关键词: 天池火山, 火山玻璃, 长石, 风化层

Abstract: The micro features of the pumice and ash produced by the eruptions at 1000a BP and 5000a BP at the Tianchi volcano in the Changbaishan Mountains are studied in the paper. TEM and SEM-EDX analyses reveal the surface characters and thickness of the weathered layers of the volcanic glass and crystal feldspar in the pumice. The result shows that the average thickness of the weathered layers on the surface of the volcanic glass for the first eruption (1000a BP) is 0.98μm. The feldspar has fresh surface and sharp edge angles, some of which present approximately parallel fissures and circle vesicles. For the second eruption (5000a BP) the average thickness of the weathered layers on the surface of the volcanic glass is 3.74μm, and the feldspar has obtuse edge angles and morel basins with 1μm depth. These features indicate that the thickness of the weathered layers on the surface of the volcanic glass and the micro structure of the feldspar from the Tianchi volcano are related with the ages of volcanic eruptions. Therefore, they can be used as indicators for determination of a sequence of volcanic eruptions. The EDX analysis also shows that the weathered layers on the surface of the volcanic glass contain increased Al and reduced Si in chemical composition with respect to the volcanic glass itself. It is inferred that such a change should have occurred in a low-temperature setting and a nearly neutral to weak acid solution. Hence it is possible to identify the dates and sequences of volcanic eruptions as well as their environments based on a systematic study of micro characteristics of eruptive materials in the Tianchi caldera.

Key words: Tianchi volcano, volcanic glass, feldspar, weathered layers