Geology of Segula, Davidof and Khvostof Islands Alaska g W. H. NELSON INVESTIGATIONS OF ALASKAN VOLCANOES - - 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 Aleutian Arc," and in 1951 in Bulletin 989-A,UGeoro~ of Bd&r Island, Aleutian Islands, 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 Unpublished results of the early work and all of the later &dies are being incorporated as parta of Bulletin 1028.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 Amchitka 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