A tower of choking ash spewed from the eruption of a volcano on White Island in New Zealand Monday, killing five people and leaving more missing.

The island some 30 miles off mainland New Zealand is uninhabited, but its moon-like landscape is highly visited by tourists, and Michael Schade and his family were among the visitors today. Schade was on a boat near the island when the blast occurred and he shared a stream of videos and photos on Twitter.

“My god,” wrote Schade, whose Twitter account states that he's from San Francisco. “My family and I had gotten off it 20 minutes before, were waiting at our boat about to leave when we saw it. Boat ride home tending to people our boat rescued was indescribable.”

His videos show a wall of ash and steam that scientists say reached 12,000 feet into the air. "This is so hard to believe," he writes. "Our whole tour group were literally standing at the edge of the main crater not 30 minutes before."

Photos show a helicopter badly damaged and covered in ash and tour operators rescuing people on the island. "Endless gratitude to that crew for stepping up as first responders," he wrote on Twitter.

Schade shared that his mom tended to a woman who was in critical condition, but "seemed strong by the end."

The terrifying disaster immediately raised questions of why people were allowed to visit the island after scientists had noted an uptick in volcanic activity in recent weeks.

The GeoNet agency, which monitors volcanoes and earthquakes in New Zealand, had raised the alert level on White Island on Nov. 18 from 1 to 2, on a scale where 5 represents a major eruption, noting an increase in sulfur dioxide gas, which originates from magma deep in the volcano. It also said at the time that over the previous weeks volcanic tremors had increased from weak to moderate strength.

Police Deputy Commissioner John Tims said the number of missing was in the double digits, but he couldn't confirm an exact number. Fewer than 50 people were on the island when it erupted, and 23 had been taken off, including the five dead, he said. Tims said there had been no contact with any of the missing.

He said New Zealanders and foreign tourists were among the dead, missing or injured. Most of the 18 who survived were injured, some suffering severe burns.

SFGATE reached out to Michael Schade for this story and will update the information if we hear back.

The Associated Press contributed to this report.

Amy Graff is a digital editor for SFGATE. Email her at agraff@sfgate.com.