Victoria's Giant's Causeway EarthCache

One of the most iconic Natural Wonders of the World is the Giant’s Causeway on the Antrim coast of Northern Ireland. Renowned for its regular array of hexagonal, basalt columns, this area has inspired mythologies and legend of the Irish giant Finn MacCool, but a more scientific interpretation relates to a series of volcanic eruptions approximately 60 million years ago at the beginning of a geological period called the Palaeogene.

Basalt columns are characteristic of a number of igneous bodies of varying compositions (sills and dykes and ash flows). In this EarthCache you will study them in a prominent lava flow exposed by wave action on a headland along the western side of Kitty Millar Bay, Phillip Island; Victoria’s own version of the Giant’s Causeway.

Victoria’s Giant’s Causeway

This EarthCache was identified on a visit to Kitty Millar Bay to log another EarthCache, a little further around the headland (GC5PHTX). You might wish to compare the columnar jointing of the igneous bodies at both these EarthCaches on the same trip.

For safety reasons it is best to attempt this EarthCache on a calm day and a falling/low tide. Even though the exposure at Location 1 is well above high water mark, Location 2 is subject to wave action in high winds and should only be attempted at low tide. To access the two locations, you will find plenty of parking at Kitty Miller Bay. It is then just over 500m of easy walking from the steps at the western end of the car park, westwards around the bay, to Location 1 and a further 50 metres to Location 2 (Figure 1).

Figure 1

The Geological Background

Basalt is an iron-rich igneous rock that forms many kilometres below the surface of the Earth from a melt that is then erupted onto the surface as a near horizontal sheet of lava before cooling to form rock. As lava cools it contracts, with the top of the lava flow becoming cooler than the bottom as it loses heat to the atmosphere. This sets up a temperature gradient between the top and bottom of the flow and the stresses produced cause the cooling rock to fracture and create joints (Figure 2). Experiments have shown that this is when the temperature falls between 90 degC to 140 degC below the temperature at which it crystallises, which is about 980 degC for basalts.

Figure 2

When conditions are right, contraction occurs towards equally spaced centres (C) in the lava (Figure 2) producing a hexagonal pattern of intersecting joints (Figure 3) that extend down through the lava as it cools further to form elongated, hexagonal (six sided) columns. The columns can vary from meters to a few centimetres in diameter, and can be as much as 30 meters tall. They are typically parallel and straight, but can also be curved and vary in diameter, with the width of the columns depending upon the rate at which the lava cools. (Figure 4)

Figure 3

Figure 4

Logging details

To log this EarthCache please send the answer to the questions by e-mail or through the Message Centre;

1. Describe the range of colours of the surface of the basalt at Location 1 compared to the surface at Location 2. Suggest an explanation for this difference.
2. Columnar jointing often develops six-sided basalt columns forming perfect hexagons with sides of equal length in cross section.
   (i) Give a range for the number of sides (smallest to largest) you can find in a random sample of about 10 columns in the area.
   (ii) Suggest why not all columns show perfect hexagons with sides of equal length.
3. Please add a photo to the gallery of the most perfect hexagonal shaped column you can find together with an item (e.g. GPS unit or coin) to give an indication of its scale.