Island Mountain is a resistant formation of volcanic and metavolcanic rock in the Franciscan Assemblage of southwestern Trinity County, California.

It was mined for sulfur metal deposits during the first half of the 20th century and was the final obstacle in the construction of a railroad line to Eureka, California in 1914.

[1] Island Mountain in Trinity County was named by settlers in the 1850s because it is nearly isolated by water from two creeks and the Eel River.

[3] Due to the resistance of the rock to erosion, the Eel River makes an abrupt "S" curve around Island Mountain in its otherwise north-northwesterly flow between the California Coast Ranges.

[10] Volcanic rock was quarried from Island Mountain by Northwestern Pacific Railroad for heavy riprap used as far away as Humboldt Bay.