The political events of the 1990âs have had significant effects on the number of Member. States of the United Nations. In 1990, the entry of both Namibia and Liechtenstein made the. number of members rise to 160; however the successive union of North and South Yemen, and.
Several types of volcanic eruptions âduring which lava, tephra (ash, lapilli, volcanic bombs and blocks), and assorted gases are expelled from a volcanic vent or fissure âhave been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed.here are two types of eruptions in terms of activity, explosive eruptions and effusive eruptions. Explosive eruptions are characterized by gas-driven explosions that propels magma and tephra. Effusive eruptions, meanwhile, are characterized by the outpouring of lava without significant explosive eruption.
A more viscous (thick) magma will produce a more violent eruption This is controlled in part by the concentration of silica in the magma. A magma with low silica (<45%) will be runny and so the eruption will not be explosive. A magma with high silica (>60%) will be quite viscous and so the eruption will be violent. The temperature of the magma is also a factor. A hotter magma will be less viscous. One other factor is the amount of dissolved volatiles in the magma (chlorine gas, water and other gases). A magma with a lot of dissolved gases will form a lot of large bubbles.