In late Cretaceous time, Central Texas was part of a vast marine shelf on which carbonate rocks were deposited with the entire area gradually subsiding as sediments were laid down. The volcano formed when magma worked its way to the surface and encountered water-laden, unconsolidated sediments with the existing water rapidly vaporizing into steam resulting in an enormous explosion that formed an explosion crater. Explosive eruptions continued at Pilot Knob as new magma encountered more water in the volcanic ash. Gradually, an ash cone was built up over the explosion crater. Eruptions of ash continued until the mound grew above the level of the shallow sea. Ash beds, now altered to clay, occur interbedded with limestone and marl of the Austin Group around Pilot Knob; these ash beds provide evidence for subaerial eruptions at Pilot Knob. The Pilot Knob ash cone eventually built an unstable slope on the sea bottom, resulting in mud flows of ash and carbonate mud which tore up the underlying carbonate mud in places and injected itself into the carbonate mud at other places. The subaerial Pilot Knob ash cone allowed the intrusion of magma into the mound without contact with sea water, resulting in quieter lava eruptions. Such magma cooled and solidified to form the core and satellite areas of the trap rock. Some of the trap rock bodies are the erosional remnants of lava flows, due to their apparent dip away from the central core area. Cooling joints exposed on a hill about west of Pilot Knob suggest a dip of that trap rock body towards the center of the core area, possibly indicating that it is the erosional remnant of a cone sheet injected outwards from a central, discordant intrusive body of magma. Exposures at other bodies of trap rock are not generally good enough to determine their exact emplacement, but some, at least, are probably plugs of solidified intrusive magma. Magnetic anomalies on the northeast flank of the core area suggest a buried trap rock body within the ash mound, possibly a cone sheet or lava flow.