Wildfires as a Weathering Agent of Carbonate Rocks

Nurit Shtober-Zisu,Lea Wittenberg

doi:10.3390/min11101091

Abstract

While most of the scientific effort regarding wildfires has predominantly focused on fire effects on vegetation and soils, the role of fire as an essential weathering agent has been largely overlooked. This study aims to evaluate rock decay processes during wildfires, in relation to ground temperatures and rock morphologies of limestone, dolomite, and chalk. In 2010, a major forest fire in Israel caused massive destruction of the exposed rocks and accelerated rock weathering over the burned slopes. While a detailed description of the bedrock exfoliation phenomenon was previously reported, here, we conducted an experimental open fire to determine the temperature and gradients responsible for boulder shattering. The results show ground temperatures of 700 °C after 5 min from ignition, while the peak temperature (880 °C) was reached after 9 min. Temperature gradients show a rapid increase during the first 5 min (136 °C/min), moderate increase during the next 4 min (43 °C/min), and slow decrease for the next 9 min (25 °C/min). After 12 min, all boulders of all formations were cracked or completely shattered. The behaviour of carbonate rocks upon heating was studied to identify the erosive effects of fire, namely the formation of new cracks and matrix deterioration.

Highlights

Fire is a significant driver of rock decay among physical weathering agents that has often been overlooked, in fire-prone environments
This study aims to evaluate the rock decay processes during wildfires in relation to ground temperatures and rock morphologies, namely over partially buried rock surfaces or over totally exposed boulders
Field observations revealed massive destruction of the carbonate rocks, especially in areas hit by high temperatures, where fire severity was “high” or “very high”

Summary

Introduction

Fire is a significant driver of rock decay among physical weathering agents that has often been overlooked, in fire-prone environments. The physical breakup and removal of rocks through mechanical weathering following a fire is a primary process that denudes and shapes mountainous regions. Fieldworks described the effects of fire on rock decay [2,3], followed by a series of laboratory simulations on rock burnings [4,5]. The effect of heat on the mechanical properties of rocks poses great significance to weathering, mainly via the following processes: (a) high thermal gradient: under the influence of high temperatures, micro-cracks form in the transition between the hot and cold material, which leads to a gradual weakening of rock strength. Rocks composed of several minerals experience inner stress resulting from differential responses to heating and cooling cycles [13]; (c) vaporization of pore fluids: rocks may contain trapped water in their pores

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