Download 11.3 Mountain Formation and more Study notes History in PDF only on Docsity!
Mountain Building 317
11.3 Mountain Formation
Reading Strategy
Outlining As you read, make an outline of the important ideas in this section. Use the green topic headings as the main topics and the blue headings as subtopics.
Key Concepts
What mountains are associated with convergent plate boundaries? What mountains are associated with divergent plate boundaries? How is isostatic adjustment involved in mountain formation?
Vocabulary
◆ accretionary wedge ◆ accretion ◆ terrane ◆ isostasy ◆ isostatic adjustment
Mountain building still occurs in several places worldwide. For
example, the Himalayas began to form 45 million years ago and are still rising. Older mountain ranges, such as the Appalachians in the eastern United States, are deeply eroded, but they have many features found in younger mountains. Many hypotheses have been proposed to explain mountain for- mation. One early proposal suggested that mountains are wrinkles in Earth’s crust, produced as the planet cooled from its early semi-molten state. People believed that as Earth cooled, it contracted and shrank. In this way, the crust was deformed the way an apple peel wrinkles as it dries out. However, this early hypothesis and many others were not able to withstand careful analysis and had to be discarded.
Mountain Building at
Convergent Boundaries
With the development of the theory of plate tectonics, a widely accepted model for oroge- nesis became available. Most mountain building occurs at convergent plate bound- aries. Colliding plates provide the compressional forces that fold, fault, and metamorphose the thick layers of sediments deposited at the edges of landmasses. The partial melting of mantle rock also provides a source of magma that intrudes and further deforms these deposits.
Figure 11 Young Mountains The Grand Tetons of Wyoming are an example of relatively young mountains.
I. Mountain Formation A. Mountain Building at Convergent Boundaries
- Ocean-Ocean Convergence
- a.
- b. B. Mountain Building at Divergent Boundaries
?
?
FOCUS
Section Objectives 11.8 Identify the type of mountains associated with convergent plate boundaries. 11.9 Distinguish between mountains formed by ocean- ocean convergence and mountains formed by ocean- continental convergence. 11.10 Identify the type of mountains associated with divergent plate boundaries. 11.11 Explain how isostatic adjustment is involved in mountain formation.
Build Vocabulary Paraphrase After students have read the definition of accretionary wedge on p. 319, but before they have learned the definition of accretion on p. 321, have them write a short paragraph describing in their own words the process of accretion. After they have read the definition of accretion , ask if they would change their definitions.
Reading Strategy I. Mountain Formation A. Mountain Building at Convergent Boundaries
- Ocean-Ocean Convergence
- Ocean-Continental Convergence
- Continent-Continent Convergence B. Mountain Building at Divergent Boundaries C. Non-Boundary Mountains D. Continental Accretion
- Terranes
- Mountains from Accretion E. Principle of Isostasy
- Isostatic Adjustment for Mountains
L
L
Reading Focus
Mountain Building 317
Section 11.
318 Chapter 11
Subduction zone
Asthenosphere
Melting
Volcanic island arc
100 km
Subducting oceanic lithosphere
Subduction zone
Melting
Volcanic island arc
Sedimentation
Asthenosphere
100 km
Subducting oceanic lithosphere
A B
Figure 12 A A volcanic island arc develops due to the convergence of two oceanic plates. B Continued subduction along this type of convergent boundary results in the development of volcanic mountains.
Ocean-Ocean Convergence
Ocean-Ocean Convergence Ocean-ocean convergence occurs
where two oceanic plates converge and one is subducted beneath the other, as shown in Figure 12. The converging plates result in partial melting of the mantle above the subducting plate and can lead to the growth of a volcanic island arc on the ocean floor. Because they are associated with subducting oceanic lithosphere, island arcs are typically found on the margins of a shrinking ocean basin, such as the Pacific. These features tend to be relatively long-lived. Here, somewhat sporadic volcanic activity, the depth of magma, as well as the accumulation of sediment that is scraped off the subducting plates, increases the volume of the crust. An example of an active island arc is the Aleutian arc, which forms the Aleutian Islands in Alaska. Some volcanic island arcs, such as Japan, appear to have been built up by two or three different periods of subduction. As shown by Japan, the continued development of a volcanic island arc can result in the formation of mountains made up of belts of igneous and metamorphic rocks. Ocean-ocean convergence mainly produces volcanic mountains.
Ocean-Continental Convergence Mountain building along
continental margins involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust. A good example is the west coast of South America. In this area, the Nazca plate is being subducted beneath the South American plate along the Peru-Chile trench. As shown by the Andes Mountains, ocean-continental conver- gence results in the formation of a continental volcanic arc inland of the continental margin.
318 Chapter 11
INSTRUCT
Mountain Building
at Convergent
Boundaries
Use Visuals
Figure 12 Have students study the diagrams of ocean-ocean convergence. Ask: What is the result of the convergence of two oceanic plates? (formation of a volcanic island arc) Why does the volume of the crust increase in this type of convergence? (Volcanic magma is added to the crust. Sediment that is scraped off the subducting plate accumulates to increase crustal volume.) Visual, Logical
Partial Melting
Purpose Students observe the process that generates magma during boundary convergence.
Materials can of frozen grape juice concentrate (inexpensive brands work best), can opener, plastic tub, rubber gloves, apron, water, plastic pitcher, paper towels for clean-up
Procedure Allow the grape juice to thaw slightly, but do not let it liquefy. Use a can opener or pull tab to remove one end of the can. Squeeze the juice concentrate out of the can through your hands, letting it fall into the plastic tub. Tell students to look for evidence of partial melting in the concentrate. Be sure to wear an apron and rubber gloves. Use the water, pitcher, and paper towels for cleaning up.
Expected Outcomes The sugary juice has a melting point of about 40°C, whereas the ice crystals in the juice melt at 0°C. If the juice is thawed to a temperature of about 5°C, part of the mixture is liquid, but the ice crystals are still solid. Explain to students that when a plate is subducted, it experiences partial melting. Sometimes the mantle wedge above the subducting plate also can melt when magma coming up from a subducted slab doesn’t get all the way to the surface immediately. This melting and migration generates magma and causes the formation of volcanic arcs. Kinesthetic, Visual
L
L
2
Section 11.3 (continued)
Customize for English Language Learners
Have students use a dictionary to look up the origin of the term isostasy. (Iso- means “equal” and stasis means “standing.” ) Ask students to brainstorm words that begin with iso-. Ask them how the words are similar or different in
meaning. ( Students may know the word isobar [equal air pressure] and isotherm [equal temperature] from meteorology, isotonic [equal concentration] from chemistry , and isometric and isosceles [equal sides] from geometry. )
320 Chapter 11
Continent-Continent
Convergence Continental
crust floats too much to be sub- ducted. At a convergent boundary between two plates carrying continen- tal crust, a collision between the continental fragments will result and form folded mountains. An example of such a collision began about 45 million years ago when India collided with the Eurasian plate, as shown in Figure 14. Before this event, India was once part of Antarctica, and it had split from that continent over the course of millions of years. It slowly moved a few thousand kilometers due north. The result of the collision was the formation of the spectacular Himalaya Mountains and the Tibetan Plateau. Most of the oceanic crust that separated these landmasses before the collision was subducted, but some was caught up in the collision zone, along with the sediment along the shoreline. Today these sedimentary rocks and slivers of oceanic crust are elevated high above sea level. A similar but much older collision is believed to have taken place when the European continent collided with the Asian continent to produce the Ural Mountains in Russia. Before the theory of plate tec- tonics, geologists had difficulty explaining mountain ranges such as the Urals, which are located far within continents.
Why can’t continental crust be subducted?
Figure 14 Continental- Continental Convergence The ongoing collision of India and Asia started about 45 million years ago and produced the majestic Himalayas. A Converging plates generated a subduction zone, producing a continental volcanic arc. B Eventually the two landmasses collided, which deformed and elevated the mountain range.
Subductingoceaniclith osph ere
India (^) Continental shelf deposits
Continental crust
Ocean basin
Continental volcanic arc
Tibet
Asthenosphere^ Melting
Developing accretionary wedge
Himalayas
India (Ganges Plain) Tibetan Plateau
Suture
Asthenosphere
A
B
320 Chapter 11
Use Visuals
Figure 14 Have students examine the diagram of continent-continent convergence. Ask: Why are there sedimentary rocks and oceanic crust high above sea level in the Himalayas? (They come from sediment and bits of crust that were scraped off an oceanic slab as it subducted under India. When India and Asia collided, this material was uplifted.) What caused the formation of a continental volcanic arc in the Himalayas? (partial melting of the overlying mantle rocks triggered by the subducting oceanic slab) Visual, Logical
Build Science Skills
Inferring Have students infer how a mountain range can occur far inland. (Two continental plates must have collided sometime in the past.) Logical
Use Community Resources
Invite a civil engineer to your class, and have students interview him or her to find out what a civil engineer needs to know about local landforms when planning new roads. Have students prepare questions for the guest speaker in advance of his or her visit. Verbal
L
L
L
Section 11.3 (continued)
Numerous earthquakes recorded off the southern coast of India indicate that a new subduction zone may be forming. If formed, it would provide a subduction site for the floor of the Indian Ocean, which is continually
being produced at a spreading center located to the southwest. Should this occur, India’s northward journey, relative to Asia, would come to an end, and the growth of the Himalayas would cease.
Facts and Figures
Mountain Building 321
Mountain Building at Divergent
Boundaries
Most mountains are formed at convergent boundaries, but some are formed at divergent boundaries, usually on the ocean floor. These mountains form a chain that curves along the ocean floor at the ocean ridges. This mountain chain is over 65,000 kilometers long and rises to 2000 to 3000 meters above the ocean floor. The mountains that form along ocean ridges at divergent plate boundaries are fault-block type mountains. The mountain chain that makes up the Mid-Atlantic Ridge is an example.
Non-Boundary Mountains
Even though most mountains are formed at plate boundaries, some are found far from any boundaries. Some upwarped mountains, fault-block mountains, and volcanic mountains are not formed at plate boundaries. Volcanic mountains such as the Hawaiian Islands are formed at a hot spot, far from any plate boundary. Many fault-block mountains occur in areas that are undergoing regional extension or stretching. These areas may possibly become a plate boundary if the plate rifts apart.
Continental Accretion
Plate tectonics theory originally suggested two major mechanisms for orogenesis at convergent boundaries: continental collisions and sub- duction of oceanic lithosphere to form volcanic arcs. Further studies have lead to another mechanism in which smaller crustal fragments collide and merge with continental margins. When the fragments col- lide with a continental plate they become stuck to or embedded into the continent in a process called accretion. Many of the mountainous regions rimming the Pacific have been produced through the process of collision and accretion.
Where is the longest mountain range?
Continentallandmass
Inactive volcanicisland arc Trench
Asthenosphere
Subd uctingoceanic lithosphere
Collision ofvolcanic island arc and continent
Asthenosphere
Continentallandmass
Asthenosphere
Subductingoceaniclithosphere
Inactive volcanicisland arc Trench
Mountain Building by Continental Accretion
Figure 15 This sequence illustrates the collision of an inactive volcanic island arc with the margin of a continental plate. The island arc becomes embedded or accreted onto the continental plate.
A B^ C
For: Links on mountain building Visit: www.SciLinks.org Web Code: cjn-
Mountain Building at Divergent Boundaries
Build Science Skills Problem Solving Have students describe a process whereby mountains can form at divergent boundaries. Students should draw a series of diagrams showing the formation of an ocean ridge and describe what is happening in each diagram. Visual
Non-Boundary Mountains Build Reading Literacy Refer to p. 124D in Chapter 5, which provides the guidelines for summarizing. Summarize Have students reread the description in Chapter 9 of how hot spots cause the formation of volcanic mountain arcs. Students should write a short summary of the process. Then they should write a paragraph about how the formation of hot spots relates to this section. Verbal
Continental Accretion
Use Visuals Figure 15 Have students examine the diagram of continental accretion. Ask: What is the crustal fragment in these diagrams? (the volcanic island arc) Will the mountains that are formed by this process be as large as mountains formed by converging continental plates? (No, there is much less crustal material in the fragment.) Visual, Logical
L
L
L
Mountain Building 321
Download a worksheet on mountain building for students to complete, and find additional teacher support from NSTA SciLinks.
Answer to...
It is too buoyant.
the Mid-Atlantic Ridge on the Atlantic Ocean floor
Mountain Building 323
Principle of Isostasy
In addition to the horizontal movements of lithos- pheric plates, gradual up-and-down motions of the crust are seen at many locations around the globe. Although much of this vertical movement occurs along plate margins and is linked to mountain building, some of it is not. The up-and-down motions also occur in the interiors of continents far from plate boundaries. Earth’s crust floats on top of the denser and more flexible rocks of the mantle. The concept of a floating crust in gravitational balance is called isostasy ( iso equal and stasis standing). One way to understand the concept of isostasy is to think about a series of wooden blocks of different heights floating in water, as shown in Figure 17. Note that the thicker wooden blocks float higher than the thinner blocks. In a similar way, many mountain belts stand high above the surface because they have buoyant (less dense) crustal “roots” that extend deep into the denser mantle. The denser mantle supports the mountains from below. What would happen if another small block of wood were placed on top of one of the blocks in Figure 17? The combined block would sink until a new isostatic balance was reached. However, the top of the com- bined block would actually be higher than before, and the bottom would be lower. This process of establishing a new level of gravitational equilibrium is called isostatic adjustment.
Figure 17 Isostatic Adjustment This drawing illustrates how wooden blocks of different thicknesses float in water. In a similar manner, thick sections of crustal material float higher than thinner crustal slabs. Inferring Would a denser wooden block float at a higher or lower level?
Isostatic Adjustment in Mountains
Mountain range Oceanic crust (^) Continental crust
Mountain remnant
Continentalcrust Continental crust
Erosion
Uplift
Deposition Deposition
Sinking
A
B C
Figure 18 This sequence illustrates how the combined effect of erosion and isostatic adjustment results in a thinning of the crust in mountainous regions. A When mountains are young, the continental crust is thickest.
B As erosion lowers the mountains, the crust rises in response to the reduced load.
C Erosion and uplift continue until the mountains reach “normal” crustal thickness.
Principle of Isostasy
Modeling Isostasy Purpose Students model the principle of isostasy by measuring how high blocks of varying density float in water. Materials large plastic container, water, balance, metric ruler, several blocks of varying sizes and kinds of wood Procedure Wear apron and goggles. Fill a large plastic container approximately half full with water. Determine the mass and volume of each of the wooden blocks. Calculate the density of each. Place each block in the water. Use a ruler to measure how much of each block is supported above the water. Stack two or more blocks on top of the other blocks in the water. Measure how much of the blocks are supported above the water. Expected Outcomes The thicker blocks floated higher than the thinner blocks. The blocks that had lower densities also floated higher than the denser blocks. Visual, Kinesthetic
L
Mountain Building 323
Archimedes, born in Sicily in 287 B.C., is credited with discovering the principle of buoyancy. He was ordered by King Hiero to determine whether a crown was pure gold or whether the goldsmith who had made it had replaced some of the gold with cheaper silver. Archimedes realized while taking a bath that he could solve the puzzle using water displace- ment. After determining the volume of water displaced by the crown, a piece of pure gold
could easily be made to match the volume of the displaced water, and thus the volume of the crown. In theory, if the volume of the crown and the volume of the gold block are the same, they should also have the same mass. The only reason they would not have the same mass is if one of them was not pure gold. Archimedes tested the crown and found that it, indeed, weighed less than the block of gold. The goldsmith confessed.
Facts and Figures
Answer to...
Figure 16 ancient ocean floor deposits
Figure 17 at a lower level
A terrane is an area that has a different geologic history from surrounding areas.
324 Chapter 11
Section 11.3 Assessment
Reviewing Concepts
1. What types of mountains are associated with convergent plate boundaries? 2. What mountains are associated with divergent plate boundaries? 3. How is isostatic adjustment involved in mountain building? 4. How is accretion involved in mountain formation?
Critical Thinking
5. Comparing and Contrasting Compare mountain building along an ocean-continent convergent boundary and a continent- continent convergent boundary. 6. Drawing Conclusions How does the theory of plate tectonics help explain the existence of marine fossils in sedimentary rocks on top of the Himalayas? 7. Applying Concepts How would the accretion of a large microcontinent affect the isostatic adjustment of the region around a mountain range?
Isostatic Adjustment for Mountains Applying the con-
cept of isostasy, we should expect that when weight is added to the crust, the crust responds by subsiding. Also when weight is removed, the crust will rebound. Evidence of crustal subsidence followed by crustal rebound is provided by the continental ice sheets that covered parts of North America during the Pleistocene epoch. The added weight of a 3-kilometer-thick mass of ice depressed Earth’s crust by hundreds of meters. In the 8000 years since the last ice sheet melted, uplift of as much as 330 meters has occurred in Canada’s Hudson Bay region, where the ice was thickest.
Crustal buoyancy can account for considerable vertical movement. Most mountain building causes the crust to shorten and thicken. Because of isostasy, deformed and thickened crust will undergo regional uplift both during mountain building and for a long period afterward. As the crust rises, the processes of erosion increase, and the deformed rock layers are carved into a mountainous landscape. As erosion lowers the summits of mountains, the crust will rise in response to the reduced load, as shown in Figure 18 on page 323. The processes of uplifting and erosion will continue until the mountain block reaches “normal” crustal thickness. When this occurs, the moun- tains will be eroded to near sea level, and the once deeply buried interior of the mountain will be exposed at the surface.
How are ice sheets related to isostatic adjustment?
Creative Writing Describe a trip through a mountain range like the Andes that has formed at an ocean-continent convergent boundary.
324 Chapter 11
Build Science Skills
Predicting When you place a block of wood in a pail of water, the block displaces some of the water, and the water level rises. Ask: If you could measure the mass of the water that the block displaces, what would you find? (The mass of the water equals the mass of the block.) If 1 million kg of ice were added to a land mass, how much mantle would be displaced? (1 million kg) Logical
ASSESS
Evaluate Understanding
Ask students to describe two events that would cause crust to subside. (Accept any event that increases the mass of the crust, such as formation of large ice sheets, accumulation of large amounts of sediments, or formation of volcanic mountains.)
Reteach
Have students design an experiment that models the formation of an uplifted mountain. Students may use a bicycle pump, balloon, fabric, and any other materials they choose when designing their model.
Students’ descriptions will vary. Be sure they correctly describe the volcanic arc and another mountain range inland of the continental margin.
L
L
3
L
Section 11.3 (continued)
4. At a subduction zone, if an oceanic plate is carrying any island arcs or small continental fragments, they will be stuck to or embed- ded into the margin of the continental plate. This process of accretion deforms the conti- nental plate and can form mountains. 5. Ocean-continent convergent boundary: subduction of oceanic crust beneath conti- nental crust, development of a continental volcanic arc, and formation of folded moun- tains as accretionary wedge is deformed and folded. Continent-continent convergent boundary: no subduction because the conti-
Section 11.3 Assessment
1. mainly volcanic mountains 2. Fault-block mountains at ocean ridges are associated with divergent boundaries. 3. As the crust is thickened due to mountain building, the lithosphere will sink deeper into the mantle. But the lithosphere is less dense than the mantle, so the lithosphere will stand higher. To balance the added thickness, the crust will rise as a new level of gravitational equilibrium is reached.
nental lithosphere is too buoyant, continental plates collide and form folded mountains with a lot of deformation, shortening, and thrust faulting, and little volcanism.
6. The ocean basin between India and the Eurasia was subducted before the collision of India with the Eurasian plate. The sedi- mentary rocks on the ocean floor and along the shoreline were scraped off and uplifted and folded up into the mountains. 7. The addition or accretion of a large frag- ment onto a mountain range would load the crust and cause the crust to subside.
Answer to...
The weight of added ice causes crust to subside. When ice melts, and weight is removed, crust rebounds.
