Geology of Segula, Davidof and Khvostof Islands g W. H. NELSON

INVESTIGATIONS OF ALASKAN VOLCANOES

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GEOLOGICAL SURVEY BULLETIN 1028-K

Pr~filzredis coweration with the Depa~e~tsof the Asmy, hwy, md Air Force

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON r 1959 UNITED STATES DEPARTMEW OF THE INTERIOR

FRED A. SEATON, -my

GIEOUK:ICAL SURVEY

Thllmt.8 8. Noh, Dhctw PREFACE In Wbr2848 the War Deparbnent (now bpartment of the by)requad the GeologicaI Survey to undertake a program of volcano investiptions in the AIentim Islands-Alaska hninwls area. The field studies were made during the years 19M-1954. The re- sults of the first year's field, laboratoq, and library work were h&iIy assembled as two Bdministrative reports, and most of thesa data haw been revised for publication in Omlegiml Survey Bulletin 1028. Part of the early work was publish4 in 1950 in Bulletin 974-B, "Volcmic Activity in the ," and in 1951 in Bulletin 989-A,UGeoro~ of Bd&r Island, , Alaskan both by Robert R Coats. Unpublished results of the early work and all of the later &dies are being incorporated as parta of Bulletin 1028. The ggolagical investi- gations covered by this report were reconnaismnm. The investigations of 1948 were supparted aIm& entirely by the Military Intelljpnae Division of the Mce, Chief of Engineers, U. S. Army* From 1947 until 1956 the Departments of the Axmy, Navy, and Air Form joined to furnish financial and lopistic tw&tmce.

CONTENTS

kbetraot,, ,, ------,,- - -,- - - -,,, , ,, , ,, ------,------Inttodudion, ,-, - - - ,- - - -,,- ,, ,- ,, , , , , , , ,, . , , , , , , , , - - - - -,,, - ,, - - - - - Geog~aphy,,,, ., , . ------,, - -,- , - - - - - ,, . , , , ,, , , , , ,------Topograp~,,,,,,------,,,,,,---,-----,------Bemls I&nd ,-,------,,,,------, Khvtwtof and Davidof Man& -----,,,,,,,,,,,---,---,-,---- Rubmarine topomphg ------,------,,,,------+- climate -,,,*,, ,----* ,--,------,,, ,,., ,---..,------Vepetatiun---,---,,,------,,,.,,.,,,,,---- Geology------,. , ,, ,, ,, - - - - - ,------,-- - ,,,, - - ,, ,, ------Generslf&um-,,--,,,..--,-,------,,-,,,,++ Composite volaanIc material ,,,,,------_-----,------Busface ppruclastic layem ,.,------,,------+- Surface lava flows- - - --,,,, ,-,- ,------,-,,,,,,,,,,,,, Unaltered flow^ ., , , ,, , . ------,--,,,, , , ,,, ,, , , Altered volcanic roab,-,-,-,------,-----,,,,,,-,- 8edimentary depoeite on Segula Island------,----,,,,,,,, Btructure, - - - - - ,-,------,- ,- ,- ,- , ------RagulaXaland------,------,,,,,,, Davidof Island and Khvostd Ieland. ,,--,------,-,

Qedo~iahiwtory -----+------totototo~tototo,tototo----to------~------Future volcanic activity------..,,.,,.,------Rafemnce~cited--.,-----,------+~+-,,-,,~~,,---,-+-~---,+-----~

[Plates are In met1 Pam PLA~26. Geologic map of Davidof and Khvostaf Islands and vicinity, Aleutian Ialanda, Alaska ---+------,-,,---- In poaket 37. Geologic map of Seguls Island, Alenhn Islanda, Almka.,, In pocket 38. SeNa Ieland, aerial view-,,,------,---+---,,.,,,, Faoing 260 Frau~m48. Location of JChvostof, Davidof, and Segula Islands,,,,-,.--, 258

fNVESTiGATIONS OF ALASKAN VOLCANOES

GEOLQGY OF SEGULA,DAVIDOF, AND KHVOSTQF ISLANDS, ALASKA

ABSTRACT Zegu~a,Dnvidot, and KbvoaW me volmnfc iehnda that hefKnn an exteo- sive submarine plauorm about 350 feet below area level. Tbe formation of thin plafform by ermjon le the oldmt geologic went mrded here. Tbe subsanent history fa largely a record of eruptions of endmltlc laoa and pyroclastic material which formed Segola Volcano and a bypothetkcal voicanlc moantafn immediately north ot Davidof Ialand. A catnetmphlc eruption. perbarn during late Tertiary time, deetroged this mountah, formed the caldera north of Davidof Island and left Ilarldof and Khvostof Islands as remnante. The moat recently wnpted lava and pymlastfc materiala on Segula Island are young ~nou~hto have kn virtually antouched by erosion. Deposits of reworked volcanic debris underlie small areas of thew idmds. INTRODUCTION Seguler, Davidof, and Khvostaf Islands are part of the Rat Idandq near the mter of the western half of the Aleatian Islands chain (fig. 481, and Iie between 51°5'7' and 52O03' north latitude snd 178O04' and 17Be31' esst longitude. Reconnaissance, upon which this report is based, was done during 3 days in Aupst 1951. Lndinp on Setqula Island and Dsvidof Idand were supplemented by observations from shipboard, Richard 62 Lewis, lhnnis El. Cox, and Edward C. Stoever participated in the field work. QEOGECAPHY TOPO~BdPHP smm XBLdWD Sqph Islmd is formed by a single mniml mountain 8,800 feet high snd about 4 milea in diameter. The slopes relatively smoothly from the summit to ma level. A cinder cone forms the higheat point on the islaad. It covers part of the rim of a prly defined crater, about onehalf mile across, on the math; younger lava flows have poud over and partly obscured the crater on the norbh. 257

OEOLl3QT OF BEQULA, DAVIMIF, AND gBP08TOF ISWR, hLASKA 259

Ae extensive lava field mvers the northern slope, and a layex of ppdastil: material the southern and eastarn dopes. Stmp diffs along much of the western side expm the layered internal structure of the mountain. A deep gully ~Htendsfrom the summit down the south slop of the monntain. A small lava field extends to the coa& from its plm of origin at B small subsidiary cone and crater &bout8. mile mutheast of the summit, The entrance of a singIe narrow cove on the north coast of the idand irt nearly closed by shallow reefs. Landings can be made in favorable weather an narrow beachss around most of the ihd,but steep cliffs almost everywhere back of the beaches make inland mss difficult. Segula has no perennial streams; although the turf bWet is com- monly saturated with moisture, the underlying volcanic material is very permeable md allows much of the surface watar to seep down- ward. KaVOBTOP X-MD AmDAVmbb EKLARD Khvostof has a rudely rectangular outline, and cons* of a gently doping cmtral upland that risas westward to the high point of the island, about 850 feat above ma level. There are mveral small lakes and strems on the island. Davidof is very irregular in outline and has a maximum elevation of about 1,050 feet. M& of its surface is steep, and it iia mmplebly bounded by very steep sea diffs. There are no lakes or streams, but mall quantities of water probably could be obtained from wepage out of the water-sodied turf bIanket. SWWmE rnPOGr&4PHP Davidof and Khvostof Islands are the highest pad of two broad, mbmerged, ddmes that rise from a platform about 250 feet blow sea level (pl. 36). Dwidof7 Pyramid, and Lopy Islands are pa& of the rim of a submerged caldera on the eastern cone. Khvmbf is the subasrial part of the western cune. About 54 miles south of Rhvmtof, ts flabtopped submarine hob*abont 975 feet blowsea level, may be another small volcanic cone. Basides the caldera, several other submarine depdons are shown on plate 36. One, about a mile west of KhdfIsland, is probably mounded by lava flows. Depressions northemst of Dapidof Island are probably of teetanic origin. -TE The Rat Inlands have 8 cool, wet, windy, marine dimah, aa does the entire Aleutian region. Ternperaturn recorded at from February 1943 through April 1949 (Arctic Weather Central, 1950) rang.ed betwmn 66°F and 15°F and averaged 3g°F. Augmt, the warneat month, had a mean tempemturn of 4B°F, and Januaq, the coldest, a mean temperature of 31°F. The annual precipitation av- eraged 34.6 inches and included 10.6 inches of snow. Precipitation during August, the wettsst month, averagd 4.82 iinchea, and during February, the driest month, 2.76 inch= Snow can be expected from October through May, the greatest amount falling in February, when snowfall averaes 15.5 inches. Snow accumulaka only at higher elevations becauae winter temperatures dm to sea level allow the snow to melt Fog and cloud am nearly always- to be reckoned with The Itat Islands are in the path of major storms that frequently bring winds in excess of 100 miles per hour. Win& exding 75 mph occur on all the islands from 2 to 15 times a year, and winas exceeding 32 mph occur about 80 times a year. Over 90 percent of the strong winds arrive during the &month parid, October though March. The maximum wind velocity mrded at AmchitIra is 115 mph. Average wind velocitim range betwmn 31 mph in krnber and 19 mph in June. ~GETATIOFJ Stable slow below about 1,600 feet are mvePBd with a deep, water- gaaked carpet of lush vegetation underlain by a thick mat of peaty plant remaina. No trees or shrubs grow in them islands. The vegetation is chiefly composed of moss, lichens, and plants of the heath family. GTrmand sedges abound, especially along the beaches, where thick stands of wild rye or strand wheat attain heights of 4 or 5 feat. Fungi, liverworts, hometails, clubmosses, ferns and a wide variety of herba- ceous flowering plants add to the varieb of the veptation. Promi- nent among the ffowarhg plants am the white-&wered narciwrrrs anemone, lupines, and several species of orchids. Luxuriant growth af msrine algae or seaweed are abundant along the show nnd on rocky reefa between high tide and shut ZQO feet below ma level.

Segnla Volcano md the caldera north of Ihvidof bland are two of s row of volcano88 that extenda &wand from Riska Volcano at least to Sembpoehnoi Island, and possibly gs far aa the (fig. 481, and that probably marks en extsnded zone of weak- ness in the ocean floor. Segul e, Island is essentially a single composite volcano mmpdof many superimposed and interbedded layere of fapa and pyroclastir, rocks all dipping radially outward from the

center of the idand {pla 37 and 88). Davidof, Khvostof, Pyramid and lropy Marids are made up of interbedded Iava and pyrmlastia deposits in eveq way similar to the rocks of Segula Island. There is no fossil evidence an which to wign mi age to these & It is infedthat the Sepla Volcano is of late Quaterhary a@ from the absance of any evidence of glaciation and the small amonnt of destruction of the volcanic cane by marine emion, The FQC~of Khvostof and Davidof are mrnewhat older than the youngest Isvrr flows on Ssgula, but probably are about the same age an the main body of Segula. The submerged caldera north of Davidof and the Iinear topographic feature that ~0868Segula bland in a north-narthwestemrd dim tian are atructum.al features formed by .rrolcanism. Except for thm, them haa been little structural disruption and most of the rocks pmb ably have their original depitional attitudes. CO-E VOXXTblPSC XATaRrAr. Xnterbeddd lava and pyrmlastic matsrial can be seen wherever eroaion has exposed thick sections, especially in the sea cIifls that bound all the islands and in the riftlike gully on the south side of Segula Volcano. The lams accumdatsd as many small, lenticular tongues and sheets from a few to about 100 bedthick. The pyrloclastic layers are comparable in thichw to the leva flows but individual unirts cover much wider areas. mACE -UPEBB A pymlagtic mantle, lithologidy similar to ths older pyroclastic deposits, forms mmtmctional surfaces on mnch of Segda Island, and eblyDavidof and Hhvostof &Ian&. On Segula the pyro- clastic mantle is largely covered at lower elevations by vepbtion, but light-gray pyroclastic debris at East 150 feet thick is well axposed at Gray Point on the e~& side of Davidof Island. The ash mantle subdues or ohumolder topography, and can be recognized by its relativdy mooth surface on aerial photographs, even though much of it is mmred by vegetation. The pymclnstic fragments range in gize hmhe eand to hrge blocks of lava =Vera1 feet across, but fragments between one-eighth inch and 2 or 3 inches predominate. The fragments varg from pumi- ceous and saori-us tm massive antiesite and glass, and mylm red, bmwn, py,tan, or black. The ejecta of individual eruptions usually abonnd in f nrprtenta of one color and, 1&se consishntly, of one texture+ at any particulsr point of ohemation. 262 INVESTIGATION OF MASKAN VOM~OEB

SVBFAeE UVd FZOWH

mPAL'mmm3 mwB &mnt lava flows analogous to the older lavm cover about one quarter of the surfam of Se@a Island. On the north dope of Segula Volcsno, Snvn apparently issud from several openings within the summit matar and from a humthat extends northward for about one third of a mile from a place 3,200 feet due north of the summit. This fidd of lava is formed of many aeparate flow tongues. Smaller lava flows that reach the sea along the gouthesst shore of %@a Idand imed from mts in two small coalesd subsidiary cones about 1$OQ feet hove sea level. At ChuguI Point two recent flows lie bath a thin cover of volcanic a&, Parts of Davidof and Khvostof appear to have constructional toppphy although none of' the volcanic material there is recent enough to b free of some vegetation. Many of the flows are C~~VB~by pegetation, but mn be identified on the aerial photographs by their lobate form, natural dikes or levees, and flow-ribbed surfaces, B-1 parts of many Aows consist of brick-md bmia. The flows are generally dark py, gl&sv or he-pined, compact or dikty- taxitic. Phenmryh of feldspar, pyroxene, and olivine are common, The unaltered, relatively young lava flows are petropphiaaIly sh- ilar. Seven specimens fiom Segula ZsIand and one from Davidof Island wem studied petrographically md two ware chemically analyzed. AU are porphyritic and feld~pathie. Phenocrysts of by - towniN clinopyroxene (tbugite), olivine, and in some specimens, orthopyroxene, are in a groundmnss of labradorite, pyroxene, olivine, magnetite, and rr~lptocryshllinem&ri al. Maximum refractive in- dim of bytownits phenocrysts axe btwwn 1.676 and 1.678. Labra- dorite phenocrysts in n specimen from SeguPa Island wem found to have d4 = 1.564. Most of the plagioclase phmocrysta have normal zoning. Small poikilitic inclusions of mafic minerals and amorphous material nrs common, generally in only one part of the phenocryst, either the care or one of the wnes. All the ~pcimenscontain phenmrpta of mono- clinic pyroxene. In one specimen the monoclinic pyroxene has a 2V of 54" and a aZ of about 1.695, indimking tlmt this pyroxene is augite. The pbenocr~rstsare euhdral to subhedral, usually twinned, and range from 0.2 to 1.5 millimeters long. They make up abut 6 to 15 pewant of the volwe of the rock. In four of the specimens some of the orthohombic pyroxene pheno- cry& are rimmed by monoclinic pyroxene, although many am not. OEOUIOP OF BEGPLA, DAYIWF, AND KEWOSTOF ISLANDS, ALAW 263

Analyea 8j pmphydic jtldapalhic lwas of SeguEa volcam ICbsarlal dmb3r L. Kahl; -~hlo anal- by P. B Brims#, Jona fw oldamaoy Wwlroad dtplw&a* chemical maly~: S.plyra+auudiw~&I halt t Si4,-,,------62.64 61.21 Alp%-,--,.,--,,-..,,,------lti. 22 18.01 Fe& ----.---.---.------2.20 2. 70 FeO------,-,------4.33 6. 21 MgO------,.-...--.-+------,------1.72 6. 81 CaO------.-.------,-, 4.62 11.24 nls0 -----+,,---.,-,- -,-----,+------4. #ti 2 67 Kg0,------1. 86 .68 1120-----,,,-,,-,,-..-*.---~------08 -00 IFpO+,------..------. 00 -00 TiQt------,,,------*------+- .88 .50 CQz------+--,----*.------.01 .01 PIOr------r----r-----.------.R4 -14 F_.--,------,,------04 -01 hfnO,,------,.,-.--,.------. 18 .17 I$aO,-.------.04 .02

Total ------+---*------100.05 100.26 lese 0------,------.02 .00 100.03 100.20 Spe&~grapblo anal-: B------..---_----_------0.005 0.0 Ba------.------.06 -03 co-----*--,------+------*-*------.oOl .00Q Cr,-,.------,,,------+------.OW3 .008 Cu-4+----,-,-++--b------+--- *,--- .003 .02 Ga-,,---..,------,------,------001 . Ooll9 Li----,------.001 .no02 Ni--,------,..,,-.------++,------,-- .O .DO3 Rb-,L_--,-,,----,.,------.-,008 .O Sc------,,------.OD2 -004 Sr------L------,+------*------.C)4 .08 V----_------_-.------+------.008 . 03 Y--,------_------,.------++-+------. 008 ,006 Tb-.------+.------,------, . 0006 -0004 k+,_------,------. 02 .005 Norm : q------+------.--.------14.58 1. 32 or ..-----+-- ..---+--,- ,,.,-,------11.12 3. 89 ab,,------,----+---- 40.87 21.48 ~n,,---,------,----+--16.96 38.09 wo-_+--,,------_.--,-----,,--,-----1.28 16-82 fk -.,------,------4------4.88 8. 32 en,.--,+.-,------,-----,------*-++ 4.30 14.50 mt,,-.-----,-,------,-,------3.25 3. 04 il-,.,.-,,--,-,++-----+--4-*------,--+- 1.87 1, 22 ap,,,,,-,,.,,-,-,------*------1.01 .34 C. I. P. W. clseeibation,,------,--,, --- ,,-- - II.(4)5-(2)5.4 IX(I1I)d-4 sndme hewwe Rittmsnn QamiiScation,-,------daeite labradorits- andcsik Theee phenware enhedd to mbhsdral, range from 0.2 to 1.5 mrn in length, and make up a smdl percentage of the rock Olivine phenocysts wem 4een in six of the specimens studied. They ere anhedral, wry from 0.5 by 2 mm in length, and make up 1 to 5 pemnt of the rock volume, The groundmass of the lava is composed of plqioclase, pyroxene, rnagnetita, cr~~ptocrystallinematerial, and olivine in some flows. The plagimlase commonly forms fine lathlike crystals, with maximum extinction angles of from 28O to 3g0, indicating compositions of about Anas. Most of those plaghlam crystals have narrow normally mned rims. The other groundmass minerals occur as amall Buhedral pins. AL- VOXCAH'TC ROCK@ Much of the rock on hpyIsland and on the north end of bavidof Island has been intensely albdto bright yellow, orme, and brown, probably by hydrothermal solutions. The- most highly sltemd rock is mapped on plab 36. In a specimen from an altered flow on Davidaf Island the only minerals that retain their original appearance are the plagioclase phenocrysts; the mafjc phenocrysts have aEI beerr albred to; chlorite, or chlorite and mrrgnetite, and the groundmaga to chlorite cryptmry&allina quartz, and mapetita The quartz, which now makes up 10 to 16 prmnt of the mkvolume, was probably introduced during the alteration.

J)ep&ts composed of more or less reworked voIcanic debris unilarlie mall areas on Segula Island. Unconmlidaed ruddy d,mdemtely we111 bedded volkmic detritus (mmtly p~lastio)at leust 600 feet thick mop out aromd the cove on the north coast, The fragments mmnb1e thm elmwhere inhrbedded with flows and range in size from hesand to blmh of lava mveral feet across. Sizes between one-eighth inch md 8 inch= pdminata Although the matsrial ie unmnsoIidatsd it stands in swp elopes where ededat sea cliffs The stratigraphic position of this deposit ia uncertain, but its poor atah of mnaolidation, as well as the fwt that it is composed mostly of pyroclastic material that muld have been derived from the volcano sbeve sngmthat it ia younger then much of the mmposite cone material. Large amounts of rddislodged fmm the mpoeib volcanic material on tha w& aide of the island form landslide cones along the coast. Poorly mrted volcanic deb& fills s channel at athe mouth of the large gully on the south shore. Wsve-romded boulders mixed with minor %mounts of fine gravel form beach= along parts of the coast. The besch at tZle back of the oove on the north coast is the only ane that cantsins appreciable qnantitiee of mud. 0mvm

Evidmw of tilting or warping of rock on Segula I~hndis lacking, but mme topographic faaturn suggest volcanic rifts. The deep gully on the south& slope of the volcano, the summit crater, and the cove on the north coast form a linear topographic deprassion that transects the idmd. A mntflaw apparantly fills parts of this depmion on the north slope of Segula Volcano. Recent conetructiond land snr- fama on either side of the rift do not appear to be displaced, and the cuspate shape of the sida of the gully suggests that a series of sxplo- sions or collapses took plam at mveral places along the rift, possibly as a mItof the stoping action of molten lavw within or below the ma Such voIcsnic rifts, without displamrnent, are common in the Aleutian Island& The -it of Kanaga Volcano is cut by a linear aeries of depmssim miated with active fnmmIw; these depm- sians ocenr within the crater and on the outer fl& of the cune but the crater rim itself is not broken (Snyder, 43. L, m1communica- tion). A curvilinear serim of thirteen sparate craters and four mall lava flows occur on the southeast flank of Gareloi Volcano as a mul t of an eruption in 1929, {Goats, in preparation, 1956.) Several -ria of alind explosion cmtm and small pymdastic cones QCCU~an the, northwe flank of Mhshin VoIano (E'rsser, E. D. oral oommurrication) . Another pomible or rift exhm& northward from point 8200 feet north of the witof Segula Volamo, and appesm ts have provided the conduits which feed some of the recant flows. Another firnure OF rift is suggested by a topographic high near the gourn of the recant lava flom on the north flank of Segala Volcano.

Attitadm of some lava 0om and pyroclastic layera on Davidof Island ltnd Rhv&f Island are ~hownon plate 36. The dip am mostly gentle or moderate md generally to the southeast and east. A large submarine caldera lies between the two dands. A lava flow that forms a dip dope extending southward hrn the highst point on avidof IsIand may have originated from an undiscovered vent near the present crest of the island; its appearance suggests, howevez; that it flowed down the slopof a wlcsnic mountsin Ul&t no longer exh Such a mombin probably was pmtimmediately north of Davidof Island prior to the emption that formed the m1dm them. The two mbmarine depressions northeast of Davidof Tslmd &re be- lieved to be at lead partly due to f adking* Steep interior north rims mag@ fault searpa. QE0M)GIU BXBFORP The oldest recorded p1ogie event in the Seela, Davidof, Khvostof amwm the development of an extensive flat platform, now about 350 feet blow sea level ; probably the platform was formed by erosion in middle Tertiary time. Sukquently, volcmjarn built the i~landsand the submerged knobs and depressions on the ~ubmarineplatform. One of the later events in the Davidof-Khvosbf amwa9 the cata- strophic eruption that resulted in the formation of the caldera north of Davidof Islmd. This eruption probably occurred during later Ter- timbq he. !!A@caldera, which is northwest of hvidof Island, ap- peam to have been LittlemodXed since its formation.

Although Segda Volcano and the caldera north of Davidof Island are now quiewenb a major eruption of lava, ash, OF both can mnr at any time. Segula Volcano haa erupted inhrmittmtly mtil mnt timas as proven by the pmnce of flows and pynxlastic materials, which can. not be mom than a few hmWsof gears.

A&Ic Weather Cent&, W,Clhmte, weather and flying cowlitlonu of Ahka and eaatem Siberla: bretlc Weather Central, 11th Weather Bqnadrm, EIm- endorf &ir Force Base, AhmBa Coatrl,RR,l~.Apeol@c~oiUanlol~d,Uega:U.8. GmL Sums BuL, 1028-J, p 24&256.