Figures 2(a)-(d) show photoemission images during the collision of the rock samples. These images show that the unique appearance and intensity of photoemission depend on each rock. Granite showed an intense whitish spark streak during impact [Fig. 2(a)]. Pyroclastic rock produced a glowing gaseous red lightning around the hotspot [Fig. 2(b)]. Biotite-bearing rhyolite showed an orange-colored lightning streak [Fig. 2(c)]. Limestone showed a weak red color on the newly created fracture surface [Fig. 2(d)]. Serpentinite showed no photoemission, and thus it is not shown in the figure. The red, green, and blue (RGB) intensities on a scale of 0 to 255 for photoemissions from the brightest part at impact for each rock and from a position off the hotspot for granite and rhyolite are also shown in these figures. The order of the photoemission intensity was granite > biotite rhyolite > pyroclastic rock > limestone, with no emission from serpentinite. The difference in the shape, color and RGB ratio of the photoemission as seen in Fig. 2 suggests that the physicochemical mechanism differs from rock to rock. Next, the mechanisms of photoemission from each rock were investigated, and are discussed in detail.
3.1 Granite
Photoemission from granite was the brightest among the tested rocks, and was the only one that could be captured upon impact using a spectroradiometer, as shown in Fig. 3(a). The spectral intensity increased continuously as the wavelength became longer, which is characteristic of black-body radiation: the peak of the spectrum should be in the far-infrared region, but the temperature can be inferred from the intensity ratio for two wavelengths, l(740 nm)/l(620 nm) = 3.6, assuming that the tail can be expressed by Planck’s law [Fig. 3(b)]. We used the results in Fig. 3(c) to estimate the frictional temperature during collision to be 1,750K.
The TDS-MS analysis showed that any flammable gases are less contained in the granite; therefore, photoemission due to degassing is not possible. At such a high temperature, minute fragments of non-stoichiometric oxides of rock constituents would be ignited when the fragments underwent oxidation reaction and show a continuous radiation spectrum.
In contrast, many spectral lines appeared in the short wavelength range around 380-420 nm, which might be attributed to photoemission from vibrationally or electrically excited states of the constituent elements of the granite; that is, when the temperature rises to 250-400°C, lone-pair electrons captured by lattice defects in the granite are thermally released. In a previous experimental study (Enomoto et al., 1993), measurements of thermally stimulated exoelectrons (TSEEs) emitted from constituent minerals, including quartz, feldspar, and biotite in granite (Inada, Ibaraki), indicated notable emissions from biotite. The energy of TSEEs should be as low as about 1 eV, but when the rock breaks, an intense electric field is likely created in the small gap between both cracked new faces due to charge separation. Also, a high electric potential might locally be generated by the piezoelectric effect of quartz. These electric fields can accelerate exoelectrons to gain enough energy to ionize atmospheric molecules and other species in the rock minerals, resulting in the generation of a plasma streak (Brady and Rowell, 1986). That is, according to Paschen’s law, which gives the voltage necessary to form a discharge between two electrodes in a gas as a function of pressure p and gap length d, the minimal breakdown voltage V for pd = 7.5 × 10-6 m•atm, where V is 327 V in air at standard atmosphere pressure (p=1 atm) at a distance of 7.5 mm. This condition could be achieved by the present Charpy impact experiment.
Japan has many mountains composed of granite, such as Mt. Kai-Koma, Mt. Yakushi, and Mt. Tsukuba in [see Fig. 1(a)]. However, our survey showed that there are few Japanese historical earthquake records on EQL due to granite landslides on such mountains (e.g., ERI, 1981–1994; Musya, 1932).
3.2 Pyroclastic rock
The pyroclastic rock was subjected to the TDS-MA analysis. The typical mass spectrum is shown in Fig. 4(a) for pyroclastic rock, showing H2O, C2H4, H2S, and CO2 peaks, which are components of typical volcanic gases. It is noted that the amount of C2H4 outgassing increases about 20 times as the temperature rises from 250 to 450°C, which indicates that it is promoted by heating. Both C2H4 and H2S are flammable gases, but the ignition temperature for C2H4 is 632°C, which is higher than that for H2S, 260°C. However, these temperatures could easily be reached by frictional heating during impact. Therefore, H2S first burns due to frictional heating and heats methane to release carbon and become hot: the carbon produces a red flame, as confirmed in the laboratory test, and shown in Fig. 1(b).
3.3 Biotite-bearing rhyolite
Rhyolite has the second-highest photoemission intensity after granite [see Figs. 1(c) and (e)]. The B value is slightly smaller than the R and G values [Fig. 1(e)]. The TDS-MA spectrum of this rock is shown in Fig. 4(b), showing that even when the heating temperature rises from 250 to 500°C, the peak intensity for C2H4 remains almost unchanged, unlike that for pyroclastic rock. This may indicate that C2H4 was strongly chemisorbed by the rock minerals. Furthermore, the peaks appear periodic with m/z differences of 14 (CH2), so this rock may contain a small amount of mixed hydrocarbons. However, the amount of photoemission from the rhyolite due to the effect of degassing and burning of these hydrocarbons might be small, unlike that for pyroclastic rock. Rather, because the rhyolite contains biotite and quartz with high TSEE activity, the cause of the photoemission is likely to be similar to that of granite, as described above.
3.4 Limestone
Limestone is a sedimentary rock composed of calcium carbonate (CaCO3) formed mainly from sea shells. Limestone does not contain quartz, biotite, or flammable gases that can cause photoemission. But it is well known that natural radiation for extended geological ages creates electronically excited (trapped) states in limestone. Heating allows these trapped states to interact with thermally activated lattice vibrations and rapidly decay into lower-energy states, causing the emission of photons, that is, TL (Kalita and Chithambo, 2019).
The fracture surface, newly created during impact tests, shows red emission due to TL, as seen in Fig. 1(c). The red light associated with the collapse of limestone in the 1909 Anegawa earthquake may be the reason why the villagers assumed that the volcano had exploded. To confirm this hypothesis, we conducted TL experiments on the limestone sample.
A thin limestone plate sliced from the as-received sample was heated on a hot plate at a rate of 5°C/min in air in a dark room, and the color and the spectra of the light emitted from the sample were observed by photography and spectroradiometry, respectively. At a temperature of 212°C, an orange spectrum around 613 nm was seen, as shown in Fig. 5(a), but the red emission at 780 nm became more intense at around 260°C. The activation energies for photons released from the trapped sites at wavelengths of 613 nm and 780 nm were determined from the Arrhenius plot to be 0.42 eV and 0.27 eV, respectively, as shown in Fig. 5(b). A temperature of 260°C could be easily reached by frictional heating in the present Charpy tests.
3.4 Serpentinite
No photoemission from serpentinite during impact could be confirmed with the present camera sensitivity, although we were able to measure emissions of negatively charged particles (fracto-emissions) accompanying fracture in previous experiments during an uniaxial compression fracture test (Enomoto et al., 1994). It is known that normally the shear-fractured surface of serpentinite contains clay minerals such as montmorillonite and smectite, and thus the rock is often smooth, such that a fault may have a mirror-like luster capable of specular reflection. In the present Charpy test, when the serpentinite was subjected to impact, the impacted surface seemed as smooth as a cleaved surface. Therefore, physicochemical mechanisms might not occur to produce photoemission by thermal stimulation.
There are mountains in Japan formed by serpentine, such as Mt. Tanigawa and Mt. Shibutsu [see Fig. 1(a)], that are apt to collapse, but there is no report of landslide EQL for these mountains in historical records.