This site provides information about the number, thickness, and grainsize of Holocene volcanic ash deposits at 50 localities in the eastern Aleutian volcanic arc. In addition, the major-element compositions of the glasses separated from more than 350 samples of tephra from these localities, determined by electron microprobe, are presented as a basis for correlating samples. Where known with reasonable certainty, the source of an analyzed sample is also identified for use in comparative studies of magma chemistry.
Abstract The Edgecumbe volcanic field (EVF) on Kruzof Island in southeastern Alaska has been the site of explosive eruptive activity within the past 11,000 yr. The EVF is complicated by having multiple vents and a variety of deposits. Early Pleistocene fissure eruptions of basalt flows built a low shield volcano; late Pleistocene eruptions of basaltic andesite, andesite, and low-silica rhyolite from vents aligned along a fissure in the southern part of EVF initiated construction of Mt. Edgecumbe cone (ME) and the domes of Crater Ridge (CR). A series of explosive eruptions occurred over a period of at least 2,000 yr, ending about 9,200 yr ago. The eruptions (1) produced hot pyroclastic flows that reached the sea on both the west and east coasts of Kruzof Island, (2) bored a crater in the CR domes by ejecting blocks of rock that were as large as 10 cm in diameter 5 km from the vent, and (3) produced a series of ashfalls that eventually totaled 1.5 m in thickness at Sitka, 20 km to the east, and which have been identified 200 km to the north near Juneau and Lituya Bay. Two small eruptions of CR produced airborne ash between about 4,000 and 6,000 yr ago. There are presently no visible signs of pending eruptive activity at the ground surface; however, periods of quiescence lasting 10,000 yr or more are not unusual for volcanoes, and so EVF cannot be considered “dead.” The geologic record indicates a trend toward explosive activity at EVF in the past, implying that any future eruptions are likely to be explosive. Moreover, a wide variety of eruptive phenomena are probable in the event of future activity. In decreasing order of likelihood, the hazards associated with such phenomena are as follows: (1) ash plumes can affect regional air traffic and ash fallout can affect residents of Sitka; larger and less frequent ash-forming eruptions would affect inhabitants further from the volcano and interstate air traffic; (2) although Kruzof Island is now uninhabited, any persons within EVF during an actual eruption could face extreme hazards such as falling blocks, asphyxiation, incineration by pyroclastic flows, and burns or respiratory irritation resulting from airfall; (3) if eruptive activity includes dome emplacement at ME or production of rhyolitic pyroclasts at CR, then the possibility of producing a volcanogenic tsunami (a sea wave caused by rapid mass movement of material into the sea) would increase. Despite the absence of eruptive activity more recent than 4,000 yr ago, EVF should be monitored for potential early signs of an eruption because of its proximity to populated areas. One possible method of monitoring would involve installation of a seismometer on southern Kruzof Island.
One of the most spectacular physiographic images of the conterminous United States, and the first to have been produced digitally, is that by Thelin and Pike (USGS I-2206, 1991). The image is remarkable for its crispness of detail and for the natural appearance of the artificial land surface. Our goal has been to produce a shaded-relief image of Alaska that has the same look and feel as the Thelin and Pike image. The Alaskan image could have been produced at the same scale as its lower 48 counterpart (1:3,500,000). But by insetting the Aleutian Islands into the Gulf of Alaska, we were able to print the Alaska map at a larger scale (1:2,500,000) and about the same physical size as the Thelin and Pike image. Benefits of the 1:2,500,000 scale are (1) greater resolution of topographic features and (2) ease of reference to the U.S. Geological Survey (USGS) (1987) Alaska Map E and the statewide geologic map (Beikman, 1980), which are both 1:2,500,000 scale.Manually drawn, shaded-relief images of Alaska's land surface have long been available (for example, Department of the Interior, 1909; Raisz, 1948). The topography depicted on these early maps is mainly schematic. Maps showing topographic contours were first available for the entire State in 1953 (USGS, 1:250,000) (J.H. Wittmann, USGS, written commun., 1996). The Alaska Map E was initially released in 1954 in both planimetric (revised in 1973 and 1987) and shaded-relief versions (revised in 1973, 1987, and 1996); topography depicted on the shaded-relief version is based on the 1:250,000-scale USGS topographic maps. Alaska Map E was later modified to include hypsometric tinting by Raven Maps and Images (1989, revised 1993) as copyrighted versions. Other shaded-relief images were produced for The National Geographic Magazine (LaGorce, 1956; 1:3,000,000) or drawn by Harrison (1970; 1:7,500,000) for The National Atlas of the United States. Recently, the State of Alaska digitally produced a shaded-relief image of Alaska at 1:2,500,000 scale (Alaska Department of Natural Resources, 1994), using the 1,000-m digital elevation data set referred to below.An important difference between our image and these previous ones is the method of reproduction: like the Thelin and Pike (1991) image, our image is a composite of halftone images that yields sharp resolution and preserves contrast. Indeed, the first impression of many viewers is that the Alaskan image and the Thelin and Pike image are composites of satellite-generated photographs rather than an artificial rendering of a digital elevation model.A shaded-relief image represents landforms in a natural fashion; that is, a viewer perceives the image as a rendering of reality. Thus a shaded-relief image is intrinsically appealing, especially in areas of spectacular relief. In addition, even subtle physiographic features that reflect geologic structures or the type of bedrock are visible. To our knowledge, some of these Alaskan features have not been depicted before and so the image should provide earth scientists with a new "look" at fundamental geologic features of Alaska.
Geologic description:MacKevett and Holloway (1977, locality 9) describe mined areas extending three to six miles downstream from the landing strip located near the head of the Lewis River on the west side of Mt.Susitna.Stream gravel carry coarse angular gold with attached quartz.Cobb (1979, OFR 80-86, p. 17-18) reports that gold is probably derived from quartz veins in local Jurassic or Cretaceous granitic bedrock or from reworked Tertiary conglomerate.Drilling for possible dredging was done in the early 1900's.Placer mining was active mostly before 1918.Bedrock is 12 to 22 feet below surface.According to Brooks (1918, p. 45-47), seven small placer operations operated in 1916 in the canyon between upper basin and the flats and produced over $2,000 in gold (worth $18.66 an ounce).One $2 nugget was found.An unknown but probably small amount of gold recovered in 1956 -57 (Cobb, 1973 , B 1374, p. 17, p. 17).
