Pillowed Lavas, I: Intrusive Layered Lava Pods and Pillowed Lavas Unalaska Island, Alaska and Pillowed Lavas, 11: A Review of Selected Recent Literature By GEORGE L. SNYDER and GEORGE D. FRASER SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 454-B, C UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 7-5, D.C. Pillowed Lavas, I: Intrusive Layered Lava Pods and Pillowed Lavas Unalaska Island, Alaska By GEORGE L. SNYDER and GEORGE D. FRASER SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 454-B CONTENTS Page Description of pillowed lava and lava pods-Continued Page Abstract_------_----------------------------------- B 1 Lava pods-Continued Introduction and acknowledgments- _ _ _ - - -- - - ---- -- -- - 1 Descriptions of lava pods in specific areas-Cont. Geologic setting-__ ................................. 1 Surveyor Bay area_-- -- - --- - ---- - - -- -- -- Description of pillowed lava and lava pods _.----------- 3 Cape Prominence area ---------..--------- Pillowed lava----------------------------------- 3 Eagle Bay area ......................... Pillows------------------------------------ 3 Lance Point area- - - .................... Matrix-_--------_--__----------------------- 4 Origin of pillowed. .lava and lava pods on Unalaska..- - _ _ - Lavapods-__---_--------,--------------------- 5 Intrusive ongin _-__-_------------------------. General features--___----__----------------- 5 Emplacement in muds and development of mud- Descriptions of lava pods in specific areas-. - - - - 8 .magma emulsion _____-_-___------------------- Tower Point area ....................... 9 $rigin of layers in lava pods and sills_----_-_-----_- Buttress Point area-__ _-_--------------- 9 Reconstruction of a typical igneous episode- - - - - - - - Tower Point-Lion Bight area ---_--------- 11 References cited .................................... ILLUSTRATIONS Page FIGURE 1. Index map-----------_----------------------------------------------------------------------------- B2 2. Sketch of altered andesitic pillowed lava ............................................................... 4 3. Section of laccolith intruding rocks exposed in sea cliff south-southeast of Spray Cape_-- - - - ----------------- 6 4. Sketch showing geologic relations in sea cliffs on south and southwest sides of Tower Point - _ _ _ - - - - - _ - - - - - - - - - 8 5. Stereophotograph of 1,200-foot sea cliff, Tower Point _-_---------_-_-_-------_--------------------------- 8 6. Drawing showing upper contact of layered lava pod exposed in small headland near head of cove west of Tower Point-------------------------------------------------------------------------------------------- 7. Sketch showing geologic relations in sea cliffs on southeast and southwest sides of Buttress Point-- -- - - - - - - - - - - 8. Photopanorama of layered lava pod in wave-cut rock bench on east side of Buttress Point- - ----------------- 9. Photograph of layered albitized dacite in active sea cliff on east side of Buttress Point ...................... - 10. Drawing showing tiny pillows in sedimentary matrix near contact 11. Photograph of layered albitized dacite lava pod in sea cliff west of Buttress Point ........................... 12. Photograph showing closeup of layered albitized dacite lava pod shown in figure 11_._ ...................... 13. Drawing showing massive and pillowed lava overlying layered lava in cliff exposure high on Buttress Point----- 14. Drawing showing derby-shaped layered lava pod exposed in three successive small headlands along coast east of Tower Point ...................................................................................... 15. Restored section of semiellipsoidal layered lava pod exposed on three successive small headlands midway between Tower Point and Lion Bight ...................................................................... -- 16. Drawing showing sheet of pillowed lava containing layered lava pods exposed in sea cliff on southwest side of Cape Prominence-____-_---------------------.----------------------------------------------------- 17. Drawing showing sheet of layered deltoid lava pods in pillowed lava in sea cliff on south side of Cape Prominence-. 18. Drawing showing association of layered lava pods with peperite and irregularly shaped pillows in sea cliff at east entrance of Eagle Bay-_----------__--------------------------------------------------------------- 19. Drawing showing pillowed lava in sea cliff exposure 154 miles west of tip of Lance Point- - - - TABLES Page TABLE1. Chemical analyses and petrographic descriptions of altered andesitic pillow lava, Reef Point - - - - - - - - - - - - - - - - - - - B 4 2. Chemical analyses and petrographic descriptions of altered dacitic lava pod layers and associated unaltered pillow lava, Buttress Point-------------------------------------------------------------------------------- v SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY PILLOWED LAVAS, I: INTRUSIVE LAYERED LAVA PODS AND PILLOWED LAVAS, UNALASKA ISLAND, ALASKA By GEORGEL. SNYDERand GEORGED. FRASER ABSTRACT and others, 1961). Most of these structures, which Extensive sills of andesite and dacite intrude a thick se- are well exposed in large sea cliffs from Surveyor Bay quence of argillites of Miocene age in the eastern Aleutians. to Cape Prominence along the southwest coast, were The sills contain large volumes of pillowed lavas that grade examined from the Motorshi~Eider. then of the U.S. into massive lava, layered lava, and breccia. Pillows range in Geological Survey ; other field observations were made diameter from 1 to 40 feet but are generally 5 to 10 feet; by landing parties. The pillowed lavas and lava pods peperite, a mixed intrusive breccia of sedimentary and primary igneous debris, commonly fills pillow interstices and locally are described in detail and their origin is explained. constitutes the entire thickness of a sill. Concentrically lay- This paper has benefited by many conversations with ered lava forms roughly equidimensional pods, as much as 600 and suggestions from U.S. GeologiEal Survey colleagues feet thick, and extensive tubular and tabular sills; these lay- in the Aleutian Islands, Alaska, in Colorado, and, in ered lava pods are larger than any reported previously. Con- Hawaii. We are particularly indebted to Harald centric layers in the lava pods are 1foot or more thick ; alternate layers differ in appearance and composition and generally Drewes and Richard Goldsmith for many critical sug- weather differentially. Most of the rocks are completely gestions, including those about the reorganization of albitized, but some glass is unaltered. parts of this manuscript. Besides Drewes the fol1o~- The sills and all the second-order structures were formed by ing people helped with the original field investigations : magma intruding semiconsolidated muds. A modified "emul- sion" theory best explains these and probably other large tracts V. E. Ames, H. I?. Barnett, Jr., W. B. Bryan, C. E. of pillowed lava comnlon in parts of the world. Tough, elastic Cl~apin,E. H. Meitzner, R. P. Platt, H. B. Smith, and chilled pillow skins maintain the droplike form of magma in L. D. Taylor. Carl Vevelstad and Charles Best fur- contact with water or mud and serve the same function as nished logistic support during the Aleutian fieldwork. fluid surface tension in a normal emulsion. Pillows were formed as unstable apophyses or immiscible droplike masses GEOLOGIC SETTING during turbulent digital advance of magma fronts and engulf- ment of mud and water in magma. The concentric layers of the The rocks of Unalaska Island comprise an older lava pods probably result from laminar intrusive flow in tubes group of sedimentary and shallow intrusive rocks, a and tongues which channeled magma from a central source to group of plutonic rocks intermediate in age, and a the pillow-forming front. Compositional differences in succes- sive layers of the layered lava pods, on the basis of evidence younger group of volcanic rocks. The older igneous from analyzed samples in one lava pod, may be due to different rocks, with which this paper is chiefly concerned, were amounts of included mud or to postsolidification differential emplaced in a submarine environment and were subse- alteration. quently altered, whereas the younger volcanic rocks INTRODUCTION AND ACKNOWLEDGMENTS were extruded subaerially and are fresh. The rocks of Unalaska are typical of those of most other Aleutian Extensive andesite and dacite sills that are abundant Islands, although some islands do not contain plutonic on Unalaska Island (fig. 1) in the Aleutian Islands con- rocks. sist of large units of pillowed lava and layered lava The oldest formation underlies most of the island; pods-larger than previously described in the geologic the texture of the sedimentary rocks and the structure literature-that grade into thick units of layered and of the igneous rocks differ from the northern to the nonlayered igneous rock. The pillowed lavas and lava southern parts of the island. Coarse conglomerate and pods were examined during reconnaissance geologic tuff breccia are abundant in the northern part of the mapping of Unalaska Island in 1953 and 1954 (Drewes island and argillite and tuffaceous argillite are rela- B2 SHORTER CONTRIBUTIONS TO GENERAL