A Study On Geologic Features And Effusive Hazards Of Shield-forming Basalts In The Tianchi Volcano

The Tianchi volcano situated on the boundary between China and the Democratic People's Republic of Korea is the most important active volcano in China for its largest magnitude, complicate history and great potential hazards. It has experienced early basalt shield, middle trachyte composite-cone and late ignimbrite-forming stages.The Junjianshan Formation was first introduced for the Neocene thick confined or unconfined basaltic lava flows around the Tianchi Cone. A representative profile for the Junjianshan basalts is located on the north bank of the Tumen River near Chongshan. Some Chinese researchers applied the Junjianshan basalts later to the basalts around the Tianchi cone in their studies of chronology and tectonic setting.The so-called Tianchi shield-forming basalts were formed from the center vent at different times and composed of the surrounding shield-like basalts. To clearly identify the time and space distribution of the Tianchi lava shield and the composite cone, and to exactly understand the volcanic activity regime, a volcanic texture mapping program was carried out on the surrounding Tianchi basalts in recent years .It reveals distinct feature differences in time, lithology and aspect ratio of the lava flows, all of which shows an obvious difference in the magma ascending, stagnation, crystallization and differentiation process. Based on mapping result of the northeastern Tianchi volcano, this revises the original Junjianshan F, and presents a new scheme for Toudao F, Baishan F and Laofangzixiaoshan F.The Tianchi shield basalts flowed a distance of a few to dozens kilometers with a wideness hundreds to thousands meters and a thickness of a few to dozens meters, comprising the composite flow units in confined or unconfined mechanism. The Baishan basalts experienced obviously a deep crystal differentiation from the early Toudao basalts as shown by the enrichment of feldspar phenocrysts in the Baishan basalts. The Laofangzixiaoshan basalt, composed of the uppermost part of the shield, flowed a much less distance than the earlier Toudao and Baishan basalts with a lower aspect ratio. It shows that the basalt shield was formed between 4.26 Ma and 0.31 Ma. A detailed...