Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Igneous Rocks

Cool from the Molten State

Large Grain Size ----> Slow Cooling

Porphyritic Texture:
Large Crystals in Fine-grained Setting

Why Igneous Rock Classification Matters

Silica Content governs viscosity or resistance to flow, because silica tetrahedra link to form chains (polymerize), and the chains tangle around each other to impede flow. Fluid lavas allow gases to escape easily so silica content governs violence of eruptions. The principal volcanic rocks are

Basalt (45-52% SiO2)

Slightly modified planetary raw material and therefore likely to be common on all planets. We know it occurs on the moon, Mars, Venus, and some asteroids. On earth basalt is derived directly from mantle and occurs in settings where magma comes straight from the mantle to the surface:

Fluid lava with little explosive activity, builds shield volcanoes and cinder cones.

Andesite (52-66% SiO2)

Mixture of mantle material and continental crust and mostly restricted tocontinental volcanic chains. Pasty lava with significant explosive activity, builds stratovolcanoes.

Rhyolite (>66% SiO2)

Mostly remelted continental crust, occurs in settings where magma has a long time to react with continental crust:

In addition to stratovolcanoes, rhyolite sometimes builds small obsidian domes. Occasionally there will be no volcano at all; instead the roof of a magma chamber simply collapses and huge volumes of ash, pumice and gas are vented suddenly. These are the most catastrophic of all eruptions. Some major examples:

Some Igneous Rocks Are Named on Textural Criteria:

Types of Volcanoes

types of volcanoes

Classes of Eruption

Eruptions can be divided into two broad families

Explosivity Index

Proposed by Chris Newhall of the U.S. Geological Survey in 1982 as a means of quantifying the violence of eruptions. Volume refers only to material ejected explosively; Hawaiian and Icelandic eruptions may erupt vast volumes of lava but eject very little material explosively. Also, the index isn't really concerned with material that falls immediately around the vent. Hawaii is noted for spectacular fire fountains and the total material ejected may be quite large, pushing the upper end of explosivity 1, but the material mostly falls back around the vent.

Like the Richter Scale for earthquakes, the Volcanic Explosivity Index is logarithmic. Each step is about 10 times larger than the previous step.

Index Description Plume Height Volume of Ejecta Classification Example
0 non-explosive < 100 m 1000's m3 Icelandic, Hawaiian Kilauea
1 gentle 100-1000 m 10,000's m3 Hawaiian, Strombolian Stromboli
2 explosive 1-5 km 1,000,000's m3 Strombolian, Vulcanian Galeras, 1992
3 severe 3-15 km 10,000,000's m3 Vulcanian Nevado Ruiz, 1985
4 cataclysmic 10-25 km 100,000,000's m3 Vulcanian, Plinian Galunggung, 1982
5 paroxysmal >25 km 1 km3 Plinian St. Helens, 1980
6 colossal >25 km 10's km3 Plinian, Ultra-Plinian Krakatau, 1883
7 super-colossal >25 km 100's km3 Ultra-Plinian Tambora, 1815
8 mega-colossal >25 km 1,000's km3 Ultra-Plinian Yellowstone, 2 Ma

Effusive Eruptions

Lava simply issues from fissures without building a volcano, though repeated activity may build shields. The greatest historic example was the Laki fissure flow of 1783, which killed about a fifth of the population of Iceland, mostly through crop and livestock destruction. Explosivity index 0. Note that merely being in Iceland doesn't make an Icelandic eruption; the Icelandic volcano Hekla has has some of the largest explosive eruptions in history.
Basalt issues from long-lived central vents and builds shield volcanoes. Explosivity index 0-1.

Explosive Eruptions

Steam explosions caused when lava or magma comes in contact with water. Large events may blast out craters called maars. The few explosive eruptions on Hawaii have been phreatic. One in 1790 killed several members of a Hawaiian war party passing close to Kilauea. Another series occurred at Kilauea in 1924 and one person was killed by flying ejecta. Explosivity index 0-1 in most cases but maar eruptions may go as high as 3 or so.
Named for Stromboli, Italy, which has been popping mildly since Roman times and is nicknamed "lighthouse of the Mediterranean." Mild, long-lasting explosive activity confined to the immediate vent area. Typically associated with small stratovolcanoes (they don't erupt much material) and basalt or andesite lava. Explosivity index 1-2
Named for Vulcano, Italy, from which we also get the term volcano. Typical explosive eruption, with a large eruption cloud but not much pyroclastic flows. Generally associated with andesite stratovolcanoes. Explosivity index 2-4
Named for Pliny the Younger, who left a description of the eruption of Vesuvius in 79 A.D., and his uncle Pliny the Elder, who died in the eruption. Large eruption cloud, pyroclastic flows, and may collapse to create a caldera. Andesite or rhyolite stratovolcanoes. Mount Pelee, 1902 and Mount St. Helens, 1980 are examples. Explosivity index 4-6
Caldera-Forming (Ultra-Plinian)
Catastrophic eruption usually associated with rhyolite stratovolcanoes or magma chamber collapse. Extremely large volume of pyroclastic flows that may travel for long distances. Tambora, 1815, Krakatoa, 1883, Katmai, 1912 and Mount Pinatubo, 1991 were historic examples. Mount Mazama 7000 years ago and Thera (Santorini) about 1500 B.C. are other famous cases. Explosivity index 6-8

Products of Eruptions

Environmental Hazards of Volcanoes

Greatest Earthquakes and Volcanic Eruptions

Pyroclastic Flow or Nuee Ardente (French: Fiery Cloud)

pyroclastic flow

  1. Gas Expands as Lava Rises
  2. Lava Breaks up into Fragments Supported by Escaping Gas
  3. Cloud Flows Downhill at 60-100 M.p.h. Temperature about 1000 C. You will not outrun this.

Nuee Ardente is a more "classical" term, but seems to be in the process of being replaced by "pyroclastic flow." Pyroclastic flow deposits are typically called just that, or also "ash flows." Really large and thick deposits, especially those with evidence of violent eruption, are sometimes called ignimbrites from the Latin ignis, fire and imber, storm - a wonderfully descriptive term. Some pyroclastic flow deposits are so thick that the hot interior fuses together into a glassy mass. Such deposits are called welded tuffs.

Volcanoes with pyroclastic flow deposits in the vicinity should always be regarded as dangerous when they erupt. Once pyroclastic flows do begin erupting, the safest procedure is to estimate the greatest likely range of the flows, add a safety margin, and evacuate. The only way to survive an oncoming flow is to outrun it or evade it. Evasion might be possible if there's a quick route off to the side or uphill. Outrunning it is only possible if there's a good straight road, and then do not stop for anything - stop signs, police cars, red lights, animals, anything. Anything that does not outrun the flow will die.

How Calderas Form

caldera formation

Calderas form when volcanoes collapse. In some cases, violent explosive eruptions (left) can empty a magma chamber enough that the summit collapses. In other cases, magma may erupt on the flanks of a volcano or drain back to deeper levels, permitting the summit to subside (right). These caldera collapses are generally not violent.

In some cases, magma may be just a few kilometers below the surface. The roof of the magma chamber may cave in, uncorking the gas pressure and resulting in colossal pyroclastic flows and Ultra-Plinian eruptions with Explosivity Index 8. Yellowstone, Long Valley Caldera and Toba are important examples in the recent geologic past.

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Created 3 Feb 2006, Last Update 14 December 2009

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