Gas Bubbles - Rock Platform, Culburra, NSW - Places of Geologic Significance on Waymarking.com

Received an email from lobr8028 a Science teacher & former geologist who provided more accurate information about this site. The email in part:

Although the circular bubbles look like popped lava bubbles, they’re actually fossils. The rock platform at Culburra Beach is part of the Wandrawandian Siltstone which formed during the Permian period over 250 million years ago. Although they don’t look like fossils, they’re actually spherical concretions that would have started off as a small fossil that acted as a sort of nucleus and then little by little, layer by layer, minerals formed around it and it grew bigger and bigger. Kind of like a pearl growing inside an oyster. It’s a similar process that gives rise to the billions of Manganese nodules that litter the Pacific Ocean floor.

Igneous rocks generally don’t preserve gas bubbles. Lava erupted from a volcano is usually quite explosive and the gas escapes quickly and violently. The only rocks where the presence of a gas is visible is when you have pumice or a vesiculated basalt. Pumice is when lava cools extremely quickly immediately after an eruption (usually by touching water) and then it forms a sort of volcanic glass (like obsidian) but with all these holes in it so that it looks and feels like a crusty sponge because the gas didn’t have time to escape.

A vesiculated basalt is much more solid but will have a decent amount of small white spots on it which is where the tiny gas holes have been infilled with a secondary mineral. So, although it makes sense in our head to imagine gas bubbles escaping from a pool of lava like you would see in a pool of hot mud or a big vat of honey, it doesn’t actually work like that. Rocks are really dense even when they’re molten and so big bubbles of gas can’t float through them like they do in water or mud. The gas either escapes explosively and rips the rock apart in doing so or it turns it into pumice where the lava freezes and preserves the spaces where the gas once was.

My original description with the inaccurate sentences removed:

These vesicles now dot the rock platform which is composed of basalt & is beside the Pacific Ocean at Culburra. There are four fine examples at the co-ordinates but many more nearby. Some are still solid & are described by locals as eggs, others have small holes in the top, others have larger holes, while others may have been vandalised to see if they contained crystals such as agate or chalcedony. They are all a deep reddish brown in colour indicating that chalcedony was the most common mineral present as they were forming.

There are no signs about the bubbles & I discovered them when completing the Earthcache "Bubble Blowin 'Burra (visit link)

You should check the tides as they may be submerged during high tide. Also as you are near the ocean always keep an eye out for large waves.

You can park your car & walk along a track & the platform to the bubbles, a distance of less than 400 metres.

Waymark is confirmed to be publicly accessible: yes Parking Coordinates: S 34° 55.950 E 150° 46.710 Access fee (In local currency): .00 Requires a high clearance vehicle to visit.: no Requires 4x4 vehicle to visit.: no Public Transport available: no Website reference: [Web Link]

Visit Instructions:
No specific requirements, just have fun visiting the waymark.

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