thumb|right|The Pæla lava field in 2009 with a lava river from the 1947 eruption The explosive eruption increased in strength from 9–12 April and then from 28 April it reduced again. On 3 May, the volcano stopped throwing out lava in sudden explosions from its craters and changed to continuously ejecting tephra and ash for long periods, until early June when this reduced. On 2 September, the Shoulder Crater had a circumference at its top and the Summit Crater a circumference at its highest point, above the ridge. Sandy tephra and ash fell over Iceland in May and June, sometimes making it dark in the daytime near Hekla. The tephra caused fluorine poisoning of grazing sheep, making them unable to walk. That winter more craters formed, building up cones. Explosive activity had ceased six months after the first eruption. Lava flowed from the Lava Crater continuously during the eruption, starting at a rate of over 100 m3·s−1, dropping to 5–10 m3·s−1 in April and early May at a speed of around 20 cm·s−1 before increasing, eventually reaching 150 m3·s−1 at the end of June and at similar levels until mid-July with a peak flow speed of 2–2.5 m·s−1. From there it gradually decreased to under 10 m3·s−1 in November. Initially the lava comprised 57–58% SiO2 and 11% Fe2O3, from the time of peak flow onwards this changed to 54% SiO2 and 13.5% Fe2O3.
thumb|right|The Pæla lava field in 2009 with a lava river from the 1947 eruption The explosive eruption increased in strength from 9–12 April and then from 28 April it reduced again. On 3 May, the volcano stopped throwing out lava in sudden explosions from its craters and changed to continuously ejecting tephra and ash for long periods, until early June when this reduced. On 2 September, the Shoulder Crater had a circumference at its top and the Summit Crater a circumference at its highest point, above the ridge. Sandy tephra and ash fell over Iceland in May and June, sometimes making it dark in the daytime near Hekla. The tephra caused fluorine poisoning of grazing sheep, making them unable to walk. That winter more craters formed, building up cones. Explosive activity had ceased six months after the first eruption. Lava flowed from the Lava Crater continuously during the eruption, starting at a rate of over 100 m3·s−1, dropping to 5–10 m3·s−1 in April and early May at a speed of around 20 cm·s−1 before increasing, eventually reaching 150 m3·s−1 at the end of June and at similar levels until mid-July with a peak flow speed of 2–2.5 m·s−1. From there it gradually decreased to under 10 m3·s−1 in November. Initially the lava comprised 57–58% SiO2 and 11% Fe2O3, from the time of peak flow onwards this changed to 54% SiO2 and 13.5% Fe2O3.