Lava flows: effusive volcanism
Lava flows are quite slow: about 10 km/h close to the volcanic vent and slower than 1 km/h with some distance to the vent. The volcanic flow is related to the force of gravity. Lava makes its way mainly through valleys and depressions/graben. Lava flows are mostly local phenomena and extend for several tens of kilometers. Large effusive eruptions can also form up to hundreds of meters thick lava fields. The outflow of lava takes place within a few days or a couple of weeks, but can also last up to decades.
Lava flows have surface temperatures of 500 to over 1000 °C. There are several factors influencing their flow characteristics: The rate, the amount of outflowing lava, the morphology of the environment and the physical parameters of the lava itself, less viscous lava flows faster and more far. Cooling effects led to different surfaces/shapes like Aa (blocky) and pahoehoe lava (flat) or pillow lava.
Lava flows are often a risk for people, infrastructures and ecosystems in the vicinity of a volcano. Mostly they are slowly enough to be able to escape. However, they can destroy environment and can cause forest fires. Therefore Civil Protection Agencies try to bypass the flows by building dams or graben around settlements.
Tephra fallout, pyroclastic currents: explosive volcanism
Pyroclastic materials develop when rising magma is fragmented during an explosive volcanic eruption. In gas-rich melts the bubbles burst, and form highly porous rock fragments such as slag, pumice and ashes. With velocities between 400 and 1000 km/h, the rock material as well as volcanic gases are blown into the atmosphere up to 40 km high. Tephra is part of the plume and classified according to their grain size. Ash is less than 2 mm, lapilli between 2 and 64 mm. Blocks (angular) and bombs (round) are bigger than 64 mm and differ in their shape.
Volcanic gases and precipitation
Volcanic gases and acid rain caused by many types of eruption. Carbon dioxide (CO2), hydrogen sulfides and chlorides are blown into the atmosphere. Sulfur dioxide, water and ash are forming aerosols, which are harmful to humans. Risky is also calm discharge of gases in volcanic regions. In volcanic lakes dissolved gases such as methane or carbon dioxide can be found in the water. Sudden movements of the water body caused by a weak earthquake for example can discharge that gas e.g. in 1986 at Nyos Lake in Cameroon. Many people lost their lives due to the gas cloud.
