Plate Tectonics and Volcanoes, Victimology , Criminal Justice
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Lab10PHSC1021PlateTectonicsandVolcanoesLab.docx14
Google Earth: Plate Tectonics and Volcanoes 1
Objectives
· Introduce Google Earth software as a tool to learn about Earth processes.
· Learn about the different types of plate tectonic boundaries.
· Understand the relationships between plate tectonics, volcanism, and earthquakes.
Introduction
Plate Tectonics:
Did you know that the earth and an apple are similar in many ways? An apple has distinctive layering composed of the skin, the meat, and the core. The earth has a similar layered structure with the outermost layer also being very thin like the skin on an apple. This “skin” is made up of several individual pieces (“plates”) that move around the surface of the earth, much like a contact lens can move around on an eyeball. The movement of these plates tells us a lot about what is going on in the interior of the earth and has led to the formation of many of the landscapes we see around the world. Between individual plates are definable boundaries with very specific types of features that can tell us many things about the plates, like: what direction they are moving, how fast they are moving, how long they have been moving, etc. Plates meet along these boundaries: sometimes they crash into each other, sometimes they move apart from one another, and sometimes they slide past one another. Each of these types of boundaries helps accommodate movement of the plates around the surface of the earth.
There are three basic types of plate boundaries: divergent, convergent, and transform. Divergent plate boundaries are those where two adjacent plates are splitting apart (See Fig. 1). Convergent boundaries are those where two adjacent plates are moving toward each other. Transform boundaries are those areas where two adjacent plates move side-to-side relative to each other. Each of these plate boundaries has unique characteristics with the combination of all three, allowing movement of the plates around the surface of the globe.
Figure 1. Plate Tectonic Boundaries (image source: U.S.G.S.)
Divergent Plate Boundaries
Divergent plate boundaries are those where two plates are moving away from each other. As these plates move apart, magma rises up to fill the void and creates new ocean floor. Through geologic time, this process happens continually and creates a measurable difference in the age of the rocks that make up the oceans (divergent plate boundaries also occur on continents, but we’re going to focus on oceanic crust in this exercise). This gradual splitting apart and “filling in” of the earth’s outer layers creates a striping effect of rock age. As you move away from a divergent plate boundary, the rocks gradually get older. In this exercise we will explore several divergent plate boundaries to understand how they help accommodate movement of the plates.
Convergent Plate Boundaries
Convergent plate boundaries are those where two plates are crashing into one another. There are three varieties of convergent plate boundaries: ocean-ocean, ocean-continent, continent-continent. In an ocean-ocean boundary, two plates consisting principally of oceanic crust crash into each other – one of the plates becomes subducted beneath the other one (see Figs. 1&2). Through time, the subducted plate travels deeper into the earth and eventually begins to melt. The melted rock (magma) is less dense than the surrounding rock causing the magma to rise back up through the crust to the surface in the form of volcanic activity. Many of these eventually form a volcanic arc that contain melted rock from previously subducted ocean floor. In the ocean-continent collision, the oceanic crust will always be subducted beneath the continental plate. Similar to ocean-ocean collision, the oceanic plate that is subducted melts at depth creating a source of magma that eventually rises back up through the outer layers of the earth and causes volcanic activity along the edge of the continent. Along the western coast of South American is an excellent example of oceanic crust (Nazca Plate) that is being subducted underneath continental crust (South American Plate). This causes volcanism along the west coast of South America and through millions of years led to the creation of the Andes Mountain chain. The final type of plate boundary we will explore are continent-continent boundaries. These boundaries occur where two plates made of continental crust collide. In this special case, neither plate is subducted. The two plates will collide causing a massive mountain building event. The Himalayas are an excellent example of this type of collision – the Indian Plate collided with the Eurasian Plate millions of years ago causing the dramatic buckling and crumpling upward of the two plates. This event caused the resulting Himalayas to rise more than 8800 m (29,000 feet) above sea level!
Continent-continent convergence
Ocean-ocean convergence
Ocean-continent collision
Figure 2. Three types of convergent plate boundaries. (image source: U.S.G.S.)
Transform Plate Boundaries
Finally, transform boundaries are where plates move side-to-side relative to each other (i.e., slip past each other). An example of this transform motion is the San Andreas Fault Zone in California. Along the San Andreas fault, the Pacific Plate moves northwest relative to the North American Plate. These two plates scrape along-side each other and are home to some of the world’s most expensive earthquakes!
Volcanoes:
The morphology (size and shape) of volcanoes around the world varies greatly. The morphology varieties arise primarily from differences in the composition of the source magma. Some magmas have high viscosity (resistant to flow, like cold pancake syrup) while other magmas have very low viscosity (flow easily, like water). Volcanoes that have low viscosity magmas tend to build more slowly as individual lava flows accumulate, while volcanoes with high viscosity magmas tend to erupt quite violently abruptly releasing magma. In addition, the dissolved gas content of magmas is also important – magmas with high amounts of dissolved gases erupt more violently than those with low relative amounts of gases.
The following table lists and briefly describes the three basic types of volcanoes:
|
Volcano Type |
Magma Composition |
Morphology (Size and Shape) |
|
Shield |
Low viscosity, tend to have low gas content. Less explosive. |
Tend to be very large, diameter at base can be 20 + miles. Low sloping flanks. |
|
Stratovolcano (composite) |
High viscosity, higher gas content. Can be very explosive. |
Many have diameters from 3-7 miles at their base. Characterized by very steeply sloping flanks. |
|
Cinder Cone |
Highly variable. Cone generally forms directly above vent and contains mostly exploded fragments from highly gaseous magma. |
Relatively quite small. Many have basal diameters from a few hundred meters to a few miles wide. Can have large associated lava flows that represent a second stage of eruption originating from much less gaseous magma. |
Most volcanoes around the world happen near plate tectonic boundaries. The most explosive type of volcano (stratovolcano) tends to form near a special type of plate tectonic boundary known as a subduction boundary. These boundaries are present where two plates are colliding into each other, with one plate riding over the top of the other. The subducted plate begins to melt at great depth, and the resulting liquid magma becomes buoyant and rises through the crust to the surface to create long chains of volcanoes known as volcanic arcs. (See Figure 1)
Figure 2, Subduction Zone. This is an example of an oceanic plate colliding with a continental plate forming a volcanic arc. The Andes Mountains of South America were formed by this type of plate tectonic collision.
Volcanoes are not always associated with plate tectonic boundaries. One unique type of volcanism happens at locations called “hot spots”. These are locations where a tectonic plate has moved over an area of the mantle that is actively producing magma (the exact mechanism is still not well understood). As the tectonic plate moves across the hot spot, it leaves a chain of volanoes in it’s path. The chain of Hawaiian Islands was formed by the Pacific Plate moving over a hot spot located deep in the mantle underneath the plate.
FEATURES OF GOOGLE EARTH
Google Earth is free software that allows users to visualize the earth’s surface in a variety of ways. We are going to use Google Earth as a tool to help us analyse volcanoes from around the world. Like most mapping software, Google Earth has simple tools (Pan, Zoom, etc.) to help us navigate, but unlike most other freeware, Google Earth also allows you to view the earth in full 3-D rendering.
Quick Intro to Google Earth Tools
Open Google Earth. When you first open the software, the earth will rotate in the background and center you over the United States. The navigation tools are located along the upper right side of the map. Starting from the bottom, the zoom tools have (+) and (–) buttons that allow the user to zoom in (+) or zoom out (-) from any location. Directly above that button is the pan button. Pressing the small arrows on this button will move the map in the direction of the arrow pressed. Directly above the pan button is the 3-D view button. This button is a little hard to figure out at first, but a little practice and you will see that you can view earth landscapes in full 3-D. Finally, along the top are a variety of buttons for adding graphics, etc. to the map. In addition, there is a ruler button along the top. This button will allow you measure distances between features on the map. Before you begin answering the questions , you will need to change the units used in Google Earth. Find the “Tools” drop-down menu at the top. Select “Options”. Then in the middle of the options window for “Units of Measurement”, select “Meters, Kilometers”. Click “Apply” and shut the “Options” window.
In this exercise and the next, we will examine many of these plate boundaries in Google Earth.
Activity 1
Download and open the Google Earth project file ‘Introduction to Plate Tectonics.kmz’ (NOTE: Be patient when opening this file – it may take as long as one minute to open). This project file contains two sets of plate tectonic data we are going to analyse. The first dataset shows the locations of plate boundaries worldwide. The second dataset shows the age of the deep ocean floor.
Helpful Hint: For each question you can simply double-click the question number in the Google Earth legend (along the left hand side).
1) Scroll around the globe. Notice there are many different plates that make up the surface of the earth each labeled accordingly. List the names of 12 of the major plates here (note that you will have to zoom in before the plate name will show up on some plates):
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1 |
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7 |
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2 |
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8 |
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3 |
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9 |
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4 |
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10 |
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5 |
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11 |
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6 |
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12 |
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2) Go to the following location: 26.89, -44.77 Zoom to Eye Alt: 6000 km
The easiest way to navigate to this location is by double-clicking on “Question #2” in the left sidebar. At this location, you will see the large divergent plate boundary that separates North and South America from Western Europe and Africa. The red line indicates the location of the divergent boundary which is most famously recognized by the Mid-Oceanic Ridge. The ridge formed from newer, hotter magma welling up along the divergent boundary. Zoom to an eye altitude of approx. 2000 kilometers and you will be able to see the transform boundaries (blue line) that help accommodate the spreading along the irregularly shaped plate boundary. Pan to the south and you will that this boundary extends all the way into the southern Atlantic until it reaches the Antarctic Plate. Pan around the world and find two other divergent plate boundaries. List the plates that are being separated.
______________/_______________ ______________/_______________
3) Go to the following location: -22.03, -71.74 Zoom to Eye Alt: 5000 km
The easiest way to navigate to this location is by double-clicking on “Question #3” in the left sidebar. Here you will see the convergent boundary (yellow line) just off the west coast of South America. At this location the Nazca Plate is actively being subducted under the South American Plate at a rate of about 3.7 cm per year. This subduction led to the creation of the Andes Mountain chain along the west coast of the continent. Note how the Andes Mountains are more than 240 km (150 miles) away from the subduction zone – as the Nazca Plate is subducted, it sinks at an angle (see Fig. 2); therefore melting significantly inward from the outer edge of the plate boundary. Move your cursor inland due east of the “Question #3” marker until you find elevations above 3500 meters. Approximately how far away is the trench area (where the subduction boundary is located) from these high altitude areas? You can measure the distance from the boundary to the location where the height is approximately 3500 meters by using the ruler at the top of the screen. Press the ruler icon. Then in the “Line” tab, change the Map Length units to kilometers. Your mouse cursor will change from an arrow to a target box. Place the target box over the green star for Question #3 and click the mouse once. Then drag the cursor to the right and a yellow line marking the path will appear. While keeping the yellow line horizontal, move the cursor inland until the elevation listed at the bottom of the screen is approximately 3500 meters. The ruler box will list the map length that you have measured. What is that distance in kilometers? ___________________________
4) (Optional) Go to the following location: 17.46, 147.52 Zoom to Eye Alt: 2,200 km
The easiest way to navigate to this location is by double-clicking on “Question #4” in the left sidebar. At this location, an ocean-ocean convergent boundary known as the Mariannas Trench is present. The Pacific Plate subducting under the Phillipine Plate has created the volcanic island arc that are known as the Marianna Islands. Note how the island arc forms nearly 100 miles away from the subduction zone.
a. Why do you think the island arc is so far away?
b. Notice how Google Earth will show you the depths of the ocean floor (using negative values). Move your cursor along the trench and list the deepest value you find: ____________
c. How does this compare to the other areas of the ocean floor that are not along a plate boundary?
d. Scroll around the globe analyzing depths of the ocean at different locations. Generally speaking, does it look like the deepest part of the ocean are the areas farthest away from continents or are they located somewhere else?
5) (Optional) Ocean Floor Ages
Zoom back to the location used for Question #2 (the Mid-Atlantic Ridge). Turn on the layer (found along the left hand side) named ‘Ocean Floor Ages’. Be patient, as it might take several seconds to turn on. This layer is color coded by age: the reds and oranges represent the youngest rocks, the yellows represent older rocks, while the blues represent the oldest rocks present here. Notice that along the Mid-Atlantic Ridge, the rocks are very young. As you move away from the ridge (either east or west), the rocks progressively get older. During the time (not so long ago) when the idea of plate tectonics was still new and was not widely accepted, this recognition of the gradual increase in age as you move away from divergent boundaries was considered a key piece of evidence to support the theory of plate tectonics. Can you explain how examining the ages of the rocks on the ocean floor could be used as evidence that plate tectonics was a real phenomena?
Activity 02
Download and open “Volcanos of the World”. This application was created by the Smithsonian Institute and contains locations of all of the world’s recently active volcanoes (NOTE: “recently” is a geologic term meaning within the last several tens of thousands of years. For example, a volcano that erupted 23,000 years ago erupted recently to a geologist). Each volcano location has the name and is linked to a variety of internet resources where more information about that particular volcano can be found. In addition to volcano locations, the plate tectonic boundaries are shown on the map. Viewing these two features in combination shows the strong relationship between volcanic activity and plate boundaries. The plate tectonic boundaries are color-coded according to boundary type. (If the “Ocean Floor Ages” box from Activity 1 is still selected, then you should unselect it.)
1) Pan around the continental U.S.
a. Where are most of the volcanoes in the continental U.S. located?
b. Approximately how many volcanoes are there in the continental U.S.?
2) Go to the following location: 46.19, -122.18 Zoom to Eye Alt: 17 km
The fastest way to go to this location is by double-clicking on Question 2 under Volcanoes of the World in the left sidebar.
a. What is the name of this volcano?
b. What is the difference in the elevation of the rim and the center of the crater?
c. What is the approximate width of the crater (in meters)?
d. Notice the NE corner of the volcano. Why is there no rim evident here? Use the navigation controls to look at the volcano in 3-D. Does this help?
3) Go to the following location: 15.33, -62.98 Zoom to Eye Alt: 1600 km
The fastest way to go to this location is by double-clicking on Question 3 under Volcanoes of the World in the left sidebar.
a. What pattern do you see in the location of these volcanoes to the east of the Question #3 marker?
b. Do you see any other features in the general area that have the same basic shape?
c. Stay at the same Eye Altitude and pan to the west and south, looking at Central and South America. Can you see a connection between the volcanoes in these areas and the plate boundaries?
4) Go to the following location: 21.51, -159.89 Zoom to Eye Alt: 1800 km
The fastest way to go to this location is by double-clicking on Question 4 under Volcanoes of the World in the left sidebar. You are looking at the Hawaiian Island chain. Notice that there are no plate boundaries near this location.
a. How else could a volcano form if there are no adjacent plate boundaries? (Hint: See page 5 of the Introduction.)
b. What kind of volcano is Mauna Kea?
5) Go to the following location: 35.58, -111.63 Zoom to Eye Alt: 8 km
The fastest way to go to this location is by double-clicking on Question 5 under Volcanoes of the World in the left sidebar. This is a cinder cone volcano named the ‘SP Crater’ in north-central Arizona.
a. What is the black material located at the base of the NNW corner of the volcano? The material extends outward to the north-northeast a great distance (zoom out to see how large it is).
b. If this is a cinder cone volcano, does this very large feature seem to fit with this type of volcano?
6) Go to the following location: 54.38, -170.59 Zoom to Eye Alt: 2,500 km
The fastest way to go to this location is by double-clicking on Question 7 under Volcanoes of the World in the left sidebar. You are looking at the Aleutian Island chain off the west coast of Alaska, just to the south of the Question #7 marker. There are more than 300 volcanoes that make up this volcanic arc.
a. Do you see any other features nearby with the same size and shape as the chain of volcanoes? If so, what is this feature and how is it related to volcanism in the Aleutian Islands?
b. There are two arrows in the vicinity of the Aleutian Islands pointing approximately NNW. What are their values?
c. Can you guess what the values are associated with these arrows (what do they measure)?
d. Quickly zoom around the planet and look for other arrows. Are the values near the Aleutians higher or lower than most others?
e. How do you think the values you’ve found in the Aleutians relative to other areas of the world would affect the relative activity of the volcanoes found there?
The original draft for this lab was created by the geology department at Arkansas Tech University. Source of Data Used in this Exercise:
Plate Tectonic Boundaries: U.S. Geological Survey
Ocean Floor Ages: Tony Carrasco, et al., San Diego State University, Department of Geological Sciences
For complete source information, simply click on the layer name in Google Earth.
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