Harrow and Hillingdon Geological Society


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Sheppy Shepherd

The speaker explained that the story of meteorites began with the origin of the solar system and recalled the talks by Anton Kearsley of the Natural History Museum in 2006 and 2012 on the analysis of material that has impacted on satellites, including the special-purpose Stardust mission which collected material from the tail of a comet. The predominant minerals found were olivine and pyroxene.

The nebula, from which the solar system formed, rotated and collapsed with materials gathering together and accreting into planets but there was still a lot of material in space, some of which has never been part of a planet.

The most common type of meteorites are chondrites, characterised by round chondrules, material that was once molten. They mainly consist of olivine and pyroxene, though others have iron silicates, bits of metal and traces of carbon.86% of the total meteorites falling to earth are chondrites. The higher percentage of carbon indicates more primitive material. The Allende meteorite from Mexico has bits of iron in it.


Chondrules in a meteorite

The continual bombardment of proto-planets generated heat, leading to melting and iron sinking to the bottom with slag at the top, resulting, for example, in a layered earth with a solid iron-nickel core surrounded by a liquid iron and iron sulphide outer core, the mantle and the crust.

Between Mars and Jupiter is the asteroid belt and many of the asteroids were parts of planets or proto-planets.

Other types of meteorites are iron meteorites, stony iron meteorites and achondrites. The nickel-iron meteorites are much rarer than chondrites (about 5% of those falling to earth) and their particular structure results from very slow cooling in an inner core at 5,000oC with cooling to the melting point of iron at 1,200oC over 0.5 million years.

iron meteorite

Iron meteorite

Stony iron meteorites (about 1% of those falling to earth) contain some silicate minerals surrounded by iron and are thought to have come from a core/mantle boundary. The silicate minerals are mainly olivine, a magnesium/iron silicate ranging from Mg-rich forsterite to Fe-rich fayalite.

Stony iron meteorite

Achondrites are material from the mantle (or crust) of broken-up planets and contain no chondrules. Of the 50,000 or so meteorites found to date, 58 are from Mars and others are from the Moon.

Meteorites are only meteorites when they land, they are meteoroids in space and meteors when they pass through the atmosphere. On passage through the atmosphere, the heat generated melts the outer zone and many have a glassy outer surface as a result. Most meteors burn up in the atmosphere, though very tiny particles do not get hot and make it down to earth.

Meteorites can land anywhere on earth but because of the arid conditions, a lot are found in Australia, where the dark meteorites stand out against the background rocks and in Antarctica, where they are distinctive against the snow.

A number of meteorite impact craters have now been identified, many of which were originally thought to be volcanic in origin. Notable events include:
• the Barringer Crater in New Mexico, which landed about 50,000 years ago;
• the Tunguska event in Russia in 1908, where an air-burst flattened millions of trees over a vast area;
• on 15 February 2013, a meteor that was described as being brighter than the sun exploded in an air burst at just over 18 miles high at Chelyabinsk in Russia, damaging 7,200 buildings and injuring 1,500 people;
• and a very large meteorite landing in Yucatan has been blamed, at least in part, for the extinction of the dinosaurs.

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