Module 1 Week 3

Question 1

1. What is the composition of the core? Does this “fit” our previous assumption that the interior of the Earth must be composed of much denser materials?

Answer 1

• The earth’s core is composed mostly of iron and nickel. The outer core is liquid while the inner is solid. Yes, it fits our assumption that the interior of the earth is composed of much denser materials since iron and nickel are both denser than the materials that make up the crust and the mantle.

Question 2

2. If the inner and outer core are composed mostly of the same material and temperature increases with depth, why is the inner core solid while the outer core is liquid?

Answer 2

• Since the temperature is increasing from the crust to the core, this increase is compensated by an increase in pressure as well.

Question 3

3. What do scientist study to determine the structure of the Earth?

Answer 3

• Scientists study earthquakes to determine the structure of the Earth. This is because earthquakes generate energy in the form of seismic waves, which provides clues to the Earth’s interior.

Question 4

4. When an earthquake occurs, in what form is energy released?

Answer 4

SEISMIC WAVES

Question 5

5. Differentiate between Earthquake focus and epicenter?

Answer 5

• The point source of the energy is the focus, while the point directly above the focus is the epicenter.

Question 6

6. What are the different types of seismic waves and what are their characteristics?

Answer 6

• P-Waves (Primary wave): Compressional type of wave; vibration direction is parallel to the direction of propagation; faster than S-waves and can travel in both solid and liquid medium
• S-Waves (Shear wave): Vibration direction is perpendicular to the direction of propagation; cannot travel in liquid medium
• Love waves: Shaking motion that is felt during an earthquake; large amplitude wave
• Rayleigh waves: heaving motion that is felt during an earthquake; large amplitude wave

Question 7

7. How are seismic waves detected?

Answer 7

SEISMOGRAM

Question 8

8. How is epicentral distance determined using a seismogram?

Answer 8

• By knowing the lag time between the arrival of the p-wave and s-wave, epicentral distance is determined.

Question 9

9. What type of seismic waves are propagated into the Earth’s interior? How are seismic waves propagated into the Earth’s interior (direct transmission, reflection, refraction)

Answer 9

• P-waves and S-waves are propagated into the Earth’s interior. Regarding direct transmission, the waves are propagated directly through a curve, and not necessarily straight. The density of the rocks usually affects the speed of the waves.
• Through reflection, waves are propagated through another curve and then reflected by surface waves, and re-propagated again.
• While refraction happens when the p-waves and s-waves intersect the mantle-core boundary. Refraction also happens due to a change in the physical properties of the rocks.

Question 10

10. In the direct propagation of seismic waves, they generally follow curve paths. Why?

Answer 10

• This is because the density of the rocks usually affects the speed of the seismic waves.

Question 11

11. Explain the presence of the P wave and S wave shadow zone. Why are they both proof of the existence of liquid outer core?

Answer 11

• Seismologists observed that P waves are not recorded at seismograph stations which are from 104 degrees to 140 degrees away from an earthquake. Gutenberg (1913) explained that this was a shadow zone with a core that slowed and bent P waves. Later on, an S wave shadow zone was recognized, meaning that no S waves were received at 104 degrees to 180 degrees from an earthquake. And S waves cannot be transmitted through liquid, meaning that the outer core is composed of liquid.

Question 12

12. What are discontinuities? Name two of the most important discontinuities.

Answer 12

• A discontinuity happens when there is a distinct change in density of the rocks. The major discontinuities are the core-mantle boundary and the crust-mantle boundary.

Question 13

13. Differentiate between compositional layering vs layering based on physical behaviour of the Earth.

Answer 13

• Compositionally, the earth is subdivided into the crust, mantle, and core. However, in terms of physical behavior, we subdivide the earth into the lithosphere, asthenosphere, and the mesosphere.

Question 14

14. What does the low velocity zone represent?

Answer 14

• From a depth of 100 to 200, there is a distinct drop in velocity, this is called the low velocity zone. This is already part of the mantle, while the top of the low velocity zone coincides to the bottom of the lithosphere.

Question 15

2. What are the evidences used by Alfred Wegener to prove continental drift?

Answer 15

• Continental Fit
• Distribution of Fossils
• Match of Rock Structures and Ages
• Stratigraphy
• Paleoclimate Indicators
• Glaciation
• Apparent Polar Wandering Curves

Question 15

15. If the mantle is compositionally homogeneous, how do you explain the discontinuities found within the mantle?

Answer 15

• The 400-km discontinuity and 670-km discontinuity in the mantle are not caused by a change in composition but because of atomic repacking. Elements could have the same composition but different physical properties as they were formed at different environments.

Question 15

1. State the theory of Continental Drift.

Answer 15

The continental drift theory proposes that the continents move around on
earth’s surface and that they were once joined together as a single
supercontinent.

Question 16

3. Pangea, the supercontinent that existed 225 millions of years ago initially broke into two large continents – Gondwanaland and Laurasia. Name the present-day continents that constituted Gondwanaland and Laurasia, respectively.

Answer 16

• Gondwanaland: South America, Africa, Antarctica, India, and Australia
• Laurasia: North America, Europe, and Asia

Question 17

4. How is the magnetic field recorded in rocks?

Answer 17

• This applies more on igneous rocks. Below curing point, these contain minerals that are strongly magnetic and tend to align themselves with the ambient magnetic field. If you cool down magma and the rocks solidify, recording in the rock is also the direction of the magnetic field at the time of inclination. If the rocks formed near the equator, the inclination would be close to zero rocks that form near the poles, the inclination will approach 90 degrees.

Question 18

5. What is meant by “apparent polar wandering”? Why is it used as proof of continental drift?

Answer 18

• By studying rocks of different ages, they were able to generate separate polar wandering curves for different continents. This is a bit unusual because the position of the north should be the same. Thus, this is used as proof of continental drift because the magnetic poles were not the ones drifting but it is the continents.

Question 19

6. Why was continental drift, given the evidence laid out by Alfred Wegener, at the time it was proposed, met with a lot of skepticism?

Answer 19

• It was met with a lot of skepticism because it didn’t provide a convincing driving mechanism. This is due to Alfred Wegener’s inspiration for the theory, which is a drifting iceberg. So, he thought that the continental crust plowed over the oceanic crust but there is no evidence.