29 January 2015
More pillow basalt at Rodeo Cove
Posted by Jessica Ball
I promised photos of the second part of my trip to see pillow basalts at the Marin Headlands, and here we are, just as the fog was lifting in the early afternoon. After exploring the Point Bonita lighthouse and its vicinity, we decided to hike down through the abundant succulents (Carpobrotus edulis, if I’ve got it right) to Rodeo Cove and its beach.
On the beach, we were treated to some lovely ‘fresh’ pillow basalts that have been eroded by wave action. In some cases, the erosion has actually emphasized the curves of the pillows.
There was a lot more mineralization in the interstitial spaces in this exposure – maybe something like calcite?
A little further up the beach I found this neat example of a pillow weathering into little chunks that looked like jointing, but probably aren’t.
The tide was just going out and made it much easier to get up onto the rocks.
But there were also lovely exposures in the cliffs directly behind us, where you could also see the transition to the overlying cherts.
While exploring one of the outcrops made accessible by low tide, we found these lovely slickensides:
Hannah obligingly took a brief nap on a pillow:
And Drew posed with some particularly ropy-looking specimens:
Down here it was easy to see what had been (relatively) recently exposed by erosion, and what had been sitting out in the elements weathering for a while. The color contrasts aren’t quite as dramatic as they were up on the cliffs, but they’re still pretty clear.
All in all, it was a lovely day for a walk on the beach amid the former ocean floor eruptive features – and we weren’t done looking at deep-sea rocks just yet!
The interstitial minerals are typically calcite and zeolites of one kind or another.
Hello Jessica I was actually out at Point Bonita today and was reading up about the pillow basalt and found these great posts. I think I’ve seen this type of basalt along the coast on the San Francisco side of the Golden Gate. But I wanted to ask you this: There are some sea stacks along the SF side that are made of a pumice-like rock that is kind of spongiform with lots of pockets in it yet the rock is very sharp. I was wondering if this is a type of volcanic rock or something else?
Hi Felicia! It’s possible that they’re made from tafoni (some great photos here), which is a weathering feature rather than a rock type. It can happen in any hard rock, so my guess would be that it’s either basalt or a well-cemented sandstone in that area.