Download Geological Sciences 101 Lab #10 - Exploring Plate Tectonics with

Geological Sciences 101
Lab #10 - Exploring Plate Tectonics with GIS
INTRODUCTION
In this lab we will use ArcView, the GIS software that you were introduced to in Lab 5, to explore
plate tectonic processes. A very powerful aspect of Geographic Information Systems is the ability
to display and analyze different types of spatial data--it allows us to see the world in new ways.
This week we will take advantage of the world wide web to access the USGS National Earthquake
Information Center and download data for recent earthquakes. This will be our principle data base
for examining plate tectonics. This lab is divided into several sections--acquiring the data,
formatting the data, and plotting & analyzing the data. We'll use the computer and different
software for each part. The first part of the lab will be done without computers; we begin with
another analog experiment, this time using wood, water and aquaria to represent the Earth's crust
and mantle.
PART I: ISOSTACY
Isostacy describes how materials float. When we explored the Earth's interior, we discovered that
the outermost layer, the lithosphere, lies above a partially molten layer, the asthenosphere. The
lithosphere is less dense and thus "floats" on the asthenosphere. The principle of isostacy is
extremely important to plate tectonics--this is what allows the plates to move.
We can define "The Law of Isostacy," which states that: A body floats when it displaces a mass of
fluid equal to its own mass.
In this experiment you have two Plexiglas aquaria, a number of blocks of wood, and some water.
Fill the cube-shaped container with water (as full as it will go) and set it inside the wide flat
container (this is the "flood-prevention container"). The water represents the partially molten
asthenosphere and the blocks of wood represent rocks of the Earth's crust. We'll use the blocks to
perform two related experiments, (1) the deposition of sediment in the ocean, and (2) the collision
of continents and the creation of mountain ranges. For both of these processes the law of isostacy
is very important.
EXPERIMENT #1: SEDIMENTATION
Note that two sides of the cube-shaped container are taller than the others. These taller walls
represent sea level. The lower walls--thus the surface of the water--in your experiment represent
the floor of the ocean (so in our experiment the "air" between the lower and higher walls of the
container represents the ocean). A wooden block will play the role of sediment dumped into the
ocean by rivers (see figure on following page). Note that the thickness of the wooden block is the
same as the distance between the upper and lower walls of the container.
Other parameters for this experiment: average depth of ocean = 5.5 km
density of water = 1.0 g/cm3
density of sediment = 2.4 g/cm3
density of continental crust = 2.8 g/cm3
density of asthenosphere = 3.17 g/cm3
Top of box = sea level
Water surface = floor of the ocean
H2O = Asthenosphere
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Add a wooden block to the apparatus. When sediments deposited in the ocean reach sea level
deposition will normally stop. Note that when you fill the ocean with sediments of thickness
equal to the ocean depth, that the upper surface of the sediment is not at sea level.
(1) Why is this?
(2) What fraction of the block lies below the asthenosphere?
(3) How many blocks can you add before the sediment reaches sea level?
(4) Measure the size of the wooden block and calculate its density using the Law of Isostacy.
EXPERIMENT #2: MAKING MOUNTAINS
When continents collide the crust is shortened and thickened. For example, the average thickness
of continental crust is 35 km, but beneath the Himalaya and Andes mountains the thickness is 70
km. This extra-thick crust is what creates mountains. Once high mountains form they immediately
begin to erode. Thus erosion works to reduce the height of mountains while collision acts to
increase this height.
Faults & EQs,
folded rocks
High mountain ranges
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*
*
*
*
Original thickness
of continental crust
Examine the wooden blocks floating in the aquarium. Think of these blocks now as representing
not sediment in the ocean, but rather, the thick crust of mountain ranges.
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"Erode" your mountains by removing one the blocks. What happens to the others?
(5) Use the measurements that you made in Experiment #1 to derive a formula that relates the
submerged thickness of a wooden block (Hs) to the total thickness of the block (H), the density of
water (ρ H2 O) and the density of wood (ρ wood). Hint: this will take the form of a proportionality.
(6) Now consider the Himalayan mountains, with a crustal thickness of 70 km. Using the
densities for crust and asthenosphere given above and your formula from (5), calculate (a) how
much of the Himalayan crust "sticks up" as mountains and, (b) how much is hidden beneath our
feet.
(7) Now erode the Himalayas. Use your answer from (6a) and erode the entire height of the
Himalayas. What is the elevation of this newly eroded surface? Is it at sea level, or some other
height?
PART II: ACQUIRING GIS DATA
Use Netscape to connect to the USGS National Earthquake Information Center:
http://wwwneic.cr.usgs.gov
Choose "Search earthquake data base," and then "Global."
This page allows you to access the USGS National Earthquake Information Center seismology
catalogues. The page is divided into sections. You will enter values in several of the available
boxes. First, in the section "Select Output File Type" choose Screen File Format. This allows us
to easily transfer the data into Excel (in Part II). Second, in the section "Select the Data Base"
choose USGS/NEIC (PDE) 1973-Present (the first option). This accesses a comprehensive global
data set. Below, in the "Optional Search Parameters" section, we will conduct several searches,
selecting different combinations of parameters in order to examine a variety of features of global
seismicity.
Lets begin by collecting earthquakes one month at a time. For example, to view all earthquakes
that occurred in January 1999:
• In the "Date" boxes, enter 1999, 01, 01 for Starting Year, Starting Month and Starting Day.
• Enter 1999, 01, 31 for Ending Year and Ending Month and Ending Day.
We'll leave the rest of the boxes blank in order to collect all earthquakes of all magnitudes and
depths for the month.
• Click the Submit Search button at the bottom of the page.
Please note: The USGS takes s few months to acquire a complete data set, so don't look for EQ's
from this month or last--the data will be incomplete.
It takes a few minutes for the USGS computer to search its data base and extract the information
you asked for. When you do get results, look at the information that precedes the earthquake list.
Note the line "Global Search Earthquakes ="
(8) (a) How many earthquakes occurred in this month?
(b) Does this number surprise you?
(c) How many earthquakes per day occurred on a global basis this month?
Now scroll down and examine the earthquakes that you requested. The first few columns of data
indicate the catalogue, then the year, month, day and time (the time is an 8-digit number giving 24hour Universal Time with an accuracy of 1/100 of a second). Following the time are the
coordinates of latitude and longitude, indicating where the earthquake took place. North latitudes
and East longitudes are positive numbers, while South latitude and West longitude are negative.
Finally, there is depth (in kilometers) and magnitude. The letters following magnitude indicate
which of several ways the magnitude was calculated.
Now that we know what we are looking at;
• Go to the File menu and choose "Save As."
• Give your file a name that you will recognize, and one that indicates what kind of data it
contains. Save it as a text file (add the extension .txt to the file name) in the folder for this
class. While we are using Netscape, we'll collect four more months of earthquakes. Use the
Back button to return to the search page.
• Request the earthquakes for any four months you wish, saving the files with different names,
as above.
PART III: FORMATTING THE DATA
We've got the data--so what do we do with it? Lists of numbers are not very exciting, nor are they
very informative. We'd like to plot the position of the earthquakes on a map of the world so that
we can see which regions are most earthquake-prone and which are not. In order to do this, we've
got to set the data up in a format that ArcView can understand. To do this, we'll use the
spreadsheet Excel. This intermediate step is necessary because the USGS provides us with more
data than we need, thus we'll have to trim the data set to just the information that ArcView
requires.
Open Excel and open the first file you saved (ex: Jan99.txt). When you open the file you should
see an "Import Wizard" box that asks for instructions on how to deal with the file. In the Original
Data Type box, click the "Delimited" option (this one is the correct choice most of the time). Click
the Next button. Choose "Space" as the delimiter. Now in the Data Preview section, scroll down
until you find the table of earthquake data (usually a lot of scrolling). Check to see that the
columns we examined using Netscape are now separated by black lines. This will place the data
into columns in Excel. Click Finish.
You should see a spread sheet with all your data, plus a lot of text at the top of the page that got
sent into different cells. We're not really interested in the text, so don't let its fragmented
appearance bother you. Find the columns for latitude (LAT) and longitude (LON). Look
carefully--most often the labels LAT and LON will not be directly above the actual latitude and
longitude data. Locate the latitude and longitude data (latitude will be numbers from -90 to 90,
while longitude will be numbers from -180 to 180).
• In the cell immediately above the latitude column type Lat.
• In the cell immediately above the longitude column type Long.
• Using the cursor in the letter and number cells at the top and left of the spread sheet, delete
everything that is not latitude and longitude (see the small example below).
Lat
Long
10.57
0.45
-14.17
60.96
12.49
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92.74
126.44
166.84
167.4
-89.05
Once the data consists only of Lat and Long values, choose Save from the File menu. Make
sure to keep the file in .txt format.
Do the same thing for your other data files. Now you can close your files and minimize Excel.
PART III: VIEWING THE DATA USING GEOGRAPHIC INFORMATION SYSTEMS (GIS)
Making Maps: Open ArcView using the shortcut on the desktop. Open a new view by clicking the
New button in the upper left corner of the project window. You should see a new box called View
1. We'll begin by using some of the files that come with ArcView to make a map of the world.
Locate the button with the big plus sign on it in the upper left corner of the button row (if you don't
see it, click in the View 1 window). Click the plus (the Insert Themes button). A theme in
ArcView is a layer of data. We'll begin with a map that has two layers (themes), a
latitude/longitude grid, and a map of the world. Navigate to the Plate Tectonics folder
(\\geoserv\home2\geo101\plate_tectonics). You should see a list of "feature data sources" in the
right-hand box. Choose WORLD30.SHP. Next add CNTRY92.SHP. Click the gray check
boxes next to the file name at the left side of the View 1 window (this is the map legend). You
might have to edit the CNTRY.92 legend to change the legent tyoe to "Single Symbol." If you've
managed to follow all these intstructions, you should see a map of the world with 30-degree
lat/long grid.
Editing Maps: The colors you choose to represent features on your map often make a big
difference in the ease of reading and interpreting the map. ArcView chooses somewhat random
colors when you add new files, so lets change the colors of the map to make them logical and easy
to read. Click the word CNTRY92.SHP once to make it active (you'll see it now has a raised
button appearance). Now double-click it to edit the legend. In the Legend Editor box double-click
the map color, and in the Fill Palette window click the little paintbrush once. Choose a pale yellow
color, then click the Apply button in the legend Editor window twice. Close the Legend Editor
window. Now double-click WORLD30.SHP to edit its color. Choose a light blue color from the
Color Palette window. Click twice on the Apply button as before; close the Legend Editor and the
Color Palette windows.
Adding Data: Now click on the Project window to make it active, and click the Tables icon on the
left half of the window. Click the Add button, and add your Jan99.txt file. (Note that ArcView
opens the Jan99.txt file into a new window. Verify that you have two columns, Lat and Long, and
close the window). Click on the View 1 window to make it active again and choose Add Event
Theme from the View menu. From the list of tables choose Jan99.txt. The X field should say
Long and the Y field should say Lat. Click OK.
The Fun Part! Click the gray check box next to Jan99.txt. You should see a map showing the
location of all the earthquakes that occurred in January of this year. Return to the Project window
and click Add to bring in your other data files. Make the View 1 window active and select Add
Event Theme from the View menu to add these files to the map legend. Click the check boxes to
add the data to the map.
PART IV: ANALYZING THE DATA
Turn the earthquake themes on sequentially and watch the pattern of seismicity that emerges. The
Earth's tectonic plates are defined by the location of earthquake epicenters. How many months of
real-time data are required for the plates to emerge?
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Print a copy of the ArcView file showing the plates. Trace the outline of the plate boundaries
on your printout.
(9) How many plates did you map?
One of the very excellent things you can do with ArcView is to take a "flat" map and turn it into a
spherical Earth. Go to the View menu and choose Properties. Click the box labeled "Projection."
Change the type to The World from Space." Click OK. You can "turn" the Earth by re-centering
the projection. Return to the Projection window and now select Custom. Enter any Latitude and
Meridian (Longitude) coordinates you wish. For example, (0,-90) will show you the South Pole.
This is a much better view of the Antarctic plate than you will get on most maps.
(10) Which plate is the biggest? Use ArcView's polygon tool to measure the size of the largest
plate in square kilometers.
(11) Which plate is the smallest? Measure this one in the same way.
EARTHQUAKE MAGNITUDES
Open the View labeled 1997 Global Seismicity. This view plots all 1997 EQs with magnitudes
between 3-8, with the points coded by color and size.
(12) How many magnitude 4,5,6,7 and 8 quakes were there in 1997? (Hint: open the .txt files in
the table window (or use Excel for easier counting)).
(13) Display only the largest magnitudes (7 & 8). What parts of the world are at high risk for
large earthquakes?
PLATE DYNAMICS
We know that earthquakes occur where blocks of rock slide past each other. Since the Earth's
tectonic plates are bounded by earthquakes, we know that the plates are all in motion.
(14) How many different ways is it possible for one plate to move with respect to another? Make
a sketch illustrating your ideas.
(15) Open the View called Earthquake Depth. Here EQs from 1996-98 have been color-coded by
depth. Where are the deepest earthquakes?
(16) Where are there only shallow (and no deep) EQs?
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•
•
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Use the measuring tool to find the relationship between distance and depth of EQs in two
different parts of the world, (a) across western South America through Chile and Argentina,
and (b) across the Tonga Islands (east of New Zealand). PS--if you don't know where these
countries are, make the CNTRY.92 theme active and choose the little i-button at the left end of
the button bar. Click on the country of your choice.
Begin by clicking on an EQ at the shallowest depth (0-50km). Write down the number 0 to
indicate your starting point, and write down 0 for its depth. Click on an EQ from the next
depth category (50-100km; don't worry if you need to skip a category, sometimes there just
isn't data available). Record the distance and depth (50km). Click on EQs from each category,
recording the cumulative distance from your starting point.
Use Excel to make a graph (an XY scatter) of these data, with Distance on the horizontal axis
and depth on the vertical axis.
Do the same for Tonga and plot the data on the same graph with South America.
0
0
200
400
Depth (km)
-100
-200
-300
-400
Distance (km)
(17) Try to interpret these data. What do you think your graphs mean? Print your chart.
(18) While you've got Excel open, make a bar chart showing the frequency of EQs at different
depths. You can use the data table on the last page. These are all EQs reported for April
1998. Do you notice any prominent breaks in the data? Where? What do you think these
mean? Print your chart.
(19) Add the theme Volcanoes to your EQ depth view (change the point shape to little triangles to
help you distinguish the volcanoes from the EQs). Is there any association between
seismicity and volcanism?
(20) Are there EQs or volcanoes that don't occur along plate boundaries? Where?
(21) Open the view called Topography. This is a DEM of the world with a resolution of 5 minutes
(about 10 km). Add some of your monthly data to this view (using Add Event Theme). What is
the relationship between topography and seismicity? Add volcanoes to this theme as well. What
do you observe?
(22) Examine the color hard copies of the map of the Age of the Ocean floor (sorry--we don't have
these geocoded yet...). What relationship, if any, is there between age of ocean crust, volcanism
and seismicity?
ANSWERING YOUR PRE-LAB QUESTION
Return to Netscape and go to Cornell's Atlas server:
http://atlas.geo.cornell.edu
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•
•
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Select Interactive Mapping and Analysis and start the application. Choose "Show Data Sets."
Draw a box on the map the encompasses both New York and Paris.
Under the Images and Grid menu, select Topography Hill Shaded.
Under the Geophysics menu, select CMT focal mechanisms.
Click Submit Request.
Focal mechanisms indicate what type of fault was responsible for each EQ. They are interpreted
on the following page:
Thrust Fault
Normal Fault
Strike-Slip Fault
Thrust faults mean that the two plates are moving toward one another. Normal faults indicate
plates that are moving away from one another, and strike-slip faults occur when the two plates are
moving sideways past each other.
(23) Can you determine whether New York and Paris are getting closer, farther apart or staying
the same distance? Explain.
(24) Finally, write up a report that summarizes your results from Part IV of this assignment. You
should incorporate the answers to the questions printed here, as well as reflect more deeply on the
significance of the observations you've made this afternoon. Include all of your maps and graphs,
and any other material that you feel would be helpful.
DATA FOR APRIL 1998
Depth
0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
Frequency
251
780
468
69
63
98
59
34
55
12
35
5
3
16
2
12
1
2
2
1
5
3
2
3
1
16
1
1
15
1
15
2
2
2
0
0