One remarkable tourist highlight on Phillip Island, near Melbourne, Australia, is Pyramid Rock, which presents a distinctive silhouette against the Southern Ocean. You can see its black triangular shape from most of the beaches and headlands along the island’s southern coast.
The boardwalk to the lookout gives an excellent view of the black basalt columns crammed along the base of the steep cliff.1 The same columnar jointing is visible in Pyramid Rock in the distance, as well as in most of the headlands and wave platforms that surround the island. And Phillip Island is only part of the area of land that was covered by hot, fuming lakes of molten basalt lava, now referred to as the Older Volcanics.2
The basalt outcrop in the foreground on Phillip Island itself and the pyramid in the ocean were once connected, but the intervening rock has been eroded away. You can judge something of the depth of a single lava flow from the length of the columns. As you look at these rock outcrops imagine the extent of the lava flow and its depth. Imagine, too, the huge volume of basalt rock that has been removed by erosion since the rocks solidified. This is just one of many lava flows stacked one upon the other that is visible on the island.
There is another fascinating geological feature visible from this lookout, one that allows you to understand something of the geological history at a glance. Notice that the rocks at the base of Pyramid Rock are of a different colour from the pyramid itself. They are pink because they are composed of granite, not basalt. They are part of a huge granite body, some of which rises high above the ocean to form Cape Woolamai—the south-eastern tip of Phillip Island.3
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