3 - Volcanoes


There are an estimated 1500 active volcanoes around the globe; 550 are on land and the rest are underwater. Some erupt frequently and quietly while others erupt rarely but explosively. Still others erupt extremely rarely, perhaps once every 100,000 years, but catastrophically with global effect. Geologists call molten rock lava when it spills out on the Earth's surface and magma when it is underground, but it is not all the same stuff. Some magma flows easily while other types are viscous. Some magma contains a lot of gas under high pressure and others types don't have much gas at all. These properties dictate whether eruptions will be peaceful or explosive and whether they are a great danger to life or not. In turn, these differences are closely related to their position relative to plate boundaries and the type of crust (oceanic or continental) that the volcano sits on. So the first part of this unit will be an examination of volcanism and how it is related to plate tectonics.

The next part will discuss environmental risks near the volcano. These include direct effects from glowing clouds, lahars (mud flows), lava flows, and the release of poisonous gasses. If the volcanic eruption is very large, effects can also be felt on a regional or even global scale so we will spend a bit of time on this. Tsunami for instance can travel across oceans to devastate distant shores.  However, the most important regional or global risk is when volcanic debris of various types reaches the stratosphere. Climate change has been observed after large eruptions in modern times (eg, Tambora) and there is also some dramatic evidence from extremely large eruptions in ancient times. Case histories, both ancient and modern, will be used throughout this unit because they are the best way to illustrate volcanic processes and risks.

Although we can't stop a volcano from erupting, most loss of life due to volcanic eruption can now be prevented by early prediction so we will also spend a bit of time on early warning systems and the like.

Learning Objectives

Upon completion of this unit you should be able to:

1. Identify the three components of magma and their importance.
2. Describe Bowen’s reaction series and the order in which silicate minerals crystallize from a cooling magma.
3. Identify the properties related to viscosity of magma and how this affects volcanism and volcanoes.
4. Differentiate between magma associated with continental crust and that from oceanic crust
5. Explain the Volcanic Explosivity Index.
6. Fully describe glowing cloud eruptions.
7. Recount the lessons scientists learned from the deadly eruption of Nevado del Ruiz.
8. List the environmental and human concerns associated with tephra falls.
9. Discuss the various volcanic gasses and their potential hazards.
10. Describe the sequence of events that resulted in the high loss of life at Lake Nyos, Cameroon.
11. Explain the volcanic events and processes can impact regional and global climate.
12. Discuss how short-term and long-term volcanic eruptions are forecast.

Required Readings

Abbott, P.L., 2014. Natural Disasters, Ninth Edition: Readings From the Old Book

Abbott, P.L., 2012. Natural Disasters, Eighth Edition:

Glossary Terms

Aerosol Pyroclastic
Andesite Pyroxene
Angle of Repose Quartz
Basalt Radiometric Age Dating
Caldera Resurgent Caldera Eruptions
Crater Lake Rhyolite
Curie Point Richter Scale
Fluorosis Rock Slide
Fossilized People Seismograph
Fumarole Shield Volcano
Global Positioning System (GPS) Silicate Minerals
Glowing Cloud Stratosphere
Hot Spot Stratovolcano
Ice Age Subduction Zone
Infrared S-wave
Jokulhlaup Tephra
Lahar Tsunami
Lava Tuff
Magma Viscosity
Magnetite Volcanic Bombs
Olivine Volcanic Explosivity Index (VEI)
Phreatic Eruption Volcanic Tremor
Plagioclase Volcanic Vent
Plate Tectonics Volcanic Winter
P-wave Welded Tuff