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Chapter 6: Volcanoes and Other Mountains

1. The Volcano Commandos2. Magma Viscosity3. Magma Sources and Magma

Composition4. The Mount St. Helens

Eruption5. Products of Volcanic

Eruptions6. Volcanoes and Volcanic

Landforms7. Mountains: Why are They

There?8. The Rise and Fall of

Mountains and Temperatures

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The Good Earth/Chapter 6: Volcanoes and Other Mountains

As a scientifically literate citizen, what 3 questions would youask about this volcano if you moved to the city in the foreground (Tacoma, Washington)?

Volcanoes and Other Mountains

The Volcano Commandos

• 1,500 active volcanoes worldwide− a third have

records of previous eruptions

− 2 or 3 eruptions per decade are major disasters

− 500 million people live near active volcanoes

− Fewer than 200 volcanoes have instruments to assess potential for eruption



• Volcanic mudflows from the eruption of Nevado del Ruiz volcano, Colombia killed more than 23,000 people

Geologists of the Volcano Disaster Assistance Program (VDAP) help identify risks of potential eruptions



• VDAP scientists traveled to Congo to examine eruption of Nyiragongo volcano (2002) − Lava flowed through the

nearby city of Goma(450,000 residents)

− Fortunately, toxic volcanic gases trapped in Like Kivu were not releasedDistribution of older lava flows gave scientists some

idea of the potential eruption products of Nyiragongo.



− Lava from Nyiragongo was several meters thick and burned and destroyed buildings in Goma



Poisonous volcanic gases from Lake Nyos, Cameroon, killed 1,700 people in surrounding villages (1986)


Go back to the Table of Contents

Go to the next section: Magma Viscosity


Magma Viscosity

• Escaping gases drive volcanic eruptions

• How easily gases escape from magma is controlled by magma viscosity

− Magma – molten rock below the surface

− Lava – molten rock at the surface

• Viscosity = resistance to flow− Viscosity depends upon temperature and

magma composition

− Magma composition varies with plate tectonic setting


Volcanoes and Other Mountains Conceptest


Place the following 4 materials – maple syrup, milk, peanut butter, frozen yoghurt – in the correct positions (A, B, C, D) for their relative viscosity.

Magma Viscosity


Viscosity = resistance to flow

− Viscosity of materials decreases with increasing temperature

− Viscosity varies with composition

Magma Viscosity

Escaping volcanic gases drive eruptions

• Gases are dissolved in magma

− Water vapor, carbon dioxide, sulfur dioxide

• Pressure on magma decreases as magma rises toward surface

• Gases are released as pressure decreases− Carbonated drink analogy




All other factors being equal, which magma would flow the fastest?

A. High viscosity magmaB. Low viscosity magmaC. Neither, magma does not have viscosity



How would the viscosity of motor oil in your car’s engine change from the time you turn the key in the ignition to when you have driven 50 km (30 miles)?

A. Viscosity would increase

B. Viscosity would decrease

C. Viscosity would stay the same

Magma Viscosity

• Silica is combination of oxygen and silicon that combines with other elements (e.g., sodium, potassium) to form minerals− Elements combine to form simple structures in

minerals with less silica = low viscosity− Elements combine to form more complex structures in

minerals with more silica = high viscosity

Viscosity depends upon magma composition (silica content)


Magma Viscosity





Which type magma most likely has the lowest viscosity?

A. High silica, high temperature

B. High silica content, low temperature

C. Low silica content, high temperature

D. Low silica content, low temperature

Magma Viscosity

More viscosity = More violent eruptions

• Gases escape easily during mild eruptions with low viscosity magma, e.g., Hawaii

• Gases escape explosively from high viscosity magma, e.g., Alaska

Mild or violent eruption?



Imagine taking a straw and bubbling air through liquids in two glasses –milk and a milkshake. Which liquid has the higher viscosity?

A. Milk

B. Milkshake


Hint: Gas (air) would escape more readily from which liquid?



How would you classify the viscosity of the magma that produced the eruption of Nyiragongo and the violence of the eruption?

A. Low-viscosity magma; violent eruptionB. High-viscosity magma; violent eruptionC. High-viscosity magma; mild eruptionD. Low-viscosity magma; mild eruption

Magma Viscosity

Q: Why does silica content vary?

A: Silica content controlled by magma source (plate tectonics)

. . . See next section




Go to the next section: Magma Sources and Magma Composition


Magma Sources and Magma Composition


• Most active volcanoes (top) found near convergent plate boundaries (bottom)− Others

associated with divergent boundaries (Iceland, East Africa) or form above hot spots (Hawaii)



Different plate settings generate magma from melting different source rocks

1. Basaltic magma –partial melting parts of asthenospherebelow oceanic ridge or hot spots− 80% of all

magma), also continental divergent boundary and hot spot

2. Rhyolitic magma -melting of parts of continental crust

12



Different plate settings yield different magmas from different source rocks

Oceanic Hot spotBasaltic magma

Divergent BoundaryOceanic RidgeBasaltic magma

Divergent BoundaryContinental Rift

Basaltic & Rhyoliticmagma



Different plate settings generate magma from melting different source rocks

3. Andesitic magma– partial melting of mantle rocks (with water) at subduction zone

3



Different plate settings yield different magmas from different source rocks

Convergent BoundaryOceanic Trench/Subduction ZoneAndesitic magma

Convergent BoundaryOceanic Trench/Subduction ZoneAndesitic magma



• Partial melting occurs when some minerals in a rock melt while others remain solid− Minerals with lowest melting temperatures will melt first

− Silica-rich minerals have lowest melting temperatures

• Partial melting generates a more silica-rich magma than the parent rock

Silica content is controlled by partial melting of rocks at magma source



Partial Melting Of Asthenosphere/Mantle wedge/Continental crust

. . . Low/Intermediate/High silica content

Three types of magma defined by silica content

Different magma types form in different plate tectonic settings

. . . generates Basaltic/Andesitic/Rhyolitic magma with



Which image best illustrates the source of most magma from active volcanoes?

Magma has a source in the molten rocks of the outer core

Magma comes from a source layer in the mantle

Magma comes from isolated sources below volcanoes

Volcanoes and Other Mountains Checkpoint 6.6


Use the Venn diagram to compare and contrast the compositions and sources of the 3 types of magma. 1. Low silica content2. From a mantle source3. Example: Aleutian Island

volcanoes, Alaska4.5.6.7.8.9.10.



1. Examine the map and identify 5 volcanoes formed above subduction zones.

2. Name a volcano that generates low-viscosity magma.

3. Other than Mauna Loa, which volcano is most likely to have formed above a hot spot?


Go to the next section: The Mount St. Helens Eruption


The Mount St. Helens Eruption


• Cascade Mountains – volcanic arc in Pacific Northwest− Major cities within 100 km of

active volcanoes− Mount St. Helens eruption of

May 18, 1980



• Cascade Mountains− Volcanoes formed above

subduction zone where Juan de Fuca plate slides beneath North America

− Mount St. Helens is most active volcano in conterminous US



What type of magma is associated with Mount St. Helens?

A. Basaltic

B. Rhyolitic

C. Andesitic



Prior Activity• Early (March) unrest featured

− Minor eruptions− Earthquakes− Release of volcanic gases

• Followed by change in shape of cone (bulge on North flank)

• Increasing frequency of earthquakes



May 18 Eruption• A moderate earthquake

triggered a massive landslide (debris avalanche) on the North side of the volcano− Debris clogged streams

− Pressure released on near-surface magma

− Lateral blast produces an initial sideways eruption to North

− Later vertical eruption



Measuring the Eruption• Volcanic Explosivity Index (VEI)

measures volume of erupted material− 8 divisions on log scale (x10

increase between divisions)



Measuring the Eruption• Mount St. Helens May 18

eruption had a VEI = 5

• Later eruptions were much smaller

• Eruption of VEI 5 or higher approximately every 22 years− Loss of life from

eruptions often associated with associated mudflows, tsunami


Go to the next section: Products of Volcanic Eruptions


Products of Volcanic Eruptions


Major products of volcanic eruptions:• Airborne – lateral blast, tephra, volcanic gases• Flows on land – lava, pyroclastic flows, lahars



• Eruption of Mount St. Helens reduced height of volcano by 400 meters

• Features near volcano were blown over or carried away by products of eruption

Geologist David Johnston (right) died at this site (Johnston’s Ridge) located 10 km from the volcano.



• Rare lateral blastscan destroy objects up to 12 km away and knock down trees more than 25 km distant− Effect of lateral

blast only seen on North flank of Mount St. Helens

Airborne Eruption Products



• Tephra represents particles blasted into air by eruption− Volcanic bombs and ash are found near and far from eruption

source, respectively


Blobs of magma solidify to form lava bombs

Wind can transport fine volcanic ash for hundreds of kilometers downwind



• Tephra represents particles blasted into air by eruption− Volcanic ash

(particles of <2 mm diameter)

− Measurable ash deposits found hundreds of km from volcano

− Compare Mt. St. Helens ash fall to the Yellowstone super-eruption 640,000 years ago.




• Volcanic gases (water vapor, sulfur dioxide, carbon dioxide) may affect climate patterns− Sulfur dioxide may

block insolation, temporarily (up to 1 year) reducing global temperatures

− Widespread release of carbon dioxide and higher temperatures due to faster rates of volcanic activity approximately 120-80 million years ago


Trees killed by excessive carbon dioxide released by magma under Mammoth Mountain, California.



Eruption Products on Land

• Low viscosity lava can flow up to 50 km from its source− Lava transported to

front of lava flows in long lava tubes

− Lava flows build up in a series of layers




• Low viscosity lava can flow up to 50 km from its source− Lava transported to

front of lava flows in long lava tubes

− Lava flows build up in a series of layers

Walter’s Kalapana store, Hawaii, was buried in lava within a few weeks in 1990




• Higher viscosity lava remains within volcano crater− Lava dome formed in crater of

Mount St. Helens



Eruption Products on Land• Pyroclastic flow – dense

cloud formed from combination of tephra and volcanic gases− Fast moving, up to 700 C




• Lahars – mudflows formed when volcanic debris mixes with streams or melting ice − Often confined to stream channels

Lahar along Muddy River reached depths of 20 meters following Mount St. Helens eruption



Predict which product of the volcanic eruption traveled farthest from Mount St. Helens?

A. Pyroclastic flow

B. Lava

C. Lahar



Compare and contrast the eruptions of Nyiragongo and Mt. St. Helens. Place the numbers in the appropriate locations on the diagram.

1. Located near a convergent plate boundary.2. Located near an early stage divergent plate

boundary.3. Produced significant lava flows.4. Eruption followed a century of inactivity.5. Several eruptions in the last century.6. Few monitoring instruments prior to unrest.7. Volcanic gases released prior to main eruption.8. Frequent earthquakes associated with unrest.9. Unrest lasted for approximately 2 months

before eruption. 10. Eruption occurred in daylight. 11. Volcano located within 20 km of large city. 12. Volcanic activity subsided after about one

week.13. Low viscosity magma. 14. USGS geologists aided in interpretation of

volcanic activity.

15. Death toll less than 100. 16. Death toll more than 100. 17. Shape of volcano changed prior to eruption.18. Eruption characterized by a massive lateral

blast.


Go to the next section: Volcanoes and Volcanic Landforms


Volcanoes and Volcanic Landforms


Three types of volcanoes• Shield, stratovolcanoes, cinder cone

• Composed of different materials

• Volcano type and eruption style varies with plate setting

• Different sizes (note size of trees on volcano slopes)

Shield volcano Stratovolcano Cinder cone volcano



Shield volcanoes (e.g., Hawaiian Islands)• Broad, gentle slopes• Built from many low viscosity lava flows (basalt)• Relatively mild eruptions associated with hot spots,

divergent plate boundaries



Stratovolcanoes (e.g., Osorno, Chile)• Most common volcano type• Steeper slopes built from alternating layers of tephra and

medium viscosity lava (andesite) • Form on plates overriding subduction zones at convergent

plate boundaries



Cinder cone volcanoes• Smallest volcanoes, up to 400 meters elevation• Built from more viscous magma products (coarse tephra)• May form on slopes of shield or stratovolcanoes

Sunset Crater volcano, Arizona



What type of volcano is Mount St. Helens?

A. Shield volcano

B. Stratovolcano

C. Cinder cone volcano



Other Volcanic Landforms - Caldera• Giant crater formed from collapse of volcano into

underlying magma chamber • e.g., Yellowstone, Wyoming

Crater of Karymskyvolcano

Caldera formed from collapse of previous volcano



Other Volcanic Landforms – Lava Plateau

• Hundreds of low viscosity lava flows stack up to on top of each other

• Individual layers of basalt 10-20 meters thick

• Plateau thousands of meters thick

• Form some of the largest volcanic eruptions

• e.g., Columbia River plateau



Lava Plateau• Massive basalt eruptions formed lava plateaus and elevated

atmospheric levels of carbon dioxide 120-80 Myrs ago to produce global “hothouse” conditions



Geysers, Hot Springs, Fumeroles, Mud Volcanoes• Form when water circulates near magma chamber

− Geyser – water heated under pressure with volcanic gases− Hot spring – heated groundwater rises to surface− Mud volcano – chemical reactions convert rock to clay− Fumerole – volcanic gases escape in absence of water

Geyser Hot Spring Mud volcano Fumerole


Go to the next section: Mountains: Why Are They There?


Mountains: Why Are They There?


• Thickest crust found below mountains along convergent plate boundaries− Himalayas, 70 km thick− Andes, up to 60 km thick− “normal” crust 40 km thick

Himalayas - thickest continental crust and tallest mountains



• Higher, younger mountains along present convergent boundaries (e.g., Himalayas)

• Lower, older mountain belts represent ancient convergent boundaries (e.g., Appalachians)



• Reverse faults stack up and thicken the crust along convergent plate boundaries − Additional

thickening of crust where the northern margin of Indian continental crust wedged below southern margin of Eurasia

India Eurasia

India Eurasia



If the crust is 30 km thicker under the Himalayas, whey are they not 30 km higher than the rest of the continents?

The elevation of mountains is tied to density and isostasy.



• Wood floats because it is less dense than water− Density of pine is half (50%)

the density of water

− Half of the pine block lies below the water surfaceWhat would happen if we used

a block of oak with a density that is 80% that of water?

Density = mass per unit volume− Density of water = 1 g/cm3

− Density of pine wood = 0.5 g/cm3



• Density of oak is 80% density of water − 80% of oak blocks lies

below the surface

− Smaller blocks don’t float as high but don’t extend as far below surface

− Much of the difference in the size of the blocks is in the submerged “root”



Different types of wood have different densities. For example, the density of pine (0.5 g/cm3) is less than ebony (0.9 g/cm3) but more than balsa wood (0.14 g/cm3). All would float in water but with different proportions of each block lying above and below the surface.

1. Draw a diagram to show what would happen if equal-sized blocks of each type of wood were added to a container full of water.

2. What would happen to the blocks if we replaced the water with a liquid with higher density like corn syrup?



• Density of continental crust is 80% density of mantle− Thicker continental

crust rises higher but also extends farther below the surface

− Much of the difference in the thickness of the continental crust is in the crustal “root”

− Similar to deeper foundation for taller buildings



• Elevation of mountains depends on − Thickness of crust− Density contrast with

underlying mantle

• Isostasy – balance between topography of Earth’s surface and thickness and density of underlying rocks− Higher mountains with

thicker or less dense rocks



The following profile shows the topography of a continent. Draw the relative position of the base of the crust taking into account the principles of isostasy. Label continental crust, oceanic crust, and mantle. Explain your drawing.


Go to the next section: The Rise and Fall of Mountains and Temperatures


The Rise and Fall of Mountains and Temperatures


The height of wood blocks and continents will increase (float higher) as mass is added

The height of wood blocks and continents will decrease (float lower) as mass is removed

• Changes in elevation depends on the relative density of crust and mantle

Elevation only changes by 20% of added/removed material

Most changes occur in “crustal root” below the surface



• Isostasy compensates for added material by building a bigger root or for lost material by raising the pile

Elevation only changes by 20% of added/removed material

Most changes occur in “crustal root” below the surface

How would the elevations of mountains differ if Earth’s crust was composed of denser rocks? Mountains would be

A. Higher

B. Lower

C. Unchanged in elevation





• For every 1,000 meters of rock eroded from mountains, isostasy results in just 200 meters decrease in elevation − 800 meters of change

accommodated by raising the crustal root

− Evenly distributed erosion causes uniform lowering of mountains

− Erosion concentrated in valleys, can cause peaks to become higher



• Mountains are long-lived features on Earth − Still forming Himalayas began to form ~50 Myrs ago − Lower Appalachian Mountains formed ~300 Myrs ago



• Mountains are long-lived features on Earth − Approximate erosion rates 0.1-0.2 mm/year

− Rate depends of rock types, climate, other factors

− 5-10 Myrs to erode 1 km of rock

− But isostasy will replace 80% of this erosion

− 200 meter change in elevation every 5-10 Myrs

− 25-50 Myrs to lower mountains by 1 km

• Mountains ranges will persist on the landscape for hundreds of millions of years



Use the information from this section to explain:

1. Why are the ten tallest U.S. peaks all in Alaska?2. Why are the Rocky Mountains taller than the

Appalachian Mountains? 3. Why we can drive across former mountains in

Canada without rising in elevation?



• Erosion of the Himalayan Mountains− Warm, moist air from

Indian Ocean rises over Himalayas to form monsoon rains

− Rain feed rivers that erode mountains

− Sediment deposited in Bay of Bengal and Arabian Sea



• Mountains influence climate patterns − Monsoon rains strip

carbon dioxide from atmosphere

− Removal of CO2 has been going on for 20 Myrs

− Reduction of this greenhouse gas has lowered global temperatures by ~5oC

Use information from this chapter to identify interactions between volcanoes and the earth

system.


The End



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