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Lab 11. Geological Mapping of the East Pacific Rise In this map you are going to be working with GeoMapApp to look at a very high resolution bathymetry map of the East Pacific Rise and at photographs taken during Alvin dives. 1. Start GeoMapApp and chose the default Mercator Base Map. 2. Zoom in on the East Pacific Rise to look at a region extending from 9°30’N-9°50’N and

104°15’W-104°20’W. 3. Select the “Show Contributed Grids” tool (3rd from right) and wait while they load (this can

take a while). 4. Deselect the Zoom In tool. 5. You should see several boxes outlined in black (because of a bug in the code you may not see

their titles). You need to use the left mouse button to select the box that extends from about 9°27’-9°51’N and 104°21-104°13’W. The selected box appears in white. You may need to click several times to get the box you want.

6. When you have the right box selected, click the right mouse button to load the grid. After waiting a while, you will get 3 windows (they may be hidden behind the main map window). The “Layer Manager” and “Contributed Grids” windows should show that you have loaded the grid entitled “High-resolution Grids (5m, from ABE) – Fornari et al.(2004)”.

7. If you wish you can uncheck “GMRT Image” in the “Layer Manager”, so that you see only the high-resolution map.

8. After organizing the 3 windows (do not close them) zoom in on the high-resolution bathymetry centered on 9°50’N, 104°17’30”W. You should get a map just like Figure 1.

(a) Working with the map (i) Create and print a ridge-perpendicular (roughly E-W) bathymetry profile for this high-resolution data set across the rise axis as follows 1. Deselect the Zoom tool on the main GeoMapApp window and select the Distance/Profile

Tool icon in the “Contributed Grids” window (and not in the main GeoMapApp window) 2. Draw the profile by clicking the left mouse button at its start and releasing at it at the profiles

end. 3. Print the profile from the Save menu 4. Once you have your print, deselect the Distance/Profile tool. The rise axis is characterized by an axial summit graben. This is a volcanic collapse feature that forms as the result of the eruption of underlying lava. During eruptions it can fill up and will then empty as the magma flows off-axis or drains back into the ground. The hydrothermal vents in this region are found in the axial summit graben. (ii) What is the width and depth of the axial summit graben? How does this compare to the axial valley’s on intermediate and slow spreading ridges? Are there any other interesting features of the profile? (iii) Using a piece of tracing paper overlaid on Figure 1 draw and label the outline of the axial summit graben and of possible lava flow fronts on the rise flanks (we walked past one on the way down from Ape Cave on the Mount St Helens field trip).

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(b) You are now going to work with Alvin photographs to look at 4 different features. To load and view Alvin photographs do the following. 1. Select the menu option “Focus Sites → NSF Ridge 2000 Program → East Pacific Rise (9N)

→ Bottom Photos → EPR:9_50”. You will get a map showing track lines just like Figure 2 and a new “Shapefile Manager” window.

2. One track will be selected by default (highlighted in white). 3. You can select a track line either by clicking on the track line in the GeoMapApp window or

by selecting a different row in the Shapefile Manager window. 4. In the Shapefile Manager window click the light bulb icon and black circles will appear

along the track – each represents a pair of photographs taken from Alvin. The first circle on the profile will be automatically selected (white)

5. Click the light bulb icon two more times and you will get two photograph windows. Organize their locations and select Camera 2 in one to see the two pictures taken from the submersible. You may see two red lasers in one photograph – these are 10 cm apart and provide a scale

6. To navigate pictures either click the arrows in the photograph windows to move one at a time or click on another circle along the profile in the main GeoMapApp window. When you want to look at another Alvin dive you have to do as follows

1. Close the photograph windows 2. In the Shapefile Manager window uncheck Visible for EPR_9_50W and Alvin_#### (where

#### is the dive number you were just looking at and then recheck Visible for EPR_9_50W. 3. Follow steps 2-6 above (Note that if you are going back to a track line you previously

selected you will have the check its ‘visible’ box in the shapefile manager). (i) Figure 3 shows examples of different lava flow morphologies. Select Dive 4057 (see Figure 2 for its location) and work in the region near its SE end at depths greater than 2525 m. To see depths you can see the color scale in the Contributed Grids window and each photograph has its depth listed in the Shapefile Manager window. Create a labeled sketch on a piece of graph paper comprising

(1) the track line (2) a scale (you can add a scale to the map with the Overlays → Distance Scale menu option) (3) ticks on the track line to mark geological boundaries and interesting features along the profile (4) labels of the lava flow morphology between the boundaries and descriptions of any other interesting features.

(ii) Select Dive 3963 and look at photographs taken near the flow front at 9°50’25”N 104°16’45”W. Describe the characteristics of the lavas on either side (flow morphology, amount of sediment) and describe the flow front (appearance, height) – to get a sense of height note that each photograph has a submersible depth and altitude (distance from the bottom) listed in the Shapefile Manager window.

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(iii) Select Dive 4264. This was obtained in November 2006 about 1 year after a volcanic eruption (dives 3963 and 4047 was from before the eruption). There is a flow boundary towards the western end of the line. What flow morphologies can you observe on the new flow. Find this boundary (it actually a boundary zone with several boundaries visible on the profile) and mark it on your overlay. Describe the difference between the lavas on either side of the boundary. How does the amount of sediment compare with the profiles to the east of the rise axis? (iv) Zoom in on the Axial Summit Graben and select a dive in this region. It is probably easiest to work with a dive with a number below about 4100 because these are before the eruption although if you prefer you can look a more recent dive. Can you find examples of active black smoker vents (chimneys and smoke) and of diffuse low temperature venting (look for biology). Mark and label them on your overlay? – Also note the dive number.

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The figure on the next page is from Marine Geophysical Researches 21: 23–41, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 23 Volcanic Morphology of the East Pacific Rise Crest 9◦49′ –52′ : Implications for volcanic emplacement processes at fast-spreading mid-ocean ridges Gregory J. Kurras1 , Daniel J. Fornari2 , Margo H. Edwards3 , Michael R. Perfit4 & Matthew C. Smith1

Marine Geophysical Researches21: 23–41, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Volcanic Morphology of the East Pacific Rise Crest 9◦49′–52′: Implicationsfor volcanic emplacement processes at fast-spreading mid-ocean ridges

Gregory J. Kurras1, Daniel J. Fornari2, Margo H. Edwards3, Michael R. Perfit4 &Matthew C. Smith11Department of Marine Geology and Geophysics, School of Ocean Earth Science and Technology, Univer-sity of Hawai‘i, Honolulu, HI 96822, USA (Ph: (808) 956-3593; Fax: (808) 956-6530; E-mail: gkur-ras@soest.hawaii.edu);2Department of Geology and Geophysics, Woods Hole Oceanographic Inst., Woods Hole,MA 02543, USA;3Hawai‘i Institute of Geophysics and Planetology, School of Ocean Earth Science and Technol-ogy, University of Hawai‘i, Honolulu, HI 96822, USA;4Department of Geology, University of Florida, Gainesville,FL 32611, USA

Received 22 February 1999; accepted 20 December 1999

Abstract

Deep sea photographs were collected for several camera-tow transects along and across the axis at the East Pacific Rise crestbetween 9◦49′ and 9◦52′ N, covering terrain out to 2 km from the ridge axis. The objective of the surveys was to utilizefine-scale morphology and imagery of seafloor volcanic terrain to aid in interpreting eruptive history and lava emplacementprocesses along this fast-spreading mid-ocean ridge. The area surveyed corresponds to the region over which seismic layer2A, believed to correspond to the extrusive oceanic layer, attains full thickness (Christeson et al., 1994a, b, 1996; Hooft et al.,1996; Carbotte et al., 1997). The photographic data are used to identify the different eruptive styles occurring along the ridgecrest, map the distribution of the different morphologies, constrain the relative proportions of the three main morphologiesand discuss the implications of these results. Morphologic distributions of lava for the area investigated are 66% lobate lava,20% sheet lava, 10% pillow lava, and 4% transitional morphologies between the other three main types. There are variationsin inferred relative lava ages among the different morphological types that do not conform to a simple increase in age versusdistance relationship from the spreading axis, suggesting a model in which off-axis transport and volcanism contribute to theaccumulation of the extrusive layer. Analysis of the data suggests this ridge crest has experienced three distinctly differenttypes of volcanic emplacement processes: (1) axial summit eruptions within a∼1 km wide zone centered on the axial summitcollapse trough (ASCT); (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3)off-axis eruptions and local constructional volcanism at distances of∼0.5-1.5 km from the axis. Major element analyses ofbasaltic glasses from lavas collected by Alvin, rock corer and dredging in this area indicate that the most recent magmatic eventassociated with the present ASCT erupted relatively homogeneous and mafic (> 8.25 weight percent {wt.%} MgO) basaltscompared to older, off-axis lavas which tend to be more chemically evolved (Perfit and Chadwick, 1998; Perfit and Fornari,unpublished data). The more primitive lavas have a more extensive distribution within and east of the ASCT. More evolvedbasalts (MgO< 8.0wt.%) are concentrated in a broad area a few kilometers east of the axis, and in an oval-shaped area southof 9◦50′ N, west of the ASCT. Transitional and enriched (T- and E-) mid-ocean ridge basalts exist in relatively small areas(< 1 km2) on the crestal plateau and correlate with scarps or fissures where pillow lavas were erupted. Mafic lavas in this areaare primarily related to the youngest magmatic events. Geochemical analysis of samples collected at distances>∼ 500 m fromthe ASCT suggests that regions of off-axis volcanism may be sourced from older and cooler sections of the axial magma lens.Analysis of these data suggests that this portion of the EPR has not experienced large scale volcanic overprinting in the past∼ 30 ka. The predominance of lobate flows (66%) throughout much of the crestal region, and subtle variations in sediment coverand apparent age between flows, suggest that eruptive volumes and effusion rates of individual eruptions have been similar overmuch of the last 30 ka and that most of the eruptions have been small, probably similar in volume to the 1991 EPR flow whichhad an estimated volume of∼ 1× 106 m3 (Gregg et al., 1996).

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Figure 4. Digital photographs showing examples of lava morphologies and surface texture, nature of the glassy crusts, and variability insediment cover. Classifications and approximate dimensions are listed for each photograph.(a) Pillow lava (∼ 4.5 m×3.0 m). (b) Hackly lava(∼ 4.0 m×2.5 m).(c) Lobate lava (∼ 4.0 m×2.5 m). (d) Lineated sheet lava (∼ 4.5 m×3.0 m). (e) Ropy sheet lava (∼ 4.5 m×3.0 m). (f)Collapse feature (∼ 4.5 m×3.0 m,∼ 1 m of relief).(g) A flow contact (∼ 5.0 m×3.5 m).(h) Heavily sediment-covered lobate flow with asmall fissure (∼ 4.5 m×3.0 m).