Phillip Island's very own Pyramid EarthCache

PYRAMID ROCK PHILLIP ISLAND

Pyramid Rock is an iconic feature of Phillip Island. Pyramid rock consists of Black Basalt and Pink Granite. A very popular feature with the tourists that visit this area. Pyramid Rock black triangular shape can be seen from most of the beaches and headlands along the island’s southern coast.

The boardwalk out to the lookout which gives an excellent view of the black basalt columns along the base of the steep cliffs in this area. 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. Pyramid rock is not only famous for its beauty but also for a New Year festival that was held in the nearby paddocks. In the beginning it was a single day event, from its second year it was run over two nights with two stages – the Main Stage, and the Pharaoh's Annex. The Main Stage hosted the major local and international acts, while the Pharaoh's Annex featured lesser-known local acts, and in the later evening, the more dance-oriented acts of the festival. There was also a variety of market stalls. With declining attendees that stemmed from the embarrassing failure at the 2009/2010 festival to perform a New Year countdown and the last-minute cancellation of numerous acts, the festival was shelved for 2013 and as of September that year its promoters were exploring options for the festival to return in 2014 but as yet it hasn’t eventuated.

Now back to the reason we are here. The basalt outcrop in the foreground and the pyramid in the ocean were once connected, but the intervening rock has been eroded away. Seeing what it is today it does make you wonder how much of the basalt has eroded away over the years due to the constant forces of nature.

In geological terms the contact between the underlying granite and the overlying basalt is called an unconformity. It signifies a time gap between the granite being emplaced and the basalt lava flowing on top. It’s from sites like this that geologists conclude that lots of erosion occurred between the time the granite was emplaced and when the basalts were erupted.

There is another geological feature visible from the lookout, one that allows you to understand something of the geological history at a glance. 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.

WHAT IS BASALT?

Basalt is the most abundant volcanic rock in the Earth’s crust. It is made up of tiny crystals of olivine, pyroxene, amphibole and feldspar. Lava that forms basalt erupts at over 1000 °C and is very fluid, so it can flow great distances from a volcano.

Basalt is a dark-coloured, fine-grained igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro the difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is a coarse-grained rock.

Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock.

WHAT IS GRANITE?

Granite is a light-coloured igneous rock with grains large enough to be visible with the unaided eye.  It forms from the slow crystallization of magma below Earth’s surface.  Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles and other minerals.   This mineral composition usually gives granite a red, pink, grey or white colour with dark mineral grains visible throughout the rock.

Granite is the best-known igneous rock. Many people recognize granite because it is the most common igneous rock found at Earth's surface and because granite is used to make many objects that we encounter in daily life. These include counter tops, floor tiles, paving stone, curbing, stair treads, building veneer and cemetery monuments. Granite is used all around us.

LOGGING REQUIREMENTS

No1   From Gz look directly down till you see the bottom of the cliff. Now describe what you see colours shapes granite or basalt?

No2   On your way to GZ you will walk past a sign. What should you be looking for in the area?

No3   When looking at Pyramid Rock is the “pyramid part” mainly Basalt or Granite?

PLEASE EMAIL YOUR ANSWERS THANK YOU .