NOT the video of the recent Semeru volcano eruption. The FACT is, the video is doctored version of Calbuco volcano eruption in 2015 located in Chile combined with the picture of Tumpak Sewu Waterfall in Indonesia.
More in the EXPLANATION and REFERENCES section.
CATEGORY: Manipulated content.
SOURCE: Twitter, https://archive.fo/R4a39 (backup archive).
NARRATIVE: “Semeru Mountain volcanic eruption from 2 days ago”.
Based on 7 Types of Mis- and Disinformation by First Draft News, classified as “Manipulated content: When genuine information or imagery is manipulated to deceive.” 
The SOURCE shared a doctored version video of Calbuco volcano eruption in 2015 in Chile combined with the picture of Tumpak Sewu Waterfall in Indonesia, which has caused a FALSE conclusion.
One of identical videos/images of Calbuco volcano eruption, Nimia: “Without previous warning, the eruption of Calbuco volcano, on April 22, 2015, surprised the whole country and the world.” 
One of identical images of Tumpak Sewu Waterfall, JOURNEY ERA: “Tumpak Sewu Waterfall (also known as Coban Sewu) is arguably Indonesia’s best waterfall. It isn’t the largest, even in East Java, but is definitely the most impressive. From above it looks like the tree of life, from below it is immensely powerful and from all angles, it is quite simply mindblowing.” 
Other Related References
Google Images search result of Mount Calbuco eruption in 2015. 
Google Videos search result of Mount Calbuco eruption in 2015.