v1 foley-geoHW3

1 Geology 2341, A Term 2023 Worcester Polytechnic Institute Homework Assignment 3, Due by Sep. 14 PART A: OBJECTIVE: TO DISCOVER SOME OF THE ASPECTS OF PLATE TECTONICS BY EXAMINING REAL DATA. Background: The Hawaiian-Emperor seamount chain stretches for 6,000 km across the Pacific Ocean. This string of more than 100 volcanoes was formed as the Pacific plate slid across a hot spot (or mantle plume) in the asthenosphere below. Some of these volcanoes extend above sea level to form islands while others are submerged seamounts. Over time the volcanoes erode, cool and subside below sea level often forming flat-topped guyots. Procedure: Step 1: Plot the age-distance data To better understand this process you will plot the age-distance relationship of 30 volcanoes listed in Table 1 on the graph provided. Mark a “ + “ at each volcano’s age-distance point. Give the plot a descriptive title and label each axis so that someone seeing the graph for the first time could understand what it represents. (Use Microsoft Excel to plot the data; and don’t forget units of the axes). Step 2: Interpret the data 1) Draw a straight line that best fits (is closest to) all the data points. The slope of this line represents the average rate at which the plate is moving (in km/Ma). Note that the points do not all lie on the line. Give some reasons why all the points don’t lie exactly on the line.

2 The points may not all fall exactly on the line because the Pacific Plate has changed its rate of movement throughtout the years. The line of best fit (trendline) shows the overall average speed over millions of years. Its rate of speed is constantly changing. 2) Calculate the rate (distance/time) at which the Pacific plate is sliding across the hot spot. a. First determine the slope of the line. If you don’t know how to do this see page 4. Plate rate = _______75.39__________ km/Ma b. The answer above is in units of km/Ma. Geologists typically measure plate motions in units of cm/year. Convert the value above to cm/yr. See page 4. Plate rate = ___________7.539____________ cm/yr In what direction is the Pacific plate moving? _____west-southest____________ 3) Try fitting the data better by putting a bend in your line thus drawing two lines with different slopes. How long ago would the bend (see the above figure) have happened?____________ What caused the bend?_____________________________________________________ 4) Examine the map on the first page of the exercise. Note that there is a bend in the seamount chain on the map. How do you explain this? ___The bend indicates that the plate was changing direction. The hot spot stayed in place while the volcanoes moved with the plate.____________ When did this happen? _______~43 million years ago (around when the plates direction changed).______________ 5) How old is the oldest volcano in the chain? ___(Meiji) 85 million_____ yrs The creation of the Hawaiian-Emperor seamount chain represents what percentage of the earth’s whole history? ____1.88____% (Take 4.5 billion years as the earth’s history) 6) Questions to ponder ( for bonus points ): • Are there other island chains similar to this one? Yes, the Cosgrove Track.

4 23 Necker 10.3 1058 0.4 7.1 26 La Perouse 12 1209 0.4 7.9 30 Gardner 12.3 1435 1 9.1 36 Laysan 19.9 1818 0.3 11.1 37 Northampton 26.6 1841 2.7 11.3 50 Pearl & Hermes 20.6 2291 0.5 13.6 52 Midway 27.7 2432 0.6 14.4 57 unnamed 2 28 2600 0.4 15.3 63 unnamed 3 27.4 2825 0.5 16.5 65 Colahan 38.6 3128 0.3 18.1 67 Daikakuji 42.4 3493 2.3 20 72 Kimmei 39.9 3668 1.2 20.9 74 Koko 48.1 3758 0.8 21.4 81 Ojin 55.2 4102 0.7 23.2 83 Jingu 55.4 4175 0.9 23.6 86 Nintoku 56.2 4452 0.6 25.1 90 Suiko 1 59.6 4794 0.6 26.9 91 Suiko 2 64.7 4860 1.1 27.2 Note: Ma = Mega annum (million years ago) Volcano Number is the volcano’s number in the chain counting NW from the hot spot. Not all volcanoes are on this list. The last two columns are estimates of the errors in the first two columns. Hints How to draw a “best fit” line Here is a website that illustrates how to draw graphics and a best-fit line in excel. http://phoenix.phys.clemson.edu/tutorials/excel/graph.html . How to calculate slope By definition, slope is the ratio of the vertical change in a line to the horizontal change. In the figure below the sloping line is defined by two points (a,b) and (x,y). The vertical change is equal to y - b and the horizontal change is equal to x - a . The slope, m, is therefore equal to: m=(y-b)/(x-a)

5 How to convert from km/Ma to cm/yr The rate you calculated from the raw data will be in the units of kilometers (km) per million years (Ma). However, geologists routinely report the rate of plate motions in centimeters (cm) per year. So you must convert the rate to cm/yr with the following conversion: Km/Ma * 1000 m/Km * 100 cm/m * 1Ma/10 6 years = cm/year PART B: THE BEHAVIOR OF MELTS Basalts (and Gabbros) are igneous rocks rich in Ca-Plagioclase, Olivine and Pyroxene. Andesites (and Diorites) are igneous rocks with Na-Plagioclase and Pyroxene. Rhyolites (and Granites) are igneous rocks with Quartz and Alkali Feldspar (rich in potassium and sodium). In general, the viscosity of a melt (a measure of its resistance to flow) is inversely proportional to its temperature. At high temperatures the melt tends to flow more readily than at low temperatures. Water can drastically modify the viscosity of a melt; the higher the water content, the lower the viscosity. Questions 1. Relate the type (quiet versus explosive) of eruption to the viscosity of a melt. 2. In your own words, compare and contrast the viscosities of rhyolites, andesites and basalts. Which, other things being equal, tend to have the highest viscosity? Rhyolites have the highest viscosity (650-800 degrees Celcius), Andesites have the middle/intermediate viscosity (800-1000 degrees Celcius), and Basalts have the lowest (1000-1200 degrees Celcius). Rhyolites contain the highest silica content, which explains the higher viscosity. 3. At what types of plate tectonic settings do you tend to find these three lava types? Ocean-ocean faults result in Basaltic, low viscosity settings due to the ocean. These are divergent plates. Ocean-lithosphere faults result in Andesite. It is intermediate viscosity due to the partial ocean involvement. Subduction is involved here and they are convergent plates. Continent-continent faults result in Rhyolite. As mentioned above, Rhyolite is the most viscious. Contient-continent faults are land-on-land and can provide high silica levels to produce these high-viscocity settings.

7 Texture: Mafic; Composition: Aphanitic; Rock name: Basalt. Texture: Glassy; Composition: Felsic; Rock name: Obsidian.

8 Texture: Phaneritic; Composition: Felsic; Rock name: Granite. Texture: Phaneritic; Composition: Intermediate; Rock name: Diorite.