Formed from an ancient volcanic eruption
Sedimentary rocks underlie the western Waikato: ancient sandstones and argillite topped by younger marine sandstones, mudstone and limestone.
These layers are now eroded and have been moulded into gently rolling hills and valleys. Rising above this low profile landscape are five dominant volcanoes running in a straight line from Karioi southwest to Tokanui.
Known as the Alexandra Volcanics, they lie along a line of weakness in the earth's crust.
Pirongia is the largest in the group and was built up mainly from thick layers of basalt lava.
Basalt lava is very fluid and it flowed swiftly from the vents near the summit to form the gentle slopes and long ridges around the mountain. Some flows travelled almost 20 kilometres to Kawhia Harbour.
Deposits of volcanic ash, tiny particles of molten rock fragmented by violent eruptions, are rare on Pirongia, suggesting most eruptions were relatively quiet. But quiet does not necessarily mean dull - spectacular fire-fountains of lava like those seen on Hawaii or Mt Etna, would have occured.
The youngest lavas on the summit of Pirongia are about 1.6 million years old - about the same age as the oldest eruptions from the Rotorua /Taupo area.
Although the summit has been eroded and there is now no sign of the original crater, its maximum height was probably no more than 100-150 m above its current level. The oldest dated lavas on Pirongia, found on the southern side, are 2.7 million years old.
The Waikato region has also been influenced by more distant volcanic activity. Rivers carried vast amounts of material from the volcanoes of the central North Island to the alluvial plains around the Waikato – creating the fertile soils that typify the area.
Airborne ash from eruptions to the south also reached Pirongia, burying much of the forest on the lower slopes of the mountain.
WINDY AND WET
The park's climate is mild and wet, with humid summers and temperate winters. Average temperatures range from 18°C to 8°C over the year. Light snow falls can occur on the summit in winter.
The topography of the area has a significant effect on the local climate. The mountains form a barrier to incoming fronts, causing these air masses to rise and in so doing, form clouds.
The clouds tend to develop continuously on the windward side of hills, but clear away on the lee side. This helps explain the difference in rainfall on the mountain and the surrounding farmland. Pirongia summit for instance, receives in excess of 2400 mm of rain per annum compared to the township of Pirongia which may only receive 1100 mm.
The mountains of Pirongia and Karioi also present a barrier to the prevailing westerly winds which can be particularly strong on Karioi. Just look at the wind-sculptured trees on the western flanks of the mountain if you are in doubt!