Volcanism & Volcanic Hazards

Summary of basic terms and concepts

William P. Leeman/Rice University



Types of


Scales of




Global Volcanism

On any given day, there are at least several volcanoes active in various parts of the world. Current volcanic activity is conveniently summarized in Earth Alert (also published weekly in the Houston Chronicle, Monday's Business section).

An index map of currently active volcanoes shows a strong correspondence with specific tectonic areas (also see other WWW sites reporting current activity). Many of these volcanoes (and particularly those associated with subduction zones pose some kind of threat to the people living nearby, and some may have global impacts. This page aims to summarize basic volcanological characteristics and to highlight relations between volcanic activity, types of hazards, and simple hazard assessment.

A nice Volcano Learning Module presents a useful overview and QuickTime animations of some of the principles covered in this page.

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Fundamental properties of magmas

The properties of magmas are closely related to their compositions, and these control their behavior, evolution, mode of eruption, and ultimately the type and degree of hazard that they may impose.

Magma formation - melting

Magmas are basically molten rock, formed deep in the Earth - either in the crust or upper mantle where temperatures exceed the melting point of rocks in those regions. Where magmas form depends on the temperature gradient and the availability of constituents like water that can affect melting temperatures of those rocks. A wide variety of magmas can be produced, and these can be loosely categorized in terms of their silica (SiO2) content. Generally, magma type corresponds to the composition of source rocks that are partially melted. Relatively silica-rich crustal rocks typically melt to form siliceous magmas, whereas silica-poor mantle rocks generally produce low-silica magmas.

However, a spectrum of magma compositions can arise due to variations in source rock composition, degree of melting of those rocks, and the specific history of a magma as it ascends toward the Earth's surface. For example, magmas formed or stored at different depths can mix to produce intermediate hybrids. Or, magmas can undergo crystallization as they rise into cooler regions; because the minerals formed in this process generally have compositions distinct from the magma itself, their removal can cause a progressive shift in the composition of the remaining magma - usually toward more silica-rich variants. Most magmas erupt carrying a few percent of mineral fragments produced by this process.

Primary magma formation mechanisms:

Important magma evolution/modification mechanisms:

Eruption & ascent

What causes magmas to rise in the first place? Magmas almost always have lower density and higher specific volume than the rocks from which they form, or than most deep crustal and mantle rocks through which they may ascend. Thus, the main driving force behind magma ascent is buoyancy. This may be enhanced in some magmas by expansion of gases released during decompression. Key processes include:

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Types of eruptions

Eruption type is a function of magma composition, viscosity, and gas content (all influenced by plate tectonic setting), recurrence time between eruptions, magma volume, replenishment of magma reservoir, strength of surrounding rocks, and access of external water.


Explosive (in order of increasing energetics):

Volcano Morphology

Volcano forms and processes are directly related to physical properties, compositions, and volumes (eruptive rates) of magmas, which in turn influence style of eruption.

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Principal Volcanic Hazards

Define terms, describe each volcanic hazard and the resulting deposits, provide a good example of a volcanic event where each was a contributing factor:

Hazardous Volcanic Events

Review descriptions of the following volcanic catastrophes (cf. Abbott, 1999). What was the principal process in each case that caused great loss of lives and/or property (in the prehistoric examples, what would have been the most devastating processes)?

See other examples and a Table of casualties.

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Scales of impact

Distances covered

Areas covered per eruptive phase

Volumes extruded

Volcanic Explosive Index [VEI] - roughly tenfold increase in ejecta volume (and energy release) for each VEI-step; applied to explosive or pyroclastic volcanoes. Certain types of eruptions (e.g., flood basalts, ignimbrite flows) are not included. Also, time-history of eruptive activity is highly variable between volcanoes - some deposits accumulated over significant time periods (years, Mazama) whereas others were formed in days (Krakatoa).

Maximum volumes for a single event generally increase with silica content and vent size - up to 100s - 1000s km3 for large caldera-related silicic eruptions; can be significant for large basalt eruptions.

Caldera sizes - range up to 100 x 35 km (Toba), 70 x 40 km (Yellowstone). There is a complete spectrum to smaller sizes. Number of examples decreases with increasing size.

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Probabilities of eruptions

Recurrence intervals [R] for eruptions increase with size of eruptions (as indicated by volcanic explosive index [VEI]). R is a function of volume of magma input, type of magma, type of reservoir, and strength of containing rocks, among other factors (tectonic activity, access of water to magma body, triggering earthquakes, etc.). Larger R generally implies higher probability of a violent eruption. See discussion of future Cascade eruptions.

Areal density of volcanoes in regions - problems involved in counting distinct volcanoes - is a parasitic cone a different volcano? (recall Mt. Shasta, which has numerous parasitic cones on the main cone; also, the main cone sits on eroded remnants of at least 2 older stratocones).

Individual volcanoes - estimates are based on relatively few events in a comparatively short period of observation; average recurrence intervals and uncertainties (one standard deviation) are as follows for selected volcanoes:

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Hazards assessment

Among the many active volcanoes in the world, some in proximity to population centers have been designated as Decade Volcanoes to promote more intensive monitoring and volcanological study. Another source of info on Earth's active volcanoes.

See basic modules concerning volcanic risk assessment and approaches to dealing with volcanic threats.

Volcano monitoring

This section addresses methods of assessing volcanic activity, pending eruptions and risk analysis; combinations of these indicators work best. See the USGS publication on this subject.

Further WWW resources provide valuable information:

Other indicators

Location, tectonic setting

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Volcano Statistics

Summaries of common magma types, and types and sizes of eruptions.

Study questions

Useful discussion and study and review questions can be found in the textbook Natural Disasters, P.L. Abbott (1999).

You should also explore additional WWW resources, particularly Tilling et al..

To test your understanding of volcanic processes and hazards, try to develop appropriate hazard mitigation plans for various real world scenarios.

Related web pages:

Volcano questions

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-----LAST MODIFIED: 22 Feb 99
-----BY: Bill Leeman