THE EMPLACEMENT OF INTERMEDIATE VOLUME IGNIMBRITES - A CASE-STUDY FROM ROCCAMONFINA VOLCANO, SOUTHERN ITALY

被引：44

作者：

COLE, PD

GUEST, JE

DUNCAN, AM

机构：

[1] School of Geological and Environmental Sciences, University of Luton, LUTON, LU1 3JU, Park Square

[2] University of London Observatory, University College London, London, NW7 2QS, Mill Hill Park

来源：

BULLETIN OF VOLCANOLOGY | 1993年 / 55卷 / 07期

关键词：

ITALY; ROCCAMONFINA; IGNIMBRITE EMPLACEMENT; CO-IGNIMBRITE LITHIC BRECCIAS; GROUND SURGES; INTERNAL SHEAR; OVERRIDING FLOWS;

D O I：

10.1007/BF00304590

中图分类号：

P [天文学、地球科学];

学科分类号：

07 ;

摘要：

A model is presented for the emplacement of intermediate volume ignimbrites based on a study of two approximately 6 km3 volume ignimbrites on Roccamonfina Volcano, Italy. The model considers that the flows were slow moving, and quickly deflated from turbulent to non-turbulent conditions. Yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows. In proximal locations, on the caldera rim, heterogeneous exposures including discontinuous lithic breccias, stratified and cross-stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows. In medial locations thick, massive ignimbrite occurs associated with three types of co-ignimbrite lithic breccia which we interpret as being emplaced by non-turbulent flows. Multiple grading of different breccia/lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower-moving front part. This overriding of different parts of non-turbulent pyroclastic flows could be caused by at least two different mechanisms: (1) changes in flow regime, such as hydraulic jumps that may occur at breaks in slope; and (2) periods of increased discharge rate, possibly associated with caldera collapse, producing fresh pulses of lithic-rich material that sheared onto the slower-moving part of the flow in front. We propose that ground surge deposits enriched in pumice compared with their associated ignimbrite probably formed by a flow separation mechanism from the top and front of the pyroclastic flow. These turbulent clouds moved ahead of the non-turbulent lower part of the flow to form stratified pumice-rich deposits. In distal regions well-developed coarse, often clast-supported, pumice concentration zones and coarse intra-flow-unit lithic concentrations occur within the massive ignimbrite. We suggest that the flows were non-turbulent, possessed a relatively high yield strength and may have moved by plug flow prior to emplacement.

引用

页码：467 / 480

页数：14

共 31 条

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