This ophiolite is a piece of Middle Jurassic oceanic crust and mantle that has been obducted onto land and largely been converted to serpentinite.

[3][4] Mount Umunhum and nearby Loma Prieta are pushed upward by local collision forces associated with a left bend in the San Andreas Fault.

Rather, the resulting transpression force pushes up the local terrain and helps explain why these are the highest peaks in the Santa Cruz Mountains.

[8] The summit was closed to the public due to hazardous materials and unsafe conditions caused by partially demolished structures from the former Air Force station.

[17] The mountain is topped by an eight and a half story (84.5 feet tall) concrete radar tower (5 floors interior), known locally as "the Cube" or "the Box".

[20] The high elevation is necessary for line of sight in the region's varied terrain, but it also limits the ability to detect storms with bases lower in the atmosphere.