文档介绍：Volcanoes and Other Igneous Activity Chapter 4
Earth: An Introduction to Physical Geology, 7e
The Nature of Volcanic Eruptions
Factors determining the “violence” or explosiveness of a volcanic position of the magma
Temperature of the magma
Dissolved gases in the magma
The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption
The Nature of Volcanic Eruptions
Viscosity is a measure of a material’s resistance to flow (., Higher viscosity materials flow with great difficulty)
Factors affecting viscosity
Temperature - Hotter magmas are less position - Silica (SiO2) content
Higher silica content = higher viscosity
(., felsic lava such as rhyolite)
The Nature of Volcanic Eruptions
Factors affecting viscosity continued
Lower silica content = lower viscosity or more fluid-like behavior (., mafic lava such as basalt)
Dissolved Gases
Gas content affects magma mobility
Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure
The violence of an eruption is related to how easily gases escape from magma
The Nature of Volcanic Eruptions
Factors affecting viscosity continued
In Summary
Fluid basaltic lavas generally produce quiet eruptions
Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions
Materials extruded from a volcano
Lava Flows
Basaltic lavas are much more fluid
Types of basaltic flows
Pahoehoe lava (resembles a twisted or ropey texture)
Aa lava (rough, jagged blocky texture)
Dissolved Gases
One to six percent of a magma by weight
Mainly water vapor and carbon dioxide
A Pahoehoe lava flow
A Typical aa flow
Materials extruded from a volcano
Pyroclastic materials –“Fire fragments”
Types of pyroclastic debris
Ash and dust - fine, glassy fragments
Pumice - porous rock from “frothy” lava
Lapilli - walnut-sized material
Cinders - pea-sized material
Particles larger than lapilli
Blocks - hardened or cooled lava
Bombs - ejected as hot