Spheroidal weathering of columnar-jointed basalt of the Ogura lava in the Takurayama Volcano, western Japan

Yasuto Hirata,Masahiro Chigira,O Chudaev,R Millot,R Harmon,Y Kharaka,O Shouakar-Stash

doi:10.1051/e3sconf/20199801021

Yasuto Hirata, Masahiro Chigira + Show 5 more

Open Access

https://doi.org/10.1051/e3sconf/20199801021

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Abstract

Spheroidal weathering is an important form of rock weathering and has been attributed to chemical weathering from joint surfaces. However, the effects of the original rock structures on the processes of spheroidal weathering have not been well addressed. A columnar-jointed basalt contains primary structures that resulted from cooling, which exerted strong controls on the development of weathering with hard crusts. The primary structures were small-pore-size layers beneath the surfaces of columnar joints and some cross joints. Weathering of plagioclase was less intense in the small-pore-size layers, resulting in the exfoliation of surface parts of the layers and internal cracks.

Highlights

Spheroidal weathering shows a simple concentric succession of weathering that propagated from surrounding joints into the parent rock, with layers of weathered rock exfoliated from ball-like fresh parts
Columnar joints indicate a rapidly cooled igneous rock, and spheroidal weathering has been recognized with columnar joints in many basaltic lavas [4], as well as in a granite porphyry [3]
The mercury intrusion tests showed that the porosity is 5.38% in the light gray zone, 5.39% in the surface dark layer, and 8.84% in the white zone

Summary

Introduction

Spheroidal weathering shows a simple concentric succession of weathering that propagated from surrounding joints into the parent rock, with layers of weathered rock exfoliated from ball-like fresh parts. It has long been recognized as a dominant weathering process in hard rocks, including most igneous rocks [1,2,3]. Most previous studies of spheroidal weathering have generally supposed the parent rock to be homogeneous [1] and exfoliation of rock to be formed during rock expansion due to chemical weathering [2, 3] but some studies have inferred that original structure due to the cooling process exerts an influence in determining the style of weathering [4]. We observed joints and weathering features in the field, collected rock samples, and analyzed their pore sizes, porosity, mineralogy, and chemistry

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