Volcanoes: Source

In Volcanoes: Source, Kinds and Eruptions, you'll learn ...

• How our modern scientific knowledge about volcanoes was developed
• Understand why, when, and how volcanoes erupt
• The internal structure of planet earth including the properties of the core, the mantle, and the crust, Sources of heat that stoke the deep seated geologic processes within the earth
• Mechanism of heat transfer by convection from the earth's interior to the surface

Overview

Credit: 4 PDH

Length: 50 pages

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 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 is 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 estimated size of their eruptions. 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. This course on volcanoes is presented as a complement to courses G-3004 Earthquakes and Tsunamis: Fundamental Concepts and G-3005 Tsunamis: Generation and Propagation for the benefit of those students who want to learn more about the deep seated processes in our earth that are responsible for the generation of these natural phenomena. However, courses C-3008 and C-3021 are not pre-requisites for taking this course. This course on volcanoes is a stand-alone presentation that can be taken independently of the other two.

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