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The Formation of Calderas

Introduction

At the summit of many of the world’s largest volcanoes is a feature known as a caldera. A caldera, essentially, is a massive crater that indicates a large historical eruption. So why is it important to understand calderas? First of all, their formation can be extremely violent and result in the most massive explosive eruptions in Earth’s history. Equally important, however, is their presence atop many of our best-known volcanoes. Hence, understanding their formation is a key part to understanding the history of any large volcano on Earth.

Physical Characteristics and Formation Basics

Calderas can range in diameter from several hundred meters to several hundred kilometers. They are usually roughly circular, although are frequently oblong (like a stretched circle). Depths from the rim to the basin are usually on the order of hundreds-of-meters and can vary from place to place. Inside the caldera of active volcanoes, one can commonly find fumaroles or hydrothermal systems. They also, frequently, contain cinder and spatter cones.

A question one might ask, however, is “other than size, how does a caldera differ from other eruption craters?” Well, other craters are formed as the magma and gases are expelled. The ground is essentially blasted skyward, and what is left is a hole that may be partially filled by the remnants of a lava dome or a congealed lava lake. A caldera does not form in this manner, however; it is formed through the removal of support from below. Let’s look at a scenario that can be applied to many of the world’s volcanoes:

We have a dormant volcano, or an active one that has not erupted in a fairly long time. Magma is rising into the base of the magma chamber, kilometers below the summit, while magma is working its way toward the surface. Meanwhile, the magma chamber is expanding slightly, and becoming highly pressurized…more so than usual. Eventually the rising magma reaches the surface and a massive eruption ensues (it does not matter what kind, as far as we’re concerned). Gas rich magma at the top of the chamber is pulled through the vent and the entire chamber begins to depressurize. The magma expands slightly, allowing more to escape the chamber. The depressurization, more importantly, facilitates the exsolution of gas. This gas migrates to the vent and causes the eruption to continue. Overall, the chamber has been pretty dramatically depressurized and, while there is no real empty space, the pressure is low enough that it can’t support the overlying rock any longer. Adding to the problem is that much, if not all, of that erupted material is laying on the surface, directly overtop of the chamber. The erupted magma not only depressurizes the chamber, but also adds to the weight pushing down on it from above!

Since the chamber can no longer support the overlying rock, it begins to collapse. First the ceiling begins to fracture dramatically. At the surface large faults occur (and may be emplaced very rapidly in dramatic earthquakes), which may reach directly to the magma chamber itself. This network of cracks grows, comprising a roughly circular shape and probably outlining the very shape of the chamber itself (to some degree). During this fault emplacement, the entire summit has already begun to subside slightly; as the network of fractures grows, however, the blocks are able to move with less difficulty. This creates an interesting situation: as the chamber is squeezed from above by the dropping blocks, it is partially repressurized. One or two things might happen. First, there will probably be a large increase in the eruption from the vent. Of course, recall the fact that the erupted material adds to the overlying weight. The second event that may occur, and is probably common only in the most massive eruptions, is that the pressurized magma forces its way through the fractures. This means the eruption becomes gargantuan, with enormous fountaining or explosive activity emanating from rifts within and along the edges of the caldera- from the very faults themselves. After a time, usually days, the subsidence slows and stops, as does the eruption. What is left is a massive collapse structure where the peak of the volcano used to be: a caldera.

This generic model can be applied to many different types of volcanoes and at many scales. For example, Kilauea contains numerous pit craters along its east-rift zone. The craters are little more than small calderas. They are typically on the order of hundreds of meters in diameter with depths of a similar magnitude; they are almost always circular, or nearly so. They occur in a linear pattern, roughly outlining the rift zone. The relatively small sizes indicate that the magma chambers for these vents lay very close to the surface and were probably not very large. Of course, there is an enormous magma chamber below the much-larger summit caldera of Kilauea; these pit craters are from the rift zone where magma is being diverted for the time being.

Detailed Characteristics and Formation Styles

While many calderas form over the course of days, still some occur at much longer time intervals: over the course of years or even millennia! It depends on a number of factors, not the least of which is the style of caldera being formed.

Plate, or Piston, subsidence is the simplest to visualize and understand. This occurs where the ring faults completely or nearly connect, allowing the block to drop almost straight down. The failing region drops downward as a single unit, all at once. It is thought that this kind of caldera forms during a single large eruption over a large magma chamber.

A Piecemeal style of subsidence is thought to occur over long periods of time, probably comprising many different eruptions. During this scenario, individual blocks drop downward one at a time, relative to each other. This style probably occurs over a large chamber where the eruptions are not very large, and where the eruptive vent moves frequently (allowing different sections of the chamber to decompress). It is important to remember that much of these explanations remain, at present, highly speculative.

Trap-Door subsidence; this requires a pop quiz. What is easier to break? A half-inch board or a block of wood three feet thick? Ok, that shouldn’t have been too hard. What’s easier to break? A layer of rock one kilometer thick or another layer that’s 10 km thick? Hopefully you were smart enough to call the thinner one. All rocks fracture, but as you might imagine, a thin layer will fracture completely much more easily than a thick layer. Hence, where the magma chamber is oddly shaped, we can have bizarre subsidence behavior. What we have is a situation where the magma chamber is much closer to the surface at site A than at site B. Consequently, the earth at site A will fracture straight to the magma chamber whereas site B will only fracture part of the way. This means that the thick section remains connected and acts like a hinge for the completely disconnected section. Thus, the caldera behaves like a trap-door!

The concept of Downsag builds on those developed for Trap-Door. In this situation, however, no side fractures completely. Here the chamber is either very deep, or the eruption was just borderline for the formation of the caldera. Anyway, the center of the chamber begins to sag, pulling down the surface. Since the fracturing is poorly developed, however, all sides act like a hinge. This means there is little faulting and that the center simply sags: downsag!

As calderas form, the faults are extremely steep; near vertical! This means the walls are extremely sharp cliffs, but we don’t see this in most of our calderas today. For example, we did not know Yellowstone was a caldera at all until just recently…and it’s the world’s largest! The problem is erosion: the steep cliffs of the caldera erode and collapse until the sides eventually become hills. After enough time, even these can wear away, making identification of the caldera difficult (again, see Yellowstone). Also, it’s important to note that calderas are not the last stage of a volcano.

Volcanoes go through many stages, which frequently repeat themselves. After a caldera has been formed, dormant volcanoes will still exhibit earthquakes, deformation, and hydrothermal activity. This is because the magma chamber below still contains magma and may still be fed by an underlying magma system (a hotspot for example). Where the caldera remains highly active, you can have all of the aforementioned in addition to eruptive vents and dome formation. Calderas can become filled with pyroclastic material and even lava. In fact, it has been suggested that caldera formation is a regular process in the growth of basalt volcanoes, such as those in Hawaii. Thus, you would have a large eruption, followed by caldera formation; the caldera is then filled over hundreds or thousands of years, built up, and then a new one is formed at a later date. It’s a very interesting thing to think about…especially if you’ve ever worked next to a caldera.

Conclusion

Here our discussion of calderas comes to a close. Like all of the subjects dealt with in this website, there was a great deal we didn’t cover. Hopefully now you understand the importance of calderas and how they form. For more information I would suggest consulting a textbook or scientific journals.