About 17 mya, thick trachyte lava welled up from a number of vents over a wide area. About 16 mya, as the volcano aged, more fluid basalt lava flowed from new vents, alternating with ash and scoria explosions. The later flows filled the spaces between the earlier trachyte domes and built a large cone rising~1000m high and nearly 50km wide. Later, the magma became less viscous, spreading further from the vents and forming thick trachyte flows such as Mt Exmouth and Siding Spring Mountain. In the final stage from 14 mya, the flows became thinner, longer and more basaltic. Since the end of volcanic pile, exposing the early hard trachyte features.