Tectonics_VL2.1-1 (1)
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School
De Anza College *
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Course
010
Subject
Geology
Date
Dec 6, 2023
Type
Pages
8
Uploaded by EarlDiscovery7530
© 2010 C. G. DiLeonardo
Objectives
By completing this exercise, you will be able to:
•
Distinguish between absolute and relative plate motions.
•
Recognize the polarity of a subduction zone from seismic data.
•
Use seismicity to determine the location and nature of plate boundaries.
Materials:
Pencil; color pencils; millimeter scale/ruler; calculator; and eraser.
Computer
connected to the internet.
Discovering Plate Boundaries
Lithospheric Plates & Plate Boundaries
Open>
Tectonic Plate Map
http://tectonics.caltech.edu/images/maps/plates.pdf
Take a moment to familiarize with the map and use the zoom feature to
look more closely. The map shows the word’s plate boundaries, using
colors to differentiate between divergent, convergent and transform
margins. Study the map noting the general location of major plate
boundaries.
1.
Place in order from greatest length on the Earth’s surface to least,
the tree general types of plate boundaries.
When Wegener devised the Continental Drift hypothesis he envisioned the continents moving
either over or through the ocean basins. This was one of his greatest difficulties. Geophysicists
correctly pointed out that the frictional resistance in such a scenario would be impossibly large
and the rocks would simply shatter.
Examine the relationship between continents and plates by looking at the map you created.
Discovering Plate Boundaries & Processes
Virtual Geology Laboratory Edition
Christopher DiLeonardo, Ph.D.
Earth & Space Sciences
De Anza College
Introductory Geology Laboratory
Plate Tectonics 2
2.
In general, do continental shorelines coincide with plate boundaries?
3.
Do lithospheric plates tend to have both continents and oceans imbedded within them?
Now look at the coastlines of North America.
Continental margins that are associated with a
plate boundary are referred to as “active margins.”
Those coastlines that are not associated with
a plate boundary are referred to as a “passive margin.”
4.
Of the Pacific, the Gulf, and the Atlantic Coasts, which are passive and which are active
margins?
If any are active, list the types of plate boundaries that are present.
Zones of Seismicity
Open>
Global Seismicity Map
https://www.nsf.gov/news/mmg/media/images/global_seismicity_h.jpg
The color of the epicenter plot is related to the depth of focus (the center point of
the area of rupture on a fault that generated the earthquake).
Look at the map
Legend
with a key relating color of the plot to depth of foci.
Our earlier studies have indicated that the lithosphere is rigid enough to store strain
and generate earthquakes.
The asthenosphere beneath is too weak to generate
earthquakes.
5.
In general, how do zones of seismicity compare to plate boundaries?
Examine the world seismicity map.
Note the key indicating location of epicenters for earthquakes
and focal depths. Note that the tectonic map and the seismicity maps are similar but vary depending
on the focal depths of the various earthquakes that are plotted and the time period of seismicity
that is displayed.
Look at areas of the Northwestern Pacific Basin.
This area is part of the
Ring of Fire
of the Circum
Pacific region.
It is dominated by subduction zones.
You can zoom in on this high resolution map.
Examine the pattern of seismicity across the subduction zones.
6.
Describe the seismicity above subduction zones.
How does it compare to the
seismicity seen along divergent and transform margins?
Introductory Geology Laboratory
Plate Tectonics 3
Earthquake Depth
Use the
Earthquake Viewer
in the virtual lab module on the class site. Follow the instructions
below to answer questions about earthquake patterns at different types of plate boundaries.
Select>
Japanese Island Arc
from the Earthquake Viewer
The parameters of the map have been set up for you. All you need do is click on the map in the lab page and then
follow the instructions below.
Look at the seismicity across the Japanese island arc plotted for this area in the western margin of
the Pacific.
Now click on the “3D” button on the panel. A new window will open with your
earthquakes plotted in 3-D. Take the cursor and using the right click grab and rotate the North-
South axis up to see the plot of earthquakes going into the earth. This cross-section-like view
across the island arc shows seismicity and focal depths of earthquakes.
Look at the pattern on the
map and compare it carefully to the 3-dimensional view.
7.
Look carefully across the Japanese Island arc.
Here the Pacific Plate is diving under Japan.
The underlying
asthenosphere
cannot generate earthquakes. The pattern of earthquakes
diving under the Japanese Island Arc is from the subducting Pacific Plate. How deep is the
deepest quake?
8.
Note the distribution of shallow to deeper focus earthquakes, is it random?
In what compass
direction do they get deeper?
The pattern you are noting shows the subduction of the down-going slab of the Pacific Plate under
the Japanese Island Arc. This pattern of seismicity is diagnostic for subduction zones. The
direction the plate descends down into the asthenosphere is known as the
polarity of subduction
.
In this case it would be towards the west/northwest. Only subduction zones show such a pattern of
seismicity with earthquakes increasing in depth in one specific direction.
Select>
Western South America
from the Earthquake Viewer
Now let’s look at a similar pattern along a continental margin by viewing seismicity along the west
coast of South America. Here the oceanic Nazca Plate is subducting under the active continental
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Introductory Geology Laboratory
Plate Tectonics 4
margin of the South American Plate. Look at the pattern in the map area and then click on the 3D
button to view the earthquake pattern in 3-dimensions.
9.
Does the seismicity get deeper under the continent? How does this pattern compare to what
you viewed under the Japanese Island Arc?
10.
What is the polarity of subduction here, what is the direction of movement of the down
going Nazca plate?
Select>
Eastern Himalaya
from the Earthquake Viewer
The Himalaya represent a
continental collision zone
. A type of convergent margin involving two
continental lithospheric plates. Look at the distribution of earthquakes in map view. Then click on
the 3-D button and manipulate the plot with your cursor so you can see the distribution of
earthquakes. Be sure to complete rotate the diagram so you can see the distribution of quakes
below the earth’s surface.
11.
Note there are mostly shallow earthquakes (depth < 70 km). But there are a number of
deeper earthquakes. How does the distribution for deeper earthquakes differ from what you
have seen in subduction zones, such as under Japan and western South America?
Both types of convergent margins have deeper earthquakes (depth > 70 km). There is no subduction
at this type of boundary. The thickening of the lithosphere here by convergence has led to deeper
earthquakes. Only convergent plate boundaries, both subduction zones and continental collision
boundaries exhibit deeper earthquakes. Let’s compare this to other plate boundaries.
Select>
Mid-Atlantic Ridge
from the Earthquake Viewer
The Mid-Atlantic Ridge has formed along a divergent margin in the sea floor. This is a location of
both earthquakes and volcanism. Examine the patter of seismicity along the length of the margin
in the plot area. Be sure to click on the 3-D plotting tool and manipulate the visualization.
12.
The earthquake pattern hree is typical of divergent margins. Are there any earthquakes
deeper than 70 km?.
Introductory Geology Laboratory
Plate Tectonics 5
Select>
East African Rift Zone
from the Earthquake Viewer
The
East African Rift Zone
is a place where a divergent margin is forming ripping Eastern Africa
away from the rest of the continent. Look at the plot of the data and visualize it using the 3-D tool.
13.
How does the depth of earthquakes here compare to those along the Mid-Atlantic Ridge?
14.
These two settings are typical for divergent margins. How do their patterns differ from
convergent margins in terms of depth of focus?
Select>
Northern California San Andreas
from the Earthquake Viewer
Note the patterns of earthquakes in terms of depth of focus. Visualize and manipulate the data
using the 3-D tool.
15.
Is there any difference between the depth of focus between divergent and transform
boundaries?
Circle One:
YES
NO
16.
Given what you’ve learned, how would you distinguish between convergent boundaries,
both subduction zones and continental collision boundaries, and divergent and transform
boundaries?
So, you can easily recognize convergent boundaries by having deeper focus (> 70 km) earthquakes.
You can also distinguish between subduction zones and continental collision boundaries by their
Introductory Geology Laboratory
Plate Tectonics 6
pattern of earthquakes. Subduction zones have earthquakes getting deeper in the direction of
subduction whereas continental collision boundaries have a large “swath” of shallow earthquakes
with a somewhat randomly distributed population of deeper earthquakes.
How then can we distinguish between divergent and transform margin based on their earthquake
patterns. The simple answer is you can’t. However, if you consider the relative motion of the plates
that the boundary is on, we can mostly figure it out without other geological observations or
geophysical data.
Identifying Plate Boundaries
Open or re-open>
Global Seismicity Map
https://www.nsf.gov/news/mmg/media/images/global_seismicity_h.jpg
Each earthquake is represented by an epicenter.
The color of the epicenter plot is related to the
depth of focus (the center point of the area of rupture on a fault that generated the earthquake).
Look at the key relating color of the plot to depth of foci.
You can zoom in on each area of the
high-resolution map as needed.
Do not look at any published plate boundary maps, you are to figure out the type of plate
boundaries for the numbered locations marked on the map below. You should be able to define
the subduction zones simply by their pattern of deeper earthquakes in the direction of
subduction. Define the divergent boundaries as they are shallow focus earthquakes generally
opposite convergent boundaries (mostly subduction zones) on the other side of the plate. They
are perpendicular to the direction of motion of the plate. Transform boundaries will also have
only shallow focus earthquakes.
Location
Type of Boundary (circle one for each map location)
Location 1
Subduction Zone
Continental Collision
Divergent
Transform
Location 2
Subduction Zone
Continental Collision
Divergent
Transform
Location 3
Subduction Zone
Continental Collision
Divergent
Transform
Location 4
Subduction Zone
Continental Collision
Divergent
Transform
Location 5
Subduction Zone
Continental Collision
Divergent
Transform
Location 6
Subduction Zone
Continental Collision
Divergent
Transform
Location 7
Subduction Zone
Continental Collision
Divergent
Transform
Open or re-open>
Tectonic Plate Map
http://tectonics.caltech.edu/images/maps/plates.pdf
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Introductory Geology Laboratory
Plate Tectonics 7
17.
After completing your answer to the plate boundaries above from the map, check each
location against the
Tectonic Map
of the world. How do your answers for each location
compare?
Acknowledgements
The digital maps are produced and provided by the California Institute of Technology Seismological Laboratory and is supported
by the National Science Foundation.
This IRIS Earthquake Viewer
is an interactive mapping and visualization tool developed by
the Incorporated Research Institutions for Seismology (IRIS).
About the Earth Discovery Project
The Earth Discovery Project is a collaborative effort to integrate hands-on discovery-based
learning with modern research tools in undergraduate geoscience education.
The approach is to
develop and disseminate a comprehensive set of learning resources and experiences supporting
systemic educational reform.
The logo of the Earth Discovery Project portrays the earth as a
three-dimensional puzzle.
The globe used in the logo is from NASA’s
Blue Marble Project
.
The
Blue Marble
is a unique view of the earth, which integrates numerous data sets to construct a
“true-color” three-dimensional globe.
Introductory Geology Laboratory
Plate Tectonics 8
Placi
c Plate
Nazca Plate
North American
Plate
Caribbean Plate
Cocos Plate
Juan de Fuca Plate
Eurasian Plate
Indo-Austrailan Plate
Philippine Plate
1
2
3
4
5
7
6
Indian Plate
South American
Plate
Subduction Zone
Continental Collision
Boundary
Divergent Boundary
Transform
Boundary