The small low coastal cliff here at Wagait beach mostly consists of Porcellanite, which is common around the Darwin area.
Porcellanite is the name given for a group of dense siliceous rocks that have the texture, dull lustre and conchoidal (smooth rounded) fracture of unglazed porcelain. The term Porcellanite has been used for: Impure chert. Baked clay or Shale often found in the roof or floor of a burned-out coal seam, in this case metamorphic.
In this case the Porcellanite is a siliceous duricrust, a duricrust is a hard mineral crust formed at or near the surface of soil in semi-arid regions by the evaporation of groundwater, in this case the duricrust has developed within altered Cretaceous mudrocks. Its distinctive geotechnical properties include low bulk density, variable strength that is highly dependent on moisture content, and relatively high but inconsistent durability becoming weaker with depth. It is also highly permeable in places, due to solution cavities. These characteristics are common to all duricrusts and result from processes of solution, (dissolving in a liquid) replacement and redeposition by silica‐laden groundwater. In this case this is likely to have occurred during the Cenozoic era and there is evidence that they continue to the present.
Porcellanite has long been used as the main building stone in Darwin, many examples can be seen around the CBD, (old town hall for example) and is now quarried for shoreline filling, because of its tendency to have pockets of inconsistencies some of the quarried Porcanilite has been a failure as a breakwater and is unsuitable for road bases.
We found the rocks here particularly attractive due to their varied colours, generally speaking it can be assumed the colours have come from the following impurities: White; Kaolinite - pure clay. Yellow; Limonite - hydrated iron hydroxide. Brown; Goethite - a partly hydrated iron oxide. Red & Purple; Hematite - an anhydrous iron oxide, particle size determines the colour. Sienna; a mix of Limonite and Manganese oxide, higher manganese content produces a browner appearance.
The posted coordinates are for the Porcellanite outcrop we would like you to visit, although low tide isn’t essential to visit GZ, the tide and weather conditions might make it difficult on some occasions to visit GZ exactly, as long as you visit the outcrop attached to GZ (within 10-20m) your observations will be valid.
Q1. Can you scratch the rock with your fingernail? What does this suggest about the hardness of the rock? Does the hardness vary from the top to the bottom of the outcrop?
Q2. Find a pebble or rock laying on the beach, has it come from the outcrop? Hypothesise about its origins in relation to the outcrop.
Q3. Describe the colours you can see, what pattern are they making and what has caused these colours?
Q4. A photo of you or something that identifies you, such as your team’s name written on a piece of paper or even the sand (not the rocks though please).
You are welcome to log your find straight away to keep your TBs and Stats in order, but please message us with your answers within a few days. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted without notice.