Beach Geology

I have such a backlog of posts to write, but I’ll start with today’s photos. We went for a little exploring on the East Side of Kaua’i, starting at the north end of the bike path by Kumukumu (aka Donkey) beach.

There is a series of little coves to explore, however the paths in between (and the inland trail back) are getting overgrown and hard to find. These coves are more isolated, though on a beautiful Saturday like today, we saw people at every remote beach. Here’s an example:


But that is not the subject of this post. We had some big swells recently, and the beach combing was quite good. But let’s just say the rest of my family has more fun looking for seashells than I do, so I started exploring the rocks instead. When I started paying more attention to the rocks and the shoreline, I noticed many different shapes and textures:



I suppose this coast was created by several lava flows thousands of years apart. The first flow of dense lava created the black boulders that were eroded by the waves. Then a secondary eruption of lighter lava covered the boulders, and now they’re just being exposed again. This little formation in particular was striking, I’ve never seen such an orange-colored rock, and the eroded shapes were unique:



When dense lava cools slowly and creates basalt, it can make hexagonal columns that look like large cobblestones from the top (like the Giant’s Causeway in Northern Ireland):


But what kind of cooling and erosion created this?


Another part of the beach had lithified sand (essentially young sandstone) that was exposed in raised walls. I think this is formed when fresh water seeps through the beach, cementing the grains of sand together. Later, when the sea level changes (or the beach recedes), the hard cemeted sand remains. But in this case, the ancient sandy beach formed over basalt rocks, then was lithified, and is now a beach again:


Here, there was a dark flow over the lithified sand, but I couldn’t tell if it was a later lava flow or a layer of lithified black sand:


And a few steps further, the lithified sand contained chunks of ancient coral and seashells. The few seashells that you could see were definitely larger than today’s shell, so I wonder if they were thousands or millions of years old.


And here, the red and orange flows contained only a few boulders and were weathered fairly smooth by the wind and waves.


What is incredible is how the 3 processes of volcanism, erosion, and lithification are all overlapping and mixed together to create so many amazing combinations in just a mile or two of shoreline.

Printed from:
© 2023.


  1. Whale says:

    Very cool! Some of that rock looks like an artist made it.

  2. Frank says:

    Glad to see and read your January ’13 blog. The hike to donkey beach looks like it was a fun time!

RSS feed for comments on this post

Leave a Comment