Countertop geology: Tiger eye, Hamersley Range, Western Australia

Some seections of the Marra Mumba tigers eye from Hamersley Range (WGS84 21° 53′ 0″ S, 116° 46′ 0″ E). Photographed on the 7/5/2015 at the Melbourne Museum by Ant.

Geology around town - Basalts at Southern Cross Station, Melbourne, Aus

 

This is a picture of something residents of Melbourne take pretty much for granted - basalt pavers. They're everywhere, we've used them in to make gutters, pave the city and build old churches and public buildings. Why you ask? Do the people of Melbourne like having a grey city or do the architects have something about igneous rocks? Nope, we've used them because there are 437 volcanoes (at last count - thanks Dr Julie Boyce!) between the CBD of Melbourne and Mt Gambier in Souther Australia. These have provided a plentiful supply of basalt which is both easy to access and a reasonably good building material (compared to limestone).

As a result, we as Melbournians, see it everywhere. Recently, the Melbourne City council decided it wanted to replicate the 'laneway' feel of the city so many internal walking zones have been paved with this stuff too.

The bubble trails in the basalt give them texture and visual interest. By looking at the bubbles you can often tell which way they were traveling and thus which side is 'up'. The gas bubbles are created when volitiles in the magma are able to exsolve due to the decrease in pressure as the magma rises to the surface. The gas bubbles then move to the surface of the magma but quick cooling will often trap some of the bubbles leaving these delightful trails behind. If you look at the bubbles they will often have a hot-balloon shape. The thin part of the bubble points to the direction it came from where the top of the balloon heads towards the area of least pressure (i.e. the top of the lava). In the image above I spotted three balloon shaped bubbles but as they are in the minority, and point in three different directions I'm of the opinion that this bubble trail was actually rising towards the viewer.

Photos taken at Southern Cross Station, Melbourne on the 2nd of September 2015 with an Iphone 4.