Earthcache: Tuoro (Black Rock) - Rarotonga - Cook Islands
Your Tasks:
To claim this earthcache, please send the following answers to Cache Owner (don’t post answers in your log):
Question 1
Take a closer look at the basalt columns. What geometric shape do you see, and how does this relate to the process of columnar jointing?
Question 2 :
Find the highest column in the Black Rock formation. Describe its texture, colour, and approximate height, and explain how these observations reflect the cooling process and mineral content of basalt?
Question 3. (PHOTO) Please take a photo of yourself or your GPS, your caching name , etc with the Black Rock in the background, attach this picture to Your LOG. This photo only need to be proof you visited,
Logging:
Do not post the answer in your log. You can go ahead and log your find (ie. you don't need to wait for permission) if you answered the questions above and uploaded the correct picture. There is no need to email me your picture. If there are any questions about your answers CO will contact you. Logs without answers to CO , with pending questions from CO or without the picture will be deleted without further notice.
New from : 2026
If you are part of a group, the person submitting the answers must mention all group members by their Geocaching usernames. Each group member must include their own photo to log this EarthCache.
Earthcache lesson: Basalt at Black Rock
Tuoro (Black Rock) is one of Rarotonga’s most spectacular and important landmarks. Culturally, it serves as the departure point for the spirits of the Maori people to Hawaiki. Tuoro is composed of basalt, an igneous volcanic rock formed from the cooling of molten magma roughly 1.9 million years ago.
Black Rock is a striking basalt outcrop on Rarotonga’s coast. Basalt forms when lava cools rapidly at the surface, creating dark, fine grained volcanic rock. At this site, the cooling process produced impressive basalt columns, giving the rocks their unique shapes.
By studying the columns, we learn about a process called columnar jointing. As lava cools and contracts, it cracks into polygonal shapes, often hexagons. This is why the rocks here appear so regular compared to most natural formations. Their texture and colour reflect the minerals in the basalt, while their size tells us something about how quickly the lava cooled.
This makes Black Rock not only a dramatic coastal landmark, but also an open-air classroom showing how volcanoes and cooling processes shape the Earth.
History & Heritage:
Basalt’s strong durability and resistance makes it an ideal rock for construction purposes; during ancient times, Maori people were heavily dependent on stone for tool, and tool-making, utilising this rock to create anchors, weapons, and knives amongst other objects. The history and geology surrounding Black Rock makes it not only a geological landmark, but also a cultural site of great importance.
Extra Notes:
- This is a coastal location, please take care on the rocks as they may be slippery when wet
- Please respect the cultural significance of this place
- Look out onto the horizon for potential whale watching!
Acknowledgements: Many thanks to local Cook Islands guides and geological records for background information on Rarotonga’s volcanic history.
Happy Earthcaching!!!
