Dashing Rocks

The lava flows here from the Timaru Basalt (aged around 2.5 million years) originate inland from a vent near Mt Horrible (north west of Timaru) where they are about 25 m thick, thinning towards the coast at a slope of around 1-2 degrees down to just over a metre thick.

The well-formed columnar joints are a result of even shrinkage in the lava flow as it cooled. When cooling rates are fairly uniform, with the heat from the lava escaping at regular intervals, it contracts and fractures evenly, leading to tall, well-developed, generally hexagonal (but can be other polygons) basalt columns.These geometric columns are seen in a variety of volcanic rocks across the Earth, the most famous of which is the Giant's Causeway in Northern Ireland (see link). The temperature at which basalt lava does this has been determined through one study to be at between 840-890C, when the lava is still hot, but solidified. The slower the cooling rate, the larger the basalt columns.

Directly on top of the columnar joints lies a thin (1-2m) layer of yellow coloured loess. Loess is wind-blown glacially derived silt, primarily eroded from the Rakaia terrane in the Southern Alps over the last ~500,000 to 120,000 years. It contains quartz and plagioclase feldspar minerals, as well as significant amounts of mica-derived clays. This high clay content is significant in that it means loess is highly prone to gullying and tunnel erosion.

What shapes are the basalt columns? How many are truly hexagonal? Can you find other polygon shapes? Have you noticed that mud does the same pattern when it dries out in a lake or river bed? Why do you think that happens? How do you think the process of forming mud cracks compares to the process of forming these columns in lava? A question of geometry - Why do you think the hexagon is the most common shape for these columns?

You can also see lots of gas holes (called vesicles) in the tops of the columns. These are caused by gas escaping from the lava as it cooled.

You can see the effects of erosion in the clay-rich loess, represented by characteristic vertical polygonal joints (mirroring the columnar cooling joints in the basalt below). In July 2007, heavy surf with 5 m swells toppled a famous arch here, leaving the lone sea stack of loess you see here today.