The eruption lasted about four minutes and generated pyroclastic flows, fast-moving avalanches of hot gas, ash and rock fragments from the volcano's summit, said Mukdas Sofian, a volcano observer at Indonesia's Centre for Volcanology and Geological Hazard Mitigation (PVMBG).
The miner, Very Irawan, 33, suffered burns to about 80 per cent of his body and was admitted to hospital for intensive treatment, local media said.
Irawan and several colleagues were collecting volcanic sand deposited by previous eruptions when a mass of unstable volcanic material suddenly collapsed into a river channel, his brother Aris Susanto was quoted as saying by Detik.com.
Indah Amperawati, the head of the Lumajang district, said authorities had repeatedly warned residents not to carry out mining activities in hazardous areas around Besuk Kobokan.
"I have already appealed for people not to conduct mining activities in the southeastern Besuk Kobokan sector within a 13km radius," she was quoted as saying.
Sand mining has become an important source of income for many communities living around Mount Semeru.
The volcano's eruptions regularly deposit large quantities of sand and stone that are highly valued by Indonesia's construction industry.
Mount Semeru, standing at 3676 metres, is the highest mountain on Java, Indonesia's most populous island, and among the country's most active volcanoes.
Indonesia sits on the Pacific "Ring of Fire," a seismically active zone where several tectonic plates meet, making eruptions and earthquakes a frequent occurrence across the archipelago.
