Maunganui Bluff basalt lava flows

Most of the Waipoua Forest area and Maunganui Bluff, on the west coast of Northland, is underlain by a thick sequence of Early Miocene basalt lava flows (Waipoua Basalt) that erupted 19-17.5 million years ago. Remote sensing techniques show that they are the eroded eastern third of a 50 km-wide basalt shield volcano, which was centred 10 km west of Maunganui Bluff. For comparison, the Miocene Banks Peninsula, Dunedin and Auckland Islands “shield” volcanoes were each 20-30 km in diameter.
The cliffs of 450 m-high Maunganui Bluff are entirely composed of a pile of 2-10 m-thick basalt flows separated from each other by thin beds of rubbly basalt breccia and red baked and oxidised volcanic ash. In the cliffs of the bluff there are many near-vertical dikes that cut through the lava flow sequence. These dikes were sheets of basalt lava that intruded up vertical fractures in the growing Waipoua shield volcano. In basalt shields, the dikes of molten lava commonly intruded fractures that radiate outwards from the central conduit. This seems to be the case at Waipoua also, where the dike orientations indicate the centre was just offshore from Maunganui Bluff. Some of these dikes could have fed young lava flows that were erupted on the upper slopes of the shield volcano. One thin dike can be seen just 50 m around the base of the bluff from the beach.
As the centre and highest point of the shield volcano was offshore to the west, all the lava flows that are now visible on land would have originally flowed down the northern and eastern slopes and had a slope in these directions of about 10-15°. On the east side of Maunganui Bluff itself, there is a small, underfit stream flowing east down a wide valley through Aranga. This valley could not have been eroded by this small stream and undoubtedly it is a remnant of one of the radial valleys that flowed down the original Waipoua volcano, but now all its higher headwaters have been eroded away by the Tasman Sea.

How many lava flows can you count in this part of the bluff?
Can you find a steep dike of lava cutting through the lava flow sequence?