Mangawhai Heads lava columns

BY MIKE ISAAC (NO AFFILIATION)
Accessibility: EASY
Dacite lava with low angle flow banding and high angle jointing, north of Mangawhai Heads
Dacite lava shows gently dipping flow banding, cut by near-vertical joint sets formed when the lava contracted as it cooled.
In places the gently dipping flow banding is folded
These lavas were erupted and intruded along a large fault called the Waipu Boundary Fault. This runs along the southern edge of the Brynderwyn Hills. The lava is a type called dacite which is intermediate in chemical composition between andesite and rhyolite. There are other outcrops of this lava between Mangawhai Head and Bream Tail.
Lava contracts as it cools, cracking to form sets of joints. Immediately north of the car park the main joint set is near horizontal, but about 2 km up the beach the cooling cracks are near vertical, splitting the lava outcrop into tall, slender prismatic columns. The lava also shows gently dipping flow banding, formed when it was still molten. In places these bands are deformed into small folds.
Dacite lava above high tide level at the Mangawhai Heads car park shows honeycomb weathering
This is a New Zealand miniature of the famous Giant's Causeway columns in Ireland, which formed in a similar way. Why are the joints near-horizontal just north of the car park bur near-vertical 2 km up the beach? Does that tell us anything about the cooling surfaces and the margins of the lava? Look at the Waipapa Terrane sandstone and argillite basement rocks which outcrop just south of the columnar dacite and small beach - do you see any signs of heat alteration?
Directions/Advisory

From the car park at Mangawhai Heads walk north along the beach past outcrops and low cliffs of dacite and to the end of the sandy beach, past the track leading up the spur to the Mangawhai Coastal Walkway. Follow the coast north (over rocks called the 'Waipapa Terrane basement ') to reach a small sheltered beach, on the north side of which is the columnar jointed dacite. At low or half tide you can continue walking north to where the Waipu Coastal Walkway descends, just south of Bream Tail. The Mangawhai Coastal Walkway is a pleasant return trip.

A safe and easy walk at low or half tide

Google Directions

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Accessibility: EASY

Easy walk along beach and non-slippery rocks to get to the columnar dacite. Moderate walk further north over boulder beach. Mangawhai Coastal Walkway is a good formed track. It ascends to about 120 m above sea level (formed steps). Spectacular coastal views make it worth the effort.

Features
Volcanic
Geological Age
Early Miocene
Zealandia Evolution Sequence
Māui Supergroup (Emergence): 25 – 5 million years ago