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Volcanoes: Origin, Types and Eruptions

Samir G. Khoury, Ph.D., P.G.


Course Outline

Following a brief introduction of the myths and fanciful speculations that surrounded the subject of volcanic eruptions throughout most of human history, this course explains how our modern understanding of this mighty natural phenomenon began to develop. It is only since the second half of the twentieth century that scientific explanations, grounded on sound systematic and technical principles, began to lift the veil of mystery that shrouded the questions of why, where and how do volcanoes erupt. As will be explained in this course, volcanic activity has in fact, over the long span of geologic time, helped to shape the surface of the earth on which we live today.

Starting with a description of the internal structure of the earth, which we now know is composed of a dense core with a radius of about 3,400 km, a lighter mantle that is about 2,900 km thick, and a still lighter crust that is mostly rigid and up to 60 km in thickness, the course proceeds to explain our present understanding of the processes that are active within the deep reaches of our planet and how these processes lead to the development and the eruption of volcanoes. The possible sources of heat within the earth are presented and the process of heat transfer by convection is explained. The concept of mantle convection is introduced and its consequence on the development of a clearly defined pattern of crustal segmentation (crustal plates) and crustal motion (plate tectonics) is introduced. Also, the process of subduction, whereas an oceanic plate is pushed under the leading edge of a continental plate, is also illustrated. The ultimate outcome of this process is the generation of volcanic chains that develop parallel to the margin of continental plates. As explained, the jetting of plumes from the mantle into the crust also results in the development of volcanic centers within continents and oceans.   

The course points out that the majority of all known active volcanoes, and earthquakes for that matter, are actually located, or occur, along well defined narrow bands that circle the earth parallel to the boundaries of the major tectonic plates. These bands form a continuous belt that rings the Pacific Ocean, from New Zealand to the southernmost tip of South America (Tierra del Fuego). It is known as the circum-Pacific belt, also known as the ring of fire. A stem, known as the Mediterranean belt, branches out from the circum-Pacific belt around Indonesia and the Andaman Islands and extends essentially east-west thereafter through the Himalayas and onto the western Mediterranean. Other thinner bands that circle the earth within the oceans coincide with the mid-ocean ridges and highlight the locations where relatively weaker volcanic (and seismic) activity occurs. These features were all formed by the active interaction between the tectonic plates that constitute the crust of the earth.

Based on their mode of origin and distinctive internal morphology, four main types of volcanoes can be recognized namely: Cinder Cones, Composite Cones, Shield Volcanoes, and Lava Domes. The four types are presented, illustrated and described in sufficient detail to characterize and classify the products of their eruptions. Next, the volcanic Explosivity Index (VEI), a scale to estimate the size of volcanic eruptions, is presented and its use by volcanologists to compare quantitatively the magnitude of volcanic eruptions worldwide explained.
  
The next section of the course describes some of the historically famous volcanic eruptions. Included are descriptions of the eruptions of Santorini (Thera), Vesuvius, Mount Tamboro, Krakatoa, and Mount St. Helens. This part of the course is followed by a section on Supervolcanoes that explains how these oversized features are identified from their geologic record and the size of their eruptions estimated. The cataclysmic eruptions of Toba, Yellowstone and Taupo (Lake Oruanui) are presented as examples and contrasted to the largest known volcanic eruptions in the historical record. Finally, the course concludes with a table that lists 89 notable eruptions sequenced in the order of their estimated Volcanic Explosivity Index (VEI). Students can access a Wikipedia description of each listed eruption by using the ctrl+click key on their computer.

Lastly, a glossary of terms and acronyms used in this course is presented following the Summary Section. It will provide the students with a handy reference to assist them in following the concepts that are presented and discussed throughout the text.

This course includes a multiple choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the end of this course, you will be able to understand how our modern scientific knowledge about volcanoes was developed. You will also be able to answer the questions of why, when and how do volcanoes erupt. In the process you will learn about the internal structure of planet earth including the properties of:

You will also learn about the:

Depending on the conduits by which the magmatic materials reach the surface you will be able to recognize and differentiate between four basic types of volcanoes:

You will also learn how to determine the size and severity of volcanic eruptions using:

Finally descriptions of notable volcanic eruptions throughout the course will help you recognize and follow the stages of volcanic activity from start to finish.

Intended Audience

This course is intended for geologists, engineers, scientists, geotechnical engineers, engineering geologists, other individuals and the general public who are interested in learning about the active and deep seated geological processes that are helping to shape the surface of our planet.

Benefit to Attendees

Students who take this course will gain an understanding of the ongoing geologic processes within the lower crust and upper mantle of the earth that lead to the formation and eruption of all types and kinds of volcanoes. They will also be able to answer the questions of why, where and how do volcanoes erupt, will learn how to determine the size and severity of volcanic eruptions and recognize and follow the stages of volcanic activity from start to finish.

Course Introduction

From the human perspective, volcanic eruptions are some of the most spectacular and destructive geological phenomena. Their onsets are usually sudden, often very violent, of undetermined duration and they are essentially uncontrollable. Because of these inherent characteristics volcanic eruptions have been shrouded throughout most of human history in a fog of mystery and fanciful speculation. Whenever and wherever they occurred they struck fear and apprehension equally into the imagination of the mighty and the humble alike.

More recently, geologists have recognized that volcanic activity has also helped shape the surface of the earth and sculpted much of the natural beauty of the land. Volcanism has also produced a great deal of the mineral wealth and the fertile soils that we now extract and exploit at and near the surface of the earth.  Despite this recognition, it may be surprising for most people to learn that it is only since the second half of the twentieth century that finally a sound and scientific understanding of this mighty natural phenomenon began to develop and be grounded on sound systematic and technical principles.

Below is a brief historical background explaining the origin of the word volcano and the mythology associated with its periodic rumblings.

Course Content

The course content is in the following PDF file:

Volcanoes: Origin, Types and Eruptions

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.

Course Summary

Following a brief introduction to the myths and fanciful speculations that surrounded the subject of volcanic eruptions the course embarked on explaining the scientific principles that undergird our present day understanding of why, where and how do volcanoes erupt. The course started by considering the internal structure of the earth, which we now know is composed of a dense core, a lighter mantle, and a still lighter crust. The processes that are active within the deep reaches of our planet were elucidated and the processes that lead to the development and the eruption of volcanoes were explained.

The concept of mantle convection was introduced and its consequence on the development of a clearly defined pattern of crustal segmentation (crustal plates) and crustal motion (plate tectonics) was clarified. Also, the process of subduction, whereas an oceanic plate is pushed under the leading edge of a continental plate, was illustrated. The ultimate outcome of this process is that volcanic chains develop parallel to the margin of continental plates. The jetting of plumes from the mantle into the crust also results in the development of volcanic centers within continents and oceans.  

The course pointed out that the majority of all known active volcanoes, and earthquakes for that matter, are actually located, or occur, along well defined narrow bands that circle the earth parallel to the boundaries of the major tectonic plates. Other thinner bands that circle the earth within the oceans coincide with the mid-ocean ridges and highlight the locations where relatively weaker volcanic and seismic activity occurs. These features were all formed by the active and ongoing interaction between the tectonic plates that form the crust of the earth.

Based on their mode of origin and distinctive internal morphology, four main types of volcanoes can be recognized namely: Cinder Cones, Composite Cones, Shield Volcanoes, and Lava Domes. The four types were presented, illustrated and described in sufficient detail to characterize and classify the products of their eruptions. Next, the volcanic Explosivity Index (VEI), a scale to estimate the size of volcanic eruptions, was presented and its use by volcanologists to compare quantitatively the magnitude of volcanic eruptions worldwide was explained.
  
The next section of the course described some of the historically famous volcanic eruptions. Included were descriptions of the eruptions of Santorini (Thera), Vesuvius, Mount Tamboro, Krakatoa, and Mount St. Helens. This part of the course was followed by a section on Supervolcanoes that explained how these oversized features were identified from their geologic record and the size of their eruptions estimated. The cataclysmic eruptions of Toba, Yellowstone and Taupo (Lake Oruanui) were presented as examples and contrasted to the largest known volcanic eruptions in the historical record. Finally, the course concluded with a table that listed 89 notable eruptions sequenced in the order of their estimated Volcanic Explosivity Index (VEI).

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.