[meteorite-list] Tektites from a Ring Arc (part 1)

From: Graham Christensen <voltage_at_meteoritecentral.com>
Date: Sun Mar 27 04:32:10 2005
Message-ID: <005601c532b0$045cca30$c3e13b8e_at_megavolt>

The Formation of Tektites from a Terrestrial Ring Arc
By J. Hayawardena
Ottawa Centre, RASC
Electronic Mail: jayawar_at_allstream.net

(received April 9, 2003; revised June 14, 2004)

Abstract. Although tektites have been scientifically studied for the last
150 years, their origin still remains unknown. The presently recognized
impact theory for their formation fails in many respects. The physical
mechanisms that govern their formation, their characteristics, their
distribution as a result of impact, and their presence during only the last
35 million years have not been satisfactorily explained. In this paper, it
is proposed that tektites were formed in a ring system circumscribing Earth.

1. INTRODUCTION

Tektites are glassy objects with sizes varying from a few microns to about
10 centimetres, and they are distributed in specific areas of the Earth's
surface known as strewn fields. The Australasian strewn field, covering
one-tenth of the Earth's surface with an estimated mass of 2.7x10^16g
(Schmidt & Wasson 1993), contains the largest distribution of tektites and
was formed 0.75 million years ago. The North American strewn field, dated at
35 million years old, is estimated to be about 3.0x10^14g (Heinen, 1998).
The two smaller strewn fields, namely the Central European and the Ivory
Coast, have been dated at 15 million years and 1.1 million years ago
respectively.

Of the four principal tektite types known, the splash-forms, shaped as
spheres, rods, teardrops, and dumbells, are the most abundant and are marked
by sculptures or corrosion, most commonly systems of pits as well as grooves
and furrows that meander over the tektite surfaces. The family of tektites
known as flanged buttons, or australites, are lens shaped with a flange
around the edge and with circular ridges on the anterior side, and were
formed from the splash-form type by aerodynamic ablation. The microtektites
are the smallest, with diameters of less than 2 millimeters; their shapes
are similar to those of splash-forms, with some also showing corrosion. The
Muong Nong-type tektites are blocky and are the largest tektites. They are
tablet shaped and show a find pattern of layers a few millimeters in
thickness.

Compared to terrestrial rocks or impact melts, tektites have different
chemical compositions, being homogeneous and deficient in water and
containing very few bubbles. Tektites are known to have been present during
only the last 35 million years of Earth's history.

Present consensus on the origin of tektites and microtektites is that they
are impact melt ejected from the target rock of a crater during an impact by
an extraterrestrial object. Although geochemical evidence favours
terrestrial sandstone as a source, many fundamental issues in relation to
the physical mechanism of formation from an impact ramain unresolved. As
stated above, a distinkt characteristic of tektites when compared with
commonly formed impact glasses is homogeneity and a deficiency of water. Any
theory supporting a meteorite-impact origin for tektites and microtektites
must explain this "glass-making problem" as stressed by O'Keefe (1976), in
which melting refining and homogenizing must all occur during the impact.
Evidence suggests, howerever that the physical processes and conditions
required to remove water (refining) from common soils and rock are not
created in an instantaneous impact event. Homogenizing requires diffusion,
mixing, and a specific time period, and the instantaneous heating and
melting seen in an impact event fails to create these conditions. The most
significant piece of evidence against the impact theory is the mechanism of
launch and passage of ejecta through the Earth's atmosphere, since the
presence of atmospheric resistance retards the velocity of ejected material
within a short distance. In order to explain the formation and the wide
distribution of each strewn field (>10% of the Earth's surface, for example,
in the Australasian field) in relation to an impact crater, increadible and
inconcievable conditions of impact are demanded (O'Keefe 1976). Also, the
shapes of australites, produced by aerodynamic ablation require atmospheric
entry velocities in the order of 10 km/s, which cannot be realized by impact
ejection due to air resistance. Another point against impact origin is the
absence of target fragments and projectile contamination in tektites, both
of which are characteristic features of most other impact melts. If tektites
are formed by oblique impacts and/or jetting at the projectile/target
interface as many argue, then tektites should be rich in these contaminents.
In addition, in spite of the young age of the Australasian strewn field and
the many geological expeditions to it and sattelite images of it (Heinen
1998), the candidate impact structure (the smoking gun!), with a proposed
diameter in the range of 90 to 100 kilometres, has not yet been found.

In the model described here, it is proposed that tektites and microtektites
were formed in a terrestrial ring system. The evolution of this ring system
is considered in relation to a past geological event on Earth, namely the
assembly of the supercontinent Pangaea. This model, in addition to
circumventing the constaints encountered in the impact theory of origin, can
also explain the unique characteristics of tektites and their strewn fields.

2. PANGAEA AND GRAVITY

Beginning in the late Devonian Period and continuing further into the
Carboniferous Period, the continents Gondwana and Laurasia converged to form
the supercontinent Pangaea. Within 200 million years it was completely
assembled, and a single ocean, Panthalessa, stretched across the rest of the
planet. It is estimated that this landmass, extending from pole to pole,
encompassed about 40% of the Earth's surface during the Triassic Period. The
presence of this continental cap would have had a significant influence on
the thermal regime of the mantle and thus its convection pattern.
Simulations done by Gurnis (1988) have shown that the heat accumulated under
such an insulating continental plate could have changed the mantle flow,
causing it to well up. The warmer mantle and the mantle upwellings could
have caused thermal expansion and topographic uplift, which in turn would
have created a long wavelength geoid high over the supercontinent Pangaea.
Just such a geoid anomaly exists on the Earth, and Anderson (1982) has shown
that the present-day long-wavelength geoid highs in the Atlantic-African
reagion can be attributed to the pre-drift position of the supercontinent
Pangaea. In the subsequent discussion, the geoid high so formed is referred
to as the Pangaean Geoid High (PGH).

3. PANGAEA AND THE RING

The model described in this paper proposed that, prior to the assempbly of
Pangaea, a ring system of tektite particles circumscribed the Earth, a
system that, by spreading inward and outward, was in quasi-equilibrium state
(Brahic 1977). As the supercontinent Pangaea was assembled, the PGH
gravitationally perturbed these particles, which resulted in their orbital
decay. The tektites in this decaying orbit fell into an orbital period that
made a small whole-number ratio with the rotation of the Earth. At this
location, the particles were in resonance with the Earth's rotation and thus
with the PGH. The net effect of this resonant interaction with the PGH was
to concentrate the tektites in a cluster or an arc at the perigee of their
orbits. In such a resonant orbit, successive conjunctions between the
tektite cluster and Pangaea would have always occurred over perigee. Such a
cluster or arc at the perigee of a resonant orbit is considered to be a
stable equilibrium configuration.

The formation of such a stable ring arc due to the resonance influence of an
external gravity field can be observed in Neptune's ring system. The Adams
ring of Neptune has three distinct arcs, and it has been proposed that this
ring material is confined by resonance with the inner sattelite Galatea
(Murray and Dermott 1999). Orbital resonances of this nature are also a
common feature in the Solar System. Regular sattelites of Jupiter and Saturn
as well as the asteroid belt exhibit similar resonances, and these are
likewise considered to be stable configurations.

Thus the net effect of the assembly of Pangaea was to shepherd the
previously circumscribing tektites into an arc. A swarm of tektites and
microtektites formed, which eventually resulted in the formation of the
Australasian tektite strewn field.

4. STABILITY OF THE RING CLUSTER

It is known that ring particles orbiting a primary will spread in radial and
azimuthal directions due to interparticle collisions that arise from the
different orbital velocities of the ring's components (Keplerian shear).
Thus, it is important to investigate the long-term stability of the tektites
in the proposed ring arc, both during its formation and also after Pangaea
dispersed and the geoid high was dissipated. Several points argue in favour
of the ring arc's stability. As discussed earlier, the cluster of tektites
and microtektites located at the perigee of their orbits was in resonance
with the PGH. This resonance capturing ensured that all the tektites had the
same orbital period and thus the same orbital velocity. These necessary
features of a resonant location are the key to the stability of the ring
arc. Since the orbital velocities of all the tektites and microtektites in
the ring cluster were the same, there would have been no interparticle
collisions and no Keplerian shear as seen in normal planetary rings. Such a
stable ring cluster is known as an epiton, and Fridman & Gorkavyi (1999)
have described its stability in detail. Thus the tektites in the ring arc
would not have spread either in radial or azimuthal directions, and its
stability was ensured. Self-gravitation of the cluster would also have
contributed to its stability.

With the dissipation of Pangaea, however, the geoid high gradually declined,
the resonant lock was lost, and the orbit of the ring arc decayed at a slow
rate due to tidal forces. Nevertheless, as shown above, the cluster of
tektites and microtektites would have maintained its configuration until it
entered the Earth's atmosphere.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Graham Christensen
voltage_at_telus.net
http://www.geocities.com/aerolitehunter
msn messenger: majorvoltage_at_hotmail.com
Received on Sun 27 Mar 2005 04:33:21 AM PST