Sakurajima, located in Japan’s Kagoshima Prefecture, is frequently cited as one of the world’s most active volcanoes. Its prominent profile dominates the landscape of Kagoshima Bay, sitting only about four kilometers from the major city of Kagoshima. The answer to the question of its type is straightforward: Sakurajima is a stratovolcano, also known as a composite cone. This classification points directly to its construction, its shape, and the nature of the magma that fuels its frequent activity.
Characteristics of a Stratovolcano
Stratovolcanoes are defined by their steep, symmetrical cone shape, built up over thousands of years. This distinctive form results from alternating layers of viscous lava flows, ash, tephra, and broken rock fragments. Unlike shield volcanoes with gentle slopes, the material that forms a stratovolcano does not flow easily, which allows the edifice to build vertically.
The characteristic steep profile is directly linked to the composition of the magma, which is generally andesitic and contains a high percentage of silica. High silica content makes the magma highly viscous, preventing it from spreading far from the vent after an eruption. This viscous lava cools quickly and solidifies on the steep slopes, contributing to the volcano’s overall stability and height.
Sakurajima is actually a complex of three peaks—Kita-dake, Naka-dake, and the currently active Minami-dake—which together form the large stratovolcanic structure. At its highest point, Kita-dake reaches 1,117 meters above sea level. The volcano is part of a larger geological structure, a post-caldera cone that emerged from the Aira caldera, which formed during a massive eruption approximately 22,000 years ago.
Tectonic Setting and Origin
The existence of Sakurajima as a stratovolcano is explained by its location within a specific geological environment, the Pacific Ring of Fire. It is positioned on Kyushu Island, Japan, where the tectonic setting is dominated by a convergent plate boundary. Specifically, the oceanic Philippine Sea Plate is actively diving, or subducting, beneath the continental Eurasian Plate at the Ryukyu trench.
This subduction process introduces water and volatile compounds into the hot mantle rock of the overriding plate. The addition of these compounds lowers the melting point of the rock, generating the magma that rises to the surface and fuels the volcanoes. The magma produced in these settings is typically the high-silica, viscous type that builds stratovolcanoes like Sakurajima.
Sakurajima’s Eruptive Activity
Sakurajima is renowned for its high frequency of eruptions, often exceeding a hundred events annually since 1955. The most common style of activity is characterized as Vulcanian, which involves relatively short, sharp, and explosive blasts of ash and gas. These events are frequently accompanied by intense air-shocks and the ejection of ballistic bombs, which are fragments of rock that follow a ballistic trajectory.
The ash plumes from these frequent explosions can rise kilometers above the summit, leading to consistent ash fall that impacts the city of Kagoshima, requiring regular clearing from urban infrastructure and affecting air quality. Historically, Sakurajima has also experienced much larger, less frequent events, such as the powerful 1914 eruption.
The 1914 event, known as the Taisho eruption, was particularly significant because the massive lava flows it produced filled the narrow strait separating the volcano from the mainland. This connected the former island to the Ōsumi Peninsula, permanently altering the local geography. Constant monitoring of the volcano is necessary because its proximity to a major population center means that even the typical Vulcanian activity poses a persistent hazard.