Lunar and Planetary Science XLVIII (2017)
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A THERMAL-PLUME ORIGIN OF LAYERED AND SPLASH-FORM TEKTITES AND LIBYAN DESERT GLASS
John T. Wasson
University of California, Los Angeles, CA 90095-1567, USA
Outline of the thermal plume model. The accretion
of a large cometary or asteroidal mass will produce a
thermal plume. If this occurs on land during dry conditions, the entrainment of soils into the plume provides
suitable conditions (a) for the formation of splash-form
tektites and the entrainment of a minor fraction of these
above the atmosphere and (b) for the formation of melt
sheets that solidified and fragmented to become the layered tektites and the Libyan Desert Glass (LDG). Thermal plume is a better term than impact-plume because the
model works best if the accretionary energy, like that at
Tunguska, is liberated in the atmosphere (rather than in a
crater).
Introduction: the three models. There is general
agreement that (LDG) is a form of tektite, the chief difference being that tektites have compositions similar to
continental soils and LDG have compositions similar to
quartz-rich sands.
Crater-ejecta model. Starting about 1960 the relative
nearness (ca. 300 km) between the Ries Crater and the
moldavite tektite field and their similarity in age (ca. 15
Ma) led to the general acceptance of the view that the
moldavites were formed as P●ΔV products of the impact
that produced the crater [1].
Problems with this general model are (1) there is no
crater known for the youngest tektites, the Australasian
(AA) tektites formed 0.8 Ma ago, even though erosion in
SE Asia has been very minor, (2) the common splashform tektites are not present in regions where AA layered
tektites are abundant, and (3 the young (AA, Ivory Coast,
Moldavite) tektites contain 10Be at soil concentrations [2]
requiring that tektites formed within a meter of the surface whereas craters produce melts near the crater floor.
Tunguska-like model. A very different tektite-formation model was the “super-Tunguska model of Wasson
[3]. At Tunguska it is believed that the projectile distintegrated during atmospheric passage and deposited
most of its energy 5-10 km above the Earth’s surface
[4].–This model was proposed to account for the formation and flow of the melt sheet that produced the layered tektites. To lower the viscosity enough to allow 10100-cm of flow the viscosity needs to have been 5 pa s or
less and thus the temperature needs to have been >2300
K for tektites (~720 mg/g/ SiO2) and >2500 for Libyan
Desert Glass (~980 mg/g SiO2). An incandescent sky
offered a way to provide such high temperatures. As
proposed, the model did not offer a detailed account for
the formation of splash-form tektites.
Thermal-plume-model. There is general agreement
that splash-form tektites formed in a hot environment that
melted the precursor minerals; these coagulated and the
resulting masses deformed while spinning; I suspect that
most researchers picture this to have occurred inside a
hot, turbulent cloud, and probably a hot plume. An ideal
way to generate such a hot plume is to liberate a large
fraction of the accretionary heat in the troposphere. This
occurs if, before and during atmospheric passage, the
projectile breaks down into fragments small enough to
slow to terminal velocity in the atmosphere.
Processes occurring within a thermal plume. The
nearest analog to an accretion-generated thermal plume is
a nuclear-weapons test detonated just above the Earth’s
surface; images and videos of these are easily accessible.
The cartoon in Fig. 1 shows a sketch of a plume generated by a weapons test. Plumes produced by the accretion
of clouds of friable rocks would be much more complex;
in contrast to weapon tests, there would be many explosive centers spread out in both time and space. It is nonetheless instructive to base a rough model on this cartoon.
Fig. 1 Thermal plume (after Glasstone & Nolan [5].
The typical accretional velocity of asteroidal materials is 18 km s-1 [6]; the velocity of a typical comet would
be about twice as high. The stopping of 1 kg of material
moving at 18 km s-1 in the atmosphere releases 160 MJ of
heat. The amount of heat required to heat 1 kg of the atmosphere (the amount above 1 cm2) from 300 to 2200 K
is about 3 MJ. Thus to heat all the air above 1 km2 requires the accretion of a mass of 2●108 kg.
The release of heat creates a mass of hot gas that
starts to expand and rise. Its ascension creates an updraft
that entrains dust, especially fine dust that has not been
compacted or cemented with secondary minerals. Loess
Lunar and Planetary Science XLVIII (2017)
deposited when the climate is dry and wind shear is high
is ideal.
Some of the entrained dust makes its way towards the
central axis and is transported high in the cloud. If the
ambient temperature is >1500 K, dust with tektite compositions will melt (but melting quartz grains requires a
temperature of ~2000 K). If the density of the droplets is
large enough, they will agglomerate to form larger (e.g.,
centimeter-size) blobs. Turbulence will cause these to
spin, forming the characteristic splash-form shapes (that
were preserved when they entered cooler parts of the
plume). Splash-form tektites have lost only the most
volatile elements such as B and Zn; they have retained
most of the common alkalis Na and K.
Some materials were entrained long enough to fully
melt the dust particles, but not long enough that they
could agglomerate to large sizes before falling out of the
cloud. These droplets would have fallen directly below
the cloud. If their surface density was high, they would
have formed a melt sheet that, when thick enough,
formed layered tektites.
The layers in tektites with 70% SiO2 are generally
relatively thin; the typical spacing of ridges (high-density
layers) revealed by weathering on the surfaces perpendicular to the layering is typically about 2-3 mm. Although the layers in LDG are commonly 1-2 cm in thickness, details visible in the lighter regions show layering
on a 2-4 mm scale.
Fig. 2 shows a 5-kg LDG mass on exhibit at UCLA. It
consists of centimeter-thick, roughly planar layers alternating between dense, dark, greenish glass and white
foamy glass. It appears that to have been produced by
the alternation of fallout of hotter material that lost its
bubbles and cooler materials that retained a high density
of trapped gas. The plume would have been highly turbulent; it seems plausible that the bimodal deposition
reflects a sheet of hot materials crossing the site follow
by a sheet moving in the opposite direction that deposited
the cooler materials. These turbulent processes would be
physically similar to those occurring in a violent rain
storm..
Fig. 2. Libyan Desert Glass fragment with a mass of 5
kg; cm-grid in background. Centimeter-size dark bands
have low, light bands have high bubble contents. At
higher mag one sees mm-scale layering in the light bands.
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Some of the entrained materials never fully melted
before the mass escaped from the plume. These would
be difficult to recognize today; weathering has destroyed
the unmelted materials and the surviving glass is small
and shapeless.
As recently discussed by Wasson [7], a tiny fraction
of AA tektites were entrained to elevations above the
atmosphere (i.e., to heights > 100 km) and accelerated to
velocities >5 km s-1 that led to ballistic trajectories to
Australia and beyond. These velocities are too high to
achieve by thermal expansion of a hot bubble; they must
have been augmented by other physical processes (perhaps a kind of atmospheric Bernoulli effect or colliding
shock waves).
Summary. Properties of splash-form tektites are inconsistent with formation during the impact excavation of
a bowl-shaped crater; 10Be and other evidence require
formation from near-surface soils. A satisfactory model
needs to account for both layered and splash-form tektites (layered appear to be more abundant in SE Asia). I
suggest that both materials formed from soils entrained
into (a series of) thermal plumes. Although most evidence stems from studies of Australasian tektites it appears best to assume that tektites in all five fields formed
under similarly dry conditions during which deposits of
aeolian dust were on the surface.
Updrafts in the plumes entrained dust which melted
and agglomerated to form droplets and centimeter-size
masses; some of the latter became splash-form tektites;
some, together with small droplets rained out to form
melt sheets below the plumes that, after chilling and
fragmentation, became layered tektites.
The layering in tektites is primarily depositional; it
sometimes occurred during periods of high turbulence
with some layers deposited from hotter, some from cooler silicate rain, with the latter trapping appreciable air.
References: [1] Cohen A. J.. (1961), J. Geophys.
Res. 66, 2521. ; [2] Ma et al. (2004) Geochim. Cosmochim. Acta 68, 3883; Serrefidden F. Herzog G. F. and
Koeberl C., (2007) Geochim. Cosmochim. Acta 71,
1574; [3] Wasson J. T. (2003) Astrobiology 3, 163; [4]
Vasilyev N. V. (1998) Planet. Space. Sci. 46, 129; [5]
Glasstone S. and Dolan P. J. (1977) The Effects of Nuclear Weapons 3rd ed. U.S Govt. Printing Office, 653pp;
[6} Shoemaker E.N., Wolfe R.F. and Shoemaker C.S.
(1990) in Global Catastrophes in Earth History. Geol.
Soc. Amer. Spec. Paper 247, 155.; [7] Wasson J. T.
(2015) Lunar Planet. Sci. 46, 2879.