Discover

The 'Wairere Boulders' are an unusually large assemblage of basalt boulders in the Hokianga, in the far north of New Zealand. The boulder area, surrounded by subtropical rainforest, has been transformed into a park area to enable visitors to view this oddity of nature. It is the only valley worldwide which is formed by basalt boulders sitting on a clay base.

Contents

Origin

Formation

Surface Erosion

External links

Origin

The boulders in the Wairere valley are the erosional remnants of a lava flow out of lake Omapere (near Kaikohe) that has been dated as approximately 2.8 million years old. There are two dates for these, so called Horeke basalts: one at 2.84 and the other at 2.67 million years. There is a slight difference in chemistry in the two rocks that have been dated suggesting that the eruption took the form of several flows that once blanketed the high ground to the east of Horeke.

Formation

Erosion of the clay underlay of the basalt plateau (cap) started to create a v-valley. The edges of the cap broke off. These blocks travelled downwards along the hill sides towards the bottom of the valley, where they accumulated.


They fill now a portion of the valley which is about 1.4 km long and up to 350 m wide. There is no evidence of another valley world wide, composed of a huge number of basalt rocks, resting on a clay base.

Surface Erosion

It is assumed that the Horeke basalt is the only basalt rock showing such accentuated surface erosion (like karst, clints or lapiez in limestone). Some of the cuts discovered, are 1000 mm deep and up to 300 mm wide, which is highly unusual for basalts. It is the result of chemical leaching by the very acid soils generated by the Kauri forests that used to exist in the area.

External links

★ Wairere Boulders Nature Park, homepage

★ Origin of the Wairere Boulders from the Geology Department of Auckland University

★ Formation of the Wairere Boulder Valley

★ Lapiez and solution pits at Hokianga

★ Fluting, Lapiez and Solution pits at Wairere Boulders