These curious structures are a phenomenon known as “Pillow lavas”, which was originally extruded under the sea on the seafloor from a volcanic vent. When the hot lava meets relatively cold seawater, it immediately cools, forming these curious structures that are later uplifted by tectonic processes over geological time. The dark rim around the outside would have cooled quickly, while the interior (the lighter part) would have stayed warm for a little while longer, insulated by the rapidly cooled rim of cooled lava around it. The light yellow rock is a sedimentary rock known as limestone, made entirely out of calcium carbonate (CaCO3). Find out more about limestone here (opens in new tab.)
These are pillow lavas found in Boatman’s Harbour in Oamaru, New Zealand, and these photos do no justice to their impressive nature. I took these photos while on a geology field trip in the first year of my geology major in 2019 (which I’m still studying–fingers crossed I graduate next year (2021)), and if nothing on the field trip had convinced me to switch over from zoology (which I was not liking) to geology, it was this.
Chai Latte and Photography 