From Miller and others (1998) 
: "Shishaldin Volcano, located near the center of Unimak Island, is a spectacular symmetrical cone about 16 km in diameter at the base. The mountain, which rises to a summit 2857 m above sea level, is the highest peak in the Aleutian Islands and has a small summit crater from which a steady cloud of steam is emitted. The upper 2000 m is almost entirely covered by perennial snow and ice. It is flanked to the northwest by 24 monogenetic parasitic cones 
. The Shishaldin cone is less than 10,000 year old and is constructed on a glacially eroded remnant of an ancestral soma and shield 
, which in turn are underlain by volcaniclastic rocks of probable late Tertiary age 
. Fournelle (1988) 
suggests that the basement may consist, at least in part, of plutonic rocks."