Colonnades and Entablature 🌏 EarthCache

The Questions:

Standing at GZ and looking at the quarry face at GZ; compare what you see in front of you to the photo on the cache page (as depicted in the photo above)

1.) Which letter/s, as displayed on the photo (A, B and/or C), best shows an Entablature?

2.) Which letter/s, as displayed on the above photo (A, B and/or C) best shows a Colonnade?

3.) In your own words give a brief description of your understanding of the difference between colonnades and entablature.

Then post a photo of you with the lake in the background with your log, (please don’t show the content of the above photo in your log). Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers.

Logs without accompanying answers sent or without a photo uploaded may be deleted without notice. Sending the answers is a requirement not a request. Please note, answering the logging tasks is based on your observations at the site and using your understanding of  content of the cache page, there’s no need to do any further research. 

The Lesson:

The basalt here is estimated to be about 2 million years old. It was formed as vast lava flows spread over the Newer Volcanics Province. The NV Province volcanic field covers approx 15,000 square kilometres and was formed by the East Australia hotspot across south-eastern Australia, at the time there were over 400 small shield volcanoes and volcanic vents. This particular flow entered what is now known as the Merri Creek Valley, most probably damming and diverting water flow.

The volcanic features at GZ normally occur one above the other, it is quite unusual to see them side by side. One is known as a Colonnade and the other is an Entablature. The names come from Greek architecture; where a Colonnade is a row of evenly spaced columns supporting a roof; and the roof is known as an Entablature: the upper part of a classical building comprising of the architrave, frieze and cornice.

In geological terms a Colonnade is fairly self explanatory; it is an an ordered grouping of regular, straight, and larger-diameter columnar jointed polygonal columns.

Entablature jointing displays a pattern of numerous, irregularly jointed, cube shaped blocks but can be totally randomly oriented cooling joints. It is definitely not ordered and is usually observed in more viscous (thicker) flows that cooled under less uniform conditions. The entablature tier generally lie above or occurs in between thinner colonnade flows. The transition zone between the entablature and the basal colonnade may be very narrow in width. Typically, the entablature is thicker than the basal colonnade, often comprising at least two-thirds of the total flow thickness.

The most famous is the Giant’s Causeway in Northern Ireland. It was here in 1940 that Prof. I. Tomkeieff (a geologist and petrologist) first used the word Entablature to describe the blocky, fractured features he saw above the colonnades. Even from a distance the colonnade tiers can clearly be distinguished from the messier entablature tiers. *See our photo of the landscape at the Giant’s Causeway. The lower colonnade and overlying entablature is easily discernible and that arrangement occurs repeatedly up the face, one on top of the other.

Colonnades and entablature are the result of lava fracturing and jointing under a certain set of conditions. The process is called Columnar Jointing.

Typically a cross section of a flood basalt flow looks like the diagram below, where the columns are more regular and larger in the bottom third of the flow.

Columnar Jointing occurs as a deep lava flow cools down and cracks as the mass starts to contract. If conditions are right the cracks become polygonal faults. The stress of contraction causes pulling to the centre of each forming column. The stress forces the joint lines to spread down through the flow, from top to bottom. The polygonal pattern arises because contractional stress is most efficiently relieved by three fractures that intersect at angles of 120°, you can see those 3 fracture angles together in the centre of the above diagram. This naturally creates polygons. The columns are often hexagonal (6 sided) but not always because nature can be messy and unpredictable! The Columns are generally oriented perpendicular to the main cooling surfaces of lava flows and can be as much as 30 meters tall. They can vary from 3 meters to a few centimeters in diameter. The columns are typically parallel and straight, but can also be curved if other external forces are working on them as they cool.

At GZ, the conditions these tiers of jointing cooled under must have been a little different, it’s unusual to see colonnades and entablature side by side. The quarrying activity has uncovered this interesting bit of geology, lucky the quarrying ceased when it did or this little piece of unique geology would have been lost to surface a road somewhere.