Don Barrie, Geoscience Educator

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Homework 1 Solutions

Chapter 4:

1. What was Wegener’s evidence for continental drift?

Wegener’s evidence included the puzzle-like fit of the continental coastlines, matching of geologic features (rock types, mountain ranges, etc.) across continental boundaries, and the occurrence of identical fossils on widely separated continents. In addition, Wegener noted that the distribution of glacial markings and deposits is consistent with drifting continents, as well as the evidence for polar wander (actually wandering continents).

3. What is the evidence that South America and Africa were once joined?

Evidence that South America and Africa were once joined includes matching rock types across these now-separated continents, similar fossils on both continents, and matching coastlines as well as matching continental shelves (submerged continental edges).

5. What is Pangea?

Pangea was a giant super continent that existed until approximately 200 million years ago, when it began to break up.

7. How does seafloor spreading account for the age of the sea floor.

Seafloor spreading implies that the youngest rocks should be found at the crest of the mid-ocean ridge. Away from the ridges, the seafloor becomes progressively older as it is carried away from the ridge via seafloor spreading, similar to the motion of a conveyor belt. Seafloor age is greatest at large distances from the mid-ocean ridge, as implied by seafloor spreading.

8. What is a plate in the concept of plate tectonics?

In plate tectonic theory, Earth’s outer, brittle shell (the lithosphere) is broken up into a series of mobile fragments called plates.

9. Define lithosphere and asthenosphere.

• Lithosphere—Earth’s outer, brittle shell; consists of the crust and uppermost mantle and extends to approx. 100 km depth.

• Asthenosphere—A mobile, highly plastic region within Earth’s mantle, that occurs directly below the plates. The asthenosphere is the weak substrate upon which the plates slide.

10. What is the origin of marine magnetic anomalies according to Vine and Matthews?

According to Vine and Matthews, marine magnetic anomalies (“zebra stripes”) form as a result of two processes, including seafloor spreading and reversal of Earth’s magnetic field. As new lava erupts from the mid-ocean ridge and cools, its magnetism aligns itself with that of Earth’s magnetic field, and a magnetic stripe is created. With additional seafloor spreading, this magnetic stripe is broken into two stripes that are symmetrical about the ridge. At some point Earth’s magnetic field reverses itself, and new lava that erupts at the mid-ocean ridge after the reversal also acquires a reversed magnetic character, creating a newer set of symmetrical (but reversed) magnetic stripes.

16. What is transform fault?

A transform fault is a zone of fractured rock that typically develops between two offset portions of the ridge crest. The relative motion across a transform fault is sideways.

17. Discuss possible driving mechanisms for plate tectonics?

Driving mechanisms include slab pull (where the gravitational sinking of subducting plates pulls on the rest of the plate), ridge push (where opposite sides of the mid-ocean ridge slide away from one another down the slope of the ridge), and trench suction (where mantle flow currents set up by a subducting plate pull on the bottom of the non-subducting plate, “sucking” it toward the trench). Mantle convection (the slow overturning of the Earth’s hot, plastic interior) may also play a role in driving plate motions.

18. Describe the various type of plate boundaries and the geologic features associated with them.

Plate boundary types include divergent (where plates move away from each other), transform, (where one plate slides horizontally past another plate), and convergent (where two plates move toward each other). Divergent plate boundaries are associated with active volcanism at mid-ocean ridges; transform boundaries are associated with seafloor fracture zones between offset portions of the mid-ocean ridge crest and also with active faults on the continents such as the San Andreas Fault; convergent plate boundaries are associated with mobile mountain belts, volcanic arc volcanism, and deep marine trenches. Convergent plate boundaries include ocean-continent convergence (subduction), ocean-ocean convergence (subduction), and continent-continent convergence (collision).

21. C

22. A

23. B

24. C

25. B

26. A

27. B

28. A

29. C

Chapter 9:

1. Compare feldspar and quartz.

• How do they differ chemically?

Quartz is made up of oxygen and silicon atoms; formula is SiO2. (silica) Feldspar has aluminum that is a substitute for some of the silicon in the linked tetrahedrons. It also contains sodium, calcium, or potassium.

• What type of silicate structure does each have?

They both have framework silicate

• How would you distinguish between them on the basis of cleavage?

Quartz has no cleavage because it is strong in all directions and bonds are equally as strong. Feldspar has two directions of cleavage: right angles, 90 degrees.

2. How do the crystal structures of pyroxenes and amphiboles differ from one another?

The pyroxenes are single-chain silicates, whereas the amphiboles are double-chain silicates.

4. Distinguish the following terms:

• Silica – a term for oxygen combined with silicon.

• Silicate – substances that contain silica as part of their chemical formula.

• Silicon – an element enriched in Earth’s crust.

• Silicon-Oxygen Tetrahedron – four-sided, pyramidal object that visually represents the four oxygen atoms surrounding a silicon atom; the basic building block of silicate minerals. Also called silicate tetrahedron.

9. What are the three most common elements (by number approximate percentage) in the Earth’s crust?

Oxygen, 46.6%, Silicon 27.7, and Aluminum 8.1% (by weight)

10. What are the next five most common elements?

Iron 5%, Calcium 3.6, Sodium 2.8%, Potassium 2.6%, Magnesium 2.1% (by weight)

11. B

12. A

13. D

14. B

15. A

16. D

17. F

18. B

19. C

20. D

21. D

22. A

23. C

Chapter 11:

1. Why do mafic magmas tend to reach the surface much more often than silicic magmas?

Mafic magmas are typically much less viscous than silicic magmas, so they can more easily reach Earth’s surface before cooling crystallizing.

3. How do batholiths form?

They are formed of numerous, coalesced plutons, typically at subduction zones, as the plutons migrate upward through the lower crust and collect below the surface, where the magma cools and crystallizes.

4. How would you distinguish, on the basis of minerals present, among granite, gabbro, and diorite?

Grabbo is formed of coarse-grained ferromagnesian minerals and gray, plagioclase feldspar. You can identify it on the basis of cleavage and verify that no quartz is present. Granite and rhyolite are composed predominantly of feldspar, usually white or pink, and quartz. Granite is coarse-grained, and contains abundant quartz. Diorite and andesite are composed of feldspar and significant amounts of ferromagnesian minerals but little to no quartz. Diorite is coarse-grained and andesite is fine-grained.

5. How would you distinguish andesite from a diorite?

Andesite is fine-grained, can usually be identified by its medium-gray or medium-green color. Its appearance is intermediate between light-colored rhyolite and dark basalt. Diorite is coarse-grained and often characterized by a salt and pepper appearance due to a roughly equal abundance of ferromagnesian and non-ferromagnesian silicates.

6. What rock would probably form if magma that was feeding volcanoes above subduction zones solidified at considerable depth?

Diorite (intermediate in composition)

9. What is the difference between a dike and a sill?

A dike is a tabular, discordant, intrusive structure. Dikes may form at shallow depths and be fine-grained, or at greater depths and be coarser-grained. A sill is also a tabular intrusive structure, but it is concordant. That is, sills, unlike dikes, are parallel to any planes or layering in the country rock.

10. Describe the differences between the continuous and the discontinuous branches of Bowen’s reaction series.

The discontinuous branch, which contains only ferromagnesian minerals. Also, earlier crystals react with the remaining melt to create new crystal structures. Olivine crystallizes before pyroxene; pyroxene crystallizes before amphibole; and amphibole crystallizes before biotite. The continuous branch contains only plagioclase feldspar. A complication is that the early crystals react continuously with the remaining melt so that the composition of the plagioclase is continuously changing as crystallization proceeds.

11. B

12. A

13. D

15. D

16. C

17. B

18. D

19. B

20. A

21. B

22. B

23. B

Chapter 10:

2. What roles do gases play in volcanoes?

During an eruption, expanding, hot gases may propel pyroclasts high into the atmosphere as a column rising from a volcano. At high altitudes, the pyroclasts often spread out into the dark, mushroom cloud.

3. What do pillow structures indicate about the environment of volcanism?

Pillow structures indicate underwater volcanism.

5. What property or characteristic of obsidian makes it an exception to the usual geological definition of rock?

Obsidian is volcanic glass, is one of the few rocks that is not composed of minerals.

6. What determines the viscosity of a lava?

Viscosity, or resistance to flow, is determined by silica content (the higher the silica, the higher the viscosity), and temperature (the lower the temperature, the higher the viscosity), and dissolved gas (the higher the gas content, the lower the viscosity).

7. What determines whether a series of volcanic eruptions builds a shield volcano, a composite volcano, or a cinder cone? Describe each.

A shield volcano is a broad, gently sloping volcano constructed of solidified lava flows. The low viscosity of its lava allows the flows to spread out into wide, thin layers, producing a flattened dome shape or shield. Cinder cone is a volcano constructed of pyroclastic fragments ejected from a central vent. Cinder cones are much smaller than shield volcanoes and are associated with various magmas types. Composite volcanoes are constructed of alternating layers of pyroclastic fragments and solidified lava flows. Composite volcanoes are built over long spans of time and are typically intermediate in size between cinder cones (which are smaller) and shield volcanoes (which are larger). Composite volcanoes are typically associated with intermediate-composition magma.

8. Explain how a vesicular porphyritic andesite might have formed.

Before erupting, a magma of intermediate composition slowly crystallizes, producing early-formed, large crystals within the magma called phenocrysts. Later, the magma erupts as lava, and cools rapidly, creating a second generation of small crystals. Together, the two generations of crystals create an intermediate volcanic rock (andesite) with two distinct sizes of crystals—a texture known as porphyritic.

9. Why are extrusive igneous rocks fine-grained?

When lava cools rapidly, atoms only have time to move for a short distance before linking up into crystals, so the resulting crystals don’t have the opportunity to grow large.

10. Why don’t flood basalts build volcanic cones?

Because not all extrusive rocks are associated with volcanoes. The mafic lava associated with flood basalts is very non-viscous and therefore flows easily, and does not build a cone around its vents.

11. D

12. B

13. D

14. C

15. E

18. A

19. E

20. B

21. D

22. B

23. A

Chapter 12:

4. Why do stone buildings tend to weather more rapidly in cities than in rural areas?

In cities, industrial atmospheric discharges are associated with the burning of coal, oil, and natural gas. Such discharges also add carbon dioxide to the atmosphere, which creates acidic rain water that chemically weathers such buildings.

5. Describe at least three processes that mechanically weather rock.

(1) frost wedging--the expansion of freezing water pries rock apart; (2) pressure release--the reduction of pressure on a brittle rock mass, which can cause it to crack as it expands; (3) plant growth--as roots grow in cracks, the rock mass breaks up.

6. How can mechanical weathering speed up chemical weathering?

Mechanical weathering can increase the surface area of a rock, accelerating the rate of chemical weathering by allowing water to percolate into the rock interior.

7. Name at least three natural sources of acid in solution. Which one is important for chemical weathering?

Natural sources of acid in solution include volcanic eruptions, hot springs, and atmospheric carbon dioxide, which can combine with water to create carbonic acid.

The most important natural source of acid rock weathering is dissolved carbon dioxide in water (carbonic acid).

12. D

13. A

14. D

15. A

16. A

17. E