1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 .5 0 1 KILOMETER

SCALE 1:24 000

CONTOUR INTERVAL 100 FEET

1 1/2 0 1 MILE

APPROXIMATE MEAN

DECLINATION, 2012

4

3 Base from Army Map Service (1:25,000), Corps of Engineers, 4 / °

compiled in 1952 by photogrammetric methods from aerial 

photography dated September 1948 and field surveys in June 1950.

Universal Transverse Mercator Projection, Zone 1 North

North American Datum of 1983

NOTE: THIS MAP IS NOT 

INTENDED FOR NAVIGATION

Geology mapped by M.L. Coombs, B.L. Browne, 

and R.G. McGimsey

Geologic map compiled 2003–2006 by M.L. Coombs 

in ArcGIS

Final map prepared in Adobe Illustrator by 

M.L. Coombs and E.E. Thoms in 2008–2010

Edited by J.L. Zigler; digital cartographic 

production by Kathryn Nimz

Manuscript approved for publication May 25, 2010

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3145

Pamphlet accompanies map

Any use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, 

Denver, CO 80225, 1–888–ASK–USGS

Digital files available at http://pubs.usgs.gov/sim/3145/

Suggested citation: Coombs, M.L., McGimsey, R.G., and Browne, B.L., 2012, Geologic map 

of Mount Gareloi, Gareloi Island, Alaska: U.S. Geological Survey Scientific Investigations 

Map 3145, pamphlet 18 p., scale 1:24,000, available at http://pubs.usgs.gov/sim/3145/.

1 U.S. Geological Survey, Anchorage, AK 99508-4626

2 Department of Geological Sciences, California State University,

 Fullerton, CA 92834-6850

Geologic Map of Mount Gareloi, Gareloi Island, Alaska

By

Michelle L. Coombs,1 Robert G. McGimsey,1 and Brandon L. Browne2

2012

2

6.1±1.0 BB38

BB41

1090±70

BB39

MC12

*

dv

b

d

csp

psp

ssp

dsf

d29

p29

t29

l29

asp

lsp

l87

pnp

lnp

pef

snw

sef

lnpu

see

ssf

anf

LIST OF MAP UNITS

[All units on Gareloi Island are Quaternary. Map units have been organized 

according to age, but also according to eruptive vent—either south peak or north 

peak. The approximate time equivalence of units from the two vents is portrayed 

in the Correlation of Map Units. Units for which chemical analyses are available 

have been assigned rock names based on the scheme of LeMaitre (2002), with 

appropriate mineral modifiers. Where available, we included ranges of SiO2 and 

total alkalis (K2O+Na2O) for each unit, in wt. percent. Descriptions of phenocrysts, which are defined as crystals ≥0.5 mm in length, were determined by 

observations of both hand specimens and thin sections. In addition, a subset of 

thin sections has been point counted]

SURFICIAL DEPOSITS

Debris-flow deposits (historic)

Beach deposits (Holocene)

Viscous debris-flow deposits (late Holocene)

HOLOCENE VOLCANIC ROCKS—SOUTH PEAK

Altered crater rocks of south peak (Holocene)

Pyroclastic rocks of south peak (Holocene)

Shoshonite lava-flow field of south peak crater (historic)

Debris-flow deposits of south flank (historic)

Debris-flow deposits (1929)

Pyroclastic deposits (1929)

Trachyte lava flows (1929)

Latite lava flows (1929)

Andesite of south peak (late Holocene(?))

Lavas of south peak (Holocene)

HOLOCENE VOLCANIC ROCKS—NORTH PEAK

Lava flow (1987)

Agglutinate, spatter, and pyroclastic deposits of north peak 

(Holocene; historic)

Sheet lava field of north peak (Holocene)

Lithic pyroclastic-flow deposits of east flank (Holocene)

Shoshonite lavas of northwest flank (late Holocene)

Shoshonite lavas of east flank (late Holocene)

Lavas of north peak, undivided (Holocene)

PLEISTOCENE VOLCANIC ROCKS

Shoshonite of early edifice (Pleistocene(?))

Shoshonite dome of south flank (Pleistocene)

Andesite of north flank (Pleistocene)

EXPLANATION OF MAP SYMBOLS

Contact—Dashed where inferred

Internal contact—Delineates flow margin

Crater rim

Dike—Approximately located

Landslide scarp

Ground crack

Normal fault—Ball and bar on downthrown block. Queried where 

uncertain. Arrows on cross section show relative motion

Flank vent

Sample locality and number—Prefix 03GR removed from sample 

numbers for clarity on map

Rock sample locality—Sample not analyzed

Chemical sample locality—See appendix 1

40Ar/ 39Ar sample locality—Showing age (ka)

14C sample locality—Showing date (yr B.P.)

Fumarole field

Crater formed during 1929 eruption

d

dv

pnp

see

lnpu

p29

ssp

asp

l29

sef

t29

l87

snw

d29

b

psp

lsp

ssf anf

dsf

lnp

pef

?

csp

Pleistocene

Holocene

CORRELATION OF MAP UNITS

[See DESCRIPTION OF MAP UNITS for specific unit age assignments]

SURFICIAL

DEPOSITS

HOLOCENE VOLCANIC ROCKS—

SOUTH PEAK

HOLOCENE VOLCANIC ROCKS—

NORTH PEAK

PLEISTOCENE

VOLCANIC ROCKS

QUATERNARY

Post-1929

1929

sef

North Peak feeder dikes

East landslide scarp

Northwest landslide scarp

psp

asp

pnp

see Landslide debris? 

lnpu

see see

lsp

South Peak feeder dikes

0

METERS

A

1,000

1,500

500

–500

0

1,000

1,500

500

–500

A'

METERS

Map showing shaded relief 

topography (in gray) and 

bathymetry (color-scaled for 

depth) for Gareloi Island and the 

surrounding seafloor. Bathymetric 

contour interval is 200 m. 

Bathymetry dataset has been 

merged from 1:400,000 scale map 

(Nichols and Perry, 1966) and 

multibeam data (Coombs and 

others, 2007). Smoother areas are 

typically lacking in the higher 

resolution multibeam data. 

Subaerial topography is from the 

Shuttle Radar Topography 

Mission (Gesch and others, 2006). 

Star marks the location of a 

submarine flank cone that was 

dredged in 2005 (G. Yogodzinski, 

personal commun., 2005). 

800

600

400

2200

2000

1600

1400

1800

1200

1000

0

2400

200

2600

2800

0

400

200

179°W 178°50'W 178°40'W 178°30'W

51°40'N

51°50'N

52°0'N

Gareloi volcano’s south crater, with shoshonite lava-flow field that formed between 

1950 and 1980. Gullies in foreground cut into pyroclastic sequence that formed during 

the 1929 eruption. The breached south crater also formed in 1929. White portion of 

the crater wall is hydrothermally altered due to persistent fumarolic activity.

Gareloi volcano’s north crater, with north rim of south crater in background. Dark 

deposit that flows down from low point on the eastern (left) rim is lava flow that likely 

formed in 1987. Persistent ice field seen at right of photograph.

71

71 72 73

35

34

45 45

44

80

34

A'

A

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

b

b

l29

l29

lsp

d

lsp ssf d29 dsf dv

d29

l29

dsf

lsp

d29

dv b

see

p29

t29

p29

p29

see

psp

p29

asp

see

see

see

see

see

asp

ssp

csp

asp

see

lsp

csp pef

pef

d

see psp

lnpu

asp pef asp

psp

ice

lnpu

l87

see

lnpu

lnpu

lnpu

lnpu

lnpu

b

lnpu

lsp

lnpu

lnpu

lnp d

pnp

sef

snw lnpu

snw

anf

see

lnpu

lnpu

see

GM08

GM43

BB56

GM39

MC09

BB02

GM32

GM30

GM20

GM17

GM10

BB59

BB47

BB25

BB23

BB21

MC36

MC11

BB55

BB54

BB50

BB49

BB45

GM04

GM02

GM01

MC08

BB17

BB04

BB03

MC07

GM41

GM38

MC33

MC32

MC30

MC25

MC24

GM37

BB34

BB33

GM24

GM23

GM33

GM44

GM35

GM29

GM19

MC23

GM12

BB41

GM40

BB43

BB31 MC27 BB30

GM18

GM15

BB42 GM13

BB36

GM25

GM22 GM21

GM31

GM11

BB27

GM28

GM27

GM26

GM14

BB57

BB52

BB44

BB39

BB38

BB32

BB28

MC15

MC14

MC13

BB08

BB07

BB06

BB05

GM03

GM07

GM05

BB16

GM16

BB12

MC21

MC20

MC16

MC05

MC04

MC35

MC31

MC29

MC28

MC26

MC22

MC12

MC01

MC03

MC06

BB01

GM42

GM36

MC34

BB29

BB26

GM1S

MC17-19

BB09

GM06

0.3±1.4

6.1±1.0

-0.5±3.0

*

1

2

3

4

5

8

6

10

13

14

16 15

15

7

9

178°44'W

178°44'W

178°45'W

178°45'W

178°46'W

178°46'W

178°47'W

178°47'W

178°48'W

178°48'W

178°49'W

178°49'W

178°50'W

178°50'W

178°51'W

178°51'W

178°52'W

178°53'W 178°52'W

51°50'N

51°50'N

51°49'N

51°49'N

51°48'N

51°48'N

51°47'N

51°47'N

51°46'N

51°46'N

51°45'N

51°45'N

Mount

Gareloi

1090±70

South Peak

PACIFIC

OCEAN

BERING

SEA