Bunbury Basalt EarthCache

There is no physical cache at this site as it is an Earthcache. Please take care when walking on the rocks and be watchful of the waves. There is no need to get wet or walk on slippery rocks.

To claim this cache you must send the answers to the questions listed below to the owners BEFORE you log the cache.

There are picnic tables, barbecues, grassed area and play equipment nearby. Dogs are allowed on the beach further to the north of the site and there is a cafe to the south.

Bunbury Basalt

The black rock on the beachfront is basalt believed to be part of a lava flow dating back to volcanic eruptions some 150* million years ago. The eruptions that lead to this formation occurred as the supercontinent of Gondwanaland (Antarctica, South America, Africa, the Indian subcontinent and Australia) started to drift apart. As the lava cooled it formed into columns giving it a similar appearance as the Giant’s Causeway in Northern Ireland. So much so that when Reverend Wollaston drew the map of Bunbury in 1843 he named this rocky outcrop ‘Giant’s Causeway’.

The Bunbury Basalt is significant in several ways. Firstly, it provides evidence supporting the theory of plate tectonics (once called continental drift). As the plates separated splitting up Gondwanaland magma welled up onto the oceanic floor resulting in the formation of the basalt. It is an extremely large flow as it extends approximately 120 kilometres south to Black Point and well out to sea to the origin of the flow. This lava flow has a maximum thickness of 174 meters. The basalt defined the shape of the Leschenault Estuary and the inlet to the east and created the natural bar at Point Casurina, which separates Geographe Bay from Koombana Bay.

Basalt is the most common extrusive volcanic rock. Extrusive means the magma (molten material) flows onto the Earth’s surface. As the magma cools quickly small crystals form resulting in the fine-grained appearance of the rock. Basalt is usually grey to black in colour. Much of the surfaces of the terrestrial planets Mercury, Venus, Mars and Earth, as well as the Moon, are composed of basalt. The Earth’s ocean floor is virtually entirely made of basalt, produced from upwelling mantle below ocean ridges. Basalt is said to be mafic which means it contains very little silica, only about 50% by weight. The three essential constituent minerals are calcium-rich feldspar, calcium- and magnesium-rich pyroxene and olivine. Olivine comes in magnesium- and iron- rich varieties.

This type of formation is called columnar basalt. During the cooling of a thick lava flow, joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns.

These structures are often referred to as being hexagonal, meaning six sided. The accuracy of this description is something to study at the site. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small columns with a diameter of less than one centimeter, whilst slow cooling is more likely to produce larger columns. The columns seen here are not as well defined as those at other locations. This could be due to erosion by the seawater.

This place was a quiet seaside setting likened, by early settlers to the Giant’s Causeway in Ireland until the late 1890s. Then the area was turned into a huge quarry for extracting basalt for local road construction.

The Giant's Causeway, Ireland

For decades noisy workers set off dynamite at 5pm to loosen basalt in the quarry to make blue metal for the cobblestones road base. People would duck for cover as workers, carrying red flags came on to the streets, signalling blasting was to commence.

The remains of the manmade basalt wall stretching across the beach in front of us, is all that is left of the barrier created to restrict natural sand movement and sea surging into the working quarry. When the quarry closed in 1951 the basalt wall was broken to allow natural sand movement back into the disused quarry.

Relics of the old pump station once used for pumping seawater into public baths located to the south of the basalt outcrop can still be found on the shore. The Bunbury Town Council built the baths in the 1930s. Large pipes extending from the pump station to the baths ensured an adequate supply of seawater remained in the structure during the summer.

1. Send us an email with the following information

a. Name of the cache (we plan on having more than one)

b. Study the shape of the columns. Are they hexagonal? Count the number of sides on three randomly chosen columns and determine the average. Send us this information.

c. Did these columns cool rapidly or slowly? What evidence supports your answer?

2. Take a photo of your GPS (and yourself if desired but not essential) at this site to prove you have been there and include it in your log entry. Please avoid including the answers to the questions in your photo or log.

3. Once you have confirmation from us, go ahead and log your find and upload the photo.

If you are caching with a number of teams, each team needs to send in their answers so as to make administering this cache easier. Log entries that don’t match up with an email of correct answers will be deleted.

* While the nearby sign states it is 40 million years old, Geoscience Australia dates the rock at Early Cretaceous. The Cretaceous Period extended from 145.5 – 65.5 million years before present. All other publications used when researching the site support this age of the formation.