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"My God, White Island volcano in New Zealand erupted today for first time since 2001. My family and I had gotten off it 20 minutes before, were waiting at our boat about to leave when we saw it," he tweeted.
"Boat ride home tending to people our boat rescued was indescribable."
Several people were killed and injured and others were reported missing when the volcano erupted apparently without warning off the east coast of the North Island.
In video Schade posted on Twitter as he sped away from the island in a boat, a huge plume of white ash soars into the sky as a group of frightened tourists huddle close to the shore.
A helicopter parked nearby on the island looks crumpled and covered in ash.
"Those are some of the people (our) boat picked up. Praying for them and their recovery. Woman my mom tended to was in critical condition but seemed strong by the end," Schade tweeted to his 2,570 followers.
"The helicopters on the island looked destroyed."
Schade describes himself in his Twitter handle as someone who likes "helping people, photography, reading, gym, travelling, and learning new things". He says he lives in San Francisco.
"My thoughts (are) with the families of those currently unaccounted for, the people recovering now, and especially the rescue workers."
He also praised tour operators who helped tend to people in the minutes after the eruption.
"Endless gratitude to that crew for stepping up as first responders," he wrote.