Chimneys are smaller than stacks, which have a shape more like a mesa (see a stack here with a sea arch in it). Chimneys are taller than skerries, which are low-standing rocks that can be covered in high water.
This chimney lies off Rodeo Beach, just north of San Francisco, and probably consists of greenstone (altered basalt) of the Franciscan Complex. It is more resistant than the graywacke around it, and wave erosion has carved it to stand alone. If it were on land, it would be called a knocker.
