辅导 GEOG1402 3.0 Physical Geography: The Dynamic Earth LAB EXERCISE # 2 Winter 2025讲解 Prolog

Physical Geography: The Dynamic Earth

EU/SC/GEOG1402 3.0

Faculty of Environmental and Urban Change

Winter 2025

LAB EXERCISE # 2

Plate Tectonics and Geohazards

INTRODUCTION

Depending on your location, the ever-present movement of the Earth's lithospheric plates may vary in its effect on your activities. In some areas, such as the heart of the large continental plates, seismic activity and the associated potential geohazards maybe very far from the mind. A person could go their entire life without feeling a single tremor of the ground shaking. In other locations, the risk of disturbance may be ever-present. In this assignment, we will consider the amazing variability in the conditions associated with the movement between different lithospheric plates on our planet and the associated risk of geohazard in some locations where plates interact.

As we discussed in lectures, the surface of the Earth is covered by several different plates of varying  sizes. These plates include the   crust and lithospheric mantle, moving along the asthenosphere's top. There are seven (7) major and seven (7) minor plates and smaller fragments of others (Figure 1). The plates interact with each other in different ways. In some locations, they come together (convergent boundary); in others, they move apart (divergent boundary); and in others, they slide in parallel (transform boundary).

Figure 1: A map showing the Earth 's tectonic plates and the approximate rates and directions of plate motions.

(https://opentextbc.ca/physicalgeology2ed/chapter/10-4-plate-plate-motions-and-plate-boundary-processes/#fig10.4.1)

The location where different plates interact represents the most common locations for significant geohazards on Earth in the form of earthquakes and volcanoes. This is not to say that these disturbances cannot occur in areas more central to plates. Earthquakes commonly happen along fault lines in the interior of our continental lithospheric plates, and we discussed the nature of shield volcanoes occurring over hot spots. However, the geologically active boundaries of the moving  lithospheric plates are the location of most earthquakes (Figure 2) and volcanoes, including those most catastrophic to human endeavours.

Figure 2: Location of Earthquakes from 2000-2008 and their depth. Image: USGS.

Part A: The Ocean Cross-Section and Features

Figure 3 below is a contour map of the ocean floor between North America and Europe.

Figure 3: Contour map of the North Atlantic Ocean. This map is available as a standalone pdf on eClass.

1.   Drawing the ocean cross-section.

a.   Draw a cross-section of the ocean along the track of the line in Figure 3 from the X on the north coast of Newfoundland to the X on the west coast of England.

Remember to label the two axes carefully. (12 marks)

b.   Calculate the vertical exaggeration of the cross section you have drawn using the following equation (2 marks):

2.   Label the following features or processes on your cross-section (5 marks total):

a.   Mid-oceanic ridge

b.   Continental shelves

c.   Deep ocean basins / abyssal plains

d.   Indicate where spreading (divergence) is occurring

e.   Indicate the pattern of convection of the mantle beneath your cross-section 3.  Name the tectonic plates on the West and East of the cross-section. (1 mark)

4.   What plates are bordering the Australian plate? Describe the movement of the Australian plate to those around it. Use Figure 1 to complete this question. (3 marks)

5.   Considering the Pacific plate, indicate which neighbouring plate(s) represent convergent, divergent, and transform. boundaries. Use Figure 1 to complete this question (3 marks)

Part B: Catastrophic Earthquake Probability

The subduction zone in the west of North America, where the oceanic crust of the small Juan de Fuca Plate and the continental crust of the North American plate are converging (called the Cascadia Subduction Zone or CSZ), is an area of intense geological activity. This includes the potential for significant geohazards, such as earthquakes and volcanoes. Nelson et al. (2006) used proxy records around the Pacific to infer the occurrence of large-scale tsunamis, which would be associated with catastrophic earthquakes  along the CSZ. Catastrophic, in this case, refers to earthquakes with a magnitude greater than 9.0. Globally, these occur approximately every ten years. The last catastrophic earthquake in the CSZ occurred in 1700 CE. Using the inferred timing of these events (Table 1), you will calculate the probability and recurrence interval of catastrophic earthquakes in the CSZ. This part of the assignment is modified from Introduction to Hazard and Risk by T. Juster (2012).

Table 1: The inferred dates of the last 9 mega earthquakes to impact the CSZ. From Nelson et al. 2006.

1.   For each of the nine megaearthquakes, calculate the years since they occurred. Don't

forget to account for the fact that some dates are "CE" and others "BCE". Calculate the probability of a mega earthquake occurring over the period covered by these inferred data using the formula, and report that in a sentence (3 marks):

2.   Convert this value to a recurrence interval and report in a sentence (1 mark):

3.   Comment on the advantages of using recurrence interval instead of probability for this type of hazard assessment. (3 marks)

Part C: Hotspot Volcanoes and Plate Movement

Before the availability of highly accurate instrumentation or satellite measurements, much of what we knew about plate motion and velocity was derived from studying volcanic island chains associated with hotspots. As the plate moves over the hotspot like a gargantuan "conveyor belt" new volcanic islands are born above the hotspot. The continued movement results in volcanoes becoming extinct, and subsequent weathering and erosion reduce their size overtime. Eventually, these former volcanoes/islands may slip below the waves and exist as seamounts. The Hawaiian islands are the best-known example of this phenomenon. The history of hotspot volcanoes over this hotspot stretches into the north Pacific Ocean, as the Hawaiian-Emperor Seamount Chain (Figure 4). This part of the assignment is modified from Hotspot Theory and Plate Velocities by J. Russell (2008).

Figure 4: Bathymetric map of the north Pacific, with the Hawaiian-Emperor Seamount Chain visible.

Researchers have used radiometric dating using Potassium-Argon (K-Ar) techniques to date the approximate age of volcanoes (currently on islands in the Hawaiian Archipelago or as seamounts). A selection of these are listed in Table 2.

1.   Plot an X-Y scatterplot of the data with distance on the X (horizontal) axis and age on the Y (vertical) axis. This plot can be made in a spreadsheet program (e.g. Microsoft Excel) or by hand. Include this plot in your report, including a descriptive caption. (4 marks)

2.   Draw a straight trendline that best fits the relationship between these two variables. Determine the slope of the line and indicate that in a sentence in the units of the original chart (Ma/km). If determining the slope by hand, you can calculate the change in y-values between any two points and the change in x-values between any two points and calculate the slope as the  (2 marks)

3.   Convert the slope of the line into units of years/cm. Note: a Ma is 1,000,000 years, and a km is 100,000 cm. Report your answer in a sentence. (1 mark)

4.   Calculate the velocity of the Pacific Plate movement in cm/year and report that in a sentence. (2 marks)

Table 2: Selected ages of Hawaiian Islands (in millions of years old; Ma) and their distance to Kilauea (km).

References:

Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006, Great earthquakes of variable magnitude at the Cascadia subduction zone, Quaternary Research 65(3), 354-365.

What Else Do I Need to Know?

•   This assignment is weighted 15%.

•   You will work on this assignment individually; no group submissions will be accepted for any reason.

•   Answers to the assignment questions are preferred when typed. Hand-drawn figures

should be scanned or photographed and inserted into your Word document. Do well to ensure all aspects of the image are included and legible.

•   Your assignments will be saved as an MS Word or PDF document and uploaded to eClass.

•   All policies related to late assignments can be viewed in the course syllabus.