In the southern Spring of 1995 (mid-October) the active volcano Mt Ruapehu in the central North Island of New Zealand erupted explosively, spreading up to 40 million m3 of rhyolite tephra over thousands of km2 of farmland during the lambing season. This ash contained a high concentration of soluble fluoride, and more than 2000 lactating ewes died of acute fluoride poisoning. To investigate the effects of this brief but acute dose on the teeth of grazing animals we examined the distributions of fluorine and calcium in the permanent incisor teeth of sheep which were one year old at the time. Where part of an incisor had been in the first (secretory) stage of calcification the erupted tooth disclosed surface pitting, a thin layer of enriched mineral across the enamel with as much as 1000 ppm F w/w, and a separate layer with 4000 ppm down the dentine. The part of an incisor which had attained the later (maturation) stage showed enriched layers only in the outer enamel and in the dentine. This study has demonstrated some important features of the calcification process, and the risk of fluoride toxicity to grazing animals.
Excerpts from article:
2. Effects of fluoride from volcanic eruptions
Fridriksson  summarised historical records from Iceland which describe serious illness in grazing animals following volcanic eruptions, usually of the notorious Mt Hekla. In the earliest account from 1693 a farmer and a clergyman described deformed teeth in sheep, cattle and horses, which they called “ash-teeth”. A destructive eruption occurred in 1783 when a 30 km long fissure opened in southern Iceland. The effects of this outbreak kiIIed about half the sheep in the county and caused a serious famine. Following a Mt Hekla eruption in 1845 a botanist and a farmer described the lesions in teeth and changes in bones of diseased animals. Bones collected at the time were analysed 90 years later and found to contain up to 20000 ppm fluoride .
The 1947 eruption of Mt Hekla was the first for which the fluoride content of the ash was measured. Some sheep died, paralysis was observed in nursing ewes, and lambs born the same Spring disclosed lesions in their permanent incisors about six months later. Georgsson and Petursson  describe the effects on vegetation, ground water and sheep of the Hekla eruption in May 1970, in which the explosive phase lasted only two hours. Analysis of the ash samples in the first few days showed up to 2000 ppm of water-soluble fluoride, which decreased to 200 ppm within two weeks. Grass analysed two days after the eruption contained 4300 ppm F, though this fell quickly over the next few weeks. During 1970 and 1971 they examined permanent incisors as they erupted and concluded that developing incisors were sensitive to high fluoride levels if the exposure occurred from 6 to 12 months prior to eruption of the tooth.
… 3. Fluoride in volcanic ash
The relationship between volcanic ash (tephra) and fluoride was made clear by Oskarsson  in a study of the 1970 eruption of Mt Hekla. During the short (two hours) explosive stage a column of tephra rose to a height of I6 km, and was deposited in a narrow band 300 km long in the direction of the prevailing wind. Since smaller particles fell more slowly the deposit became progressively finer with distance from the volcano. From leaching experiments he concluded that the fluorine is adsorbed in highly soluble form on the surface of the tephra particles; the amount leached per unit mass of tephra increased as the particle size decreased, explaining why the fraction of soluble F increased to IO0 km from Hekla then stayed constant to 300 km.
Analysis of the leachate, as well as laboratory experiments on washed samples of tephra, showed that at temperatures above 700°C the adsorbed compound was calcium fluoride, but at lower temperatures was catcium fhtorosilicate. The reactions with the acid magmatic gases SO?, HCI and HF occurred in the cooling eruption column, in which conditions vary rapidly. Since the largest particles cool more slowly, condensation takes place preferentially on the smallest particles. Tephra is rapidly leached by surface waters so that streams may become contaminated with F for a short time and stagnant water for much longer.
… Publications show that the danger from soluble fluoride in tephra is well recognised in Iceland, but this is apparently not so in New Zealand: a report on Volcanic Hazard Awareness Week (held in October 1995)  does not mention fluoride, and farmers were apparently unaware of the danger until their ewes began to die in hundreds. In contrast, the potential dangers to air and road traffic, and economic effects on towns which cater to skiers are common knowledge. In areas close to a volcano, farmers should be warned that prompt action may sometimes be needed, especially if an eruption occurs in the Spring. In an eruption which lasted for more than a few days water and milk should be monitored…
References cited in excerpt:
. S. Fridriksson. In: J.L. Shupe, H.B. Peterson and N.C. Leone, Editors, Fluorides — Effects on Vegetation, Animals and Humans, Paragon Press (1983), pp. 339–344.
. K. Roholm. In: (2nd Ed.),Fluorine Intoxication: A Clinical Hygienic Study, Nyt Nordisk Forlag, Copenhagen (1937) and H.K. Lewis, London .
. G. Georgsson and G. Petursson. Fluoride 5 2 (1972), p. 58.
. N. Oskarsson. J. Volcanol. Geotherm. Res. 8 (1980), p. 251.
. D.M. Johnston and D.E. Riley, Ruapehu ’95 Volcanic Hazard Awareness Week (comps) . Institute of Geological and Nuclear Sciences Information Ser. 41 (1996).