The Earth Story

Fukuoka Sinkhole

This massive sinkhole opened up in the middle of a road in Fukuoka, Japan on Tuesday. This hole formed as workers were digging a tunnel to extend a nearby subway.

Sinkholes like this are common in areas where limestone rocks outcrop – natural acidity in groundwater can eat away at limestones, causing hidden caves to form at depth and grow until the weight above causes them to collapse. However, those sinkholes take centuries to form – they wouldn't be associated with construction.

The workers reportedly saw water appearing in the tunnel they were digging, tried to reinforce the walls, and then had to shift their priorities to protecting the nearby buildings as the sinkhole widened. In this case, water flowing in likely disrupted the sediment supporting the tunnel, causing the walls to cave in as it flowed. Stabilizing the hole now will involve either cutting off the water flowing to the site or stabilizing the walls themselves to prevent additional collapse inward.

-JBB

Image credit: Press report/fair use/BBC http://bit.ly/2fetWrw http://bit.ly/2fRlvEv

Meet the Manhattan Schist

Have you ever noticed that the high-rise buildings in New York City cluster into two areas; near Midtown and the Financial District? Turns out the reason for this clustering isn’t just “money”, it’s also bedrock.

Skyscrapers in New York are cheaper and easier to build when there is solid bedrock to support them. There is some topography to the bedrock unit and it pretty well matches where the skyscrapers are. That rock unit underlies much of the island, including a number of outcrops in Central Park. It shows up behind this worker of the NY Metropolitan Transportation Authority, and is known as the Manhattan Schist.

The Manhattan Schist is a product of hundreds of millions of years of tectonics. Its story started innocently enough 600 million years ago as part of the floor of the Iapetus Ocean; the predecessor to the modern Atlantic Ocean. At the time, the rocks sat offshore away from the heart of the Laurentian continent (the continental fragment that would evolve into North America), and sediments from Laurentia piled on top of igneous rocks on the seafloor.

Then something happened…a piece of land came slamming in. In the first case, it was an island arc; the plate that made up the Iapetus Ocean was subducted, forming an island arc offshore, and eventually that arc ran into Laurentia. When that arc slammed in, pieces of the seafloor were broken off, stuck in the middle, and accreted to the continent.

If this sounds like a violent process for the rocks, it is. This event was known as the Taconic Orogeny, and it was one of the major events that helped build the Appalachian Mountains. During this orogeny, the rocks that would become the Manhattan Schist were heated, twisted, and folded. They stopped being sediments and became metamorphic rocks. New minerals such as garnet and kyanite grew when the rocks were heated at pressure, changing their color and texture. That’s why its now called a schist; new minerals grew to the size where they could be seen and the rock developed a foliation (layering formed due to metamorphic mineral growth).

From there, the rocks that are today were buried to shallow depths and eventually exhumed following several other mountain-building events farther out along the coast.

Today the Manhattan Schist forms the backbone of Manhattan itself. In this image you can see a portion of the schist being torn apart by man. The NY MTA is building a new subway tunnel on the East side of the island, and that requires digging into the schist, identifying structures within it, and mapping out the fine scale details. In this case, of course, using a Brunton compass.

More than half a billion years of geologic history has played its part in building this great city, from the subways to the skyscrapers.

(Meet the schist, meet the schist, step right up and greet the schist…)

-JBB

MTA Photo, Metropolitan Transportation Authority / Patrick Cashin. http://www.flickr.com/photos/mtaphotos/8639449015/

SUNY Stony Brook field guide: http://www.geo.sunysb.edu/reports/ny-city/

BBC News Article: http://www.bbc.co.uk/news/science-environment-22798563

The Subway (or should it be sewer?) Garnet

Though this whopping crystal has been given a plethora of names (including the Manhattan and Kunz garnets), it's most evocative name is a misnomer, changed by Victorian sensibilities, since it was in fact excavated while digging a sewer.

The latter name might have been more appropriate, since the gem started life as disseminated atoms amongst the ooze lying over the volcanic rocks at the bottom on the Iapetus Oceanic plate, which once separated America from Europe some 600 million years ago. The sediments were eroded from the continent of Laurentia and the fine particles settled out of the water to the deep bottom.

It was found in a rock called the Manhattan schist (see http://on.fb.me/1OKsOYH) that underlies the skyscrapers of the island. As the ocean closed some 440 million years ago, and an island arc (volcanoes overlying a subduction zone, where oceanic crust is sinking into the mantle) crashed into Laurentia, it squished up some of this ooze and basalt and welded it to the continent.

The Appalachians started growing, and the intense heat and pressure transformed the rocks into different minerals, in a process called regional metamorphism. Some of the new minerals included garnets, and the rock became foliated, a series of lines developing in the rock in response to directed pressure. Finally a granite intruded in, baking the schist some more, and this garnet was found in the metamorphic halo surrounding this interloping rock.

The specimen was found in 1885, near the intersection of 35th Street and Madison Avenue. The size of a bowling ball (15cm across), it weighs around four kilos. After some time as a media sensation, it faded from the swift changing attention of the city, and ended up as a doorstop in the Department of Public Works.

It was bought by a dealer, and ended up in the hands of George Kunz, a mineralogist who was a curator of the American Museum of Natural History's collection, and a famous dealer in his own right who helped JP Morgan amass the collection that would later form the core of the museum's own (the mineral Kunzite is named after him, see http://on.fb.me/1Xm9G4H). The Subway is a typical reddish brown almandine garnet, coloured by iron. The crystal faces on the dodecahedron are well formed with clear edges, except where it was once attached to the matrix.

Loz

Image credit: American Museum of Natural History.

http://bit.ly/1hWI0TT http://bit.ly/1RYeXfD http://bit.ly/1LOllW1 http://nyti.ms/1PCq6o8

The Crossrail project is building a new 120 kilometer tunnel in southern England. The project has been in the works for decades and so scientists and engineers have drilled hundreds of vertical pipes to extract samples from the ground. These core samples are absolutely necessary to be able to determine what types of material the tunnel is going to be carved through.

When you have hundreds of cores and you need to keep them in tact for decades, what do you do with them? They have to stay dry, they can’t chemically interact with the environment or their physical properties might change before they’re tested. 

The Deepstore facility is a core repository placed in a mine beneath Cheshire. By being stored in this old salt mine, the cores are kept at a constant temperature and a low humidity, which has been enough to prevent their deterioration for decades. This one room holds a repository of geology from throughout southern England.

Metro in the Desert: Utah’s Zion Subway Tunnel

For those daring enough to venture nine miles through deep canyon and desert landscape, here is a must-see. The Zion Subway, located in Zion National Park, is nestled in the desert-ridden terrain of southwest Utah. The tunnel is named after its similar appearance to urban subway tunnels; however there is no direct correlation between the two. The Zion Subway tunnel has been carved from the Left Fork of North Creek where churning water sculpts the tunnel between two peaks called North and South Guardian Angels. In order to reach this majestic and incredibly unique landscape, hikers must first trek through 14.5 km-15.28 km (9-9.5 miles) of trails, canyons, and small river beds, but the payoff is most definitely worth the adventure.

Within the hollowed out sandstone, rivers and pools of water are met with little sunlight; conditions that allow algae to flourish and turn the water into its greenish/turquoise hue. Careful to be too lured in by the algae though, as it also coats much of the surrounding sediment, making for a slippery obstacle to dance around while hiking. Other wildlife within the tunnel includes an abundant population of dragonflies, frogs, lizards, and some species of birds. In fact, the surrounding Zion National Park is home to more than 270 species of birds, including the infamous roadrunner.

As beautiful as the Zion Subway may be, caution must be taken when hiking through, as conditions in the tunnel are known to escalate quickly. Danger from flash-floods, extreme heat, and hypothermia does put adventurists at risk but shouldn’t deter everyone from venturing into the Zion Subway tunnel. Although the hike may require extensive preparation and training, the payoff of visiting one of the desert’s best kept secrets is well worth while.

-Pete D

Photo Credit: Christian Bothner, National Geographic

References: 1. http://www.nps.gov/zion/index.htm 2. http://travel.nationalgeographic.com/travel/national-parks/zion-photos/ 3. http://www.canyoneeringusa.com/utah/zion/visitor-info/geology/ 4. http://www.nps.gov/zion/planyourvisit/southwest-desert-trails.htm 5. http://www.zionnational-park.com/zion-national-park-subway.htm