Phillip Island Excursion Site 8: The Colonnades

From Phillip Island Rd, at the township of Cape Woolamai, turn into Woolamai Beach Rd and then into Lantana Rd to the right (west) which heads south-west to The Colonnades. There is plenty of parking at the end of the road and you can walk down steps to the beach.

Visible along the beach on the cliff face is an unusual rock formation resembling organ pipes. It is best seen at low tide. The columns are typical of the joints that form in a flow of basalt after it has solidified and as it cools. The contraction of the solid rock produces cracks to form polygons. The length of the columns is indicative of the thickness of the lava flow. Here at the Colonnades the basalt has been extensively altered and it is crumbly and soft. Notice the altered basalt has a soft and pale texture and shows onion skin weathering in places. Nevertheless it still retains the original columnar texture although the face of the cliff is regularly falling onto the beach and being washed away. This alteration likely occurred soon after the basalt solidified and may have been caused by hydrothermal chemical processes.

Check the shape and size of the columns. How long are they? Each column represents the depth of one lava flow. Look for remnants of basalt rock and for minerals. Check the alteration products; look for minerals in small pieces of rock.

Significance

These deposits illustrate the extent of the volcanic eruptions and the thickness of each lava flow. You can also see something of the alteration that has occurred on the basalt after it was deposited, producing the pale, soft material as well as rich red and brown soil.

Guide available as a booklet
This entry is taken from the geological excursion guide for Phillip Island. The complete guide is available as a booklet on creation.com Aussie store.