Wednesday, November 29, 2023

Geology in Pop Culture - The Rescuers Down Under

 Geology in Pop Culture

Released in 1990, The Rescuers Down Under continues the escapades started in 1977's The Rescuers. However, in this adventure our favorite mice, Bernard and Bianca, travel down to the Australian outback (not the steakhouse). Here, they find that the villain of the story, the poacher Percival C. McLeach has kidnapped a boy, Cody, while hiding out in some abandoned opal mines.


So what exactly is opal anyway?

Different types of Australian opal varieties. Image courtesy of BlackOpal Direct

Opal (SiO2.H2O) is a mineral that forms from the packed spheres of silica (SiO2), also known as the mineral quartz. Opal is a hydrated form of silica where water has been shown to include between 3 and 9% of the total mineral structure. Unlike many minerals, because of how it is formed, opal is amorphous, or without form. This means that there is no crystal structure or cleavage that is seen in most other minerals. Opal forms through the processes of solidification of gelatinous or liquid silica within cracks and voids of other rocks. 

Since the opals are created by spheres of silica and water, the size of the sphere's dictates the colors that are produced. These colors are refracted through the opal like a prism, with larger spheres yielding red or orange, and smaller ones radiating blue. However, this is only the case with precious opals, the more gem quality ones. Most opals, ~95%, are referred to as common opals. These are opals that do not have the "play-of-color" expected in opals as seen in the image above. Although still beautiful, they are harder to identify than the precious opals. 

Location of Australian Opal Mines. Image courtesy of Opals Down Under

Despite being found around the world, it turns out that ~95% of the world's supply of precious opals comes from Australia. The opals started to form during the Cretaceous period when there was a large inland sea across Australia. During this time silica rich sands were deposited along the shorelines, then 30 million years ago, during the Tertiary, deep weathering of these sediments within the Australian Artesian Basin released large amounts of soluble silica into the groundwater. This silica traveled into the cracks and fissures in the Cretaceous age rocks where it was stopped by an impermeable layer below. Staying in the cracks and fissures, the opal deposits formed veins, often trapping fossils that also happen to be in the sedimentary rocks such as leaves, dinosaurs, small mammals, and marine reptiles such as Eric the Pliosaur

Eric the opalized pliosaur. Image courtesy of Opal Auctions

Most of the opal mines in Australia are a type known as open-cut. This means that they basically dig down to the source of the deposit, in this case opal, and clear out anything above it, forming a quarry or pit. This limits the dangers of enclosed mine spaces, allows you to find smaller deposits easier, is much faster with heavy machinery, but is also much, MUCH, more damaging to the environment, destroying literally everything to get to the deposits. 

In The Rescuers Down Under though, they are clearly not in an open-cut mine, they are underground mines. There are some underground opal mines that are found within Australia. One of these locations is the town of Mintabie, found near the north-central portion of the South Australian state. And although they did use open-pit mining in Mintabie, they also had quite a number of underground mines as seen in the picture below.

Mintabie Opal Mines. Image courtesy of Opal Auctions

What makes Mintabie interesting is that the mining heyday here was in the 1980's. Even though opals had been know from here since the 1920's, production increased starting in 1976 with the addition of mining equipment able to break through the hard sandstone. During the 1980's, production here was the largest in all of Australia with a peak during 1988. However, after 1988, production quickly declined resulting in lots of abandoned opal mines. 

Warning sign in The Rescuers Down Under

This timing of the opal mines being abandoned during the late 1980's correlates strongly with The Rescuers Down Under, which would have been starting to be in production probably ~1988. Seeing these abandoned opal mines pop up all over the place would have been a good impetus to a setting for their movie. 

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