AN EXOGENIC FULGURITE OCCURRENCE IN OSWEGO,

OSWEGO COUNTY, NEW YORK

 

 By Michael Walter

 

 

When lightning strikes the earth’s surface and melts the materials it contacts, the remains are collectively known as fulgurites.  Their classification has generally been limited to forms that include self-descriptive titles such as ground or soil fulgurites, sand fulgurites, rock fulgurites, and clay fulgurites (Galliot 1980; Petty 1936).  All of these classes of fulgurites form at or below the surface of the ground.  In 2004, Mohling published the first documentation of exogenic fulgurites produced from a lightning strike.  This new class of fulgurites was described as liquefied materials resulting from a powerful lightning strike that were thrown into the atmosphere above the lightning’s point of impact and solidified in the air.  This unusual occurrence is located in the Elko Hills of northeastern Nevada just south of the town of Elko.  Here I document a second such occurrence, which was discovered in the city of Oswego, New York, on 2 August 2008.  The information that follows should add further insight into the nature of this rare phenomenon.

 

History

On Saturday, 9 August 2008 Jeffrey Shallit approached me at the East Coast Gem, Mineral & Fossil Show in West Springfield, Massachusetts, with an offer to sell fulgurites he, his sons and nephew had dug from a lightning strike site in Oswego, New York, the day before.  The strike had occurred seven days prior at 1 P.M. on 2 August two short blocks from his nephew’s home on a residential side street in downtown Oswego.  He had a handful of specimens, several of which I recognized as being exogenic in nature.  After reviewing the several boxes of specimens he had at his car, I purchased the entire lot, excluding only several he wished to keep as souvenirs. 

I visited the location for the first time on 25 August 2008 (see fig. 1 for map).  Upon arriving at 6 A.M., I determined that the site was well preserved even though it was in the green space between the sidewalk and street in a busy residential area.  There had been a recent rain, and the areas where soil was exposed showed additional fulgurite fragments not collected by previous visitors.

That day, I took pictures, made measurements and collected soil samples from the strike site.  In addition, I removed an additional 20 gallons of soil, which contained about 15 kilograms of ground fulgurites.  Finally, I filled in the excavation with garden soil brought specifically for the purpose, and reseeded it with grass.

 

Location

The strike occurred at 43°2739.65 N latitude by 76°302.22 W longitude between the roadway and sidewalk on 100 East Cayuga Street at an elevation of 88.4 meters above sea level.  This is approximately six city blocks from the southeastern edge of Lake Ontario.  The site was composed of four distinct impact spots with the central two being larger than the two on the periphery.  These four spots represent the effusion craters, the interface between the exogenic and ground varieties of fulgurites (Mohling 2004).  This conclusion is based on burn-mark radii, size and locations of the ground fulgurites networks, and characteristics of the two huge surface entry tubes recovered by the Shallit family (but not purchased by me).  Figure 2 shows the Shallit children on 7 August beside the two central impact spots.  Figure 3 shows the entire strike site as viewed from East Cayuga Street.  Figure 4, the westernmost peripheral strike, occurred in the corner created by a blacktop driveway and the concrete sidewalk and actually contacted the concrete of the sidewalk there and at the far right of the figure.  The eastern most impact was less than a meter from the base of a residential transmission line pole.  This transmission line was also hit during the lightning strike (Shallit, pers. Comm., 2008), but there were no observable burn marks or serious damage to either the pole or the metal guide support although the pole is darker on the side that faced the lightning strike.  The four impacts form a crude line running parallel to and below the transmission line, which runs approximately east- west down Cayuga Street.  The entire strike site was approximately five meters in length and two meters in width.

Soil Conditions

The grass-covered topsoil at the site was 40 cm in depth, highly consolidated, dark in color, and rich in organics and typical debris associated with human activities in a residential area.  The undisturbed soil below the topsoil was composed of glacial till.  This sand/clay-rich subsoil contained many cobbles and boulders to 30 cm each.  Glass, ceramic shards, nails, wire, buttons, coins, and coal represented some of the transported debris found within the topsoil.  All the grass surrounding the impact locations was dead, probably because of recent digging activity and sterilization of the surrounding soil by the strike itself.  The soil was dry at the time of inspection on 25 August but was wet on the day of the lightning strike.  Although this soil was dark black and obviously rich in organic matter, none of the expected microscopic or macroscopic organisms that inhabit fertile soils were observed.  Samples from several spots that were part of the strike zone were inspected using magnification to 30x and not a single lifeform was found. 

Exogenic fulgurites

Exogenic fulgurites of all forms and combinations of forms were present on the surface of the ground surrounding the impact points of the lightning.  They extended outward for an estimated radius of no more than a meter (much smaller than the Elko occurrence) from each impact point, and there were fewer specimens of this class.  None were found deposited on any of the surrounding structures as at Elko, and there was no noticeable crater. 

Filaments with and without attached droplets were found to 13 cm in length (figs 6 and 7).  Individual and small groupings of this form were some of the most aesthetic specimens recovered from the find.  A few of these were larger aggregates, such as the one in figure 8 that reached 20 cm.  These groupings were helpful in determining the order in which the material landed on the ground after the lightning impact.

Spinose and flagellated varieties of droplets, as described by Mohling (2004), were relatively common (fig. 9) as were small droplets that were roughly spherical.  The droplets were mostly green in color and ranged in size from 1 mm to 3.5 cm.  To the casual observer all these green spheres (fig. 10) appear similar, but about a third of these specimens are completely hollow, with no external openings, and they float in water.  Microscopic investigation showed that even the smallest droplets and spheres to be highly vesicular having between 20 and 70 percent open space.  Less common were clusters of these droplets or clusters connected by filaments (fig. 11).

Drapery like forms or sheets of material were also found beside, and attached to, ground fulgurites (fig. 12).  These forms were found at the effusion craters of this occurrence but were not described by Mohling, although they seem to appear in his “conceptual genesis” drawing (Mohling 2004, fig. 4, p. 338).  It is unclear if this variety of exogenic fulgurite was actually found at the Elko strike. 

The surfaces of the exogenic fulgurites vary in texture.  Those that appear to have solidified in the air have smooth, lustrous surfaces on all sides.  Most, however, appear to have only been partially solidified upon their impact with the ground, with one side being smooth and lustrous and the opposing side having a texture reflecting the soil surface, sometimes with small bits of soil attached. 

The assemblage of colors in order of decreasing frequency of occurrence are: (1) green; (2) gray to black; (3) white; (4) silver; and (5) brown.  Examination and analysis of the superposition of forms in the more complex networks of agglomerated specimens such as the one shown in figure 8, suggest a sequence in the order that the cooling molten materials fell to the ground.  The order from first to last is:

(1)    Green spheres and flagellated droplets;

(2)    Drapery or sheet forms (only present close to the effusion craters);

(3)    Darker gray/ black spheres, flagellated droplets, and filamentous forms;

(4)  White spheres.

There were too few brown and silver-colored spherules to accurately determine how they would fit into this sequence. 

             SEM/EDS analyses of various materials yielded the following semi-quantitative estimates of composition which can be seen in table 1.  The silver-colored spheres are the most interesting of the materials investigated.  They are composed of four different reduced metal alloys:  (1) lath shaped crystals of metallic Si;  (2) blocky crystals of an Al-Si-Fe alloy, iron silicide, with an approximate formula of Al4Si3Fe, which were isolated to the periphery of the sphere; (3) acicular crystals which were not tested and;  (4) a fine- grained matrix of Al-Si metal.  The semi-quantitative estimates for elemental composition (weights percent) of the three materials tested can be seen in table 2.

            Figure 13 shows images of the materials tested with corresponding SEM images.  These silicides, compounds of metal and silicon, are known to form in extreme reducing environments, such as lightning strikes and meteorite impacts, but are unknown in other terrestrial settings (Sheffer et al. 2003).  The compounds present in this preliminary analysis demand temperatures in excess of 1,400O C at the point of the formation of the exogenic fulgurites (Daehn and McGarry 2003)  The identification of the specific silicides and analysis of their required formation conditions remain to be done.

The green material is likely representative of the overall soil composition at the site, without the organic component.  The white spheres are unusual in that one would expect a material with this high an aluminum content to be reduced to a metallic state as it was in the silver-colored spheres.

Ground Fulgurites

This strike produced an estimated 40 kilograms of subsurface, or ground, fulgurites.  Individual specimens to approximately 6 kilograms were observed and collected.  The recovered tubes reached diameters to 30 cm but were seldom any longer than this because the very compact nature of the soil made their removal in longer sections difficult.  These were larger in diameter than fulgurites from most other locations (Galliot 1980; Petty 1936; Wright 1999).  However, they were typical in form to those from other locations, possessing central vesicular glass tubes with coarse exteriors composed of partially melted and loosely attached soil and accessories.  Because of the soil’s high content of “junk” accessories, these fulgurites had a diversity of materials incorporated, fully melted, into their interiors (fig. 14), and attached, partially melted, to their exteriors (figs. 15 and 16).  At the easternmost impact and one of the central impact spots the fulgurites reached and fused into the sidewalk.  Minor fusion crusts formed on the surface of the sidewalk in those areas as shown in the westernmost peripheral impact.  Figure 16 shows where the edge of a ground fulgurite goes beneath that same area of the sidewalk coming into contact with its aggregate-rich underside.   This tube continued for an undetermined distance below the sidewalk.  All of the ground fulgurites at this site were found above the glacial subsoil within the fertile topsoil.  They radiated out from each impact point, usually in a single direction, in the typical branching form associated with other fulgurites from around the world (Galliot 1980; Wright 1999).

Several fulgurites which formed at the interface between the air and soil showed the following characteristics:

(1)  Concave formation (fig. 18);

(2)  Convex formation   (figs. 19 and 20).  Note that the top of the fulgurite in figure 20 is hollow.

(3)  Drapery or sheet like attachments (see section on Exogenic Fulgurites)

 

The three fulgurites in figures 18-20 were collected by the Shallit family prior to my investigation, so their exact placement within the strike area is unclear.  It is clear, however, that they represent entry points for the lightning discharge.  Jeffrey Shallit noted four impact points at the site as diagrammed in figure 5; however, at least six specimens clearly formed at the air/soil interface due to lightning impacts.

Color variations and banding was evident in many of the exogenic and ground fulgurite specimens.  When examined closely, some of the gray and black exogenic fulgurites can be seen to range from dark gray to black within the same specimen.  The lighter-colored specimens, mostly those of a green color, show more dramatic color variations and banding within a single specimen.  This can be seen clearly in figures 19 and 20 but is also evident in many other specimens collected at the site.    

Conclusions

The  2 August, Oswego, New York, exogenic fulgurite occurrence represents only the second occurrence of these unusual fulgurites to be documented, the first being the Elko, Nevada, occurrence, found in 1997.  Many similarities between the strikes exist, but it is the differences that are of interest.  Further, the strike documented here was discovered and excavated only days after its discovery vs. years after its discovery which seems to be the case in the Elko example.  

Ground fulgurites formed in the top soil and did not extend deeper than 40 cm, into the subsoil.  Instead, they all ran parallel to the surface of the ground.  This observation would seem to indicate that the subsoil was not as conductive as the top soil.  This top soil which the ground fulgurites formed in was compact, wet, and high in organics.  Large amounts of residual carbon in the form of dust like particles were mixed within this topsoil.  In general the subsoil was dryer, more siliceous in nature, and contained only trace amounts of organic material. 

In the Elko occurrence it was stressed that the loosely compacted top soil at that strike site may have contributed to the formation of the exogenic fulgurites, “enabled by the low confining pressure of the thin, poorly consolidated soil-gravel host,” by allowing these melted materials to be thrown into the air above the strike for an estimated distance of five meters.  Unlike Elko, the Oswego occurrence shows that densely compacted soils and those of compositions other than mixed quartzitic gravels can produce exogenic fulgurites.  Further, ground fulgurites of exceptional size and mass can also occur in soils entirely different from those found in the arid conditions near Elko.

It seems clear from the large quantities of exogenic and ground fulgurite material recovered that the lightning strike must have been unusually powerful.  There was, however, less material produced than the Elko strike and the exogenic material seems to have been expelled for a shorter distance.  This may have been a contributing factor in explaining why much of the exogenic material produced by this strike was not fully solidified upon impact with the ground.  The ground fulgurites extended below the sidewalk for an undetermined distance, perhaps exiting, subsurface, on the other side (which was in the yard of 100 East Cayuga Street and was not excavated).  The estimate for the total mass of ground fulgurites could be low because there could be many ground fulgurites that were not recovered.  The thorough sterilization of the soil for such a large area also supports the belief that this strike was powerful.

Additional investigation of the formation of fulgurites is needed, including a fulgurite recovery that maps the surface dispersal of exogenic fulgurites and a subsurface mapping of ground fulgurites using standard archaeological techniques or ground penetrating radar.  Also, the various species of silicide minerals that crystallized within these fulgurites should be identified, and the carbon remains within the surrounding soils should be tested for the presence of fullerenes, which have also been found in association with lightning strikes at other locations.  

Acknowledgments

Many thanks go to Prof. Jeffrey Shallit, his nephew Joseph, and his sons Jonah and Arlo for bringing this occurrence and its specimens to my attention.  Thanks are also extended to Dr. David Bailey of Hamilton College who performed the SEM work on the specimens discussed, and Dr. Steven Chamberlain, Arthur Smith, and Dr. Peter Modreski for their editorial review of this manuscript.  Steven Chamberlain drafted the map.

References

Daehn, G. and D. McGarry 2003.  Processing and mechanical behavior of materials.  Ohio State University Materials Science Lab.

Galliot, M. P. 1980.  Petrified lightning: A discussion of sand fulgurites.  Rocks & Minerals LV:13-17.

Mohling, J. W. 2004. Exogenic fulgurites from Elko County, Nevada: A new class of fulgurites associated with large soil-gravel fulgurite tubes.  Rocks & Minerals LXXIX: 334-40.

Petty, J. J. 1936.  The origin and occurrence of fulugurites in the Atlantic coastal plain.  American Journal of Science, 5th series, XXXI:188-201.

Sheffer, A. A., H. J. Melosh, B. M. Jarnot, and D. S. Lauretta 2003.  Reduction of silicates at high temperature: Fulgurites and thermodynamic modeling.  Lunar and Planetary Science, XXXIV.

Wright Jr, F. W. 1999.  Florida’s fantastic fulgurite find.  Rocks & Minerals LXXIV:157-59.

 

                    Table 1.  Oxide weight percent of spheres.

 

     Oxide

  Green Sphere

   White Sphere

    SiO2

      69.70

       21.35 

    Al2O3

      11.80 

       70.81 

    CaO

      11.16

         5.08 

    FeO

        3.14

             - 

    MgO

        2.02

         1.43

    Na2O

        0.88

         0.57

    K2O

        1.29

         0.75

 

                   Table 2.  Elemental composition percent within silver colored sphere.

 

   Element

  Lath-shaped Crystals

      Blocky Crystals

 Fine-grained Matrix

       Al

               -

           39.24

           69.05

       Si

          100.00

           35.75

           30.95

       Fe

               -

           23.43

               -

       Ti

               -

             0.88

               -

      Mn

               -

             0.70

               -

 

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