HR: 1330h
AN: V32B-1011    [PDF]
TI: Geological Characteristics of the 2003 Eruption at Anatahan, Northern Mariana Islands
AU: * Nakada, S
EM: nakada@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Matsushima, T
AF: Inst. Seism. Volcanol., Kyushu Univ., Shimabara, Nagasaki, 855-0843 Japan
AU: Kato, T
AF: ERI, Univ. of Tokyo, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Camacho, J T
AF: Emerg. Manag. Office, CNMI, Caller Box 100007, Saipan, MP96950 Northern Mariana Islands
AU: Chong, R
AF: Emerg. Manag. Office, CNMI, Caller Box 100007, Saipan, MP96950 Northern Mariana Islands
AB: During GPS campaign of the Northern Marianas, we carried out geological inspection about the May-June eruptions at Anatahan, landing on 8 points within the island in mid-July. The active crater, located at the bottom of the eastern caldera, has the dimension of about 300 m across and 100 m deep, and wreck of a lava dome, whose appearance was witnessed first in early June, was observed in the northern periphery. The highest temperature measured within the crater was as high as about 300 C, the crater bottom was filled with mud poured into from the surrounding steep walls. Tephra of the eruptions is thicker in the western part: up to about 1 m thick in the western caldera. Small rills and gullies grow extensively over the deposits. Total volume of eruption products can be roughly estimated at a few tens of million m3. Deposits are composed basically of lower brown and upper gray tephra layers. Both consisting of pyroclastic fall and surge deposits. Brown tephra layer contains various colored vesicular pumice, and larger and denser pumice is accumulated in the uppermost part. Gray tephra is very fine ashes, even on crater rim, deposited by falling including a lateral movement from the crater side. Though some of them were burnt in surge deposits near the crater, vegetation had stood during deposition of brown tephra and was toppled down by deposition of gray tephra. A block probably of broken lava dome is covered only by gray tephra. All pumices, block of lava dome, and masses of brown and gray tephra are andesitic in composition (60-61 % SiO2). Following the eruptive activity of brown tephra since mid-May, another explosive event of phreatomagmatic eruptions occurred probably in mid June, the latter was just preceded by breaking of newly-formed lava dome. As the active crater probably was deepened with time during eruptions, magma reacted with water invaded from aquifer or sea, resulting in the explosive eruptions. Only finer particles of quenched and fragmented magma spilled over rims of deepened crater.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting