Darwin glass is a natural

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

found south of Queenstown in

West Coast, Tasmania The West Coast of Tasmania is mainly isolated rough country, associated with wilderness, mining and tourism. It served as the location of an early convict settlement in the early history of Van Diemen's Land, and contrasts sharply with the mor ...

. It takes its name from Mount Darwin in the

West Coast Range The West Coast Range is a mountain range located in the West Coast region of Tasmania, Australia. The range lies to the west and north of the main parts of the Franklin-Gordon Wild Rivers National Park. The range has had a significant number ...

, where it was first reported, and later gave its name to

Darwin Crater Darwin Crater is a suspected meteorite impact crater in Western Tasmania about south of Queenstown, just within the Franklin-Gordon Wild Rivers National Park. The crater is expressed as a rimless circular flat-floored depression, in diamet ...

, a probable

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...

, and the inferred source of the glass.

Occurrence

Fragments of Darwin glass are found scattered over a 410 km² (160 miles squared) area. Such an area is called a

strewn field The term strewn field indicates the area where meteorites from a single fall are dispersed. It is also often used for the area containing tektites produced by large meteorite impact.''Tektites in the geological record: showers of glass from the sky ...

. On slopes and flat ground between 250 and 500 m elevation, the glass occurs with

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

fragments buried under

peat Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficien ...

and soil. The peat is normally around 20 cm thick, and the quartzite fragment horizon is typically 30 cm thick. On mountain peaks higher than 500 m, the bedrock is directly exposed to the air, and Darwin glass occurs occasionally on the surface. In valleys below 220 m the Darwin glass is buried below peat and sediments. The glass occurs north, west and south from the crater. Its distribution extends to

Kelly Basin Kelly Basin is a bay on the south eastern side of Macquarie Harbour on the West Coast of Tasmania. It was named after James Kelly an early explorer of the Tasmanian coastline. It was the location of the terminus of the North Mount Lyell Railw ...

and the lower northeast shore of

Macquarie Harbour Macquarie Harbour is a shallow fjord in the West Coast region of Tasmania, Australia. It is approximately , and has an average depth of , with deeper places up to . It is navigable by shallow-draft vessels. The main channel is kept clear by the ...

. Northwards it extends almost to the

Lyell Highway The Lyell Highway (Route A10) is a highway in Tasmania, running from Hobart to Queenstown. It is the one of two transport routes that passes through the West Coast Range, the other being the Anthony Road. Name The name is derived from ...

and

Crotty Dam The Crotty Dam, also known during construction as the King Dam, or the King River Dam on initial approval, is a rockfill embankment dam with a controlled and uncontrolled spillway across the King River, between Mount Jukes and Mount Huxley ...

. Darwin glass is rare in the crater itself. In controlled excavations of gravel deposits the abundance of Darwin glass was found to vary from 0.3 to 47 kg/m³. The highest abundance was found about 2 km from the crater, with the average abundance estimated at 3.4 kg/m³ of gravel over a 50 km² study area near the crater. From this it can be estimated that about 25000 tons of Darwin glass, or about 10000 m³, occurs in this 50 km² area. The amount of glass is large compared with the size of the crater. Preservation is helped by acid ground water which does not dissolve the glass, but this alone cannot explain the glass abundance. There is so much glass present that the glass must have been more copiously produced than in other meteorite impacts of similar size.

Nature

The glass is light to dark green, white or black. The glass takes the form of twisted masses, fragments or chunks up to 10 cm. Internally it has a flowing texture defined by lines of elliptical bubbles. There are two kinds of Darwin glass when composition is measured. Type 1 is normally white or green whereas type 2 is normally black to dark green. The dark glass contains less

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

and more

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

and

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

than the light green glass. The dark glass is also enriched in

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...

and

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...

. A possible explanation for the chemical differences is that, in addition to being mainly composed of melted local

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...

s, the type 2 glass also contains a component of extraterrestrial material from the meteorite. Darwin glass has been dated at about 816,000 years old using

argon–argon dating Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer ...

method.Ching-Hua Lo et al., 2002, ''Laser Fusion argon-40/argon-39 ages of Darwin Impact Glasses'', Meteoritics and Planetary Science 37, p 1555-200
paper
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Crater

The glass is an

impactite Impactite is rock created or modified by one or more impacts of a meteorite. Impactites are considered metamorphic rock, because their source materials were modified by the heat and pressure of the impact. On Earth, impactites consist primarily of ...

resulting from the melting of local rocks due to the impact of a large

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...

. The assumed source is a 1.2-kilometer-wide topographic depression known as

. The crater is filled with 230 m of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

s and

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

. A crater of that size would be created by a meteorite 20 to 50 m in diameter and its impact with Earth would release 20

megatons TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. The is a unit of energy defined by that convention to be , which is the approximate energy released in the detonation of a m ...

of energy.

References

External links

{{Commons category, Darwin glass * http://www.meteorite-times.com/Back_Links/2002/June/colectors_corner.htm Glass in nature Impact event minerals Pleistocene impact craters Western Tasmania