Geologic Hazards: Volcanoes and Volcanism

A. Introduction

Eruptive styles of volcanoes and their relation to physical and chemical properties of magmas

• Non-explosive (or effusive) eruption--when magma flows out of a vent as molten (generally incandescant) coherent liquid. Flows of liquid magma called are lava flows.  Mafic magmas generally erupt as lava flows with little associated explosive activity (CP&M, Fig. 10.17, 10.18, p. 252).  Intermediate magmas frequently erupt as lava flows but may also erupt explosively.  Felsic magmas erupt as lava flows least frequently--more commonly they erupt explosively. Rhyolites that do erupt as lava flows form slow-moving, "pasty" flows  that cool very rapidly, forming a jet black glass called obsidian (note Fe and Mg are very low relative to Si and Al despite the dark color of the glass; CP&M, Fig. 10.9, p. 249).
• Explosive eruption--When magmas erupt violently they get blown  apart into magma droplets--droplets cool or quench quickly to form tiny bits of glass called volcanic ash. Sometimes drops may be larger  pieces filled with gas (usually water vapor)--once cooled they form pumice (CP&M, Fig. 10.12, p. 250--very low density because of air pockets--floats on water and felsic to intermediate in composition; air-pocket-rich fragments of mafic and intermediate composition magma that are larger than ash are called scoria). Ash, pumice, and any rock fragments picked up from the volcanic vent are called pyroclasts.
  
• As ash/pumice/rock  fragments exit from volcano they form column of hot, very gas-rich, pyroclastic material. Once pyroclasts are high enough in atmosphere, some material rains  down out of column to produce pyroclastic falls.
• If the erupting column of pyroclasts is not continuously recharged with more hot magma from below, entire column of ash  and pumice can collapse catastrophically to form hot, gas-rich, fast-moving flows called pyroclastic flows (Fig. 4.6; 4.2; Box 4.5--Fig.1). Pyroclastic flows especially dangerous because of mobility (can climb over ridges) and very fast (100-200 km/hour).  Pyroclastic flows can also form as volcanic domes of extruded lava (see below) break apart or collapse (CP&M, Fig. 10.7, p. 247).
• Ash and pumice deposits that form from either the material raining down out of the cloud from the atmosphere, or from the hot flows of ash moving across the ground become lithified produce a rock called a volcanic tuff.
  

What controls eruptive style?  Controlled primarily by viscosity and the water content of the magma.

• Viscosity--measure of how resistant a material is to flow when force is applied. Qualitatively, if a liquid has low viscosity, it flows easily. Conversely, if  a liquid has high viscosity, it does not flow easily. Honey is more viscous than water, for example. Magmas which have higher viscosities are more  likely to erupt explosively because they do not flow easily.  Viscosity is in turn dependent on:
  

• composition--as silica (Si) and aluminum (Al) content increases, the viscosity increases.  Magmas which are silica-rich therefore tend to resist being deformed (i.e. will not flow easily).
  
• temperature--as temperature increases, viscosity decreases.  Magmas which are erupted at higher temperature are able to flow more easily.  Water is also more likely to remain dissolved at high T (see next bullet).
  
• dissolved water in the magma--magmas have the capacity to dissolve small amounts of water (a few percent by weight).  Water is more easily dissolved in magma at higher pressures and at high temperatures. As long as magma stays deep within the crust and hot then the water will stay dissolved in it. As magma rises pressure decreases and magma will lose heat to surrounding cooler rocks.

• At low enough pressure and temperature water exsolves (i.e. it "un-dissolves") from magma--this can happen in shallow magma bodies or conduits beneath volcanoes.  This process of exsolution causes bubbles to form--and formation of bubbles does two things

• causes dramatic increase in viscosity of magma and
• water vapor (bubbles) creates large amount of pressure on walls and roof of shallow magma chamber-- this is a very unstable  state for the magma. Eventually, the top of the magma chamber can fail, or crack and magma propagates to the surface, where it fragments and forms explosive eruptions of hot gas and pyroclasts.

C.  Volcano Effects & Hazards

• Source of energy--geothermal power is largely generated near long-lived active volcanoes.  Many of the Pacific Rim countries as well as Iceland, several European countries and California use geothermal power.  Steam or hot water or both mined out of ground to power turbines.
• Soil development--large volcanic eruptions produce abundant ash, and weathering of the ash and also weathering of lava flows produces rich volcanic soil.  Thus traditionally humans have farmed along the flanks of volcanoes.
• Hazards/fatalities--there are numerous sorts of phenomena associated with volcanoes that can cause fatalities (CP&M, Fig. 10.6, p. 246).  The primary hazards are that result directly from the eruption of materials from the volcano.  The most deadly and dangerous are pyrocalstic flows (see above).  Large ash clouds from erupting volcanoes can be a signifcant hazard to planes because the ash can clog and shut down jet engines.  Mudflows (or lahars) have also caused many fatalities and these are secondary hazards that can occur when hot lava or ash from an erupting volcano flows on to wet and/or snow-covered ground.  Even a small eruption can produce enough heat to melt lots of snow and ice which can mix with the rocks and soil and debris on the flanks of the volcano and produce deadly flows of mud.  These flows may travel great distances (many tens of kilometers) down valleys along the flanks of the volcanoes.  Tsunamis can also be associated with volcanoes that erupt in the ocean.

D. Types of volcanoes and their relation to eruptive style  (Note: we did not specifically go over all these specific forms of volcanoes in class but you are resopnsible for reading this material and for knowing it for the test!)

• Shield volcanoes--(CP&M, Fig. 10.16, p. 252; Box 10.3 Fig. 1, p. 254) the largest types of volcanoes (many thousands cubic km in volume on Earth as well as in Solar System--Olympus Mons on Mars)--broad volcanoes with very gentle sloping sides--shape of a round  shield lying face up, hence the name shield volcano--made almost exclusively of basalt -- erupts as hot, low-viscosity lava flows--flows move quickly  and easily, laterally away from vent. Hot fluid lava will not easily build a high cone but will build a broad, laterally extensive, gently sloping volcano.  Ability of lava to travel great distances is enhanced by formation of lava tubes (CP&M, Fig. 10.29A,B, p. 261).  The enormous volcanoes of Hawaii (Mauna Loa and Kilauea)  are typical shield volcanoes.
• Cinder cones--(CP&M, Fig. 10.20, p. 253). Steep-sided,  symmetrical cones made up of successive layers of pyroclastic debris ejected from the vent. The coarser material accumulates nearest the vent.  The finer material travels farther. When mafic composition magmas erupt  explosively they form cinder cones--mafic magmas do often erupt explosively  in earliest stages of eruption but explosive activity usually quckly diminshes  and is subordinate to eruption of lava flows. The layers build up at steep  angle dipping away from the vent--Paricutin, Mexico, S-P Crater, Arizona.
  
• Composite volcanoes (or stratovolcanoes)--also steep-sided, generally symmetrical cones--made of ejected pyroclastic material but also lava flows hence "composite" Examples--Mt. Pinatubo,  Philippines, Mt. Fuji, Japan (CP&M; Fig. 10.24, p. 256), Popocatépetl, Mexico (CP&M, Box 10.4, Fig. 1, p. 257), and Mt Rainier and Mt St. Helens (CP&M; Fig. 10.1A, p. 240; Box 10.1, p. 242) in the Cascades of Washington (CP&M, Fig. 10.5, p. 245) are all good examples of  large composite volcanoes. Soufriere Hills volcano on island of  Montserrat in Caribbean (Box 4.5--Fig. 1) is a smaller composite volcano that has been very active since mid-90's.  Often have tremendous heights (hence term "stratovolcanoes"). Mt Shasta, California (CP&M, Fig. 10.21, p. 255)--second tallest peak in the lower 48 states after Mt Whitney.  Eruption of pasty andesite to rhyolite lava may also form domes of round steep-sided volcanic rock that do not flow easily away from vent--can plug vent and keep it sealed until pressure builds up enough that it  is torn apart by explosive eruption--lava dome has also filled much of crater of Mount St. Helens since it erupted explosively in 1980 (CP&M, Fig. 10.25, p. 258)--makes it a very dangerous volcano still  because now very difficult for lava to flow out of vent. At some point (maybe real soon!), Mt St. Helens will erupt again.  Be sure to read up on this here.
  
• Fissures--linear vents up to several tens of kilometers long--erupt  fluid basaltic lava flows which spread out laterally from fissure(s)--form large basaltic plateaus and also commonly open and erupt lava along mid-ocean ridges. Largest volcanic eruptions in the world  have produced plateaus by fissure eruptions ("flood" basalts). The Columbia Plateau of Washington, Oregon, and Idaho covers 200,000 square km's (CP&M, Fig. 10.27, p. 260)
  
• Calderas--large (a few km to as much as 50 km in diameter), circular or oval shaped depressions with steep surrounding walls of rock--formed when roof rocks above magma chamber fracture and collapse downward into underlying magma (Fig. 4.3). May form in very large shield volcanoes (e.g. Hawaii; Fig. 4.15) but most commonly associated with large intermediate to felsic volcanoes, including some stratovolcanoes) [Yellowstone, Wyoming; Crater Lake, Oregon (CP&M, Fig. 10.3, p. 243).  Lots of attention directed at understanding calderas because most,  but not all have erupted very large-volumes of felsic pyroclastic material including large-volume particularly pyroclastic flows.  The last eruption (600,000 years ago) of Yellowstone produced almost 1000 cubic km's of pyroclastic flows  in a single eruption.  No one has ever witnessed such  enormous volcanic eruptions--eruption of Krakatoa in 1883 (off the  coast of Java in Indonesia) produced caldera--island of Krakatoa almost  completely vanished once eruption ceased. Thousands died from tsunami waves.  Formation of Crater Lake in Oregon (CP&M, Fig. 10.4, p. 244) probably witnessed  by Native Americans living in that area some 6600 years ago (ancient legends seem to indicate). Yellowstone and Long Valley caldera near town of Mammoth Lakes in east-central California went through periods  of inflation in the early to middle 80's.  Both calderas have abundant earthquake activity associated with them, which indicates they are in restless state--both produced cataclysmic eruptions of hundreds to thousands of cubic km's of pyroclastic material.  Lots of CO₂ (carbon dioxide) is currently being emitted from Long Valley-- gas is coming from magma a few km's down in crust.  Several ski patrol team members at Mammoth Lakes were killed three weeks ago when they fell into a pit in the snow that had filled with CO₂.