How Do 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 aren1t 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.

Crater lake in the making

We pick out four key stages in the development of a caldera lake:

1. Volcano – All volcanoes feature a crater to some extent at their peak, but lakes rarely get the chance to form because of geothermal activity.

2. Mega-eruption – If a volcano has lain dormant for a long time, or if there is dramatic tectonic activity, a much bigger eruption than normal might occur.

3. Collapse – Such a climactic event at the very least expands the size of the crater, however in more extreme cases the volcano’s entire cone collapses inwards to leave a caldera.

4. Lake – Over centuries, the magma chamber below the caldera turns solid. In the cooler basin, rain and snow have an opportunity to build up and form a lake.

Some like it hot…

Volcanic activity can continue to simmer under the crater, which affects the chemistry of the lake. A lack of productivity often means the water is very clear, hence why jewel-like greens and blues are common. This doesn’t mean crater lakes are barren though. Some are a lot more hospitable than others, supporting insects, fish, right through to apex predators.

But even ones spewing out deadly gases and minerals can still support ecosystems. For instance, the water of hyper-alkaline (pH 11) Laguna Diamante in the Andes contains arsenic and is five times saltier than seawater, but a research team in 2010 found ‘mats of microbes’ living on the lake bed, which served as food for a colony of flamingos.