Darwin glass is a natural glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 found south of Queenstown

Queenstown, Tasmania

Queenstown is a town in the West Coast region of the island of Tasmania. It is located in a valley on western slopes of Mount Owen on the West Coast Range.It had a population of 5,119 people . At the 2006 census, Queenstown had a population of 2,117....

 in West Coast, Tasmania

West Coast, Tasmania

The West Coast of Tasmania is the part of the state that is strongly associated with wilderness, mining and tourism, rough country and isolation...

. It takes its name from Mount Darwin

Mount Darwin (Tasmania)

Mount Darwin, Tasmania is a mountain in the West Coast Range, Tasmania, named after Charles Darwin.-Location:On the eastern side of the mountain sits Darwin, a long-abandoned town site. Mount Darwin is the southernmost mountain of the West Coast range...

 in the West Coast Range, where it was first reported, and later gave its name to Darwin Crater

Darwin Crater

Darwin Crater is a suspected meteorite impact crater in Western Tasmania, Australia. It is expressed as a rimless circular flat-floored depression, in diameter, within mountainous and heavily forested terrain south of Queenstown. It lies east of the West Coast Range and just within the...

, a probable impact crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

, and the inferred source of the glass.

Occurrence

Fragments of Darwin glass are found scattered over a 410 km² area. Such an area is called a strewn field

Strewn field

The term strewnfield indicates the area where meteorites from a single fall are dispersed.It is also often used with tektites produced by large meteorite impact.-Formation:There are two strewnfield formation mechanisms:...

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

Quartzite

Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...

 fragments buried under peat

Peat

Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...

 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 and the lower northeast shore of Macquarie Harbour

Macquarie Harbour

Macquarie Harbour is a large, shallow, but navigable by shallow draft vessels inlet on the West Coast of Tasmania, Australia.-History:James Kelly wrote in his narrative "First Discovery of Port Davey and Macquarie Harbour" how he sailed from Hobart in a small open five-oared whaleboat to discover...

. Northwards it extends almost to the Lyell Highway

Lyell Highway

The Lyell Highway is a highway in Tasmania, running from Hobart to Queenstown. The name is derived from Mount Lyell, the mountain peak where copper was found in the late 19th century, and the site of the Mount Lyell copper mine, and the sole reason for the existence of Queenstown...

 and Crotty Dam. 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 and more magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 and iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

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

Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

, nickel

Nickel

Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

 and cobalt

Cobalt

Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

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

Metamorphic rock

Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...

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 dating

Argon-argon dating

Argon-argon dating is a radiometric dating method invented to supersede potassium-argon dating in accuracy. The older method required two samples for dating while the newer method requires only one...

 methods.

Crater

The glass is an impactite

Impactite

Impactite is an informal term describing a rock created or modified by the impact of a meteorite. The term encompasses shock-metamorphosed target rocks, melts and mixtures of the two, as well as sedimentary rocks with significant impact-derived components .-Some localities:*Nördlinger Ries...

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

Meteorite

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

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

Darwin Crater

Darwin Crater is a suspected meteorite impact crater in Western Tasmania, Australia. It is expressed as a rimless circular flat-floored depression, in diameter, within mountainous and heavily forested terrain south of Queenstown. It lies east of the West Coast Range and just within the...

. The crater is filled with 230 m of sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s and breccia

Breccia

Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....

. 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

TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

of energy.

External links

• http://www.meteorite-times.com/Back_Links/2002/June/colectors_corner.htm