Umnak Island is a dumbbell-shaped island in the eastern part of the Aleutian Islands. The island is 70 miles long and trends northeast-southwest. During 1946 volcano investigations were begun on the island and geologic mapping of most of northeastern Umnak Island was completed.

Okmok Volcano, a large, broad volcanic mountain rising to altitudes of 3,000 to 3,500 feet, occupies the central portion of northeastern Umnak Island. Fort Glenn, and Army airbase, is situated on the eastern end of the island, approximately 9 miles east of Okmok Volcano.

The central part of Okmok Volcano is indented by Okmok caldera, a large cliff-rimmed volcanic depression, 7%, miles in maximum diameter. The floor of the caldera is 1,500 to 2,500 feet below the caldera rim. Nine large cinder cones and many small ones lie on the caldera floor, chiefly along two arcuate zones. The caldera is drained by Crater Creek, which flows through a deep gorge cut in the northeastern wall of the caldera, and into Bering Sea.

Mount Tulik (4,111 feet altitude) and Mount Idak (1,918 feet altitude) arc important centers of ancient volcanism on the flanks of Okmok Volcano.

The geologic history of Okmok Volcano falls into three stages: The first includes the upbuilding of an ancient cone—Mount Okmok—to an altitude of at least 6;500 feet on the site of the present caldera; the second encompasses the destruction in a castastrophic eruption of the summit cone and the formation of the caldera; the third comprises events since the great eruption.

The earliest activity at Mount Okmok probably dates hack to the late Tertiary period. A composite cone, concave-sided in profile, was built by the alternate eruption of ash, coarse pyroclastics, and basalt flows. During the late Pleistocene, volcanic activity at Mount Okmok was greatly reduced and a topography of late youth was carved on the lower slopes by streams and valley glaciers. The summit of Mount Okmok was upwarped and dikes and necks were injected into the resulting fractures.

A large volcano at the site of Mount Idak was active during part of the period of upbuilding at Mount Okmok but became extinct during the middle Pleistocene. A parasitic vent, Mount Tulik, became active during the late Pleistocene and built a steep-sided cone before it became extinct, shortly before the formation of Okmok caldera.

A cataclysmic eruption terminated the period of dissection at Mount Okmok, 10,000 or more years ago. Part of the summit was blown away by the explosive violence of this eruption. Nuees ardentes (glowing clouds) and mudflows deposited tuff-breccia and agglomerate in the glacial valleys; later phases of the eruption blanketed the landscape with ash. Near the end of the eruption, the remaining upper part of the volcano collapsed along arcuate fractures: large blocks subsided several thousand feet and are now concealed in the caldera Poor beneath later deposits. A large arcuate fault block which subsided less than other blocks stands above the floor in the northeastern part of the caldera.

After the eruption, water collected in the caldera, forming a lake. Small but frequently active cones built islands in the lake and covered its bottom with pyroclastic debris. The lake eventually overflowed the lowest point in the rim of the caldera, and Crater Creek Gorge was carved, draining the lake and dissecting its deposits. Renewed movement along faults at the head of Crater Creek Gorge later raised a harrier which temporarily dammed the drainage and formed a second caldera lake.

Readjustments among subsided blocks in the caldera floor resulted in the folding of postcaldera deposits at several localities.

Much of the caldera floor has been covered by lava flows extruded from several cones since the draining of the first caldera lake. In general, however, volcanic activity seems to have declined since the great caldera-forming eruption. Seven eruptions from cones on the caldera floor have been recorded since 1817; the latest occurred in 1945.

As part of the geochemical program for the study of Okmok Volcano, temperatures of fumaroles were measured and samples of the products of the volcanic activity were analyzed. Average temperatures of fumaroles at one source of the 1945 lava low dropped from 320° C. on July 19 to 90° C. on September 5. The temperatures of fumaroles associated with the crater vents on both Cones A and C ranged from 95° C. to 97" C., which is slightly below the condensation point of steam, indicating the presence of minor quantities of gases other than steam. The magmatic gases of fumaroles on Cone A consisted of carbon dioxide and sulfur dioxide in about equal amounts. The lack of halogen acid gases in the fumaroles and the steadily dropping temperatures arc interpreted as indicating that the present quiescence of Cone A will continue for many months. Solid reaction products from areas of fumarolic activity on Cone A are sulfates of sodium, calcium, and iron. The presence of hydrogen sulfide as the dominant sulfur gas at Cone C is interpreted as indicating the dying stages of the present cycle of activity of Cone C.

Thermal springs along the north base of Cone D have a total discharge of 115 cubic feet per second. Their average temperature is approximately 7° C. above the annual mean. From these figures it is calculated that about 21,000 kilogram calories per second are being given off by Cone D. The spring waters contain minute quantities of boron which is indicative of a magmatic source for a small part of the water. Hence, Cone D though quiescent is not extinct. Evidence is presented to show that most of the spring water from Cone D is meteoric in origin.

Thermal waters in the southwestern part of Umnak Island at Hot Springs Cove and south of Geyser Bight contain lithium, boron, arsenic, and antimony in solution. These elements are regarded as derived from underlying magmas that are in an advanced state of crystallization and hence not likely to give rise in the near future to large-scale volcanic activity.

Three portable seismographs were placed on the flanks of Okmok Volcano and were in operation during most of the period from June 1 to October 1. During this period several slight tremors and one moderate tremor, all of distant origin, were registered, but no tremors attributable to Okmok Volcano were recorded. The lack of tremor records, however, may have been due more to the insensitivity of the instruments than to the absence of tremors.
Earth-current investigations were carried on during August and September by comparison of records obtained from a base station at Fort Glenn with those from a station 1 V2 miles southwest of Mount Tulik. The records obtained indicate that no difference in magnitude or direction of earth currents existed between the Fort Glenn and Mount Tulik areas. The similarity of record obtained is indicative of the absence of a disturbing factor such as a large body of live magma beneath Okmok Volcano.

Future eruptions of Okmok Volcano are expected to he of mild to moderate intensity, and will he chiefly in the form of ash falls from vents inside the caldera. There would be a great menace—in the form of lava flows, nuees ardentes, and mudflows—to installations at Fort Glenn if a new center of volcanism came into existence on the east slope of Okmok Volcano. Small postcaldera cones now exposed there indicate that extra-caldera eruptions have occurred in the recent past and can be expected in the future. The possibility of another catastrophic eruption of the caldera-forming type, however, is remote.

Okmok Volcano should be kept under close observation, partly because of its possible threat to Fort Glenn and partly because it is a readily accessible locale for accumulating information on details of volcanic processes, applicable to other volcanoes in the Aleutian arc and elsewhere.