Introduction:

In lecture and from your work in Lab 1 you became familiar with the spatial and depth distribution of earthquakes on the Earth and their relation with tectonic plate boundaries. In this lab you will add the distribution of volcanoes to the picture and observe where they occur with respect to the main plate boundaries. You will learn how to recognize the nature of plate boundaries based on their attributes in the topography, seismicity, and distribution of volcanoes around them.

The map you will use displays 3 types of volcanoes: Composites, shield volcanoes, and Cindercones. They appear in various geologic environments at the surface of the Earth.

Composite -or stratovolcanoes- are tall and produce explosive eruptions. They are mostly found in volcanic arcs along subduction zones. Famous volcanoes in North America include Mount St. Helens and Mount Rainier in Washington.

Shield volcanoes produce effusive eruption of fluid lava flows called basalt lava that spread widely over great distances. Some of the largest volcanoes in the world are shield volcanoes. In Hawaii, Mauna Loa, the largest of the shield volcanoes (and also the world’s largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor. Shield volcanoes are mostly found in rift systems along divergent boundaries and in hot-spots (Hawaii).

Cinder cones are the simplest type of volcanoes. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone.

(https://pubs.usgs.gov/gip/volc/types.html)

In the following we will use an interactive Geographic Information System (GIS) built by the Environment Literacy and Inquiry Group supported in part by the Leheigh Environmental Initiative (https://eli.lehigh.edu) to assess the relationship between tectonic plate boundaries, seismicity, and volcanoes.

Open a browser and navigate to the site: http://gisweb.cc.lehigh.edu/tectonics/. The page shows a map of the world with a set of tools and menus on the right-hand side. The map includes an actionable cursor on the upper-left corner to zoom in and out of the map. The lower left corner shows a scale bar and the coordinates of the cursor as you move it around the map.

1. Volcanoes distribution:

In the Dynamic Layers menu, select the Volcanoes and the Plate Boundary buttons and zoom out to see the entire world. Select Map Legend to see the color code of volcano types.

1a. (1 pt) Where are volcanoes located with respect to plate boundaries?

1b. (1 pt) Are volcanoes evenly distributed along plate boundaries? What is different about Hawaii? (You may zoom in and out of the map or use the Map Navigation tool to zoom to Hawaii or other parts of the map).

2. Aleutian plate boundary

Use the Map Navigation Tool and select Aleutian Islands from the list of locations.

Use the Dynamic Layers menu to add the Earthquakes M>4.0 (9/08-9/11)

Open the Map Legend to see the color code for depth.

2a. (1 pt) In which direction does the zone of seismicity tilt toward? Circle one.

NortheastNorthNorthwestWest

2b. (1 pt) What type of plate boundary is along the Alaska coast?

Use the Dynamic Layers menu to add the Subducting Slab Depth to the map. Open the Map Legend to see the color code for depth.

2c. (1 pt) How does the dip angle of the slab varies from East to West along the Aleutian Trench?

Hint: You may use the Subduction Zone Profile tool and click successively on the Atka Island and the Kodiak Island buttons to see the dipping slab in cross-section. The location of the profile will be shown on the map as a straight line.

3. Other plate boundaries

Let’s look at other plate boundaries around the globe. In the Map Navigation Tool select the East-Pacific Rise. The map shows the eastern part of the Pacific Ocean and South America.

3a. (1 pt) Based on what you learned on the Aleutian trench, look at how the seismicity depth varies across the western margin of South America. What is the type of boundary you can infer along the South America west coast?

ConvergentDivergentTransform

3b. (1 pt) In the Map Navigation Tool select the North Atlantic Ocean. The map centers itself on the Mid-Atlantic Ridge. Open the Map Legend for earthquakes depth color code.

What is the depth range of the seismicity along the Mid-Atlantic Ridge?

Using the Elevation Profile tool, draw a profile from the Sohm Plain in the West to the Canary Basin in the East of the North Atlantic. Double click to end the profile and look at the elevation profile in the Profile Tool.

3c. (1 pt) What is the depth of the abyssal plains at both ends of the profile?

3d. (1 pt) What is the depth in the axis of the Mid-Atlantic Ridge?

3e. (1 pt) What is the height of the axial ridge over the abyssal plains?

3f. (1 pt) What type of plate boundary is along the Mid-Atlantic Ridge?

ConvergentDivergentTransform

4. New Zealand

Using the Find Locations tool, enter New Zealand in the Find My Address box. The map will center itself on New Zealand.

Open the Map Legend to see the seismicity depth color code.

Note: In this exercise, ignore the Plate Boundary color legend, which is not accurate.

4a. (1 pt) Describe the depth of the seismicity on the northern vs. the southern islands of New Zealand.

4b. (1 pt) Describe the occurrence of volcanoes in the northern vs. the southern islands of New Zealand.

4c. (1pt) Observe where the plate boundary is located with respect to the continental masses in both the northern and southern island areas. Circle or check the corresponding plate types in contact at the plate boundary for each region:

Northern Island : continent/continent continent/ocean ocean/ocean

Southern Island : continent/continent continent/ocean ocean/ocean

4d. (1 pt) Use your observations in questions 4a, b, c to determine the type of plate boundaries for each region.

Circle (or check) one for each case:

Northern Island : convergent divergent transform

Southern Island : convergent divergent transform

5. North America

5. Use the Find Location Tool and enter Cascadia Basin in the Find My Address box, click the box. The map centers itself on the Juan de Fuca Plate between the Pacific Plate and North America Plate. Compare the map with the map shown in the figure below. Observe the depth of seismicity and the distribution of volcanoes with respect to the plate boundaries.

5a. (2 pts) Which of the four diagrams on the right best represents a cross section through the outer layers of Earth along the line X-Y? Circle (or check) the correct answer and explain your answer in simple terms (1 or 2 sentences).

Answer: abcd