The Stone - Taranaki Andesite

Te Kupenga Stone Sculpture Society uses Taranaki Andesite as the principle media for sculptural work. The andesite is obtained as large river-worn boulders, and is the predominant stone found in all the rivers and streams draining the Taranaki Peninsula as well as along the coastline.

Taranaki Andesite is derived from Mt Taranaki, the 2518m high volcano that overlooks the province and is an iconic and much revered part of the Taranaki landscape. Mt Taranaki is a classic andesite stratovolcano having a steep conical form made up of layers andesite lava and volcanic debris, resting on an extensive ring plain of volcanic material.

Sculptural properties of Andesite

Taranaki Andesite is classified as a hard rock in terms of its sculptural properties, although hardness can vary considerably between individual stones depending the original composition and degree of weathering.

Fresh stone with higher silica content can be very hard and can take a high polish, while more weathered stones are considerably softer with a more porous texture due to the selective weathering of some minerals and often have reddish colourings due to the oxidation of iron-rich minerals.

Most Taranaki Andesite stones are fairly uniform in texture, but some have subtle banding (probably due to the effects of flow within the lava) and small clasts of other rocks (xenoliths) are sometimes present.

How is Andesite formed?

Andesite is the typical volcanic rock erupted from volcanoes associated with convergent plate boundaries. The convergent plate boundaries are defined where the oceanic crust is pushed beneath either oceanic crust or continental crust by a process of subduction. Andesites are the principal rocks forming the volcanoes of the “ring of fire,” the chains of volcanoes which rim the Pacific Ocean basin.

The Marianas and Izu-Bonin islands, the islands of Japan, the Aleutian Islands, the Cascades Range of the northwest United States, the Andes of South America (for which andesite is named), and the Tongariro Volcanic Centre (TVC) of New Zealand are all examples of subduction related andesitic volcanism.

Local Andesite

Both Taranaki and the TVC are active volcanic systems associated with the Tonga-Kermadec-New Zealand convergent plate margin. The Taranaki Volcanics represent the most westerly manifestation of subduction related volcanism, and Mt Taranaki is located 180km west of the Hikurangi Trough where the oceanic Pacific Plate begins subduction beneath the continental crust of New Zealand. The oceanic crust underlies Taranaki at a depth of approximately 180km.

The volcanic centres that make up the Taranaki Volcanics are a sequence of andesitic volcanoes that get progressively older and more deeply eroded to the northwest.

Mt Taranaki is the youngest and best preserved, having commenced activity approximately 120,000 years ago it has been through a number of phases of build-up and collapse. The present cone has been constructed over the last 7,000 years and last erupted in 1755AD. It is still considered active, but is currently dormant. The moderately eroded remnant cone of Pouakai lies 7km northwest of Mt Taranaki, is about 250,000 years old and rises to 1399m. The Kaitaki range a further 10km northwest is a deeply eroded stratovolcano made up of a series of radial ridges and a central plateau at 684m, and dated to 575,000 years. The oldest and most deeply eroded centre is represented by the nearshore islands of Ngamotu and the onshore volcanic spire of Paritutu which overlooks the Te Kupenga clubrooms and Port Taranaki on the edge of New Plymouth. This centre is dated at 1.7 million years and these remnants are thought to be part of the feeder system to now eroded vents.

Composition of Andesite

Andesite is a grey to brownish-black volcanic rock, ranging between 52 to 63 weight percent silica (SiO2), and therefore of ‘intermediate composition’ between more silicic granites and less silicic (mafic) basalts. Textures are usually porphyritic (containing larger crystals set in a fine groundmass).

The lavas of Mt Taranaki are classified as high potassium, low-silica andesite (relative to other andesites), and have plagioclase feldspar as the dominant phenocryst (larger crystals). Clinopyroxene is the most common ferromagnesium mineral present, with lesser hornblende and olivine. Titanomagnetite occurs as both phenocrysts and inclusions in clinopyroxene. The groundmass comprises small plagioclase crystals, fine iron-titanium oxides, clinopyroxene and glass.