# Rates of Plate Motion

Instructions

The main aim of Part C of this mid-term exercise is to answer a series of questions based on compilation of data that enable evaluation of the following hypothesis:

1. Are the rates of production and loss of oceanic crust equivalent?

This page also provides links to the resources, previously utilized in Module 5 and in the Phase I Group Activities, that help accomplish the task.

Resources

Use of the NUVEL-1 website (http://ofgs.aori.u-tokyo.ac.jp/~okino/platecalc_new.html (Links to an external site.)Links to an external site.) is explained in a video on the Canvas page that introduces Part C of the Plate Tectonics module exploring Tectonics & Volcanism. In effect the website requires a response to the various options regarding choice of plates and location to compute the rate and direction of spreading, as follows:

• From the options in the Plate Model pull-down menu select the first, Nuvel-1.
• The left-hand map shows the geographic locations of individual plates.
• Select plates that border the chosen mid-ocean ridge or plate boundary (Given in the questions below). Options are listed on pull-down menus: Africa, Antarctica, Arabia, Australia, Caribbea, Cocos, Eurasia, India, Nazca, North America, Pacific, South America, Juan da Fuca, and Philippine Sea.
• The second plate chosen is assigned a fixed position, and the calculator determines the motion of the first moving plate relative to it. (Links to an external site.)Links to an external site.
• For reference to the topographic features related to these plate boundaries the map of topographic features of the ocean floor with latitude and longitude (TopoMaps (Links to an external site.)Links to an external site.) can be found at the Satellite Geodesy operated by Scripps Institution of Oceanography.
• Enter values for the latitude (lat) and longitude (long) on (or near) the ridge of interest – some examples are suggested below. Note that they should be described with reference to North latitude and East longitude. Thus, locations in the southern hemisphere or West of the prime meridian must be entered as negative numbers (i.e. 20°S = -20 and 115°W = -115)
• Select the “Execute calculation” button, and the output will provide (among other information) the velocity of spreading in cm/year and the azimuth in degrees from North (e.g. East = 90°).

## Mid-Term Exercises Part C: Rates of Plate Motion

Question 1: This question explores four sets of data that characterize the rates of spreading and subduction of the Pacific plate (two locations), the Indian plate, and the North American plate. The locations for these measurement are as follows:

1. Calculate (a) the rate and direction of spreading of the Pacific Plate on the southern East Pacific Rise (Plate 1 = Antarctica; Plate 2 = Pacific) at 35°S 115°W and (b) the rate and direction of plate motion of the Pacific Plate near Tonga (Plate 1 = Australia; Plate 2 = Pacific) at 20°S 175°W.
2. Calculate (a) the rate and direction of spreading of the Pacific Plate on the East Pacific Rise (Plate 1 = Nazca; Plate 2 = Pacific) at 10°S 110°W and (b) the rate and direction of plate motion of the Pacific Plate near Japan (Plate 1 = European; Plate 2 = Pacific) at 37°N 142°W.
3. Calculate (a) the rate and direction of spreading of the Indian Plate on the Antarctic-Indian Ridge (Plate 1 = Antarctica; Plate 2 = Indian) at 30°S 75°E and (b) the rate and direction of plate motion of the Indian Plate south of Java (Plate 1 = European; Plate 2 = Indian) at 5°S 100°E.
4. Calculate (a) the rate and direction of spreading of the North American Plate on the Mid-Atlantic Ridge (Plate 1 = Africa; Plate 2 = North American) at 15°N 45°W and (b) the rate and direction of plate motion of the North American Plate in the Lesser Antilles Arc (Plate 1 = Caribbean; Plate 2 = North American) at 15°N 60°W.

Describe the rates and directions of the plate motion at these four sets of locations of seafloor spreading/subduction (southern East Pacific Rise/Tonga, Antarctic-Indian Ridge/Java, East Pacific Rise/Japan, and mid-Atlantic Ridge/Caribbean) documenting the data obtained from use of the NUVEL-1 calculator for the 8 locations.

A strong answer will provide the values for plate motion (rate and direction), including units, at all four paired sets of locations.

Question 2: Do the data for the rates of production of oceanic crust at mid-ocean ridges and the rates of loss of the same plate at subduction zones support or repute the hypothesis that these processes are equivalent?

A strong answer will compare the rate of production of oceanic crust and the rate of loss of that same plate by subduction for each of the four paired data sets, referencing the data compiled, and evaluate the relationship between these rates, noting whether they provide evidence that supports or refutes the hypothesis that they are equivalent (i.e. that the rate of production of oceanic crust matches its rate of subduction).

Question 3: Bonus Question (5 pt)

What would be the likely result, in terms of plate tectonics, of a significant increase in the rate of production of ocean crust at mid-ocean ridges?

A strong answer will consider the consequence of an increase in production of ocean crust in terms of other changes in the rates of tectonic processes that would ensue.

Last Updated on March 11, 2019

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