The highway adjacent to this earthcache can be very busy. Please use the off-road parking coordinates provided and take care crossing the road to GZ.
The 'Margate Meteorite'
Around 20 Km south of Hobart, adjacent to the Channel Highway where it crosses the North West Bay River near Margate, sits a large, nondescript rock that has captured the imagination of generations of Tasmanians, with many believing it is a meteorite.
The rock is about the size of the old weatherboard house that has been built next to it, and sits a little lower than the road.
The idea that the lump of rock is a meteorite was perpetuated by a group of geology students at the University of Tasmania in 1960. They wrote a prank letter to the local paper including fanciful tales of the so-called ‘Margate meteorite’ and gave it an extraordinary, extra-terrestrial origin
Of course the 'Margate Meteorite' is a myth. The rock is actually made of breccia - fragments of stone that were thrown from a volcano that was active in the region around 20 million years ago. It has been in the same spot for millions of years, since well before the last ice age.
These days it is marked only by an old sign, the interpretative panel no longer visible — if present at all — and overgrown with fennel and blackberries.
What is breccia?
Breccia is a rock formed from angular gravel and boulder-sized clasts cemented together in a fine-grained matrix. Clasts are fragments of rock that have been broken from a larger rock
Breccia can be formed in several ways. Cataclastic breccias are produced by the fragmentation of rocks during faulting. Volcanic breccias comprise blocks of lava in an ash matrix and are the product of an explosive eruption. Hydrothermal breccias are formed when hydrothermal fluid fractures a rock mass. Impact breccias are formed when a meteor impacts the Earth's surface, fracturing rock at the site of the impact. The rock at this earthcache is volcanic breccia.
The cement that binds the clasts in a breccia is generally calcite, silica or iron oxide. The matrix can consist solely of the cementing material, but may also contain sand and/or silt sized clasts cemented together among the coarser clasts.
Breccia can be further described according to:
Class - breccia can be divided into two broad classes:
~ Clast supported - where the clasts touch each other and the matrix fills the voids; and
~ Matrix supported - where the clasts are not in contact and the matrix surrounds each clast;
Clast size - fine (2 - 6mm), medium (6 - 20mm), coarse (20 - 60mm), very coarse (> 60mm);
Sorting - a breccia comprising a mixture of clast sizes is poorly sorted, while one comprising mostly clasts of the same size is well sorted;
Lithology - a breccia where the clasts represent more than one rock type is termed polymictic, while one where the clasts are of a single rock type are monolithic.
There were a series of volcanoes on the plains around what are now the banks of the Derwent River and further south down the D'Entrecasteaux Channel. The breccia here is the product of one of those volcanoes. Other volcanic stones are still visible on the banks of the Derwent River on Long Beach in Sandy Bay, near Kingston as you head down the Southern Outlet, and on the Tinderbox headland.
How do Meteorites differ from ‘earth rocks’ such as breccia?
Meteorites are space rocks that fall to Earth’s surface. Most meteorites that fall to Earth come from the Asteroid Belt.
Dust-sized particles make up 99 percent of the approximately 50 tonnes of space debris that falls on the Earth’s surface every day. However some meteorites are as large as boulders.
The largest meteorite found on Earth is the Hoba meteorite discovered in Namibia in 1920. It measures 2.7m × 2.7m ×0.9m and is estimated to weigh over 60 tonnes The Hoba meteorite is so big, and so heavy, it has never been moved from where it was found!
Large meteorites have enough energy to make an impact crater when they hit the Earth, and pieces of meteorite may be scattered around and within the crater.
There is one known meteorite crater in Tasmania, the Darwin crater on the west coast.
Meteorites may resemble Earth rocks, but they usually have a burned exterior that can appear shiny. This “fusion crust” forms as the meteorite’s outer surface melts while passing through the atmosphere.
Meteorites tend to look different from the ordinary terrestrial rocks around them. They do not contain the common earth mineral quartz, and in general do not contain vesicles (small holes caused by gas bubbles when the rock was formed). If a suspected meteorite looks like a sponge, with lots of tiny holes, it is probably volcanic rock or slag of earthly origin.
Most meteorites contain a significant amount of extraterrestrial iron and will stick strongly to a magnet. Many earth rocks will also attract a magnet, so this is not a definitive test.
Most meteorites feel much heavier in the hand than an ordinary earth rock would. A softball-sized iron meteorite will likely weigh 2-3 Kg, making it seem unnaturally dense.
To log this earthcache...
Please send me a message or email addressing each of the following questions and tasks based on your observations at GZ and the information supplied in the Cache Description above:
1. Describe the size, colour and texture of the rock at GZ. Are your observations consistent with it being breccia?
2. Using the criteria in the cache description above, classify the breccia that is visible at GZ. i.e.
(a) Is it Clast supported or Matrix supported?
(b) Is the Clast size fine, medium, coarse or very coarse?
(c) Are the clasts poorly sorted or well sorted? and
(d) Is the lithology polymictic or monolithic?
3. Based on the information in the cache description above and your observations at GZ, provide two reasons why this rock is unlikely to be a meteorite.
4. Take a photo from GZ, including either yourself or something showing your caching name and include it in your log.
You may log your visit once you have submitted your answers to me. I will respond confirming permission to log, or seeking clarification if I feel that you have not properly answered the questions. Logs with no answers sent or photo included will be deleted.
Please do not post your answers or spoiler photographs in your log.
Enjoy the 'Margate Meteorite'.