Krakatoa is known for its catastrophic 1883 eruption. Located in the Sunda Strait between the islands of Java and Sumatra in Indonesia, the volcano is classified as a stratovolcano, also known as a composite volcano. This designation refers to volcanoes with a characteristic steep, conical profile built up over time by layers of solidified lava, ash, and other ejected material. Krakatoa sits within a highly active volcanic region, and its explosive nature is tied directly to its fundamental structure and geological forces.
The Stratovolcano Classification
Stratovolcanoes are recognized by their distinct, symmetrical cones and steep slopes, which can range from 10 to 35 degrees. Their structure is built from alternating layers, or strata, of hardened lava and tephra, which includes ash, pumice, and rock fragments. The classification as a composite volcano highlights this mixture of materials, which contrasts sharply with the broad profile of shield volcanoes.
The formation process involves repeated cycles of both explosive and effusive activity over thousands of years. Because the magma is generally viscous, it does not flow easily and solidifies quickly near the central vent. This results in the material piling up around the vent, forming the characteristic steep-sided shape. This viscous magma also traps gases, leading to the highly explosive eruption style for which stratovolcanoes are known. Krakatoa’s pre-1883 structure was a complex of three volcanic cones—Rakata, Danan, and Perboewatan—that had merged into a single island.
The Role of Tectonics and Magma Viscosity
Krakatoa’s existence as an explosive stratovolcano is governed by its tectonic setting along the Indonesian island arc system. The volcano lies directly above a convergent plate boundary where the denser Indo-Australian Plate is actively sliding beneath the Eurasian Plate, a process known as subduction. This subduction zone is part of the Pacific “Ring of Fire,” a belt characterized by frequent seismic and volcanic activity.
As the subducting oceanic plate descends, the increase in temperature and pressure causes the slab to release volatile compounds, such as water. These fluids rise into the overlying mantle wedge, lowering the melting point of the rock and generating magma. This newly formed magma then rises through the continental crust, where it often becomes enriched in silica.
The resulting magma is high in silica content, making it highly viscous. This high viscosity prevents dissolved gases—like water vapor and carbon dioxide—from escaping easily as the magma rises toward the surface. Instead, the gases become trapped, leading to an immense buildup of pressure within the magma chamber. This pressure drives the violent, explosive eruptions associated with stratovolcanoes.
Catastrophic Eruption and Caldera Formation
The extreme pressure generated by the highly viscous, gas-rich magma culminated in the catastrophic 1883 Plinian eruption, one of the most powerful volcanic events in recorded history. The initial eruptions began with the release of a shallow, silicic melt-rich region, but the climactic phase saw a massive, rapid decompression of the underlying magmatic system. This immense release of energy propelled an estimated 20 cubic kilometers of tephra high into the atmosphere.
The most significant geological consequence was the formation of a large submarine caldera, a massive depression in the crust. This occurred because the underlying magma chamber was evacuated so quickly that the roof structure could no longer be supported. The overlying volcanic edifice, including the two northern cones, Danan and Perboewatan, collapsed inward into the void, sinking beneath the sea.
The collapse event triggered devastating tsunamis, which killed tens of thousands of people in the surrounding coastal areas. The resulting caldera was approximately six kilometers wide and lay mostly submerged, with only the remnant of the Rakata cone remaining above the water. Since 1927, a new volcanic cone, Anak Krakatau, or “Child of Krakatoa,” has been actively growing within the center of the 1883 caldera.