CHAPTER 6: MATERIALS, HAZARDS, AND ERUPTIVE MECHANISMS
CHAPTER 7: TYPES, BEHAVIOR, AND RISKS
• Volcanism. Refers to the rise of magma onto the earth‟s surface.
• Magma - a mixture of liquid rock, mineral crystals, and dissolved gases.
• Volcanoes are conical or dome shaped landforms built by the emission of magma and its contained
gasses onto the earth‟s surface.
Locations of Volcanic Activity and Volcanic Hazards
Volcanic activity is controlled by plate tectonics, because plate movements relate to where sources
of magma originate inside the earth. Nearly all active volcanoes are located in one of three plate
Subduction zones at convergent plate boundaries Example: Volcanoes lining the trenches of
the Pacific Ocean, forming the Pacific “Ring of Fire”.
Rifting and sea floor spreading at divergent plate boundaries Example: Volcanic eruptions at
mid-ocean ridges, and in some rift zones on the continents, like the East African Rift Valley.
Hot spots Example: The Hawaiian Islands and the Galapagos Islands.
The majority of above sea active volcanoes (about 66%) occur in the Pacific Ring of Fire.
Magma and the Driving Force Behind Eruptions
• Magma may be ejected onto the earth‟s surface as:
– Pyroclastics or tephra- flour sized to boulder sized particles which are thrown in the air due
to the built up pressure of gasses.
• The violence of a volcanic eruption depends on the magma‟s viscosity and gas content. The more
viscous (thick) and more gaseous the magma, the more explosive the eruption.
Viscosity is a measure of a fluid‟s resistance to flow. The main factor that determines the viscosity
of magma is its silica (SiO2) content. The more silica in the magma, the more viscous it is. The
more viscous (silica-rich) the magma, the more violent the eruption.
We are concerned, then, with three types determined by the chemical composition of the magma
(Pgs. 146 -148).
Mafic or Basaltic magma – low silicon and oxygen, high iron and magnesium. Simple silicate
minerals. Dark magmas. Low Viscosity.
Intermediate or andesitic magma - intermediate silicon and oxygen, intermediate iron and
magnesium. High Viscosity.
Felsic or rhyolitic magma - high silicon and oxygen, low iron and magnesium. Complex silicate
minerals. Pale magmas. High Viscosity.
The gas content of a magma also relates to its behavior. A magma with low gas content will tend to
flow out of a volcano as relatively quiet lava. A magma with high gas content will tend to blow apart
violently upon erupting. The higher the gas content, the more violent the eruption.
The composition of the gases in magma are:
Water vapor (pg. 148).
Carbon Dioxide (pg. 145)
Minor amounts of sulfur dioxide, hydrogen sulfide, chlorine, and flourine gases (pgs. 145-146).
But it is the amount of dissolved water that typically inspires a volcano to violence (pg. 148).
Steps to a Volcanic Eruption
Volcanoes form wherever rock melts at depth and the magma can rise to erupt at the surface.
Rock deep in the earth may melt by
o Increasing its temperature
o Decreasing its pressure
o Adding water (to lower the melting temperature).
Magmas that are generated deep within the Earth begin to rise because they are less dense than
the surrounding solid rock.
As they rise they may encounter a depth (or pressure) where the dissolved gas no longer can be
held in solution in the magma, and the gas begins to form a separate phase (makes bubbles) and
will continue to grow in size as pressure is reduced.
• If the magma has a low viscosity, the gas will easily expand to atmospheric pressure at the
earth‟s surface and simply burst, and a non-violent eruption will occur, usually as a lava flow.
• If the magma has a high viscosity the gas will not be able to expand very easily creating a high
pressure inside which will cause them to burst explosively on reaching atmospheric pressure.
This will cause and explosive volcanic eruption.
Volcanic Explosivity Index
• the volume of material (lava and particles) erupted
• the height of the eruption column
• how long the eruption lasts
The larger the VEI value, the larger the eruption.
Two main factors determine the size and explosivity of a volcano‟s eruption:
The amount of time that has passed since the last eruption.
The viscosity (thickness) and gas content of the magma. Both relate to plate tectonic setting:
Divergent boundaries (MOR) and hot spots both draw their magmas from the upper
mantle. This magma is called MAFIC (basaltic) MAGMA, and is characterized by
relatively low silica and low gas content. (the eruptions have low VEI).
The magma at convergent boundaries comes from melting of subducted oceanic plates.
Subduction forms FELSIC (rhyolitic) to INTERMEDIATE (andesitic) MAGMA, and is
characterized by relatively high silica and high gas content. (the eruptions have high VEI).
Bottom line: The world’s most dangerous volcanoes are those at convergent plate boundaries!
Types of Volcanic Hazards and Products are:
Lava is molten magma that flows out and onto the Earth‟s surface.
Lava flows are typically formed from low viscosity mafic magma that erupts at divergent
boundaries and hot spots.
o The lower silica content (and therefore low viscosity) of mafic magma allows the lava to
run down slopes easily.
o Lava flow eruptions are fairly gentle and quiet. They may cause property damage, but
o Lava flows destroy whatever they overrun. Everything in the path of an advancing lava
flow will be knocked over, surrounded, buried, or ignited by the extremely hot
temperature of lava.
Ash falls form when an eruption column of tephra and gas is blown into the air by an explosive
eruption. The eruption column can rise up more than 20 km into the atmosphere.
Tephra is a general term for any size of fragmental material blown out of a volcano.
Large-sized tephra typically falls back to the ground on or close to the volcano.
Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers
downwind from a volcano. Volcanic ash can be hazardous for a variety of reasons...
o Ash spreads in upper atmosphere around the globe. The suspended ash can decrease
the insolation from the sun and lower global temperatures!
o Daylight turns into darkness
o Roofs may collapse from added weight.
o Machinery and vehicles will be abraded and engines may seize.
o Farmland will be covered.
o Roads will be slippery or blocked.
o Waste-water systems may clog.
Pyroclastic flows (Ash Flows) are avalanches of a very hot (1300-1800F) mixture of hot rock
particles and hot gas that are blown out of the vent of the volcano as an eruption column which
subsequently collapses and moves very rapidly down the flanks of the volcano at speeds from 50 to
over 200 km per hour and can travel for 10‟s of kms burning, burying and suffocating everything in
their path. Pyroclastic flows
Destroy by direct impact.
Bury sites with hot rock debris.
Burn forests, crops, and buildings.
Mudflows or Lahars
Lahars have caused more fatalities than any other volcanic hazard because they are more common,
and they can occur at any time.
Volcanic Mudflows or Lahars form by mixing water with loose volcanic ash and debris on the flanks
of a volcano. As the mud moves downslope, it gathers rocks of all sizes accelerating as it goes.
The water can come in several ways including:
- A major rainstorm.
- An eruption melts large amounts of snow and ice on the flanks of the volcano.
When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from
clay to boulders. Most lahars travel much too fast for people to outrun.
Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of
gravity. A number of factors can trigger a landslide:
intrusion of magma into a volcano
intense rainfall that saturates a volcano or adjacent tephra-covered hillslopes with water
Volcanic landslides can trigger volcanic explosions.
Gases emitted during volcanic eruptions may be toxic and/or corrosive.
The most common hazardous gas is CO2. Carbon Dioxide is deadly to people, animals, and trees
in high concentrations. Being heavier than oxygen, it can pour downslope and displace oxygen at
the surface. Because it is colorless and odorless, it can suffocate without warning.
Types of Volcanoes
Composite or Stratovolcano
They differ in igneous rock chemistry, eruption style, physical features and geographic location.
Shield Volcano Stats
Found along hotspots and divergent boundaries.
Mafic (basaltic) magma
Low gas content (no water because no subduction).
GENTLE eruptions of lava flows.
Landform composed almost entirely of relatively thin lava flows.
Gently sloping due to the low viscosity of the magma which allows lava to flow great distances
before it cools.
The largest structures on earth! 9 km from seafloor to summit!
Basalt is the most common rock type.
Most of the lava of a shield volcano flows down the flanks of ridges that radiate outward from the
volcano summit. Because they form from relatively quiet eruptions of low viscosity, fluid magma,
shield volcanoes are very large, with gently sloping sides and a convex shape.
Composite or Stratovolcanoes
• Found along convergent boundaries paralleling subduction zones.
• Andesitic (intermediate) to rhyolitic (felsic) magma.
• High viscosity
• High gas content (subduction drags down water)
• VIOLENT eruptions. Dangerous and Explosive.
• Landform composed of alternating layers of tephra and lava.
• Steeply sloping due to the piling up of tephra around the central vent and the high viscosity of
the lava that glues it together.
• Can reach heights of roughly 3500 meters.
• Andesite and rhyolite are the most common rock type.
The main hazards of a stratovolcano are:
• pyroclastic flows
• ash falls
• Poisonous gas emissions
The majority of above sea active volcanoes (about 66%) are stratovolcanoes produced by
subduction in the Pacific Ring of Fire.
Lava domes are steep sided structures that form when viscous lava is erupted slowly and piles up in
and around a volcanic vent. As it grows its outer surface cools and hardens, then shatters, spilling
loose fragments down its sides.
• Cinder cones form when fluid basaltic magma rises along a fracture and encounters groundwater.
The steam generated blows fragments of lava violently into the air which solidify and fall as
cinders around the vent forming a circular or oval cone.
• Cinder cones are commonly found on the flanks of shield volcanoes and stratovolcanoes.
• Most cinder cones erupt for only a few months to a few years and rarely rise more than 300-500
m above their surroundings. Being unconsolidated they tend to erode rapidly.
Some eruptions of stratovolcanoes are so violent that after the eruption ends the volcano collapses
into the empty magma chamber beneath, leaving a large crater behind called a caldera.
Supervolcanoes – Giant Continental Calderas
„Supervolcanoes‟ are giant rhyolite volcanoes that form over continental hotspots.
Resurgent caldera eruptions are by far the largest and presumably most destructive of all types of
Hot Springs and Geysers
• Hot springs are areas where heated groundwater reaches the surface of the earth.
• Geysers eruptions occur as a consequence of groundwater being heated to its boiling
temperature in a confined space (ex. a fracture). The resulting steam forces water and steam up
through the fractures and onto the ground.
Mitigation and Prediction of Volcanic Hazards
There is no effective way to mitigate most volcanic hazards after an eruption has occurred.
Mitigation before an eruption can be done by assessing a region for its potential volcanic hazards
and creating a hazard-zone map, which indicates the type and degree of risk in particular areas.
Involves studying the geology of the volcano to determine the types of hazards posed by the
volcano and the frequency at which these types of hazards have occurred in the past (radiometric
To predict an eruption, geologists depend on various precursors: events that occur prior to an
eruption. The main precursors that signal an impending volcanic eruption are:
1. Tilting and swelling of the volcano‟s sides. As magma rises below a volcano, it forces the
earth‟s surface upward and outward, causing it to bulge.
2. Increased seismic activity. Many small earthquakes -- called earthquake swarms or
harmonic tremors -- caused by fresh magma rising below the volcano.
3. Seismic exploration. Monitoring the movement of the s-wave shadow zone to determine the
position and movement of magma.
4. Increased steam and gas emissions caused by rising magma emitting gas as it approaches
5. Changes in Groundwater System - As magma enters a volcano it may cause the water table
to rise or fall and cause the temperature of the water to increase.