Tektites are small, pebble-sized objects made of natural glass that is black, green, brown, or gray. They form when pieces of Earth are melted and thrown into the air during meteorite impacts. The word "tektite" was created by Austrian geologist Franz Eduard Suess, who was the son of Eduard Suess. Tektites usually measure from a few millimeters to several centimeters in size. Tektites that are less than a millimeter wide are called microtektites.

Characteristics

Although tektites look similar to some types of volcanic glass found on Earth, called obsidians, they have unique physical features that set them apart. First, tektites are entirely glass and do not contain tiny crystals or larger crystals, which are found in volcanic glass. Second, even though tektites have a high amount of silica (more than 65 weight percent), their overall chemical and other properties are more similar to those of shales and other sedimentary rocks, and very different from those of volcanic glass. Third, tektites have almost no water (less than 0.02 weight percent), unlike volcanic glass. Fourth, the flow bands in tektites often include particles and bands of a mineral called lechatelierite, which are not found in volcanic glass. Finally, some tektites have partially melted pieces of shocked and unshocked mineral grains, such as quartz, apatite, zircon, and coesite.

The difference in water content helps scientists tell tektites apart from volcanic glass. When heated to their melting point, volcanic glass becomes a foamy glass due to its water and other gases. In contrast, tektites create only a few bubbles when heated because they contain very little water and other gases.

Classification

Tektites are divided into four main groups based on their shape and physical features. Tektites found on land are further divided into three groups: (1) splash-form tektites, (2) aerodynamically shaped tektites, and (3) Muong Nong-type tektites. Splash-form and aerodynamically shaped tektites are separated mainly by their appearance and some physical traits. Splash-form tektites are small, about the size of a centimeter, and have shapes such as spheres, ellipsoids, teardrops, and dumbbells. These tektites are believed to form when molten material cooled while spinning, not from melting caused by entering Earth’s atmosphere. Aerodynamically shaped tektites, mostly found in the Australasian strewn field, are splash-form tektites with a ring or flap. This feature is thought to form when a solidified splash-form tektite re-enters Earth’s atmosphere at high speeds, causing it to melt and reshape. Muong Nong tektites are larger, usually over 10 centimeters long and weighing up to 24 kilograms. They have irregular shapes, a layered structure, many small bubbles, and contain minerals such as zircon, baddeleyite, chromite, rutile, corundum, cristobalite, and coesite.

Microtektites, the fourth group, are smaller than 1 millimeter in size. They come in many shapes, including spheres, dumbbells, discs, ovals, and teardrops. Their colors range from clear and transparent to pale yellow and brown. Microtektites often have bubbles and tiny pieces of lechatelierite inside them. They are commonly found in deep-sea sediments that are the same age as the four known strewn fields. Microtektites from the Australasian strewn field have also been discovered on land in Chinese loess deposits, in sediment-filled cracks, and in small weathered pits within glacially eroded granite rocks in Antarctica.

Occurrence

Most tektites have been found in four large areas called strewn fields: the Australasian, Central European, Ivory Coast, and North American. According to Koeberl, tektites within each strewn field share similar features, including their composition, shape, chemical makeup, and age. Three of these four strewn fields are connected to impact craters based on these same features. Recognized types of tektites, grouped by their strewn fields, associated craters, and ages, are:

• Australasian strewn field (age: about 0.77 to 0.78 million years, no confirmed crater): Australites (Australia, dark, mostly black); Indochinites (Southeast Asia, dark, mostly black); Philippinites (Philippines, black).
• Central European strewn field (Nördlinger Ries impact crater, 24 km in Germany, age: 15 million years): Moldavites (Czech Republic, green).
• Ivory Coast strewn field (Lake Bosumtwi impact crater, 10 km in Ghana, age: 1 million years): Ivorites (Ivory Coast, black).
• North American strewn field (Chesapeake Bay impact crater, 40 km in the United States, age: 34 million years): Bediasites (Texas, black to dark brown, some with a metallic look); Georgiaites (Georgia, green).

When comparing the number of known impact craters to the number of strewn fields, Natalia Artemieva noted that craters must be at least 10 km in diameter and less than 50 million years old to produce tektites. This study identified 13 possible craters, with the eight youngest listed.

Preliminary research from the late 1970s suggested that either Zhamanshin or Elgygytgyn might be the source of the Australasian strewn field.

Povenmire and others proposed the existence of a fifth strewn field, the Central American strewn field. Evidence includes tektites found in western Belize near Bullet Tree Falls, Santa Familia, and Billy White villages. These tektites, some dated to about 820,000 years old, were discovered near Tikal, Guatemala. Limited evidence suggests the Central American strewn field may cover Belize, Honduras, Guatemala, Nicaragua, and parts of southern Mexico. A proposed impact crater called Pantasma in northern Nicaragua might be the source of these tektites.

Age

Scientists have used special dating methods to find the ages of tektites from four different areas where they are found. Moldavites, which are a type of tektite found in the Czech Republic, are about 14 million years old. This age matches closely with the age of the Nördlinger Ries crater in Germany, which was also dated using a method that studied Suevite, a type of rock formed by an impact. Similar matches have been found between tektites from the North American area and the Chesapeake Bay crater, as well as between tektites from the Ivory Coast area and the Lake Bosumtwi crater. Scientists usually determine the ages of tektites using methods such as the K-Ar method, fission-track dating, the Ar-Ar technique, or a mix of these methods. Tektites found in layers of rock or soil have sometimes been used to help determine the ages of those layers, but some scientists disagree about this practice.

Origins

Earth and planetary scientists agree that tektites are made from material from Earth that was thrown out when a meteorite hit the planet and created an impact crater. During the powerful force of a meteorite impact, rocks and soil near the surface were melted or turned into vapor and then thrown out of the crater. After being thrown out, this material formed small, molten pieces. As these pieces fell back to Earth, they cooled quickly and became tektites, which landed far away from the crater, forming layers of debris hundreds or thousands of kilometers from the impact site.

Evidence supports the idea that tektites come from Earth. Their chemical and isotopic makeup shows they were formed by melting silica-rich rocks and soil, which are not found on the Moon. Some tektites also contain tiny pieces of minerals like quartz, zircon, and chromite, which are common in Earth’s rocks and soil. Three of the four known tektite strewnfields (areas where tektites are found) match the age and chemical makeup of known impact craters. Studies of tektites from the Australasian strewnfield show they were made from Jurassic-era sedimentary rocks, which were deposited about 167 million years ago. This suggests a single source of material, making it unlikely that multiple meteorite impacts caused tektites.

Although scientists widely accept that tektites form when near-surface rocks and soil are melted during an impact and then ejected at high speed, the exact process is not fully understood. One idea is that molten material is forced out during the first stage of an impact. Another possibility is that shock-melted material is spread by a vapor plume created by the impact. Any explanation must account for the fact that tektites come from near-surface rocks at the impact site. Also, the limited number of tektite strewnfields compared to the many known impact craters suggests that special conditions are needed for tektites to form.

The idea that meteorite impacts create tektites is widely accepted, but there was once debate about their origin. In 1897, Dutch geologist Rogier Diederik Marius Verbeek proposed that tektites came from the Moon. Austrian geologist Franz E. Suess later supported this idea, suggesting tektites were ejected from the Moon by volcanic eruptions and drifted to Earth. Scientists like John A. O'Keefe, Dean R. Chapman, Darryl Futrell, and Hal Povenmire also supported the lunar origin theory, arguing that tektites’ chemical and physical traits could not be explained by Earth impacts. They claimed Earth impacts could not produce the low water content or certain features found in tektites.

During the 1960s, the lunar origin theory was widely discussed. However, research on Moon rocks brought back by astronauts shifted scientific opinion toward the idea that tektites come from Earth impacts. Studies showed that Earth impacts could create the low-volatile melts seen in tektites. Scientists now agree that the chemical and isotopic evidence clearly shows tektites are made from Earth’s crustal rocks, which are different from any known lunar rocks.

Literature

• Barnes, V., and M. Barnes (1973) Tektites. Dowden, Hutchinson & Ross, Inc., New York, New York. 444 pp. ISBN 0-87933-027-9
• Bouska, Vladimir (1994). Moldavites: The Czech Tektites. Stylizace, Prague, Czechoslovakia. 69 pp.
• Heinen, Guy (1998) Tektites – Witnesses Of Cosmic Catastrophes. Guy Heinen, Luxembourg. 222 pp.
• McCall, G.J.H. (2001) Tektites in the Geological Record. The Geological Society of London, London, United Kingdom. 256 pp. ISBN 1-86239-085-1
• McNamara, K., and A. Bevan (1991) T