How crater lakes form

When you look out across a mountain lake it can be easy to think it was always so serene, but this couldn’t be further from the truth. From the shifting of Earth’s tectonic plates to glaciers gouging out the land, the majority of these tranquil sites are the result of epic geological events.
Crater lakes have perhaps the most epic beginnings of them all. While maar lakes are also the result of volcanism, forming in the fissures left behind by ejected magma, they are generally quite shallow bodies of water; indeed, the planet’s deepest – Devil Mountain Maar in Alaska – is 200 metres (660 feet) from surface to bed. In terms of scale, maars aren’t a patch on their bigger cousins.
Crater lakes have very violent origins. During a mega-eruption, or series of eruptions, the terrain becomes superhot and highly unstable. In some cases the volcanic activity is so intense that once all the ash and smoke clears, the cone is revealed to have vanished altogether, having collapsed in on itself. This leaves a massive depression on the top of the volcano known as a caldera.
In the period of dormancy that follows, rain and snow gather in this basin, generally over several centuries, to create a deep body of water; Crater Lake in Oregon is the deepest of any lake in the USA, plunging to 592 metres (1,943 feet). Over time a caldera lake will reach a perpetual level that’s maintained by a balance of regional precipitation and annual evaporation/seepage.