Geo Lecture 8: Volcanism & Volcanic Hazards
Volcano architecture and eruption style
Magma may crystallize at depth in the Earth (“intrusive” igneous rock) or extrude from a volcano as lava or pyroclastic debris (both “extrusive” igneous rock)
Magma from a magma chamber reaches the surface either through a vertical chimney to the crater at the top, or through long cracks called fissures where the lava erupts from a ‘vent’ along the flanks of the volcano.
Eruptions from the crater are called summit eruptions whereas eruptions along the flanks are called flank eruptions or fissures eruptions.
Composition of Magma – source of the shape and eruption style of volcanoes
All magma are different. They each have a distinctive chemical makeup, based on their particular tectonic setting and their pathways upward toward the surface. The most important chemical component is the relative amount of silica (SiO2). Magmas are distinguished on the basis of their silica (SiO2) content.
*The shape of a volcano and the composition of the lava or pyroclastic debris that is erupted is directly related to its silica content.
Silica controls the viscosity of the magma within the chamber – viscosity is simply a measure of a fluid’s resistance to flow. (ex. Water has low resistance, thus low viscosity; honey has high resistance, thus high viscosity).
Magma consists of 1) the liquid melt, 2) any solid crystals of minerals that might have formed from the melt as it cooled, and 3) abundant gases such as H20, CO2, SO2, and H2S.
-up to 9% of a magma’s composition may consist of gases
-These gases are extremely important since they provide the ‘gas pressure’ that drives the explosive force of volcanic eruptions. (The more gases trapped in the magma, the more powerful the potential eruption.)
-The higher the viscosity, the greater the concentration of trapped gases and thus the higher the potential exclusivity of eruption.
1) Low-silica magma – the lower the silica content, the...