Mount Semeru, the highest peak on Java Island, possesses unique geological characteristics that make it far more dangerous than typical volcanoes. According to recent research findings, the magma beneath the volcano has a crystal content reaching up to 50 percent, making it a highly viscous mixture of hot liquid and solid material.
This high crystal fraction prevents the magma from flowing smoothly upon reaching the surface. Instead of spreading out, the material tends to accumulate in the crater area, forming a mound-like lava dome. This lava dome structure, continuously expanding due to magma pressure from below, serves as the main source of vulnerability.
The combination of heavy dome mass accumulation and Semeru's steep slope contours creates highly unstable conditions. As magma pressure from deep within the earth continues to rise, the lava dome can collapse instantaneously. This collapse triggers pyroclastic density currents, locally known as "wedhus gembel."
In addition to magma pressure factors, extreme weather conditions play a crucial role. Heavy rainfall can lubricate the material around the summit, significantly increasing the risk of lava dome landslides even without a drastic surge in volcanic activity.
Besides the threat of pyroclastic flows, Semeru actively produces Vulcanian-type eruptions. Although their impact tends to be limited to the crater area, gas explosions occurring every few minutes require continuous, close monitoring of this volcano to ensure the safety of communities living around the hazard zone.
