EXPLANATION VOLCANIC ASH FALLOUT AND DIRECTED BLAST

LAHAR HAZARDS

PYROCLASTIC-FLOW HAZARDS

DEBRIS-AVALANCHE HAZARDS

VOLCANIC GAS HAZARDS

VOLCANIC ASH HAZARDS

Areas likely to be inundated by lahars, lahar-runout flows, andfloods during eruptions of all sizes, but expecially during pyroclasticeruptions. Extent of inundation depends on the volume of the snow-pack and the duration of the eruption.

Areas that could be swept by pyroclastic flows to small to moderateeruptions from the present crater. Hazard boundaries correspond toH/L = 0.3 (inner zone) and H/L = 0.2 (outer zone).

Areas likely to be traversed by debris avalanches. Most likely aresmall-scale collapses of lava-flow fronts and spatter piles.

Areas where volcanic gas may be emitted.

Known pyroclastic-flow deposits.

Likely flow paths for pyroclastic flows.

Lava domes and possible vent areas that could be a source ofpyroclastic flows.

Thickness of <300-year-old pumice fall deposit, in centimeters.

Crater rim

Note: All areas of Great Sitkin Island are susceptible to ash fall.

26

REGIONAL VOLCANIC ASH FALLOUT

+

Akuyan Creek

Glacier Creek

Sitkin Creek

Sand Bay

Yoke Bay

BERING SEA

Middle Yoke Creek

Big Fox Creek

1000

4000

2000

3000

Great Sitkin Volcano5,710 ft 1,740 m

0

0

5 KILOMETERS

5 MILES

CONTOUR INTERVAL 1000 FEET

176o30' 176o20'

52o

05'

52o

00'

Teapot Rock

43

60

30

26

View of Great Sitkin Volcano from the north shore of Adak Island.

Present edifice

Crater Quaternary lava flows

Tertiary volcanic rocks

Great SitkinVolcano

Areas that could be affected by adirected blast similar to the blastgenerated by the 1980 eruption ofMt. St. Helens, Washington. Thisis a worst-case condition forGreat Sitkin Volcano.

0

0

10

50 MILES

70 KILOMETERS

10

20

20

30

30

40

40

50 60

TanagaVolcano Bobroff

VolcanoKanagaVolcano

MountMoffett

MountAdagdak

KasatochiVolcano

KorovinVolcano

A n d r e a n o f I s l a n d s

BERING SEA

PACIFIC OCEANTanaga Island

Kanaga Island Adak Island

Great SitkinIsland

Atka Island

178o 176o

52o

52o

30'

51o

30'

174o

Adak

Atka

Shaded area shows zonemost likely to receive ashfall during future eruptions.

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?

?

?

Approximate area that could beaffected by ash fallout during largeeruptions of Great Sitkin Volcano.Ash clouds will move with the prevailingwinds, which are generally westerly.

Shaded area shows zonemost likely to receive ashfall during future eruptionsof Great Sitkin Volcano.

TIMING OF VOLCANIC ACTIVITY EVIDENCE

<280 yr B.P.

550-700 yr B.P.

1150-1300 yr B.P.

34-3700 yr B.P.

3600-4000 yr B.P.

5900-6600 yr B.P.

7300-7600 yr B.P.

8400-8900 yr B.P.

>9500 yr B.P.

Lahar deposits in Sitkin and Akuyan Creek valleysPyroclastic-flow deposits in Glacier Creek valleyPumice lapilli tephra deposits

Lahar deposits in Sitkin Creek valleyPumice lapilli tephra deposits

Pumice lapilli tephra depositsLahar deposits in Sitkin Creek valley

Pumice lapilli tephra deposit (Sandwich ash?)

Pumice lapilli tephra deposit

Pumice lapilli tephra deposit

Pumice and lithic lapilli tephra deposit (Main ash?)

Lahar deposits in Sitkin Creek valley

Pumice lapilli tephra depositsLahar deposits in Sitkin Creek valley

Pumiceous pyroclastic-flow deposits north of Sitkin Creek valley

HOLOCENE ERUPTIONS

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

PRELIMINARY VOLCANO-HAZARD ASSESSMENT FOR GREAT SITKIN VOLCANO, ALASKAby

Christopher F. Waythomas, Thomas P. Miller, and Christopher J. Nye2003

OPEN-FILE REPORT 03-112PLATE 1

USG

S - UAF/GI - ADG

GS

Ala

ska

Volcano Observatory