Egmont National Park is a mountainous area that encompasses three volcanic cones. The main peak of Mount Taranaki (2,518 m) forms the nucleus of the park. It is the most recent and only active volcanic peak in the park, although it is now considered dormant. Because it is considered one of the most symmetrical mountains in the world, it is a very distinctive landmark. The other two volcanic cones, which are now extinct, form the basis of the Pouakai and Kaitake Ranges in the north-western part of the park.

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Lava flows, ash showers and lahars (debris flows) have transported volcanic material away from the peaks. The oldest lava flows on Mount Taranaki are preserved in the west, but erosion has removed a number of older flows, resulting in the picturesque forms of Humphries Castle, Lion Rock and Warwick Castle (Tahuna a Tutawa). Fanthams Peak was formed when magma found a weak point in the volcano and created a secondary vent on the side of the mountain.