GEOL101 Lab 3 Plate Tectonics Handout
docx
keyboard_arrow_up
School
San Diego State University *
*We aren’t endorsed by this school
Course
101
Subject
Geology
Date
Apr 3, 2024
Type
docx
Pages
12
Uploaded by AmbassadorLobsterMaster1052
GEOL101 Dynamics of the Earth Name:
Laboratory 3: Plate Tectonics
Section:
Learning Outcomes:
●
Identify the internal layers of the Earth based on composition and physical properties. ●
Describe how regional lithosphere density and thickness affect topographic elevation.
●
Identify major geographic features resulting from differing plate tectonic boundaries.
●
Calculate seafloor spreading rates based on distance and age date.
Slicing Up Planet Earth
The Earth’s interior can be subdivided
into concentric layers or shells based on chemical composition
or physical properties
(Figure 1). Let’s explore each of these
classifications . . . in depth <insert student groan
here>.
Based on chemical composition, the
Earth can be subdivided into three layers. The crust
is the outermost compositional layer of the
Earth and has two different “flavors.” Oceanic
crust comprises most of the global ocean basins
of the world, averages ~7 km thick, and is
generally composed of basalt and similar rocks.
Basalts are typically composed of 45% to 52%
silicon dioxide (SiO
2
), and tend to be enriched in
magnesium (MgO) and iron (FeO) compared to
continental crust. In contrast, continental crust
is
much thicker, averaging 45 km and up to 75 km
in places, and is generally composed of granite
and similar rocks. Compared to basalts, granites
have more silicon dioxide (i.e., 65% to 72%) and
less magnesium and iron. Because iron and
magnesium are much higher mass atoms,
oceanic crust tends to have a higher density
(i.e., ~3.0 grams per cubic centimeter) than
continental crust (i.e., ~2.7 grams per cubic
centimeter). We will return to the thickness and
density differences between oceanic and continental crust as they reflect and
explain some major plate tectonic processes as well as some major global elevation patterns. The mantle
extends from the base of the crust down to ~2,885 km (~1,792 miles) and is lower in silicon dioxide (typically <45%) and higher in magnesium and iron than oceanic crust, which results in a density of ~4.5 grams per cubic centimeters. Much of the mantle is composed of peridotite and other “ultra-mafic” rocks (where the term mafic represents a blend of “magnesium” and a contracted form of “ferric,” which means related to iron). The core
is the third and deepest compositional layer, extending from ~2,885 km down to the center of the Earth at ~6,371 km (~3,959 miles), and is composed largely of iron with nickel and sulfur.
All three of these layers are based on composition as summarized on the left side of Figure 1, but note
that the term “core” serves “double-duty” as it is also used for a different complementary classification based on physical properties as described below.
Figure 1: Cross-section of the Earth illustrating the two
ways to subdivide its interior. Divisions on the left side
are based on chemical composition, where divisions
on the left are based on physical properties.
The Earth can also be subdivided into five concentric layers based on physical property of the material, specifically whether the layer shows a rigid, plastic, or liquid response to an applied force (Right side of Figure 1). Note that these physical-property-based layers do not necessarily coincide with the composition-based layers. The rigid lithosphere
consists of the outermost ~100 km of the Earth, which includes the continental and oceanic crust along with the upper portion of the mantle. Starting at ~100 km and proceeding down to ~2,885 km, the mantle material remains similar in composition, but due to increasing temperature and pressure transitions in behavior from the rigid lithosphere to the plastic asthenosphere
to the rigid mesosphere
. Note that plastic is not
being used here in the “recycle your plastics” sense, but to describe the behavior of a material that flows
and deforms
in response to an applied force, but doesn’t
return to its original form after removing the force (think toothpaste). Our exploration of plate tectonics will largely focus on the compositional layers of crust and mantle and physical property layers of lithosphere and asthenosphere. Below the rigid mesosphere, continuing increases in temperature and pressure produce a liquid
outer core and a solid inner core. Note that circulation within the liquid outer core creates the Earth’s magnetic field, which not only allowed humans to invent compasses for navigation, but also protects life on our planet by deflecting harmful radiation from our Sun.
Putting Planet Earth in Motion
Now that we understand the basic nature of the lithosphere and asthenosphere, let’s take a closer look
at the lithosphere. The rigid lithosphere encircling planet Earth is broken into a number of tectonic plates
. In fact, there are seven major (large) plates and a number of smaller “microplates.” Each plate is in close physical contact with the surrounding plates, but the nature of this contact can change over space and time. Scientists have identified three fundamentally different types of plate boundaries, each based on the nature of the movement between the two plates: Convergent
is used when plates are moving toward one another, divergent
is used when plates are moving away from one another, and transform
is used when plates are grinding past one another.
Let’s use Google Earth
to get better acquainted with where these boundaries are located and how to identify them. (If you need help using Google Earth
, go to the end of the lab for tips.) 1.
If you don’t already have Google Earth Pro
desktop installed on your computer, download and install it for free or navigate to www.google.com/earth/versions.
2.
Download the GEOL 101 Dynamic
Earth.kmz
file from Canvas and open the
file in Google Earth Pro
by choosing File
then Open
and navigating to your
downloaded file. The file will appear in Places
under Temporary Places
. Click the
black arrow to the left of the file to open all
its layers. Turn off everything by
unchecking boxes except for the Dynamic
Earth
and Plate Boundary Model
layers. To
the right is a screen shot of what the Dynamic Earth.kmz
file should look like in Google Earth
.
3.
For the purposes of this lab in the Dynamic Earth
file, convergent boundaries are in blue, divergent boundaries are in red, and transform are orange and green.
Convergent Boundaries
: Convergent boundaries (blue lines) occur when two plates are moving towards and produce one of two possible outcomes depending on whether continental or oceanic crust is associated with the leading edge of the lithospheric plates (Figure 2). What to look for where continental
and oceanic crust meet at a plate
boundary
: Where the leading edge of
one lithospheric plate with continental
crust meets the leading edge of
another lithospheric plate with oceanic
crust, the oceanic-crust-bearing plate
will be “subducted” beneath the
continental-crust-bearing plate
because the former plate is denser
than the latter plate (Figure 2a). This
subduction will produce a trench
where
the two plates meet and a continental
volcanic arc
range (i.e., a mountain
range made up of volcanoes) will form
on the overriding plate along the
trench. A prime example of this is the
Andes Mountains along the western
margin of South America. Turn on layer
Part 3
in Google Earth
and double click
the Andes Mountains, South America
pin in the Convergent Boundaries
folder (you may need to expand this
folder) to be taken there. In this
example, the Nazca plate is composed
of oceanic-crust-bearing lithosphere
and is subducting beneath the South
American plate. The South American
plate, like most plates, is composed of
both continental and oceanic
lithosphere.
What to look for where oceanic crust and
oceanic crust meet at a plate boundary
: In the case where the leading edge of both lithospheric plates include oceanic crust, then the older, more dense plate will descend or subduct beneath the less dense and younger plate (Figure 2b). This is because the older plate will tend to be relatively denser. A deep bathymetric furrow, or trench, commonly marks the position where the overriding and subducting plates first contact each other. Parallel to the trench and located on the overriding plate will be an oceanic
volcanic arc.
An excellent example of this is the Island of Japan. In Google Earth
, double click the Japan
pin and you will be taken to that island arc. In this scenario, the Pacific Plate is subducting beneath the Okhotsk plate. We
know the Island of Japan is a volcanic arc and it is located on the Okhotsk plate, meaning it must be the overriding plate.
What to look for where continental crust and continental crust meet at a plate boundary: In the case where the leading edge of both plates includes continental crust, then neither plate really subducts; instead they continue to be forced into each other causing the crust to thicken (Figure 2c). The Himalayan Mountains formed in this manner and as a result contain the tallest mountain on Earth,
Mount Everest. Double click the Himalayas
pin in Google Earth
to be taken there. Figure 2: Block diagrams of the three
convergent plate boundaries.
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Divergent Boundaries: Along diverging
boundaries (red lines), plates are moving
away from one another and new oceanic-
crust-containing lithosphere is forming.
Such boundaries are commonly marked by
mid-ocean ridges and a rift valley, and are
sites (Figure 3). Double click the Mid
Atlantic Ridge
pin located in the Divergent
and Transform Boundary
folder in Google
Earth
to be taken to this divergent
boundary in the middle of the Atlantic
Ocean. Here African plate material is
moving east and South American plate
material is moving west. Zoom in to the pin
and turn the Plate Boundary Model
off by
clicking the check mark to its left. You
should still be able to recognize the
boundary between the two plates.
Transform Boundary: Transform
boundaries (green and orange lines) are
prominent in ocean basins where they
appear to offset ridge axes. However,
transform boundaries actually act to link up mid ocean ridges and allow segments of plates to move past each other laterally. Let’s stay at the Mid Atlantic Ridge
pin. The transform boundary in green is working to link up the motion of the divergent (red) boundary. If you zoom out and spin the Earth, you will see that divergent and transform boundaries are working together all through the ocean basins. Transform boundaries are not only found in the Earth’s oceans; in fact, the closest plate boundary to San Diego State University is a transform boundary and is called the San Andreas Fault to our east!
Figure 2: Block diagrams of divergent plate
boundaries.
Lab Activity
Note that the questions start with Question 7 . . . there is no Question 1 through 6!
In Google Earth
, turn on the folder entitled Question 7
by clicking the box to its left and expand the folder. Double-click on the Question 7
polygon to be taken to the location shown in the figure below. Use Google Earth
and the reading above to help answer the following questions. Tip: If you are having trouble viewing the area, turn off layers, such as volcanoes of the world or sea floor age by unchecking the box next to them until they are needed for later questions. Question 7.1: Zoom out in Google Earth
to identify and name the tectonic plates labeled A and B in the
Question 7
location. Make sure the layer Plate Boundary Model
is turned on. You may need to rotate the Earth around to see the plate names that are shown in orange.
The Pacific Plate and North America Plate
Question 7.2: What type of boundary lies between the two plates shown in the Question 7
location? Which plate do you think is being subducted? Outline your reasoning for both answers by referring to relevant seafloor features and the earlier discussion and figures on plate boundaries. The boundary between the Pacific Plate and the North America Plate is likely a convergent boundary, with the Pacific Plate being subducted beneath the North America Plate. This is supported by the presence of trenches, volcanic activity, and deep-focus earthquakes characteristic of subduction zones.
Screen shot of the Question 7 location in Google Earth.
North arrow and scale have been added here to help orient
you.
Turn on the folder entitled Question 8
and then double-click on the polygon entitled Question 8
to be taken to the location shown below. Use Google Earth
and the earlier discussion and figures on plate boundaries to help you answer the following questions. Question 8.1:
Zoom out in Google Earth
to identify and name the tectonic plates labeled A and B. Make sure the layer Plate Boundary Model
is turned on. You may need to rotate the Earth around to see the plate names that are shown in orange.
African plate and South American plate Question 8.2:
What type of plate boundary is found between these two plates (A and B)? Both divergent and transform Question 8.3:
Based on your understanding of seafloor spreading, draw an arrow in each of the circles
to indicate the relative motion of the underlying lithosphere. What direction (e.g., east, west, north, or south) is Plate A moving? Plate B?
The African and South American Plates are diverging in the Atlantic Ocean and sliding past each other along their eastern boundary. The African Plate moves east or northeast, while the South American Plate moves west or northwest.
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Question 9: Now turn on the layer entitled Seafloor
Age
; a screenshot of what you should see in Google
Earth
is shown to the right. The oceanic crust will be
color-coded based on its age, with warmer colors
(reds) represent younger (i.e., more recently formed)
oceanic crust and colder colors (blues and purples)
represent older oceanic crust. Follow the steps below to complete the table below.
●
Expand the Seafloor Age
folder and navigate to
the Mid Atlantic Ridge by double clicking the pin
labeled A
.
●
Go to the Atlantic Ocean and use the seafloor age color scale to determine the youngest oceanic crust located at Pin A and the oldest oceanic crust located at Pin B.
●
Use the ruler tool to measure the distance in km from Pin A to Pin B and round to the nearest
whole number (i.e., 4,792.5 km = 4,793 km). If you don’t know how to use the ruler tool, then go to the end of this handout for help.
●
Calculate the seafloor spreading rate using your measured distance and the formula of Spreading Rate = Distance (km) / difference in age (yr)
●
Repeat the above steps for the East Pacific Rise at C.
○
Measure to the oldest crust at D.
Hint: [1 km = 100,000 cm] and [1 my = 1,000,000 yr]
Oldest crust
age (yr)
Approximate distance from
ridge to oldest crust (km)
Spreading rate (cm/yr)
A. Mid-
Atlantic
Ridge
170 – 180 ma
2963 km
1.74 cm/yr
C. East
Pacific
Rise
170 – 180 ma
9379 km
5.52 cm/yr
Question 10: Based on your calculations in the table above, which ridge is moving faster?
The East Pacific Ridge is moving faster
Question 11: Notice the seafloor age pattern in the Atlantic Ocean, now navigate to the Pacific Ocean and notice its age pattern. With the seafloor ages and these patterns, do you need to calculate the spreading rate to conclude what you did in question 10, why?
No, spreading rates don't need to be calculated to confirm that the East Pacific Ridge moves faster each year compared to the Mid-Atlantic Ridge. Seafloor age patterns show asymmetry in the Pacific, meaning faster spreading on one side of the ridge, while the Atlantic displays symmetrical patterns, suggesting more uniform spreading rates. Question 12: Turn off the Seafloor Age
layer and turn on Question 12
. Double-click on Question 12
to move your view to the west coast of the United States. A screenshot of this view is shown below. Use it
and Google Earth to answer the following questions.
Question 12.1: Using your knowledge of plate boundaries, show the most likely relative direction of plate motion in each white circle using arrows. Remember: blue = convergent, red = divergent, green and orange = transform.
Question 12.2: Which plate shows the greatest difference between its relative motion and its absolute motion as shown by the gray arrows?
The blue one
The western North America and eastern Pacific Ocean is comprised of three
lithospheric plates (i.e. North American, Pacific, and Juan de Fuca). Their absolute
rates and directions are shown in the grey arrows. These three plates produce all
three types of plate boundary types (blue = convergent, red = divergent, green
and orange = transform).
Question 12.3: Assuming the plate motions continue into the geologic future, predict how the distance between Oakland and San Diego will change in geologic time? Will they get closer together or farther apart? How do you know?
If plate motions continue, the distance between Oakland and San Diego will likely increase. This prediction is based on the relative motion of the plates as shown by the arrows: if they are moving away
from each other, it suggests that the distance between these locations will grow over time due to divergent plate boundary motion.
Question 12.4: What type of plate boundary is located between Oakland and San Diego? This boundary is also a fault, a pretty well-known one actually. What is the name of this boundary or fault? Hint: you can find the name on page 9 of this lab. The type of plate boundary located between Oakland and San Diego is a transform boundary, also known as the San Andreas Fault.
Question 13.A-I: Now double-click on Question 13 in Google Earth and travel to the southern tip of South America. Use Google Earth and the knowledge you’ve gained in this lab to answer questions A through I below. For example, Question A below corresponds to where A is pointing on the figure below, and so on (ignore location E). Also note that the sketch beneath the Google Earth screenshot is not drawn to scale.
A. Type of crust? Continental B. Type of crust? Oceanic C. What would you call
this interval based on
physical properties? Asthenosphere D. What would you call this interval based on physical properties? Mantle or Mesosphere
F. What is the name of this tectonic plate? Nazca Plate G. What is the name of this tectonic plate? Antarctic Plate H. Based on the boundary type between Plates E and F, what would you expect to find on the seafloor?
Trenches, subductions zones or volcanic activity
I. Based on the boundary type between Plates E and F, what would you expect to see here?
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Trenches, subductions zones or volcanic activity
Using Google Earth Pro Desktop:
When Google Earth (GE) first opens all the ‘Layers’ are turned on. This can be distracting and make finding what you are looking for hard. Turn off everything in ‘Layers’ to make your life easier. When a layer is on, a check mark will be in the box to its left. To turn it off click the box and the check mark will disappear.
To expand a folder to see its subfolders/layers, click the black arrow located to its left. An expanded folder has a black arrow pointing down. A collapsed folder has a black arrow pointing to the right.
Using the ruler tool in Google Earth Pro Desktop:
The ruler tool is located in the menu bar at the top of Google Earth, to the right of the search bar
and it looks like a small blue ruler.
Click the ruler tool once to get the ruler tool dialog box. Make sure the line tab is open. You can change the units by clicking the drop-down box located to the right of ‘Map Length’. Once you have the units set, click once on one of the locations you want to measure between and then click on the other location. You do not click and drag between the two locations. The distance between your two locations will be displayed next to ‘Map Length’. To measure another distance, click ‘Clear’. To close the ruler tool, click the red x located in the top right corner of the ruler dialog box.
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help