A peculiar cooling process
Hanging Rock is the geological remainder of a volcanic event that occurred approximately 6.25 million years ago. It is not a volcanic plug or crater but a mamelon, a term for a steep-sided mound formed when very stiff, viscous magma oozed from a narrow vent and congealed in place instead of flowing away. This particular structure rises 105 meters above the surrounding plain. The magma that formed Hanging Rock was a specific type of soda trachyte. As this dense lava cooled and contracted, it fractured into the rough, vertical columns visible today.
The rock's composition is part of its story. It is a solvsbergite, a rare variety of trachyte with a high concentration of sodium. This specific mineralogy is found in only a few other locations globally, including Norway and Sweden. The interaction between the original soda trachyte and sodium-rich groundwater contributed to its final composition. Hanging Rock is the southernmost volcano in a chain formed as the Australian tectonic plate moved northward over a stationary volcanic hotspot over a period of 27 million years. This same hotspot created other volcanic provinces, including the Glass House Mountains in Queensland and the Warrumbungles in New South Wales.
Unanswered geological questions
The precise conditions that led to Hanging Rock's unique formations remain a subject of scientific discussion. While the general mamelon formation process is understood, the extreme weathering into distinct pinnacles and shapes suggests factors beyond simple cooling. The high viscosity of the soda trachyte lava is the primary reason it did not form a typical cone, but the forces that sculpted such defined columns and balanced boulders are not fully modeled.
Weathering over millions of years has carved the rock into numerous distinctive shapes. The most famous is the "Hanging Rock" itself, a large boulder suspended between two other columns over a main path. Other named formations include the Colonnade, the Eagle, and the Cathedral. The surveyor Robert Hoddle first mapped the formation in 1844, giving it the official name of Mount Diogenes. Archaeological evidence shows human use of the site dating back at least 10,000 years. For thousands of years, the area was an important ceremonial and meeting place for the Dja Dja Wurrung, Woi Wurrung, and Taungurung peoples, who know it as Ngannelong.