KEYLab2-PlateTectonics-21F-1
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LAB 2 – Plate Tectonics Name______________________________________________________
Fall 20
21
Section______ Lab Partners ________________________________________________________________ After completing this lab, you will understand: -
The differences between brittle and ductile behavior.
-
How density and brittle/ductile changes influence P
late T
ectonics.
-
How to calculate a density
from independent measurements
of mass and volume
.
-
Features and measurements indicative of the different plate boundaries.
-
The relationship between plate boundaries and hazards.
Introduction Plate T
ectonics forms the continents, ocean basins, mountain ranges, and rift valleys. It is the major underlying force that causes
earthquakes, volcanoes, and tsunami
. In this lab you will use measurements, calculations, and observations to learn the processes of plate tectonics and properties of the plates and the plate boundaries. You must show your work *and units* for full credit. I. Properties of the Plates
A. Effects of Brittle and Ductile Behavior
Both the lithosphere and asthenosphere contain portions of the mantle, yet one is rigid and brittle
(breaks under stress) and the other is plastic or ductile and flows under stress. The mantle rock in
both layers is solid; however, the mantle rock changes behavior due to increasing temperature and
pressure at depth.
It is impossible to reproduce these conditions in a classroom, but we can demonstrate how a substance can behave both rigidly and plastically using a substitute, also called a proxy: Silly Putty. Some precautions for using Silly Putty: Silly Putty is non-toxic but is also adhesive and notorious for picking up dirt and ink and sticking to hair and clothes. 1.
Your TA will give you samples of Silly Putty. Follow the directions below and record your
observations.
a.
Take your sample of Silly Putty and mold into a rough rectangle.
b.
Grab your sample on two sides. Then very quickly pull it apart. Describe what occurs in
detail. Does it snap or keep stretching?
LAB 2 – Plate Tectonics 2
c.
Remold your Silly Putty into a rough rectangular shape and pull it very slowly apart.
Again describe what occurs in detail. Does it snap or keep stretching?
d.
Which demonstrated brittle behavior and which demonstrated
ductile?
Explain your reasoning.
B.
Effects of Density
Another important property in Plate Tectonics is density, or how much mass occupies a given volume. Density is an important property in science and engineering. Lower density objects will rise to the top of higher density substances (i.e. oil floating on water). Directly measuring density of an object can be done with special instruments, but it can also be calculated from two characteristics
we can measure: mass and volume. Your group will determine densities from samples of oceanic crust, continental crust, and "mantle" rock. Make sure to show your
units.
1.
Use a scale to measure the mass of each sample in grams. Your TA will demonstrate how
to use the scale in lab. Record that information in Table 1
.
2.
Next, determine the volume of each sample. However, volume of an irregularly shaped object, like your samples, cannot be easily measured with a ruler. Another volume-
measurement method is to see how much water the object displaces. We have measured the volumes of the rock samples ahead of time; they are provided for you. Record the volumes in cubic centimeters (cm
3
) in Table 1
.
3.
Calculate the density for each of your group’s samples by dividing the mass by the volume and record your answer in Table 1
. Note that units of density are mass units/volume units.
4.
The most statistically reliable answers are usually a mean or average calculated from a range of measurements. Your TA will organize a table of density measurements for the whole class on the blackboard. Add your group's density measurements for each sample. Calculate the overall class average for oceanic, continental and mantle rock and record this information in Table 1
.
5.
On average, which is denser -- continental, oceanic, or mantle rock?
LAB 2 – Plate Tectonics 3
II. Plate Boundaries
F
igure 1. Major tectonic plates of the world. A.
Look up a plate tectonic map of the world online (i.e. https://pnsn.org/outreach/about-
earthquakes/plate-tectonics) to help you f
ind and label the following plates on the map above
:
North America (NA), South America
(SA), Pacific (Pa), Cocos (Co), Africa (Af), Eurasia (Eu),
Indian (In), Australia (Au), Philippine
(Ph), Arabia (Ar), Antarctica (Ant), Caribbean (Car),
Nazca (N), and Scotia (Sc).
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LAB 2 – Plate Tectonics 4
Figure 2. Top: The locations of earthquakes over a 30-year period. Bottom: The locations of active volcanoes.
Image credit: Prof. Charles J. Ammon, Dept. of Geological Sciences, Penn State University.
B.
Figure 2 compares two maps of the world: the top map showing the locations of earthquakes
over a 30 year period, the bottom map showing the locations of major active volcanoes.
Summarize
what these two maps tell you about the relationships between plate tectonics,
earthquakes, and volcanoes.
LAB 2 – Plate Tectonics C
.
Plate Boundaries and the Oceanic Crust
Most divergent boundaries are in the ocean basins. As the plates pull apart, hot mantle rises, partially melts, and when it solidifies, produces new oceanic crust. This new crust is continually pulled away from the divergent boundary as the plates continue to spread apart. For this reason, these boundaries are known as oceanic spreading centers
. The youngest oceanic crust is thus always found at oceanic spreading centers.
Oceanic spreading centers were the first plate boundaries to be recognized for what they truly were. The evidence came first from maps of the topography of the seafloor (called bathymetry
), collected during World War 2. On Google Earth, bathymetry is shown in shades of blue: the darker the blue, the greater the depth. Use Google Earth to tour the bathymetry of the seafloor.
1.
Ocean floor topography
a.
In Google Earth, examine the Atlantic Ocean between South America and Africa. Note that the deepest part is not in the middle; instead there is an underwater
mountain range that runs down the middle of the ocean -- the mid-ocean ridge
, or spreading
center. Zoom in enough to see that although the ridge is a topographic high, it also has a
valley (the “rift valley”) running along the middle of it. In the space below, sketch a cartoon
topographic profile of the Atlantic Ocean floor between South America and Africa. Use
Figure 1
to label the tectonic plates that meet here. b.
If the earth’s lowest spots aren’t in the middle of the ocean, where are they? Fly to the west
coast of South America. About 600 miles (1000 km) off the coast into the Pacific Ocean, find
a deep linear feature called an ocean trench, among the lowest points on Earth. Sketch a
cartoon topographic profile of the Pacific Ocean floor from South America westward. Use
Figure 1
to label the tectonic plates that meet here.
c.
Fly to a site called Challenger Deep, the deepest spot on Earth. Challenger Deep reaches 11
km (36,000 ft) below sea level. Which is greater, the elevation of Mt. Everest above sea level
(Google it!), or the depth of Challenger Deep below sea level? By how much?
Scan around to see the ocean ridges in the Indian, Pacific and Southern Oceans.
Credit: Laurel Goodell, Dept of Geosciences, Princeton University. "Using Google Earth to Explore Plate Tectonics." https://serc.carleton.edu/sp/library/google_earth/examples/49004.html
5
LAB 2 – Plate Tectonics Figure 3.
Age of the oceanic crust. Image from NOAA (https://www.ngdc.noaa.gov/mgg/image/images/g01167-pos-a0001.pdf)
6
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LAB 2 – Plate Tectonics 7
D
.
Paleomagnetism and seafloor spreading
Oceanic crust rock contains
minerals that preserve the direction and strength of the E
arth’s magnetic field as they crystallize. Interestingly, the Earth's magnetic field has not been constant throughout Earth's history. It has changed polarity over time, meaning that the North magnetic pole flips to the South pole, and then eventually flips back. A magnetometer can detect changes in the paleo
magnetism preserved in the minerals
of the oceanic crust. These changes are small (~1% of the total field strength of the Earth), but altogether the data reveal a pattern. By dating
the rocks on the seafloor, scientists have recognized that the age of ocean crust increases with distance from the axis of the mid-ocean ridge. By correlating dates with magnetic reversals, a time scale for normal
(same magnetic polarity as today) and reversed polarity was
developed.
1.
To get some idea of how symmetric (or not) sea floor spreading is, measure the distance
from
the mid-oceanic ridge axis to the odd numbered peaks on Figure 4
, and fill in Table 2
below.
Show your calculations
.
2.
Age of the ocean floor. Use Figure 3
,
depicting the age of the oceanic crust, along with Google
Earth to answer the following questions.
a.
Approximately how old is the oldest oceanic crust (seafloor)? Considering that the earth is
4.6 billion years old what could have happened to older crust?
b.
How young is the youngest seafloor in the Atlantic Ocean? Where is it located? Does its
location correspond with any of the earthquake activity or volcanism shown in Figure 2?
c.
How old is the the oldest ocean crust in the Atlantic Ocean? Where is it located? Does its
location correspond with any of the earthquake activity or volcanism shown in Figure 2?
d.
Using the difference in seafloor ages between the mid-ocean ridge (assume new crust is forming now
)
and the edge of the ocean basins,
when did the northern Atlantic Ocean begin to open?
e.
Assume that the distance between Delaware and West Africa is 5200 km. Calculate the
spreading rate (in km/Myr) for this part of the Atlantic Ocean. Show your work.
LAB 2 – Plate Tectonics 8
Table 2 Peak#
Distance
from ridge, left side (km)
Age
of ocean crust (Myr)
Spreading
rate
(km/Myr)
Distance
from ridge, right side (km)
Spreading
Rate
(km/Myr)
Spreading
Rate
Difference
3 17
5 32
7 46
9 54
11 67
5 5 3 3
7 7 9 9
11
11
Figure 4.
Magnetic sea floor anomalies in the Atlantic Ocean near Antarctica. Distance from the mid-oceanic ridge (designated as 0 km) is compared with detected magnetic field strength (in nanoTeslas). Dates in million-years are given atop peak magnetic anomalies. The bottom profile is the same as the top but is flipped end-for-end for comparison (from Pitman and Hertzler, 1966). 2.
Based on your measurements of the distance from the ridge axis of similar-aged magnetic
anomalies, does new crust form symmetrically at mid-ocean ridges? Why or why not?
9
E.
Transform Plate Boundaries: The San Andreas Fault
Figure 5
is a highly simplified geologic map of southern California showing the San Andreas Fault, a transform boundary between the North American plate and the Pacific plate. In transform boundaries, the crust of two plates moves in parallel to the plate boundary. Different rates of motion
between the two sides of the boundary over time can result in features being split apart, physically moved away from each other by the slow but ongoing motion of the plates. When transform faults pass through older features, such as a unit of bedrock, they create an offset, where one side is offset, or appears to have shifted position. We can use the distance between the two once-joined features today to calculate how much the plates have moved over time.
1.
Put arrows along the two sides of the fault (shown in Figure 5
) to show the relative
motion between the plates. Mark your measurements.
2.
Your map shows that Miocene rocks (25 million years old) have been offset by the San Andreas. Measure the distance of that offset and record that distance here:
Figure 5.
Simplified geologic map of southern CA showing displacement on the San Andreas Fault. Adapted (2012) by Tobias Hasse from the Geologic Map of California (2010).
http://www.conservation.ca.gov/cgs/cgs_history/Pages/2010_geologicmap.aspx
Pacific Ocean
LAB 2 – Plate Tectonics
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LAB 2 – Plate Tectonics 10
3.
Calculate the rate of motion in terms of km/Myr, and then mm/yr.
4.
Given that the last big earthquake in 1906 caused about 5 meters of offset, how many earthquakes of the same magnitude would you need to get the observed offset of the Miocene rocks? 5. Imagine that the entire distance of the offset has been caused by repeated large earthquakes (like in 1906) since the Miocene. Calculate a recurrence interval -- the average time between these large earthquakes -- from the Miocene to the present day.
F.
Convergent Plate Boundaries: Caribbean Plate subduction zones
1.
Using Figure 1
as a guide, label the North American (NA), Cocos (Co), South
American (SA),
and Caribbean (Ca) plates on the map in Figure 6. 400
depth(km)
400
Figure 6.
Map and cross-section of earthquake epicenters around the Caribbean Sea.
1
1
LAB 2 – Plate Tectonics 2. Use the density of earthquakes plotted on Figure 6 to sketch the boundary of the Caribbean Plate on the map.
3. How deep are the deepest earthquakes?
4. Notice that, for the Caribbean Plate, a plot of earthquake position vs depth makes a curving surface. What does this curving surface represent?
5. Is the Cocos Plate subducting beneath the Caribbean plate or the other way around?How do you know?