Abstract
Jeju Island is situated in Korea Strait to the, south of the main Peninsula at the geographical coordinated between longitude $E\;126^{\circ}10'\;to\;126^{\circ}58'$ and latitude $N\;33^{\circ}12'\;to\;33^{\circ}34'$. Also the Island covers an area of approximately $1,792.06km^2$, 31km northsoutherly and 74km. eastwesterly in the short and long axial distances respectively, and the perimeter being about 263km. The main aim of this paper is in describing the geologic evolution and in discussing mainly problems of petrochemistry of volcanic rocks in Jeju Island. Geology of Jeju Island is mainly composed of trachyte, trachy-andesite and basalt with some sedimentary rocks, exactly tuffaceous. The basalt is most. widely distributed, the trachyte and trachy-andesite are in the next order, and the sedimentary rocks are least abundant. These volcanic flows were erupted over more than 79 times. The island has begun to exist from Pliocene to Quaternary in geologic age. There is a couple of old manuscripts which date the volcanic activities of the island. They say there were several eruptions in 1002 and 1007 A.D.. The external conditions of lava eruption were different from place to place. For example some basalts can not be found in the south side of Mt.Hanla but in the north side. Therefore the order of eruption and the geologic structure ,are different locally. The eruption cycles of 5 stages determined by probably from eruption of basalt to trachyte or trachy-andesite. The volcanic rocks erupted during 1st. and 2nd. stage are distributed in the coastal areas, forming lava plateau. It might be the areal eruption. Whereas the lava flows erupted after 2nd stage had come out from the Baegrogdam crater, the present crater lake of Mt. Hanla and its neithboring craters, acted as a site of central eruption. Baegrogdam basalt is the last of all in the order of eruption. After the eruption of Baegrogdam Basalt, the eruptions transformed to parasitic volcanic activities. According to the geological field survey, the eruption conditions of basalts erupted before Pyoseonri Basalt are not known exactly, but Basal Basalt (olivine basalt) is found by boring core samples under the Seogwipo Formation and on the tidal zone neighboring Seogwipo. Judging from the rock phases and petrographic provinces, the volcanic rocks of Jeju. Island are situated in the inner zone of Island arc. But they are equivalent to those found. on the volcanic islands scattered in the Pacific Ocean Basin, so called "Intra-Pacific Pro-vince". The volcanic rocks of the island lie within the Oceanic Type which is characterized. by rich in olivine and alkali composition. But feldspathoid minerals are not found and rich sanidine in the volcanic rocks of the island. As far as the composition of olivine contained in the rocks of the island is concerned, Fe content, is higher compared with the Hawaiian Type in. which the olivine is rich in Mg content. These evidences. suggest. that the lavas of the island derived from a olivine-basaltic magma of alkali rich. phase. It can be suggested from these facts that Jeju Island is seemed to situate at the continental arc which is lied above the Benioff zone near Japanese Island Arc.