Geology
The Sugar Loaf Islands, along with onshore pinnacles such as Paritutu (153 m or 502 ft), represent the oldest volcanic activity on the Taranaki peninsula. Dating between 1.7 and 1.74 million years of age, the islands are believed to be the remains of a ring fracture or feeders to eroded volcanic vents, and are composed of a porphyritic hornblende andesite. Volcanic activity in Taranaki subsequently shifted to Kaitake (580,000 years ago), then migrated southeast to Pouakai (230,000 years ago) and the current centre of activity, Taranaki (last erupted in 1755).
The Sugar Loaf Islands are the type locality of taranakite, a phosphate mineral that forms from the reaction of bird guano with the aluminous rocks comprising the islands. This was the first new mineral species to be discovered in New Zealand.
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