Meteorites from Earth:

An angry false dawn looms over the southwestem sky. Moments earlier, a flash brighter than a thousand suns illuminated the Cretaceous landscape as an asteroid the size of Manhattan Island slammed into a shallow sea. The blast has the strength of 250 billion Hiroshima atomic bombs, temporarily ripping a hole in the atmosphere and gouging out an immense crater. Even a thousand miles away, the ground shakes with such violent intensity that the entire surface undulates like a wave-swept ocean. The sky erupts with heaven-born fire, as blobs of partially molten rock strike and ignites the vegetation below. A massive firestorm now sweeps across the wrecked plain, killing the earthquake survivors. It is the death of one age and the birth of another.

Sixty-five million years later, the great Chicxulub crater lies buried in the Yucatan Peninsula. The main structure is nearly 200 miles across and evidence of its strike can be found worldwide. Supporting the isotopic, stratigraphic and petrographic evidence, molten 'splash' deposits or tektites have been recently found in Haiti. The term tektite is a derivative of the Greek word tektos, meaning 'molten' or 'melted". Tektites are the product of large asteroid (or comet) impacts. They are thought to be the 'splash" or ejecta that is blasted into space and then re-enter's the atmosphere to strike the surface. In a sense, tektites are earth-borne meteorites Tektites are usually greenish to black glassy objects that superficially resemble obsidian. Compositionally, tektites have lower water and alkali content than obsidian and have very distinctive surface textures.

Size and Texture:

Generally, tektites are fairly small objects ranging from only a few millimeters to chunks up to 20 cm (8-inches) in diameter. Most tend to be only a centimeter or so across and weigh a few grams. Tektites can be classified into three main forms (Glass, 1982). Splash forms look like solidified drops of liquid material. They can appear as spheres, teardrops, dumbbells, and disks. This is the most common form of tektite. Ablated forms are splash forms that have been modified by passage through the Earth's atmosphere. Some of these have an appearance similar to the heat shield on the Apollo space module. As a tektite falls through the atmosphere, compression heats up and melts the forward surface. Material from the heated side often flows up to form a ridge or flange. Protected in the rear is the unablated tektite surface. Some of the best examples of this form have been found in the Australasian strewn field.

The last major main is called the Muong Nong- type tektites (Glass, 1982). These are chunks of tektite glass with a layered structure. These can be quite massive, and the largest recovered has a mass of 12.8 kilograms (= 28.2 lbs). This form is found primarily in SE Asia, but others have been reported in Texas and Central Europe.

Tektites have unusual surface features - many are pitted, have furrows or groves, or have fine striations (Baker, 1963). Some of these features are the result of etching while on the Earth's surface. Others are the result of ablation and stresses as the tektite passed through the atmosphere. The front of ablated tektites have a smooth, almost polished surface.

Many tektites show evidence of flow structures and layering. Certain types of these structures are particular to the form of tektite. Splash forms have layering that is quite distinctive when compared to ablated and Muong Nong forms. Also present in almost all tektites are 'bubble cavities'. These can have samples of trapped gas. This gas has a chemical composition close to current atmospheric values, and represents air that was trapped by the molten tektite material.

Composition:

In general chemical composition, tektites are similar to silica-enriched igneous and certain types of sedimentary rocks. Depending on the type of tektite, SiO2 ranges from 64 to 84% with a mean of 73.09% (Glass, 1982; Bagnall 1991). However, unlike "normal' terrestrial rocks they are extremely 'dry' having far lower volatile content. Tektites bear a close resemblance to terrestrial obsidian, but their inherent dryness allows for an interesting test method. When heated by a blowpipe, the water in the obsidian cause it to froth, while the tektite will simply melt (Bagnall, 1991).

Tektites also have other structural and chemical peculiarities that set them apart from 'normal" terrestrial rocks. Most have 'bubble cavities', formed by gases evolving from the molten parent rock. These vesicles range from a few micrometers to up to one centimeter in diameter and can be either spherical or elongate in shape. Nearly all tektites have pure silica glass particles known as Lechatelierite. The size and shape of these particles is highly variable and is indicative of the temperature of formation (Glass, 1982). Other tektites have nickel-iron spherules (as inclusions) typical of iron-nickel meteorites and coesite, a high pressure form of SiO₂ (quartz) often associated with impact craters.

Finding Tektites!

Tektites are not found randomly on the Earth's surface, but in well defined regions known as strewn fields. Just what are strewn fields? It is the material excavated and hulled from the impact crater. Also known as ejecta, this material can be thrown hundreds or even thousands of miles and cover millions of square miles. The names of the various types of tektites are derived from the strewn field of origin.

We can easily visualize the size and shape of unmodified strewn fields by looking at the Moon. The bright streaks radiating outward from the crater Tycho are ejecta deposits from the main impact. These bright rays are easily visible even in small telescopes and binoculars and most prominent around the time of the full moon. Other relatively recent lunar craters have bright rays radiating from them. If the impact is at a high angle, the ejecta will be distributed in a circular fashion. Low angle or grazing impacts have a much more elliptical shape. On the Earth, the placement of oceans and erosion/deposition processes have modified the strewn fields into more irregular shapes.

The Main Strewn Fields

Of the seven known strewn fields, the largest is the Australasian Strewn Field. It covers most of Australia and Southeast Asia and a large portion of the Indian Ocean. This field is huge, estimated to cover almost 20 million square miles! Tektites associated with this field include Australites, Javanites, lndochinites and Philippinites. In central Europe, there is the famous Czechoslovakian Strewn Field. The greenish Moldavites (named after the Moldau River) come from this field. Covering most of the southeastern United States is the North American Strewn Field. It is the second largest strewn field and one of the oldest (table 1). It includes the rare Bediasites (Texas) and Georgiatites - found mostly in the coastal plain of southeastern Georgia. Other important strewn fields include the Ivory Coast, Libyan Desert, and lrgiz (Central Asia) (Glass, 1982; Bagnall, 1991). It is important to note that not all strewn fields have source impact crater(s). For example, no major crater has been tied to the North American field, though the newly discovered structure in lower Chesapeake Bay may change this perception. In contrast, the Australasian field may have several source craters.

Name	Location	Age (106 years)
Australasian	Australia, Tasmania, Indonesia, Southeast Asia	0.75
Czechoslovakian	Czech Republic	14.8
North American	Texas, Georgia, Martha's Vineyard, Cuba	35
Ivory Coast	Ivory Coast of Africa	~ 1.0
Libyan	Libyan Desert	28.5
Irgiz	North of the Aral Sea	1.07
Aouelloul	Mauritania (West Africa)	3.5

Collecting Tektites

Generally, most tektites offer a less expensive alternative over collecting meteorites. Prices quoted from Bagnall for 1989 range from 1.00 to +10.00 dollars a gram, depending on the rarity and condition of the tektite. The common Indochinites are relatively inexpensive, while pretty greenish Moidavites are more moderately priced. Extremely rare types such as Georgiatites can command much higher prices. Unusual shapes, sculpturing and colors can also affect the market value.

Tektites can also be found in the field, as they are quite distinctive (though small) and often lie on the surface. For example, the Georgiatites are found along the coastal plain in the southeastern third of the state. Because they are much harder than the surface sediments, they are often left as erosional remnants on top of the sandy soil. They may be found by simply walking and looking for an oddly shaped, olive green 'rock'. But be warned, prepare to do a lot of walking! And if you are lucky enough to find one, remember you have found an earthly 'meteorite.'