Mechanical erosion of xenoliths by magmatic shear flow

Abstract

We focus on the role of mechanical erosion by magmatic shear flow in the formation of xenoliths occurring in lava flows. The process is analyzed by combining the physics of fragmentation and erosion to the concept of rock mass. The conditions for the country rock fragmentation are analyzed as a function of the magma viscosity, strain rate and tensile strength of the rock mass. In reservoirs, mechanical processes play a subordinate role and thermal erosion processes prevail. In conduits, intermediate and silicic magmas may erode and, eventually, fragment good to poor quality country rock masses. Basalts may erode poor quality country rocks. A crystal-rich magma has more chance to break up the conduit walls with respect to a vesiculated melt. The variety of xenoliths of a lava reflects a set of wall-rocks with similar mechanical properties and may not mirror the stratigraphy of the substratum of a volcanic area.