Learning Objectives

1. There is a strong correlation between magmatic (lava) chemistry and the violence of an eruption. Size and shape of volcanoes and associated features also reflect the composition of lavas. Lava is magma that has reached the earth's surface.

2. Volcanic activity can have a beneficial or catastrophic effect on humans. Weathered lavas produce fertile soils, lava fields may provide geothermal energy, eruptions may produce global cooling by reducing solar radiation.

3. Catastrophic eruptions have killed thousands in places such as Pompeii and Krakatoa, and threaten the Cascade region. Fatalities have increased in recent centuries because of increased population. Pyroclastic flows represent the greatest threat causing buildings to collapse or hitting people with fragments. Famines may follow eruptions from destruction of crops and animals.

4. Viscosity of lava is its resistance to flow. Viscosity reflects gas content and its ability to escape the molten rock. Temperature at extrusion, silica content, and amount of dissolved gas also control viscosity. Felsic lavas are very viscous, and are associated with the most violent eruptions. Mafic lavas have low viscosity and produce flows. Observation of active volcanism provides insight into past events.

5. Explosive volcanic eruptions produce great quantities of pyroclastic material, that may be released in dangerous flows. Water vapor is the most common gas released by volcanoes. Flows form from either outward exploding froth of gas and magma or gravitational collapse of a vertical column of gas and pyroclastic debris.

6. Igneous rocks are composed primarily of silicate minerals. Felsic rocks form from high viscosity magmas that are silica-rich, light and high in potassium, sodium and aluminum; rhyolite is the most common example. Mafic rocks form from low viscosity magmas that are silica-poor, dark, and exhibit high abundance of magnesium, iron and calcium; basalt is the most common example. Intermediate rocks are, of course, intermediate; andesite is the most common example.

7. Texture (size, shape and arrangement of grains). Grain size is the most important textural characteristic and it is controlled by cooling history and viscosity (Table 4.1). Fast cooling = fine-grained texture, and distinguishes extrusive rocks. Obsidian is volcanic glass that is not composed of minerals and reflects extremely rapid cooling of very viscous lavas. Porphyritic textures exhibit phenocrysts from the intrusive, slow-cooling magmatic stage, and matrix from the extrusive, rapid-cooling eruptive stage. Extrusive rocks are typically vesicular because decreased pressure releases gas from solution within the magma. Explosive eruptions produce significant pyroclastic material in the form of dust, ash, cinders, bombs and blocks (increasing size) that can form rocks tuff and volcanic breccia.

8. Volcanoes have a characteristic geomorphology including the cone, vent, and crater. Flank eruptions and caldera formation may occur. The three major types of volcanoes - shield, cinder cone, composite - also reflect composition of the lava (Table 4.2).

9. Shield volcanoes have low flank slopes that reflect low viscosity, quiet eruptions of basaltic lavas. Aa and pahoehoe are typical expressions of the basaltic composition of flows forming these cones.

10. Cinder cones have very high flank slopes that reflect pyroclastic debris formed because of the high gas content in magmas of any composition. Composite cones are constructed of alternating pyroclastic layers and lava. Most are composed of andesite and reflect the circum-Pacific belt. Their eruptions can be very violent. Volcanic domes may form from felsic lavas that are very viscous and are preceded by violent eruptions (e.g. Mt. St. Helens).

11. Not all volcanic eruptions result in cones. Plateau basalts form from very low viscosity lava floods and may exhibit columnar jointing. Submarine eruptions produce pillow basalts, particularly along mid-oceanic ridges.