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Little Sitkin Island () is a volcanic island in the Rat Islands archipelago of the Western Aleutian Islands, Alaska, about three miles east of Davidof Island. Great Sitkin Island (which is essentially the same size as Little Sitkin) lies 185 mi (295 km) further east in the Andreanof Islands. Little Sitkin is roughly circular and about 6 miles in diameter. The interior is extremely rugged and mountainous; only the lower slopes are grass covered. There are two prominent peaks, one 3,898 feet high in the northeast part of the island and the other 1,960 feet high in the southern part. Numerous streams are on the island but no lakes or ponds. The coast is generally bold, rocky, and precipitous, with a fringe of kelp 200 to 400 yards wide. Small steam jets and hot springs are in the valley at the head of William Cove on the northwest of the island. ==The volcano== Little Sitkin is a stratovolcano that erupted probably in 1776, 1828, and 1900.〔(Alaska Volcano Observatory - Little Sitkin )〕 The 1776 eruption was probably explosive. The volcano has had three cone building episodes separated by the formation of two calderas. A summit crater caps the youngest cone, about 3 miles in diameter at the base. The active stratovolcano on Little Sitkin Island occurs within the eroded remnants of a nested double caldera of probable late Pleistocene age. The older caldera (Caldera One) is about 4.8 km in diameter and is centered slightly northeast of the island's midpoint. The caldera formed at the site of a large stratovolcano, the remnants of which are the oldest rocks exposed on the island. A second stratovolcano was constructed almost entirely of lava flows within Caldera One and attained a height of about 900 m. A cataclysmic eruption, possibly in early post-glacial time, resulted in the formation of a second, smaller caldera (Caldera Two) that partially destroyed this cone. Caldera Two is elliptical in outline and measures 2.7 by 4 km; the inferred eastern and southern margins are coincident with those of Caldera One. Field relations suggest that the northern boundary of Caldera Two is a hinge along which a large block, comprising most of the Caldera One stratovolcano, was tilted southward during the caldera eruption. The highest peak on the island is on the post-caldera remnant of the second cone. A deposit of partly welded tuff up to 100 m thick extends from the remnant cone northwest across the Caldera Two boundary fault, to slightly beyond the inferred location of the Caldera One boundary fault. The deposit is thought to have been emplaced by one or more pyroclastic flows, possibly associated with formation of Caldera Two. Post-Caldera Two deposits are mainly lava flows. Two relatively recent aa flows have well developed levees; one originated from the breached central crater of Little Sitkin volcano, and the other from a fissure along the western trace of the Caldera One boundary fault.〔http://www.avo.alaska.edu/pdfs/cavwaleu_west.pdf Catalog of the Historically Active Volcanoes of Alaska, USGS, T.P. Miller et al, 1998〕 On June 23, 2014, a magnitude 7.9 earthquake struck near the island. Citizens were ordered to evacuate. Tsunami warnings were soon issued.〔("Big earthquake triggers tsunami warnings in Western Alaska" ), Anchorage Daily News, Laurel Andrews, June 23, 2014. Retrieved 23 June 2014.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Little Sitkin Island」の詳細全文を読む スポンサード リンク
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