Assignment2_ESC1000 2024 Spring PlateTectonics
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Feb 20, 2024
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Calculating Rates of Plate Motion from Geologic Features The basic equation used to calculate rates of plate motion using geologic features such as hotspots, seafloor ages (along with magnetic anomalies/stripes), and magnetic inclination is: Distance traveled (D) = average Rate (R) of travel
x length of time (T) traveled i.e., D = R x T Rearranging, we can calculate the rate from R = D/T When calculating rates of plate motion over a given time interval, regardless of the geologic feature being used to calculate motion, you need to ask two sets of questions: 1) Where did the feature form, and Where is it now? (Or at the end of the time interval in which you are interested
) 2) How long did it take to get there? If you draw an arrow with the tail at the location where the feature formed and the arrowhead at its current location, the direction of the arrow tells you the direction of plate motion, and the length of the arrow gives you the distance the feature has traveled (typically you need to use the scale of a map to get the distance). Once you have calculated the distance traveled, you need to answer the question (2), i.e., how long did it take to travel from the tail of the arrow to the head of the area? Then the rate can be calculated by dividing the distance the feature traveled by the time it took to travel that distance. Note that the rate you calculate is an average rate over the given time interval, even though rates may have varied during that time.
Hotspot Example The figure below shows the age progression of volcanic islands (blue spots) produced from the movement of a plate over a hotspot (Discussed in Module 3 Lecture 3F and Ch.4, p112-113 in your textbook). Since hotspots are assumed (for our purposes) to remain in a fixed position
, all the volcanic islands originally formed above the hotspot that is currently situated in the southwest part of the map.
We will calculate the average direction and rate of plate motion over the past 30 million years for the geologic features in the map below.
The volcanic island X in the northeast formed at ~30 Ma (Ma = mega annum or millions of years ago). 1) Where did the feature form?
Above the hotspot (which has not moved) --
‐
so place the tail of the arrow there. Where is it now?
Where it is shown on the map --
‐
so place the head of the arrow there. The red arrow shows that the plate has moved toward the northeast over the last 30 million years.
Using the scale of the map and the length of the red arrow shows that volcanic island X has traveled ~450 km since its formation above the hotspot. (i.e., the length of the arrow is ~3x the length of the bar scale, which represents 150 km distance on the ground)
2) How long did it take to get there?
30 m.y.
So it moved at an average rate of 450 km/30 m.y (i.e., Distance/Time) or ~15 km/m.y. toward the northeast between 30 Ma and today.
A similar approach can be used for other geologic features
. For example, all oceanic crust is made at mid-ocean ridges and then moves away. Thus, we can draw an arrow with a tail on the ridge and the head on some location away from the ridge (depending on the age interval in which you are interested). You may need to get the age of the crust based on using magnetic anomalies and the time of magnetic reversals. (See Module 3 Lecture 3C and Ch.4, p.114-116 in your textbook
) Lastly, magnetic inclinations can be viewed the same way for determining northward or southward motions. Simply, use the latitude at the time a rock formed (derived from the magnetic inclination Fig. F) as the starting point and its current location as the ending point. In other cases you may have two rocks at the same location with different ages, in which case you can use the latitude of the oldest rock (again, from the inclination) as the starting point
and the latitude of the youngest rock as the ending point. (
See Module 3 Lecture 3B
)
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Questions for Assignment 2: Plate Tectonics These questions refer to Figures that accompany the questions below. Although some of these figures are similar to those in the book, for questions referring to the ages of ocean floor please use only Figure B provided here as there may be some slight variations. You will need to watch the plate tectonics video lectures in Module 3 before beginning this assignment. All of the information needed is in these sources –
I strongly discourage Googling as you may get confused by what you find. Answers will be input via a quiz in Canvas –
The quiz will be posted a couple of days before the due date.
Figure A
Refer to Figure A for questions (1) through (4) and state whether the margin is a passive or active margin. Recall that margin refers to the boundary between a continent and the ocean. (1 point each)
1) East margin of South America (Refer to Figure A) (1 point)
a. passive b. active 2) East margin of North America (Refer to Figure A) (1 point)
a. passive b. active 3) North margin of Antarctica (Refer to Figure A) (1 point)
a. passive b. active 4) South margin of Alaska (Refer to Figure A) (1 point)
a. passive b. active Recall that the lithosphere comprises the uppermost mantle in addition to the crust and therefore a tectonic plate may have oceanic crust, continental crust, or both in different parts. For each of the following plates in questions (5) through (8), state whether the plate contains only oceanic crust, only continental crust, or has both types. Refer to Figure A. 5) South American plate (Refer to Figure A) (1 point)
a. oceanic crust only b. continental crust only c. both oceanic and continental crust 6) North American plate (Refer to Figure A) (1 point)
a. oceanic crust only b. continental crust only c. both oceanic and continental crust 7) African plate (Refer to Figure A) (1 point)
a. oceanic crust only b. continental crust only c. both oceanic and continental crust 8) Cocos plate (Refer to Figure A) (1 point)
a. oceanic crust only b. continental crust only c. both oceanic and continental crust
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Figure B: Ma is for mega-annum and refers to millions of years before present
A A
’
B B’
C C’
D D’
F F’
9) Refer to Figure B. What is the age of the OLDEST oceanic crust in the southern ocean along section B-B
’ (between Australia and Antarctica) ? (Ma = mega-annum and refers to millions of years before present) (2 Points)
a. >180 Ma b. ~140-154 Ma c. ~68-84 Ma d. ~33-48 Ma e. ~20-33 Ma f. ~11-33 Ma 10) The time at which two continents separated in the past is determined by? (2 Points)
a. the thickness of the sediment on top of the oceanic crust near the mid-ocean ridge b. the age of the oceanic crust at the present-day mid-ocean ridge c. the average age of the oceanic crust between two passive margins d. the age of the oldest oceanic crust that is adjacent to the passive margins at the edges of the ocean basin 11) Except for the Mediterranean Sea, the oldest oceanic crust found in the oceans is around 180 Ma. Why is there no older oceanic crust than this? (2 Points)
a. older crust has been covered by too much sediment b. 180 Ma is when plate tectonic processes, such as seafloor spreading, began c. we have no means by which to date rocks older than this age d. all older oceanic crust has been subducted into the mantle e. all older oceanic crust has been turned into liquid iron metal 12) Refer to Figure B. Which of the following are true regarding the relationship between the mid-
ocean ridges and the age of the oceanic crust? Choose all that apply
. (2 Points)
a. it is oldest at the mid-ocean ridge b. it is youngest in subduction zones c. it is youngest along the edges of continental shelves at passive margins d. bands of similar age are parallel to mid-ocean ridges e. it gets older in both directions as you move away from the mid-ocean ridge 13) The relative rates of spreading at mid-ocean ridges are determined by comparing what features? (2 Points)
a. the age of the oceanic crust at the present-day mid-ocean ridge b. the width of the age bands as viewed on a map c. the age of the oldest oceanic crust that is adjacent to the passive margins at the edges of the ocean basin d. the average age of the oceanic crust between two passive margins e. the thickness of the sediment on top of the oceanic crust at the mid-ocean ridge
14) Refer to Figure B. Which of the following mid-ocean ridges has been spreading slowest over the last ~67 million years? (2 Points)
a. The ridge between Africa and Antarctica (between A and A') b. The ridge between Australia and Antarctica (between B and B') c. The ridge between South America and Africa (between C and C') d
. The ridge between Greenland and Great Britain (between F and F’)
e. All of the above have been spreading at the same rate 15) Refer to Figure B. During the breakup of the supercontinent of Pangea, which of the following continents were the last to separate from each other? (2 Points)
a. Africa and Antarctica (between A and A') b. Australia and Antarctica (between B and B') c. South America and Africa (between C and C') d. North America and Africa (between D and D') e. Greenland and Great Britain (between F and F’)
16) During what approximate time period did Greenland separate from Great Britain (transect F-F
’)?
(2 Points)
a. ~22-33 Ma b. ~48-68 Ma c. ~68-83 Ma d. ~120-147 Ma e. ~147-180 Ma
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Figure C: Magnetic anomalies at mid-ocean ridges
17) Refer to Figure C (Magnetic anomalies at mid-ocean ridges). Using the pattern of seafloor magnetic anomalies (stripes) and the timescale of magnetic reversals to the right, what is the age of oceanic crust at point X ? (2 Points)
a. 0 Ma b. 1 Ma c. 2 Ma d. 5 Ma e. 11 Ma f. 13 Ma 18) Refer to Figure C (Magnetic anomalies at mid-ocean ridges). Using the pattern of seafloor magnetic anomalies (stripes) and the timescale of magnetic reversals to the right, calculate the average
rate at which point A has moved away from the ridge axis
since 2 Ma ? (2 Points)
a. 20 km/my b. 40 km/my c. 60 km/my d. 80 km/my e. 100 km/my f. 120 km/my 19) Refer to Figure C (Magnetic anomalies at mid-ocean ridges). Using the pattern of seafloor magnetic anomalies (stripes) and the timescale of magnetic reversals to the right, calculate the average
rate at which point A has moved away from point B in the last 2 million years ? (2 Points)
a. 20 km/my b. 40 km/my c. 60 km/my d. 80 km/my e. 100 km/my
Figure D: Hawaiian-Emperor seamount chain
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20) Refer to Figure D. What is the direction of movement of the Pacific plate between 80 Ma and 42 Ma? (2 Points)
a. ~Southeast or east-southeast b. ~West-northwest c. ~North d. ~South e. ~East
Figure E
21) Refer to Figure E. On the basis of the distributions and ages of the volcanoes, which of the following describes the motion of the plate that is moving over the hotspot (which is assumed to be stationary)? (2 Points) a. the plate moved toward the SE from 90 Ma to 50 Ma and then moved south until the present day b. the plate moved toward the NW from 90 Ma to 50 Ma and then moved north until the present day c. the plate moved toward the north from 90 Ma to 50 Ma and then moved SE until the present day d. the plate moved toward the south from 90 Ma to 50 Ma and then moved SE until the present day e. the plate moved toward the north from 90 Ma to 50 Ma and then moved NE until the present day 22) Refer to Figure E. On the basis of the distributions and ages of the volcanoes, what is that rate at which the plate moved over the stationary hotspot between 70 Ma and the present day? (2 Points)
a. 20 km/my b. 40 km/my c. 60 km/my d. 80 km/my e. 100 km/my
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Figure F: Graph showing the relationships between magnetic inclination and latitude 23) Refer to Figure F (Magnetic inclination vs. latitude graph). What is the expected magnetic inclination recorded by a rock formed at a latitude of 10°? (2 Points)
a. ~5° b. ~10° c. ~20° d. ~30° e. ~40°
Figure G
Questions (24) and (25) refer to Figure G, which shows a continent (in gray shading) that is cut by a transform fault boundary. The locations of two granite bodies that formed at 200 Ma are shown by the red dots/circles, along with the magnetic inclinations recorded when the granites crystalized. Also shown is the line of present-day 30° N latitude. (Refer to Figure F as needed to answer the questions.) 24) Which of the following is a true statement regarding the positions of the east and west sides of the continent at 200 Ma, when the two granite bodies formed. (The east and west sides are separated by the transform fault that is oriented North-South) (2 Points)
a. the granite on the west side formed at its present-day latitude, but the east side did not b. the granite on the east side formed at its present-day latitude, but the west side did not c. the granites on both the east and west sides formed at the present-day latitude d. neither the granites on the east or west sides formed at the present-day latitude 25) Given the inclinations in Figure G, which of the following is a true statement regarding the motion along the transform fault boundary after formation of the granite bodies at 200 Ma? Assume all rocks have remained in the northern hemisphere. (2 Points)
a. there has been no latitudinal motion along the transform (neither side has moved) since formation of the granite bodies, therefore all motion must have stopped before 200 Ma b. the west side has moved southward relative to the east side after 200 Ma c. the west side has moved northward relative to the east side after 200 Ma d. all motion on the fault occurred in an east-west direction e. there is insufficient information given to assess the motion after 200 Ma
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Figure H
26) Figure H shows two segments of mid-ocean ridges separated by a transform boundary. The green arrows show the direction of spreading and the age of the oceanic crust is shown for both sides of the transform fault. Given this setting and, in particular, the ages of the oceanic crust, which of the following would be expected regarding the elevation of the seafloor at points A and B? (2 Points)
a. Points A and B would be at the same elevation/depth b. Point A would have a lower elevation (greater depth) than Point B c. Point A would have a higher elevation (shallower depth) than Point B d. there is insufficient information given to address this question