Objectives

• Describe factors that affect the formation of magma.

Magma

• Compare and contrast the different types of magma.

– viscosity

Vocabulary

• In the last 10 000 years, more than 1500 different volcanoes have erupted, providing evidence that Earth is indeed geologically active.

Magma

• The ash that spews from some volcanoes can form billowy clouds that travel around the world before raining back down to Earth.

Magma

How Magma Forms

• Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases that fuels all volcanoes.

Magma

• Magma forms when temperatures are high enough to melt the rocks involved, usually between 800°C and 1200°C.

• Such temperatures exist at the base of the lithosphere and in the asthenosphere.

– Pressure, which increases with depth, is one factor that determines whether rocks will melt to form magma.

– As pressure increases, the temperature at which a substance melts also increases.

How Magma Forms

Pressure

Magma

– Due to the effects of pressure, most of the rocks in Earth’s lower crust and upper mantle do not melt to form magma.

– The presence of water also influences whether a rock will melt.

How Magma Forms

Water

Magma

– At any given pressure, a wet mineral or rock will melt at a lower temperature than the same mineral or rock under dry conditions.

Types of Magma

• The three major types of magma are: basaltic magma, andesitic magma, and rhyolitic magma.

Magma

– Basaltic magma has the same composition as basalt and fuels the volcanoes that make up the Hawaiian Islands and Surtsey, which is south of Iceland.

– Andesitic magma has the same composition as andesite and fuels Mount St. Helens in Washington State and Tambora in Indonesia.

– Rhyolitic magma has the same composition as granite and fueled the dormant volcanoes in Yellowstone National Park.

Types of Magma

Magma

– A number of factors determine the composition of magma.

Types of Magma

Magma Composition

Magma

– Viscosity is the internal resistance to flow; the higher the viscosity, the thicker the magma.

– Basaltic magma typically forms when rocks in the upper mantle melt.

– Most basaltic magma rises relatively rapidly to Earth’s surface and reacts very little with crustal rocks because of its low viscosity.

– The volcanoes fueled by basaltic magma erupt relatively quietly.

Types of Magma

Basaltic Magma

Magma

– Andesitic magma is found along continental margins, where oceanic crust is subducted into Earth’s mantle, and is formed from oceanic crust or oceanic sediments.

– Andesitic magma contains about 60 percent silica, resulting in an intermediate viscosity.

– The volcanoes it fuels are said to have intermediate eruptions.

Types of Magma

Andesitic Magma

Magma

– Rhyolitic magma forms when molten material rises and mixes with the overlying silica- and water-rich continental crust.

– Rhyolitic magma has high viscosity and fuels very explosive volcanoes.

Types of Magma

Rhyolitic Magma

Magma

Viscosity

• The viscosity of magma and lava depends on both temperature and composition.

Magma

– The hotter the magma or lava, the lower the viscosity.

– Magmas and lavas high in silica have higher viscosities than magmas and lavas low in silica.

Section Assessment

1. Match the magma types with their characteristics.

___ basaltic

___ andesitic

___ rhyolitic

Magma

A. intermediate viscosity content, forms from oceanic crust and oceanic sediments

B. low viscosity and gas content, forms from rocks in the upper mantle

C. high viscosity, forms from continental crust materials

B

A

C

Section Assessment

2. What would be the likely effect if the volcano at Yellowstone National Park were to erupt? Why?

Magma

It would most likely be a devastating eruption because it would be fueled by rhyolitic magma, which has a very high viscosity and gas content.

______ It is unlikely that Mount Kilauea in Hawaii will explosively erupt.

______ Wet granite will melt at a lower temperature than dry granite.

______ A liquid with a high viscosity will also have a high flow rate.

______ Major eruptions of Mount St. Helens in Washington state and Mount Fuji in Japan would probably be similar in nature.

Section Assessment

3. Identify whether the following statements are true or false.

Magma

true

true

false

true

End of Section 1

– vent

– crater

– caldera

– shield volcano

Objectives

• Describe the major parts of a volcano.

• Compare and contrast shield, cinder-cone, and composite volcanoes.

• Contrast the volcanism that occurs at plate boundaries.

• Explain the relationship between volcanism and hot spots.

Vocabulary

Volcanoes

– cinder-cone volcano

– composite volcano

– tephra

– pyroclastic flow

– hot spot

Anatomy of a Volcano

• When magma reaches Earth’s surface it is called lava.

Volcanoes

• A vent is where lava erupts through an opening in the crust.

• As lava flows out onto the surface, it cools and solidifies around the vent, eventually accumulating to form a mountain known as a volcano.

• A crater is a bowl-shaped depression at the top of a volcano that is connected to the magma chamber by a vent.

Anatomy of a Volcano

• Volcanic craters are usually less than 1 km in diameter.

Volcanoes

• Calderas are large depressions up to 50 km in diameter that can form when the summit or the side of a volcano collapses into the magma chamber that once fueled the volcano.

Anatomy of a Volcano

Volcanoes

Types of Volcanoes

• The appearance of a volcano depends on two factors:

Volcanoes

• Based on these two criteria, three major types of volcanoes have been identified:

– The type of material that forms the volcano

– The type of eruptions that occur

– Shield volcanoes

– Cinder-cone volcanoes

– Composite volcanoes

Types of Volcanoes

Shield Volcanoes

Volcanoes

– A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base that forms when layer upon layer of basaltic lava accumulates during nonexplosive eruptions.

Types of Volcanoes

Cinder-Cone Volcanoes

Volcanoes

– A cinder-cone volcano is a generally small, steep-sided volcano that forms when material ejected high into the air falls back to Earth and piles up around the vent.

– The magma that fuels these volcanoes contains more water and silica than shield volcanoes, which makes them more explosive in nature.

Types of Volcanoes

Composite Volcanoes

Volcanoes

– Composite volcanoes are large volcanoes that form when layers of volcanic fragments alternate with lava.

– The magma that forms composite volcanoes commonly contains large amounts of silica, water, and gases, making these volcanoes violently explosive.

Types of Volcanoes

Sizes and Shape

Volcanoes

– Shield volcanoes are by far the largest, and cinder-cone volcanoes are the smallest.

– Cinder-cone volcanoes have the steepest slopes, while shield volcanoes have the gentlest slopes.

– The slopes of cinder-cone and composite volcanoes are concave, and the slopes of shield volcanoes are straight.

Volcanic Material

• Tephra are rock fragments thrown into the air during a volcanic eruption.

Volcanoes

• Tephra are classified by size, the smallest being dust (less than 0.25 mm) and ash (0.25–2 mm).

• Somewhat larger fragments are called lapilli, or “little stones” (2–64 mm in diameter).

• The largest tephra thrown from a volcano include angular volcanic blocks and rounded or streamlined volcanic bombs, both of which can be the size of a house or larger.

Volcanic Material

Pyroclastic Flows VIDEO

Volcanoes

– Some tephra cause tremendous damage and kill thousands of people.

– A pyroclastic flow is a cloud of volcanic gas, dust, and other tephra traveling at speeds of nearly 200 km/h.

– The temperature at the center of a pyroclastic flow can exceed 700°C.

Where do volcanoes occur?

• Most volcanoes form at plate boundaries.

Volcanoes

– About 80 percent of all volcanoes are found along convergent boundaries.

– About 15 percent are found along divergent boundaries.

– Only about 5 percent of extrusive igneous activity occurs far from plate boundaries.

Where do volcanoes occur?

Convergent Volcanism

Volcanoes

– Convergence involving oceanic plates creates subduction zones, and the magma generated is forced upward through the overlying plate and forms volcanoes when it reaches the surface.

– The volcanoes associated with convergent plate boundaries form two major belts:

• The larger belt, the Circum-Pacific Belt, is also called the Pacific Ring of Fire.

• The smaller belt is called the Mediterranean Belt.

Where do volcanoes occur?

Volcanoes

Where do volcanoes occur?

Divergent Volcanism

Volcanoes

– At divergent plate boundaries, magma is forced upward into the fractures and faults that form as the plates separate.

– These areas of major faults and fractures are called rift zones.

– Most of the world’s rift volcanism occurs under water along ocean ridges.

Where do volcanoes occur?

Hot Spots

Volcanoes

– Some volcanoes are located far from plate boundaries and form as the result of hot spots.

– Hot spots are unusually hot regions of Earth’s mantle where high-temperature plumes of mantle material rise toward the surface.

– A plume does not move laterally, which results in a trail of progressively older volcanoes that formed as a plate moved over a hot spot.

Where do volcanoes occur?

Hot Spots

Volcanoes

– The Hawaiian Islands continue to rise above the ocean floor as the Pacific Plate moves slowly over a hot spot.

– The rate and direction of plate motion can be calculated from the positions of volcanoes in a chain that has formed over a hot spot.

Where do volcanoes occur?

Hot Spots

Volcanoes

– Hot spots can result in the formation of flood basalts.

– Flood basalts erupt from fissures rather than a central vent and form flat plains or plateaus.

– Volcanic activity is proof that Earth is a dynamic planet.

Section Assessment

1. Match the following terms with their definitions.

___ vent

___ crater

___ tephra

___ pyroclastic flow

___ caldera

Volcanoes

A. an opening in the crust from which lava flows

B. depression caused by a collapsed magma chamber

C. a bowl-shaped depression around an opening in the crust

D. volcanic materials that are thrown into the air during a volcanic eruption

E. a cloud of rapidly moving, extremely hot volcanic material

A

C

D

E

B

Section Assessment

2. How can chains of volcanoes that form over a hot spot track plate movement?

Volcanoes

The hot spot is in a fixed location. All of the volcanoes in the chain were over the hot spot when they formed. The volcanoes’ movement and the direction of the chain’s alignment indicates the movement of the plate.

Section Assessment

Volcanoes

3. Identify whether the following statements are true or false.

______ Many cinder-cone volcanoes are less than 500 m high.

______ Volcanism is more common along divergent boundaries as compared to convergent boundaries.

______ The Mediterranean Belt is also known as the “Ring of Fire”.

______ It can be 700ºC in the center of a pyroclastic flow.

true

false

false

true

End of Section 3

Chapter Resources Menu

Study Guide

Section 18.1

Section 18.2

Section 18.3

Chapter Assessment

Image Bank

Section 18.1 Main Ideas

• Temperature, pressure, and the presence of water are factors that affect the formation of magma.

• As pressure increases, the temperature at which a substance melts also increases. At any given pressure, the presence of water will cause a substance to melt at a lower temperature than the same substance under dry conditions.

• There are three major types of magma: basaltic magma, andesitic magma, and rhyolitic magma. These magmas differ in the source rock from which they form, viscosity, silica content, gas content, and explosiveness. Basaltic magma is the least explosive magma; rhyolitic magma is the most explosive.

Section 18.1 Study Guide

Section 18.2 Main Ideas

• Magmatic intrusions affect the crust in several ways. Magma can force overlying rock apart and enter the newly formed fissures. Magma can also cause blocks of rock to break off and sink into the magma chamber. Magma can melt the rock into which it intrudes.

• Batholiths, stocks, sills, dikes, and laccoliths are plutons that are classified according to their size, shape, and relationship to surrounding rocks. Batholiths are the largest plutons and often form the cores of many of Earth’s major mountain chains.

Section 18.2 Study Guide

Section 18.3 Main Ideas

• Lava flows onto Earth’s surface through a vent. Over time, multiple lava flows may accumulate to form a volcano. A crater is a depression that forms around the vent at the summit of a volcano. A caldera is a large crater that forms when a volcano collapses during or after an eruption.

• There are three types of volcanoes: shield volcanoes, cinder-cone volcanoes, and composite volcanoes.

• Rock fragments ejected during eruptions are called tephra.

Section 18.3 Study Guide

Section 18.3 Main Ideas

• Most volcanoes form along convergent and divergent plate boundaries. Volcanoes also form over hot spots, which are unusually hot areas in the mantle that are stationary for long periods of time.

• Flood basalts form when lava flows from fissures to form flat plains or plateaus.

Section 18.3 Study Guide

1. Andesitic magma has a silica content of about ___ percent.

a. 50 c. 70

b. 60 d. 80

Multiple Choice

Chapter Assessment

Basaltic magma has a silica content of about 50%. Rhyolitic magma has a silica content of about 70%.

2. Which of the following is the largest type of pluton?

a. stock c. batholith

b. laccolith d. sill

Batholith plutons are common in the interiors of many mountain chains and are the result of the magma generated during subduction at convergence zones.

Multiple Choice

Chapter Assessment

Multiple Choice

3. Shield cone volcanoes are fueled by what kind of magma?

a. basaltic c. tephra

b. andesitic d. rhyolitic

Chapter Assessment

Basaltic magma has a low viscosity and erupts with low explosiveness. This allows the lava to flow down the mountain’s slopes and continually add to the mountain.

Multiple Choice

4. Where is divergent volcanism generally found?

a. ocean rifts c. ocean trench

b. mid-plate d. Abyssal plains

Chapter Assessment

Divergent volcanism is almost entirely found underwater (Iceland is the major exception) at ocean rifts. Through divergent volcanism, new sea floor is created as the plates spread apart.

Multiple Choice

5. Where are most active volcanoes located?

a. divergent c. hot spotsboundaries

b. convergent d. transform boundaries boundaries

Chapter Assessment

Most active volcanoes are in the either the Circum-Pacific or Mediterranean Volcanic Belts, which are located on convergent boundaries.

Short Answer

6. How does the presence of water influence whether a rock will melt?

Chapter Assessment

At any given pressure, a wet mineral or rock will melt at a lower temperature than the same mineral or rock under dry conditions.

Short Answer

7. What is the relationship between viscosity and the temperature of magma or lava?

Chapter Assessment

The hotter the magma or lava, the lower the viscosity.

True or False

8. Identify whether the following statements are true or false.

______ Andesitic magma has more explosive potential than rhyolitic magma.

______ A dike forms when magma intrudes parallel to layers of rock.

______ Cinder-cone volcanoes can form on the flanks of a shield volcano.

______ Many of the volcanoes in the Cascade range are composite volcanoes.

______ The hot spot under Hawaii has existed for at least 75 million years.

Chapter Assessment

false

false

true

true

true

Image Bank

Chapter 18 Images

Image Bank

Chapter 18 Images

Image Bank

Chapter 18 Images

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