Volcano Explorers is a subtheme of City that was introduced in 2016 and currently consists of 8 sets. As of today, the average annual growth of all the sets in City Volcano Explorers is around 2.6% per year.
The LEGO Volcano Explorers was a City subtheme released in 2016. From LEGO's description: "A volcano is about to erupt on the edge of LEGO City! A team of top scientists and volcano explorers have been dispatched to the scene with high-tech equipment, power drills and helicopters to survey the danger and save the City! Join the action – if you dare!"
The LEGO City theme is based on city life with the sets depicting city and emergency services, police, fire, airport, train, construction, and civilian services. The City theme proceeded LEGO Town which was replaced with LEGO World City in before it was rebranded as City in 2005.