# Characterizing Plate Boundaries

Once you feel that you have a sense of how different types of plate boundaries are likely to differ in terms of their basic geologic characteristics, follow the steps below:

• Note the locations of the three sites you'll be studying on the large map below and then carefully review the small maps presented for each data set (earthquakes, volcanoes, etc.) below. The maps in the top row of each set show the plate boundaries you'll be studying in purple. The maps in the the bottom row show the distributions of earthquakes, volcanoes, seafloor ages, and elevations for the same areas as the maps in the top row. You can left click, hold, and drag to place one map on top of the other so that you can see how the features line up.
• Be sure that you understand what the color schemes mean for each type of data and note that on all these maps north is at the top, east on the right, south on the bottom, and west on the left.
• Next, briefly answer the questions posed for each type of data. Organize your answers by filling in the accompanying table that lists the sites across the top and the questions down the left side. The initial goal of your work is not to classify the different types of boundaries but, instead, to look at how each differs from the others in terms of earthquake distribution, seafloor age, and so on. Focus on making accurate observations and don't hesitate to ask questions if you're not sure about something.
• Finally, referring to the index map below and this plate boundary map determine which type of boundary (i.e. divergent, convergent, transform) sites 1, 2 and 3 are most likely to be and add this information to your table. Then log onto Etudes and use the data in your table to answer the questions in Homework 1.

#### Earthquakes

Compare the maps showing the locations of the plate boundaries (purple lines) and earthquake epicenters (colored dots) for each of the three sites. Note that the colors assigned to the earthquakes indicate their depths:

• red = 0-33 km;
• yellow = 33-70 km;
• green = 70-300 km;
• blue = 300-700 km.

For each site answer the following questions:

• Do most of the earthquakes lie pretty much right along the boundary itself or do they lie mostly off to one side of the boundary? If they lie mostly off to one side, which side are they on?
• Are the earthquakes associated with the boundary shallow (red), deep (yellow, green and blue), or are they both shallow and deep?
Site 1 Site 2 Site 3

#### Volcanoes

Compare the maps showing the locations of the plate boundaries (purple lines) and locations of young volcanoes (red dots) for each of the three sites. Most of the boundary at site 1 is underwater, so we rarely witness volcanic eruptions along it. Therefore, assume that the distribution of volcanoes in Iceland is typical of that elsewhere along this boundary. For each site answer the following questions:

• Are the volcanoes associated with the boundary closely spaced (dense), widely spaced (sparse), or are there few or no volcanoes associated with this boundary?
• If there are volcanoes associated with the boundary do they lie pretty much right along the boundary itself or do they lie mostly off to one side of the boundary? If they lie mostly off to one side, which side are they on?
Site 1 Site 2 Site 3

#### Seafloor Age

Compare the maps showing the locations of the plate boundaries (purple lines) and the ages of the adjacent seafloor crust (colored bands) for each of the three sites. Note that "warmer" colors indicate younger crust and "cooler" colors indicate older crust according to the following key:

• red = 0 to 9.7 Ma (millions of years old);
• shades of orange = 9.7 to 47.9 Ma;
• shades of yellow = 47.9 to 67.7 Ma;
• shades of green = 67.7 to 131.9 Ma;
• shades of blue = 131.9 to 180 Ma.

Dark gray indicates seafloor crust for which no age data are available and light gray indicates the approximate location of continental crust.

For each site answer the following questions:

• Does seafloor occur on both sides of the boundary or only on one one side?
• Does the age of the seafloor remain the same (uniform) or does it vary as you walk along the length of the boundary? If it varies does it get older in the same direction on both sides of the boundary or in opposite directions? (For example, it might be older to the east on one side of the boundary but older to the west on the other side.)
Site 1 Site 2 Site 3

#### Surface elevation

Compare the maps showing the locations of the plate boundaries (purple lines) and the elevation of Earth's surface (colored patterns) for each of the three sites. Note that on the elevation maps "warmer" colors generally indicate higher elevations and "cooler" colors indicate lower ones according to the following key:

• above sea level the highest elevations are shown in white and purple, moderate elevations in brown and yellow, and the lowest elevations in green;
• below sea level the shallowest depths (highest elevations) are shown in turquoise, moderate depths in blue and purple, and the greatest depths (lowest elevations) in pink.

For each site answer the following questions:

• Is the elevation along the boundary generally higher, lower, or about the same as that of the surface on either side?
• Does the elevation vary symmetrically or asymmetrically as you walk across (perpendicular to) the boundary from one side to the other? If it's asymmetrical, which side is higher and which is lower?
• Symmetrical means that elevation chanes in the same way on opposite sides of the boundary and gives rise to a "mirror image" pattern of elevation as you walk across the boundary. For example, the elevation might be high along the boundary and become progressively lower as you walk away from it on either side.
• Asymmetrical means that the elevation is different on opposite sides of the boundary, like a step. For example, the age elevation might become higher as you walk away from the boundary on one side but lower as you walk away from it on the other.

Surface elevation is complex near Iceland (site 1), so focus on the part of the boundary south of the island. Surface elevation is also complex near the ends of boundary 2, so focus on elevations along and across the middle part of this boundary.

Site 1 Site 2 Site 3