The emerald lakes were formed when water filled explosive craters on Mt. Tongariro. The steam in the picture shows the geothermal activity on the mountain. Learning about life in extreme environments has showed us that there are lots of things that can surprisingly live in these high-heat areas. While we did not look specifically at the life that occupies these lakes and craters, we can guess that there's probably lots of cyanobacteria, making the water look so green/aquamarine in the Emerald Lakes.
Mt. Tongariro first erupted about 260,000 years ago. It is an important mountain for Maori people living in the area, and is the namesake of Tongariro National Park, a World Heritage Site as well as the first national park in New Zealand.