Australian Meteorites

What is a meteor?
Almost everyone is familiar with the short-lived streaks of light that zip across the night sky. We call them falling or shooting stars. Each of these flashes of light is caused by a piece of interplanetary matter, called a meteor, entering the Earth's atmosphere and 'burning up' at about 100 km above the Earth's surface. The meteor is slowed down by the atmosphere and becomes very hot so that its surface starts to vaporise; it also ionises the gas it passes through. It is this vaporisation and the emission of radiation from the ionised gases in the atmosphere which gives the streak of light across the sky.

Usually meteors vaporise completely because most are small, but when larger chunks of material enter the atmosphere, it is unlikely that they will completely vaporise. If they do not, they will pass right through the atmosphere and hit the surface of the earth, they are then called meteorites.

Where do meteorites come from?
Meteorites come from within our Solar System and are probably derived from the asteroid belt which lies between the orbits of Mars and Jupiter. This part of the Solar System may have contained several small planets which collided and broke up into smaller fragments.

The term asteroid is used for objects larger than 1.6 km across, whereas meteoroids can range from pebble size up. It is estimated that there are about one thousand trillion asteroids of over 50 kg mass in this belt. Some of these are undoubtedly perturbed by the gravitational fields of Mars and Jupiter so that they fall into potential Earth-collision orbits.

How many meteorites have been found?
Even though it has been estimated that about 6000 meteorites hit the earth every year; only 2000 or so have been found. Of these, the falls of only about 800 have been witnessed.

Tektites
Tektites are dark coloured, rounded silicate glass particles several centimetres in size and are tear-drop shaped, button shaped or dumbell shaped and are found only in certain areas of the globe, called strewn fields. There are four well known strewn fields:
> Australasian, in which the tektites are 770,000 years old;
> Ivory Coast, at 1 million years old;
> Central European at 14.7 million years old and
> North American at 34 million years old.

Tektites are the solidified splashes of molten rock ejected by the impact of very large, high velocity meteorite. However, only two of the strewn fields have been connected with impact sites. The Ivory Coast field is associated with the Bosumtwi Crater in Ghana, while the Central European field is associated with the Reis Crater in Germany.

The Australasian strewn field encompasses most of Southeast Asia including Vietnam, Thailand, South China, Laos, Cambodia, the Philippines, Indonesia, Malaysia as well as western and southern Australia. The source of the tektites in the Australasian strewn field is believed to lie somewhere on or near the Indochina Peninsula.

What happens when a meteorite hits the ground?
When a small meteorite (up to about 1 tonne) hits the ground a hole or pit is formed at the point of impact in soft ground; the diameter of the pit being similar to that of the meteorite.

Larger and consequently faster (up to about 4 km/sec) meteorites tend to break up when they hit the ground. Fragments of the meteorite as well as soil and rock from the surrounding ground will fly in all directions, producing a fragmentation crater whose size will be considerably larger than that of the meteorite.

When the terminal velocity of a meteorite exceeds 4 km/sec (only possible for masses of over 10 tonnes) the impact generates a powerful blast which creates what is called an explosion crater. These are the large craters that most people are familiar with (e.g. Wolf Creek Crater, Western Australia; Gosses Bluff, Northern Territory; Meteor Crater, Arizona).

The largest meteorite impact structure preserved in Australia is Gosses Bluff. This structure was formed about 142 million years ago. The bluff itself is only the eroded remnant of the central core of the crater. Because it is so old, the rim is only preserved as a ring of low hills. It is about 22 kilometres in diameter.

The Barringer Crater (also known as Meteor Crater), in Arizona, USA is a more recent crater, having been formed about 49 000 years ago. It about 1.2 kilometres in diameter.

How big are meteorite craters and how old are they?
The Chicxulub crater in the subsurface of the Yucatan Peninsula, Mexico is 170 km in diameter and is 65 million years old. This crater is considered by some geologists to be the trace of the impact which completed the extinction of the dinosaurs.

The Manicouagan impact structure in Quebec, Canada is deeply eroded and one of the largest impact structures still preserved on the surface of the Earth. It has a diameter of about 100 km and is 212 million years old.

Perhaps the best known crater in Australia is the Wolf Creek Crater in northern Western Australia, south of the town of Halls Creek. It is about 880 metres in diameter. The impact that created this crater occurred about 300 000 years ago.

Henbury meteorite
Large numbers of fragments of this iron meteorite were found in 1931 in and around the Henbury Meteorite Craters, a series of at least 13 small craters, the largest one of which is 200 metres in diameter. They are located about 125 km south west of Alice Springs, Northern Territory. These meteorites fell to earth about 4 700 years ago. So far, about 680 kg of meteorite fragments have been recovered.