AngelIslandconsists mainly of rocks of theAngelIslandterrane of the Franciscan. These rocks differ from much of the Franciscan in that they are metamorphosed to the blueschist facies. Blueschists form only under the unusual conditions of extremely high pressures and relatively low temperatures that are characteristic of subduction zones.

The metamorphic rocks onAngelIslandare called theAngelIslandterrane. These rocks were originally sandstone, pillow basalt and serpentine, but were altered to schistose sandstone, schists, blueschists, metamorphosed pillow basalt, and metamorphosed serpentine. The blueschists are of special interest because they form only in subduction zones, and are thus considered as an indicator of subduction zones. After the  rocks of theAngelIslandterrane were metamorphosed, they were thrust over unmetamorphosed sandstones of theAlcatrazterrane. This must have occurred later when the subduction zone had become cooler.

Rocks of theAngelIslandterrane occur along the southern shoreline of theTiburonPeninsulaand in the Berkeley Hills as well as atAngelIsland. Metamorphism of a widespread rock unit like theAngelIslandterrane is rare in the Franciscan. Metamorphosed Franciscan rocks usually occur as individual boulders in melanges or as small isolated areas of moderate- to high-grade metamorphism.

Sandstone, Pillow basalt and Serpentine cruhing pant

On the western part ofAngelIslandthere is a northwest-trending belt of serpentine that is several hundred feet thick. This belt of serpentine separates the pillow basalts that form the westernmost part of the island from the schistose sandstones that form the central and eastern parts of the  island.  One  of  the  best  places  to  see  this  serpentine  is  at  the serpentine quarry next to the rock crusher near Perle’s Beach. The serpentine is also exposed along the shoreline east of Perle’s Beach, but access to this part of the beach is difficult and not recommended.