Eruptive and Geomorphic Processes at the Lathrop Wells Scoria Cone Volcano

1. Eruptive and Geomorphic Processes at the Lathrop Wells Scoria Cone Volcano Greg. A Valentine, Donathan J. Krier, Frank V. Perry, Grant Heiken

2. DEM of Yucca Mountain • Pliocene Volcanoes • TM Thirsty Mountain • BM Buckboard Mesa • PCF Pliocene Crater Flat • Volcanoes in Crater Flat • ~1 million years • LC Little Cones • RC Red Cone • BC Black Cone • MC Makani Cone • LBP Little Black Peak (~.32Mya) • HC Hidden Cone (~.37 Mya) • Lathrop Wells Volcano (.08Mya) • Repository Footprint

3. Lathrop Wells Volcano • Trachybasaltic lava and Scoria • Mg~54 indicates fractionalization of parent magma • Mostly porphyritic with plag ground mass • 2 lobes of early lava fields and early cone deposits have plag crystals up to 1 mm long. • Later flows are depleated and enriched

4. Distribution of Volcanic Products at Lathrop Wells

5. Inferred Fallout Distribution • Contours represent fallout isopachs. (cm) • Lettered stations are stratographic columns (Figure 8) • Inset shows possible regional extent based on thin ash remnants

6. Relative timing of emplacement of scoria cone, fallout and lave fields at Lathrop Wells

7. Cone building deposits associated with Strombolian phase activity.

8. Western Part of South Lava Field • Lobate flow margins (solid line) • Pressure ridge and squeeze up features (dashed lines)

9. Mound of broken masses of partly welded agglutinate. Notice narrow squeeze up or rootless dyke along the leading edge. • Pyroclastic mound composed of intact tilted block of bedded spatter and agglutinate (Eastern margin of south lava field) • Internal structure of intact pyroclastic mound with steeply tilted bedding of partly welded agglutinate. Notice thin layer of spatter near stick. This was covered by fallout lapilli beds from Strombolian eruptions at the cone

10. Cross stratified ash deposit interbedded with scoria lapilli deposits in upper zone • Grain size distribution for cross stratified ash deposit ( 074) and overlaying scoria lapilli bed (076)

11. Fallout deposits from ten sites up to 2 km from the vent. Inferred with a common datum

13. Scanning Electron Photomicrograph of ash from the laminated and cross laminated sequence of fallout deposits `200-300 m NW of the cone (column D). Note abrasion of some grains.

14. Medial and distal parts of the northeast lava field, illustrating lobate nature of flow margin. Dotted pattern indicates possible ripply surface ridges along with what might pf been a main surface lava channel.

15. Down stepping lobes representing successive tube fed breakouts of lava from the toes of lobes along eastern margin of lava field. • Stacking of the flow units, dashed lines indicate outer margins of two flow units. Lower unit formed a 100m wide, 3-5 m high platform where contacting a hill of Miocene ignimbrite. • Low lying flow units on the northern edge of the northeast lava field partly buried by violent Strombolian fallout deposits, sediment accumulation from lava dams to the north, and eolian sands. Arrows show tops of flow edges or squeeze ups that protrude above the pyroclastic and sediment units.

16. Xenolith concentration (volume fraction) plotted against elevation of the Lathrop cone. Lucero volcanic field (Mexico) and Hopi Butte fields (Arizona) used for comparison.

17. 1987 Lathrop Wells aerial photo showing erosional rills extending toward and away from the crater. Dark colored features are garlands produced by creep of loose scoria deposits. Light colored area on the southeast slope of cone is caused my accumulation of eolian sand and silt at a high rates.

18. Cross section to illustrate evidence for the erosional history of the Lathrop Wells cone. • Through the crater center • Cross section through western crater rim. • Dashed line extrapolates basal contact of black tephra unit above the modern rim.

19. Mechanical weathering of a small lava crag on the northeast lava field. Fragment produced by freeze thaw and thermal splitting of the lava, especially on the south side of the crag. The eroded clasts accumulate around the base and will eventually turn into desert pavement.

20. In interpretation of the effusive and explosive processes that formed the Lathrop Wells volcano. Earliest plagioclase phenocryst bearing lava flows to the southwest of the developing cone were part of Strombolian explosions that ejected ballistic bombs that formed welded deposits. Strombolian eruptions continued through construction of the south lava field where material was rafted on top of the flow. Additional strombolian activity produced multiple eruption columns and buried the south lava field and early parts of the northeast field. The north lava field continued to form after strombolian activity had ceased.

21. Conclusion • Nearly all eruptive products have been modified by a variety of geomorphic processes. • If a future volcanic event occurs at Yucca Mountain and intersects the proposed repository (250-350 m deep) bad things will happen based on the processes that formed Lathrop Wells Volcano. • Total volume of radioactive eruptive products will be around .1 cubic kilometer, with eruptive columns a few kilometers high. • The event will be fed from one main feeder dike and conduit up to 20-25 m in diameter at the depth of the repository with shallow lateral breakouts. • The violent strombolian nature of many of the eruptions we can anticipate the explosivity of the initial interactions with the repository tunnels and subsequent eruptive dispersal could be high if the trachybasaltic magmas have significant volatile content. • Fallout tephra contaminated with entrained radioactive waste will be spread across the land, water and atmosphere. We will be screwed BAD IDEA!