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Figure 4 shows the relationship between the RTC of the rocks and the AE amplitude as well as frequency of the AE for each RTC. In Figure 4, the rate of the temperature increase is shown as a plus, and the rate of the temperature decrease is shown as a minus.

Figure 4: The AE amplitude versus the rate of temperature change (RTC) of the rock samples and its frequency by RTC. A: granite, B: marble, C: sandstone.

In the case of the granite, the maximum AE amplitude was recorded when the RTC = 1.83 °C/min and the frequency of the RTC = 1.5–2.0 °C/min was approximately 20 %. In the granite, a relatively large AE amplitude is generated as the RTC increases. However, in the marble, the maximum AE amplitude occurred when the temperature decreased. The frequency of the RTC < −1.5 °C/min accounts for approximately 45 % of the whole. In the case of the sandstone, the maximum amplitude is generated at RTC = 1.5 °C/min, though the frequency of the AE during the temperature decrease is high.

Thus, the AE signal occurred when the temperature increased above RTC = 1.5 °C/min in the case of the granite and sandstone and when the temperature decreased below RTC = −1.5 °C/min in the case of the marble.

There have been many field observations of the RTC, but during recent years, it has been reported that large temperature changes have instantaneously occurred. McKay et al. (2009) measured the surface temperature of basalt using thermocouple sheets in the Atacama Desert and the cold deserts 184of Antarctica. It was found that the RTC of ≥ 2 °C/min appeared approximately 8 % on average, and the RTC of ≥ 8 °C/min appeared 0.02 % on average. In addition, Molaro & McKay (2010) measured the surface temperature of basalt and sandstone samples using a 0.375-s interval in Death Valley (USA) during April 2009. As a result, the RTC at 2 °C/min or higher accounted for 71.6 % of the basalt and 66.3 % of the sandstone, respectively.

These studies suggest that rocks subjected to rapid temperature changes due to solar radiation may form microcracks and fracture via thermal shock. In this study, it was presumed that microcracks occurred in the rock samples at an RTC above ±1.5 °C/ min. For this reason, it is believed that long-term continuous temperature change due to radiation, which is effective for microcrack generation, leads to stone deterioration.