Volumes of Yellowstone s Rhyolite Lava Flows

Transcription

1 Volumes of Yellowstone s Rhyolite Lava Flows An ArcGIS Study of Pitchstone Plateau, Solfatara Plateau and Douglas Knob Dome Matt Williams December 2, 2011 GEO 327G Dr. Mark Helper

2 Williams 1 I. Introduction Pitchstone Plateau, Solfatara Plateau and Douglas Knob dome are members of the Central Plateau Rhyolites in Yellowstone National Park. They are all related to volcanism that occurred within Yellowstone Caldera approximately 70,000, 100,000 and 114,000 years ago, respectively. Each varies greatly in size and are generally of the same composition except for Solfatara which is differentiated by presence of granophyre in part of the flow and absence in the other. II. Purpose The purpose of this project is to calculate the volume and surface area of each of the above rhyolite lava flows and divisions within each to determine if viscosity changed over time. III. Problem Formulation This semester, I started to assist Dr. Jim Gardner, graduate student Kenny Befus, and undergraduate Robert Zinke with research within the Department. The main flows that they are interested in are Pitchstone, Solfatara and Douglas Knob. It was only natural that this project coincides with my new research opportunities and pertains to Yellowstone lava flows. My main goal was to test the published volumes of these 3 lava flows using ArcGIS software. IV. Data Collection Luckily, Dr. Helper has done lots of work with Yellowstone in the past and made his data available to me for use with the project. I determined that for this project, I would need a digital elevation model for Yellowstone and a geologic map of Yellowstone to determine flow edges at the very least. Yellowstone DEM from class files on R: drive with path of: R:\geo-class\GEO- 327g_386g\Project_Data\Yellowstone\Elev_and_Fires\elev_yel Cenozoic Geologic Map of YS from class files on R: drive with path of: R:\geo-class\GEO- 327g_386g\Project_Data\Yellowstone\Geology\CenozoicGeology Flow Boundary Lines from Geology Shapefile from class files on R: drive with path of: R:\geo-class\GEO-327g_386g\Project_Data\Yellowstone\Geology\ofr99174\shape\geol83ar Dr. Helper also ed me a point file of the Central Plateau Rhyolite vents and flow features representing pressure ridges from the Yellowstone literature written by Christiansen. Kenny Befus ed me a.jpeg of Solfatara that included the line representing the division between presence of granophyre and no granophyre. V. Data Processing 1) The DEM, Cenozoic Geology layer, flow features, rhyolite vent point layer and Geology shapefile are first added to a new document in ArcMap. All data was set to NAD 83, UTM Zone 12N coordinate system.

3 Williams 2 2) A new feature class was created to digitize the flow boundaries from the geol83ar. An example of the digitized flow boundary of Pitchstone is below: Figure 1: Red lines are the digitized flow boundaries for Pitchstone Plateau. Douglas Knob is the small feature on the northwest edge of Pitchstone. 3) Another new feature class was created for flow divisions that were then digitized from the flow features class. Divisions for Pitchstone roughly correlate to 25%, 50% and 75% of the flow; Solfatara divisions roughly correlate to 50% and the granophyre line. Douglas Knob was too small to create flow divisions as it is significantly smaller than the other flows and contains no pressure ridges. An example of digitized flow divisions is below: Figure 2: Orange lines represent digitized flow divisions within Pitchstone

4 Williams 3 The granophyre line of Solfatara had to be digitized from a.jpeg after using the georeferencing tool. Figure 3: Red line represents the divisions of Solfatara that contain granophyre from the volume that doesn t. 4) Each individual flow division was then made into a polygon using the Feature to Polygon tool (Toolbox\Data Management\Features\Feature to Polygon) Solfatara s divisons are shown here. Individual polygons where further extracted into their own feature classes to make it easier to calculate volume later. Figure 4: Left: Feature to Polygon tool. Right: Flow divisions of Solfatara

5 Williams 4 5) The next step is to infer the flow base beneath each flow. This is done using the Extract by Mask tool. (Toolbox\Spatial Analyst\Extraction\Extract by Mask) This tool extracts the elevations of the flow boundaries from the YS DEM which I have assumed to be the base of the flow. (Note that the Spatial Analyst extension must be on to perform this) Figure 5:Left: Extract by Mask tool. Right: Result of Extraction by mask. Pitchstone and Douglas Knob shown 6) This raster was the converted into points. Each cell from the raster created in the last step will become a point with elevation data using the Raster to Point tool. (Toolbox\Conversion tools\from Raster\Raster to Point) Figure 6: Left: Raster to point tool. The field is set to transfer raster elevation values to point elevation values. Right: Result of Rater to Point tool. Pitchstone and Douglas Knob shown.

6 Williams 5 7) From this point file, kriging was then performed to interpolate the base elevations of the flows. Kriging creates a raster from points, interpreting the values between known points. (Toolbox\3D Analyst\Raster Interpolation\Kriging) The default Search Radius # of Points value of 12 points was increased to 200 as there are a very large number of points and 12 did not yield good results. Figure 7: Left: Kriging tool. Note the Z-field value and # of points within search radius. Right: Resulting raster produced by kriging. White is the highest elevations, green is the lowest. (Note that the 3D Analyst extension must be on to perform this action) Before settling on kriging for interpolation, I experimented with spline as well, but did not use the tool because it yielded obviously incorrect elevations of 35,000 meters to - 26,000 meters within the flows themselves. 8) Both the Yellowstone DEM and the new kriging raster were clipped to the flow boundaries to make the later steps of calculating polygon areas easier. This is done using the Clip (Data Management) tool. (Toolbox\Data Management\Raster\Raster Processing\Clip) Shown is the result of the clip performed on the kriging raster. (Note that Use Input Features for Clipping Geometry is checked. This is necessary otherwise it clips to a rectangle that surrounds all flows.) Figure 8: Left: Clipping tool. Note that Use input features for clipping geometry is checked. Right: Clipped kriging raster. All 3 flows shown.

7 Williams 6 9) From the newly clipped YS DEM and Kriging clip, raster calculation was performed to find the flow elevation above base height to later calculate volumes. (Toolbox\Spatial Analyst\Map Algebra\Raster Calculator) The Map Algebra expression entered is: elev_yel Krig_clip and yields an image that looks like this with ArcScene. Figure 9: Left: Raster calculator tool with Map Algebra expression. Right: All 3 flows shown in ArcScene. Clipped YS DEM elevations shown above (outlined in red) while heights above 0 are shown below. All 3 flows are shown; Pitchstone is the closest flow. This final step contains 2 actions. 10a) From this newly calculated raster, clipping was once again performed to each flow division polygon. (Toolbox\Data Management Tools\Raster\Raster Processing\Clip) Figure 10: Clipping toolbox showing division of Pitchstone being clipped from the height above 0 raster.

8 Williams 7 10b) The clipped-to-flow raster is now used to calculate the volume of the particular flow division using the Surface Volume tool. (3D analyst\functional Surface\Surface Volume) Figure 11: Left: Surface Volume tool showing calculation of 2 nd Pitchstone division. Right: Visual representation of 2 nd Pitchstone division. Flow boundary in red. (Note that 3D Analyst Extension must be on and plane height MUST be 0. Otherwise the volume will be given from the automatically generated plane height will be <0 and thus give an incorrect volume) This produces a table including values for 2D Surface Area, 3D Surface Area and Volume in meters. Figure 12: Table showing surface area and volume produced by the Surface Volume tool. 3D surface area and volume are then entered into Microsoft Excel to calculate square kilometers for surface area and cubic kilometers for volume.

9 Williams 8 VI. Results From calculations to convert square and cubic meters to kilometers, Excel yielded the following data. Pitchstone Division Color 3D Surface Area (km^2) Volume (km^3) Volume/Surface Area 1 Blue Yellow Orange Red Total All Douglas Knob Total Solfatara Division Color 1 Blue All outside Blue Granophyre Yellow No Green Granophyre Total All Table 1: Surface Area and Volume data collected from ArcMap and entered into Excel. Figure 12: Flow divisions of Pitchstone Plateau and Solfatara Plateau. Not to scale or geographically oriented.

10 Williams 9 VII. Conclusions From a quick review of literature, Pitchstone has a volume of ~70 km 3, Solfatara has a volume of ~7 km 3 and Douglas Knob Dome has a volume of.01 km 3. The data that my methods produced were close to these published values. Total Volume (km 3 ) Published My Findings Pitchstone Solfatara Douglas Knob Table 2: My findings using ArcMap versus published volumes for Pitchstone Plateau, Solfatara Plateau and Douglas Knob Dome. The data shows that Pitchstone Plateau did not change viscosities during the time it was erupted. Solfatara, however, does seem to have changed viscosities during its eruption as evidenced by the change in volume to surface area ratio in both divisions 1 and 2 and the granophyre/no granophyre differentiation. This may be due to the presence of granophyre within the flows. Due to the nature of how the flow base elevations were calculated, this may not actually be the case. With further refinement of the method to interpolate the flow base, it may be possible that the viscosity did not actually change. Volumes may also be better calculated with a more refined method of kriging.