Why Does a Volcano Collapse Into a Bowl Big Enough for Towns?

A Caldera Forms When the Ground Sinks Into an Emptied Magma Chamber

Most volcanoes are built upward. A cinder cone or a shield volcano grows as lava and ash pile up. A caldera works the other way around. The U.S. National Park Service describes calderas as collapse features: during a large eruption the magma chamber underneath is partly emptied, and the ground above it subsides into the space [3]. The USGS puts it the same way: a caldera is a large crater that results from collapse into an evacuated magma chamber [2].

A simple way to picture it is a blanket spread over a slightly inflated balloon. Let the air out and the blanket sags into a bowl. The picture is only a picture: real ground is rock, and what leaves is molten rock, not air.

The word itself comes from Spanish, where caldera means a cooking pot or cauldron. The Japan Tourism Agency uses that image for Lake Towada in northern Japan, which fills a caldera formed about 13,000 years ago [5].

A Caldera Is a Crater That Is Much Bigger and Comes From Collapse

The USGS explains that “crater” is a general word for an enclosed hollow in the ground, and that by strict definition a caldera is a kind of crater. What sets a caldera apart is its size and its link to volcanic collapse. A collapse a few hundred meters (yards) across is usually called a crater, and one generally more than 1 kilometer (0.6 mile) across is a caldera. The USGS adds that these size guidelines are vague [2]. The National Park Service gives the same rough limit and adds a second test: a caldera is much wider than the vents inside it [3].

So the size cutoff is a habit among scientists, not a law of nature. The better thing to remember is the cause: a big hollow made when a chamber of magma was largely emptied.

Crater Lake in Oregon Shows What Happens When a Tall Volcano Collapses

The clearest picture comes from the United States. The National Park Service says Mount Mazama in Oregon was once about 3,700 meters (12,000 feet) tall, among the highest peaks in the Cascades. Around 7,700 years ago a rapid series of explosive eruptions emptied the volcano and its structure collapsed inward [4]. What remains is a basin roughly 8 by 10 kilometers (5 by 6 miles) across and more than 1 kilometer (0.6 mile) deep. Water filled it, and the lake reaches 1,943 feet (about 592 meters) at its deepest point [4].

The USGS notes the confusing name: Crater Lake sits in a caldera, so “Caldera Lake” would be more accurate. It adds that ash from the eruption covered much of western North America [2].

Why Aso’s Bowl Holds Farms and Villages Instead of a Lake

Aso is the same kind of landform, but its bowl has an opening that lets water flow out. The Kumamoto Prefecture government describes it as about 18 kilometers east to west and 25 kilometers north to south, with an area of about 380 square kilometers (about 147 square miles). The floor lies about 400 to 500 meters above sea level, and the steep walls rise roughly 300 to 600 meters (1,000 to 2,000 feet) above it. Villages are scattered along the foot of the inner slopes [1]. One note on the source: it is a Japanese-language planning document for landslide control, and I used it for the shape of the land only.

Why is the floor not a lake? The rim has one opening: a gorge at Tateno on the west side. Water inside the bowl flows out through it, and the Kurokawa river joins the Shirakawa river and runs on toward the city of Kumamoto [1]. Crater Lake’s basin, by contrast, holds a lake [4]. I did not find a source here that explains the whole water balance of either place, so this outlet is one difference, not a full explanation. Aso seems to have had lakes in the past: a Wikipedia summary says sediment research shows lakes once filled parts of the floor and later dried up [6].

Aso’s Caldera Is Reported to Come From Several Eruptions, With New Volcanoes Inside

Not every caldera comes from a single collapse. Wikipedia’s Aso article describes Aso as the product of several large eruptions, the latest of the big caldera-forming ones, called Aso-4, around 90,000 years ago, and reports that its ash covered much of the Japanese islands [6]. I could not confirm those dates or the count of four from a primary source, so treat them as reported rather than settled.

What is easier to verify is what sits on the floor. Mount Takadake, at 1,592 meters (5,223 feet), is one of a line of smaller volcanoes running roughly east to west across the caldera floor [1], and the same Wikipedia article says one of them, Nakadake, is still active [6]. Lake Towada tells a similar story: after its bowl formed, more volcanic activity built a smaller volcano inside it, and the walls of that one rise from the water as two peninsulas [5].

Some Calderas Form Without a Giant Explosion

It is easy to assume every caldera needs a catastrophic blast. The National Park Service says otherwise: calderas at the tops of shield volcanoes such as Kilauea and Mauna Loa in Hawaii form without explosions, when large lava eruptions drain a shallow magma chamber [3]. The USGS describes Kilauea in 2018, when a large lava eruption partly emptied its magma chamber. Over about three months the summit sank by more than 500 meters (1,600 feet), in a series of drops rather than all at once [2]. Because modern collapses like this can be watched, scientists can use them to understand how older calderas formed.

Try It: Watch a Cloth Sag Over a Balloon, Then Hunt for Calderas on a Map

Blow up a balloon a little and set it on a table. Lay a light tea towel over it, then let the air out slowly. Watch the middle of the cloth sink while the edges stay put. Then ask where the model is wrong: real ground is heavy rock that can crack, and in a real volcano the empty space is made by molten rock leaving, not by air.

Next, look at a satellite map of Crater Lake in Oregon and of Aso in Japan. Look for the near-circular rim, a floor much wider than any single vent, and where water goes in or out. The USGS and National Park Service pages listed below are good places to read more.

Sources

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