Download Chapter 7 Volcanoes Notes

Chapter 7: Volcanoes
Section 1: Volcanoes and Plate Tectonics
Volcano: a weak spot in the crust where molten material, or magma, comes to the surface
Magma: a molten mixture of rock-forming substances, gases, and water from the mantle
Lava: magma that reaches the surface
1. Volcanoes and Plate Boundaries
a. About 600 active volcanoes on land
i. More lie beneath the oceans
b. Volcanoes occur in belts that extend across continents and oceans
i. Ring of Fire: a major volcanic belt formed by the many volcanoes that rim the Pacific Ocean
ii. Volcanic belts form along plate boundaries
iii. Most volcanoes form along diverging plate boundaries such as mid ocean ridges and along
converging plate boundaries where subduction takes place
1. Ex: Mt. Etna
c. Diverging Boundaries
i. Volcanoes form along the mid ocean ridges, which marks divergent boundaries
ii. Rift valleys are sometimes down the center of ridges
1. Here, lava pours out of cracks in the ocean floor, building up new mountains
iii. Volcanoes form along diverging plate boundaries on land too
1. Ex: There are several large volcanoes along the Great Rift Valley in East Africa
d. Converging Plate Boundaries
i. Volcanoes occur where oceanic plates return to the mantle
1. May occur where two oceanic plates collide
2. May also occur where an oceanic plate collides with a continental plate
3. The melted plate and rock are less dense than the surrounding rock so it rises
toward the surface
a. When the magma breaks through the surface it causes volcanoes
ii. Island Arc: a string of islands resulting from volcanoes
1. The curve of the arc echoes the curve of its deep-ocean trench
2. Examples: Japan, New Zealand, Indonesia, the Philippines, the Aleutians, and the
Caribbean islands
iii. Volcanoes occur where oceanic plates collide with continental plates
1. Has produces the volcanoes of the Andes Mountains in South America and the
volcanoes of the Pacific Northwest in the US
2. Hot Spot Volcanoes
a. Hot Spot: an area where material from deep within the mantle rises and then melts, forming
magma
b. A volcano forms above a hot spot when magma erupts through the crust and reaches the surface
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i. Some lie in the middle of plates far from any plate boundaries
ii. Others occur on or near plate boundaries
c. Hot spots can gradually form a series of volcanic mountains
i. Ex: Hawaiian Islands formed one by one over millions of years as the Pacific Plate drifted
over a hot spot
d. Hot spots can also occur under the continents
i. Ex: Yellowstone National Park in Wyoming
Section 2: Properties of Magma
1. Physical and Chemical Properties
a. Magma and Lava are made up of elements and compounds
i. Element: a substance that cannot be broken down into other substances
ii. Compound: a substance made of two or more elements that have been chemically
combined
iii. Each substance has a particular set of physical and chemical properties. These properties
can be used to identify a substance or to predict how it will behave.
b. Physical Properties: any characteristic of a substance that can be observed or measured without
changing the composition of the substance
i. Ex: density, hardness, melting point, boiling point, and magnetism
ii. A substance always has the same physical properties under particular conditions
1. Ex: Water’s freezing point is 0°C and its boiling point is 100°C, and it’s a liquid in between
c. Chemical Properties: any property that produces a change in the composition of matter
i. Something new is produced…a gas, change in color, new solid substance
ii. Ex: ability to burn and ability to combine, or react
2. What is Viscosity?
a. Viscosity: the resistance of a liquid to flowing
i. Liquids differ in viscosity, some flow more easily than others
ii. The greater the viscosity, the slower it flows
1. Ex: Honey
iii. The lower the viscosity, the faster it flows
1. Ex: Water
b. Different liquids have different viscosities based on the movement of the particles that make up
each type of liquid
i. Some liquids have a greater degree of friction among the particles and have a higher
viscosity
3. Viscosity of Magma
a. The viscosity of magma depends upon its silica content and temperature
b. Silica Content
i. Silica is the main ingredient of magma
1. Silica: made up of the elements silicon and oxygen
2. One of the most abundant materials in Earth’s crust
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3. Silica content of magma ranges from about 50% to 70%
ii. The more silica magma contains, the higher its viscosity
1. Magma high in silica produces light-colored lava that is too sticky to flow very far
2. This type of lava cools to form the rock rhyolite, which has the same composition as
granite
iii. The less silica magma contains, the lower its viscosity
1. Low silica magma flows readily and produces dark-colored lava
2. It cools to form rocks like basalt
c. Temperature
i. Viscosity increases as temperature decreases
ii. The temperature of lava can vary from about 750°C to 1,175°C
1. The hotter the magma, the lower its viscosity
iii. Pahoehoe: fast-moving, hot lava that has low viscosity
1. Looks like a solid mass of wrinkles
iv. Aa: lava that is cooler and slower-moving
1. Higher viscosity than pahoehoe
2. Forms a rough surface consisting of jagged lava chunks
Section 3: Volcanic Eruptions
1. Magma Reaches Earth’s Surface
a. Inside a Volcano
i. All volcanoes have a pocket of magma beneath the surface and one or more cracks
through which the magma forces its way
ii. Magma Chamber: a pocket beneath a volcano where magma collects
iii. The magma moves upward through a pipe
1. Pipe: a long tube in the ground that connects the magma chamber to Earth’s
surface
iv. Vent: an opening where molten rock and gas leave the volcano
1. There is often a central vent at the top of a volcano
2. Many volcanos also have other vents that open on the volcano’s sides
3. Lava Flow: the area covered by lava as it pours out of a vent
4. Crater: a bowl-shaped area that may form at the top of a volcano around the
central vent
b. A Volcanic Eruption
i. Dissolved gases are trapped in magma (Similar to carbon dioxide in pop)
1. The gases are under tremendous pressure
2. As magma rises toward the surface, the pressure of the surrounding rock on the
magma decreases
3. The dissolved gases begin to expand, forming bubbles
4. As pressure falls within the magma, the size of the gas bubbles increases greatly
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5. When a volcano erupts, the force of the expanding gases pushes magma from
the magma chamber through the pipe until it flows or explodes out of the vent.
6. One magma becomes lava, the gases bubble out
2. Kinds of Volcanic Eruptions
a. Geologists classify volcanic eruptions as quiet or explosive
i. The physical properties of its magma determine how a volcano erupts
ii. Whether an eruption is quiet or explosive depends on the magma’s silica content and
viscosity
b. Quiet Eruptions
i. Low silica content in magma
ii. Low-silica magma has low viscosity and flows easily
iii. Lava oozes quietly from the vent and can flow for many kilometers
iv. Can produce pahoehoe and aa
1. Hawaiian islands formed from quiet eruptions
2. Lava pours out of the crater near the top of Mt Kilauea
3. Lava also flows out of long cracks on the volcano’s sides
c. Explosive Eruptions
i. High silica content in magma
ii. High-silica content magma has high viscosity, making it thick and sticky
1. Does not always flow out of the crater
2. It builds up in the volcano’s pipe, plugging it like a cork in a bottle
3. Dissolved gases cannot escape
a. They build up pressure until they explode
b. The erupting gases and steam push the magma out of the volcano with
incredible force
c. Ex: Mt. St. Helens May 19, 1980
iii. An explosive eruption creates fragments of cooled lava of all sizes
1. The smallest are volcanic ash—fine, rocky particles as small as a speck of dust
2. Pebble-sized particles are called cinders
3. Bombs may range from the size of a baseball to the size of a car
4. Pyroclastic flow: occurs when an explosive eruption hurls out a mixture of hot
gases, ash, cinders, and bombs
iv. Pumice and obsidian form from high-silica lava
1. Pumice forms when gas bubbles are trapped in fast-cooling lava
2. Obsidian forms when lava cools very quickly, giving it a smooth, glossy surface
d. Volcano Hazards
i. During a quiet eruption, lava flows from vents, setting fire to, and then burying,
everything in its path
1. Can cover large areas with a thick layer of lava
ii. During an explosive eruption, hot clouds of deadly gases as well as ash, cinders, and
bombs are released
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1. Can bury entire towns
2. If the ash becomes wet, it can be heavy enough to collapse roofs
3. Ash may stall jet plane engines
4. Can cause landslides and avalanches of mud, melted snow, and rock
3. Stages of Volcanic Activity
a. Activity of a volcano may last from less than a decade to more than 10 million years
b. Life Cycle of a Volcano
i. Geologists often use the terms active, dormant, or extinct to describe a volcano’s stage
of activity
ii. An active, or live, volcano is one that is erupting or has shown signs that it may erupt in
the near future
iii. Dormant Volcano: a “sleeping” volcano that may awaken in the future and become
active
1. May become active at any time
iv. Extinct Volcano: a “dead” volcano that is unlikely to erupt again
c. The time between volcanic eruptions may span hundreds to many thousands of years
d. Monitoring Volcanoes
i. Geologists are more successful at predicting volcanic eruptions than earthquakes
ii. They use instruments to detect changes in and around a volcano that may give warning
before a volcano erupts
1. Geologists cannot be certain about what type of eruption will occur
2. Use tiltmeters and other instruments to detect changes in elevation and tilt
caused by magma moving underground
3. They monitor any gases escaping from the volcano and temperature increases in
underground water
4. Monitor small earthquakes that occur around a volcano that may lead to the
upward movement of magma
Section 4: Volcanic Landforms
1. Volcanic eruptions create landforms made of lava, ash, and other materials.
a. Include shield volcanoes, cinder cone volcanoes, composite volcanoes, and lava plateaus
b. Shield Volcanoes
i. A wide, gently sloping mountain
ii. Formed with thin layers of lava pour out of a vent and harden on top of previous layers
iii. Created the Hawaiian islands
c. Cinder Cone Volcanoes
i. Ash, cinders, and bombs build up around the vent in a steep, cone-shaped hill or mountain
ii. Paricutin in Mexico built up a cinder cone about 400 meters high
d. Composite Volcanoes
i. Tall, cone-shaped mountains in which layers of lava alternate with layers of ash
ii. Formed when lava flows alternate with eruptions of ash, cinder, and bombs
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iii. Mount Fuji in Japan and Mount St. Helens in Washington
e. Lava Plateaus
i. Some eruptions of lava form high, level areas
ii. Lava flows out of several long cracks in an area, the thin lava flows a long way before
cooling and solidifying, and the layers flow on top of each other forming a high plateau
iii. Columbia Plateau in Washington, Oregon, and Idaho
f. Calderas
i. A huge hole left by the collapse of a volcanic mountain
ii. The hole is filled with pieces of the volcano that have fallen inward
iii. Form when an enormous eruption empties the main vent and the magma chamber beneath
a volcano causing the mountain to become hollow
1. The top of the mountain collapses inward
iv. Ex: Crater Lake in Oregon
1. The caldera that formed eventually filled with rain and snow
g. Soils from Lava and Ash
i. People live near active volcanoes to take advantage of the fertile volcanic soil
ii. When the hard lava, ash, and cinders break down to form soil, it releases potassium,
phosphorus, and other substances needed by plants
iii. Some volcanic soils are among the richest in the world
2. Landforms from Magma
a. Magma sometimes tries to force its way through cracks in the crust, but fails to reach the surface
i. The magma hardens and forces of erosion eventually strip away the layers above the
magma and expose it
ii. Features formed include volcanic necks, dikes, and sills, as well as batholiths and dome
mountains
b. Volcanic Necks
i. Forms when magma hardens in a volcano’s pipe
ii. The softer rock wears away leaving the hard rock of the volcanic neck
iii. Ex: Ship Rock in New Mexico
c. Dikes and Sills
i. A dike is formed when magma that forces itself across rock layers
ii. A sill is formed when magma squeezes between horizontal layers of rock
1. Ex: Palisades in New York and New Jersey
d. Batholiths
i. A mass of rock formed when a large body of magma cools inside the crust
ii. Form the core of many mountain ranges
e. Dome Mountains
i. Forms when uplift pushes a batholith or smaller body of hardened magma toward the
surface
ii. Ex: Black Hills in South Dakota
3. Geothermal Activity
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a.
b.
c.
d.
Geo means “Earth”
Therme means “heat”
Magma a few kilometers beneath Earth’s surface heats underground water
Geothermal activity includes hot springs and geysers
i. Hot Springs
1. Forms when groundwater is heated by a nearby body of magma or by hot rock deep
underground
2. The hot water rises to the surface and collects in a pool
ii. Geysers
1. A fountain of water and steam that erupts from the ground
a. Rising hot water and steam become trapped underground in a narrow crack
b. Pressure builds until it eventually sprays above the surface
iii. Geothermal Energy
1. Energy source produced when water is heated by magma
2. In Iceland, people pipe this hot water into their homes for warmth
3. Can also be used as electricity
4. Steam from underground can be used to spin wheels in turbines which will generate
the energy of motion into electrical energy
5. Produces some of the energy in California and New Zealand
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