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.
Heading down to the beach
A typical west coast scene with succulents providing the greenery
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.
More pillows!
There was a lot more mineralization in the interstitial spaces in this exposure – maybe something like calcite?
Mineralization surrounding the pillows
Here you can see some of the overlying outcrop, which is pretty oxidized.
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.
Crunchy pillow
A closer look at the crunchy pillow
The tide was just going out and made it much easier to get up onto the rocks.
Scenery
But there were also lovely exposures in the cliffs directly behind us, where you could also see the transition to the overlying cherts.
Wall o’ pillows
While exploring one of the outcrops made accessible by low tide, we found these lovely slickensides:
Slick slicks
Hannah obligingly took a brief nap on a pillow:
Good for napping
And Drew posed with some particularly ropy-looking specimens:
Pillow fort?
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.
Weathered and fresh pillows
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!
Sea stacks
Jessica Ball is a volcanologist at the U.S. Geological Survey, researching volcanic hydrothermal systems and stability, and doing science communication for the California Volcano Observatory. She previously worked at the Geological Society of America's Washington DC Policy Office, learning about the intersection of Earth science and legislative affairs. Her Mendenhall postdoc and PhD focused on how water affects the stability of volcanoes, and involved both field investigations and numerical modeling applications. Her blogging covers a range of topics, from her experiences in academic geosciences to science outreach and communication to her field and lab work in volcanology.
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?
Thanks,
Felicia
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.