White Phosphorus people: San Clemente is at the northern edge of Camp Pendleton. All sorts of miltary smoke projectiles use WP to make fluffy white Phosphorus pentoxide smoke. If it lands in water (i.e., ocean f'rinstance) it quenches the flame. When the "rock" comes out of the water and the crust cracks to allow oxygen to reach the WP, she starts burning.
I found a 25 pound WP bomb washed up on the beach a few mile south of San Clemente a few years ago (a Navy smoke float).
WP burns are not a all funny and I have see some horrific stuff during my career..
Thanks. It’s obviously not a natural phenomenon. I was wondering if some jerk had been fooling around with phosphorus on the beach. But your explanation makes more sense.