Howells Point Volcanics

This volcanic sequence is part of what geologists call the Permian Brook Street Terrane. It was deposited as part of an oceanic island arc subduction zone. The pillow lavas formed when lava erupted effusively underwater, and the outer surface of the lava was rapidly chilled while the inside continued to flow, breaking this chilled rind in places to produce a network of interlocking lobes of lava. In cross-section these have a blobby, pillow-like shape. Also present at Howells Point are the products of more explosive eruptions, called volcaniclastic lapilli tuff breccias. These have clasts (pieces) consisting of fragments of pillow lava, which shows that they fragmented during eruption. The breccia (rock made of angular fragments) also shows weak bedding. Cutting through both pillow lavas and breccia are several dikes. These are intrusions of magma that cut through the surrounding rock as a near vertical sheet before cooling and solidifying. The whole section has been hydrothermally altered, resulting in the green colour and amygdales (bubbles that have been infilled with light coloured minerals) within the pillow lava.

The pillow lavas form a prominent, though highly weathered, cliff. See if you can spot the outlines of individual pillows. Take a look at the amygdales, notably rounded, and filled with white and pale green minerals such as quartz, prehnite, or pumpellyite. These were vesicles (gas bubbles) in the lava, and the infilling is due to hydrothermal alteration. Further along the beach there is an outcrop of pillow lava where the elongated sides of the pillows are visible.

Some of the dikes are black and unaltered, while others have been hydrothermally altered along with the other volcanics. Watch out for places where the edges of the dikes form fingers, or apophyses, into the surrounding breccia, indicating that they intruded into it before it was consolidated.