The Horeke basalts is a disused formation that contained

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

s that covered a large area in central

Northland Region The Northland Region ( mi, Te Tai Tokerau) is the northernmost of New Zealand's 16 local government regions. New Zealanders sometimes refer to it as the Winterless North because of its mild climate all throughout the year. The main population ...

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...

, and in places forms a high plateau around Okaihau. Wairere Boulders is a nature park providing walkways through boulders of basalt, derived by erosion of one of these older basalt flows that used to be included in the Horeke Basalt and are now included in the Kerikeri Volcanic Group.

Origin

The boulders in the Wairere valley are the erosional remnants of a lava flow out of a volcano near Lake Ōmāpere (near

Kaikohe Kaikohe is the seat of the Far North District of New Zealand, situated on State Highway 12 about 260 km from Auckland. It is the largest inland town and highest community above sea level in the Northland Region. With a population of ove ...

) that has been dated as approximately 2.8 million years old. There are two dates for these 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.

Surface erosion

Some of the channels discovered in the basalt are 1000 mm deep and up to 300 mm wide, which is highly unusual for basalts. It is the result of chemical leaching by acidic seepage from the humus on top of the boulders.Hayward, B.W., Kenny, J.A. 2011. Karst in basalt. Geoscience Society of New Zealand Newlsetter 3: 12-15.

References

Hokianga Rock formations of the Northland Region {{Northland-geo-stub