For better understanding of volcanoes, I want to write this blog about them so that I have full knowledge of the hazards, gases, viscosity, and types of volcanoes for the upcoming test.
There are about 1,500 active volcanoes on Earth. About 400 of these 1,500 erupted in the last century which gives us an estimated 50 eruptions per year. Volcanoes are hills, ridges, or high mountains formed by the accumulation of lava flows and pyroclastic deposits around the conduit, or crater depression, from which they erupted. The size of the volcano relates to the volume of extruded volcanic materials. The shape of the conduit and the types of eruptions determine the type of the volcano.
Cinder/Scoria cone- relatively small volcanoes, composed of basaltic to andesitic scoria. THe slope of the mountain side is usually 30-35 degrees, the angle defined by the loose bombs when they come to rest. Typically produces by a single, prolonged eruptions lasting 1-20 years. These types of volcanoes have low silica and low viscosity. When it erupts tephra (mostly ash) ejects. Example: SP Crater.
Shield volcano- Small to some of the largest volcanoes in the world. These volcanoes are composed of many thin widespread basaltic flows. Have a definite shield shape that is characterized by less than 15 degrees. Active for centuries to a few million years. Example: Mauna Loa, Hawaii.
Composite volcano- small to large volcanoes composed of interlayered lava flows, lava-flow rubble, and pyroclastic deposits. Lava flows are typically thicker and shorter than those on shield volcanoes. Most are dominated by andesite, basalt, and/or dacite. Slopes are generally greater than 25 degrees. Example: Mt. Fuji.
Calderas- form in composite volcanoes when large volumes of magma explosively erupt in just hours to a few days to form thick, widespread pyroclastic deposits. In other words, it forms from a massive eruption of rifting. Once the eruption occurs the roof of the magma chamber subsides as it loses its support.
There are many hazards that relate to volcanoes. Pyroclastic Flows are fast-moving and commonly cause horrific casualty tolls. With temperature greater than 300 degrees celsius it easily wipes out cities. Most human deaths, however, result from the rapid asphyxiation that occurs in the choking cloud of ash and gas generated by these flows. Lava flows can also by a hazard as well. Though you can outrun a lava flow it is very destructive to cities and neighborhoods. Lahars are the rapid flow of water and loose debris down steep volcanic slopes. This is caused the by hot pyroclastic deposits melting snow and glacier ice. Pyroclastic-fall deposits can also be considered as a hazard. These deposits have the potential to be heavy enough to cause buildings to collapse. In the eruption of 1991 of Mt. Pinatubo, the 350 people who lost their lives were victims of falling roofs weighed down by the ash of pyroclastic-fall deposits.
How do we monitor volcanoes?…We can look at the increase of seismic activity or the increase of gas activity. Another new way to detect an eruption is the laser the minute topography changes (tilting and bulging). And thermal readings is a simpler to detect volcanic activity.