals are large enough to be seen with the naked eye. Its composition is intermediate between granite and diorite, containing more plagioclase feldspar than orthoclase feldspar. Important minerals include quartz, plagioclase, biotite, and hornblende. The darker minerals, like biotite and needle-like hornblende crystals give the granodiorite a characteristic salt-and-pepper appearance, distinguishing it from the more uniformly light-colored granite. This entire geological feature, known locally as Roca Oceánica, was officially protected as a Nature Sanctuary in 1990.