Maitai Bay igneous intrusions

Most of the area at the end of Kairkari Peninsula comprises two silica-rich plutons (masses of magma that crystallised under the earth surface) of Early Miocene age. These are known as the Karikari Plutonics and they have intruded into much older Early Cretaceous country rocks of the Mount Camel Terrane. The same rocks also occur at Paritu (Moehau) at the northern end of Coromandel Peninsula. Although they were once deeply buried, these rocks have now been uplifted by powerful tectonic forces, and erosion has exposed them at the surface.

At Maitai Bay the roof zone of the older pluton is well exposed, especially in the area of Joliffe Point. The surrouding (country) rock is also interesting, consisting of sedimentary beds, as well as volcanic pillow basalts and rhyolites.

Can you see where the hot igneous intrusion has interacted with the country rock, causing what is known as contact metamorphism? Have a look at the textures of the different rock types; think about how long it may have taken for crystals to have grown in the magma. The igneous rocks tell geologists that active volcanism occurred in this part of Northland in the past; why are there no such volcanoes here now?