EarthDate

Sinkholes

In China, sinkholes are called “heavenly pits.” And China has the largest one in the world, named Xiaozhai. It’s two thousand feet deep and just as wide. Sinkholes occur where Earth’s surface collapses into an underground void. It could be sudden, when a giant hole opens in the ground. Or sinkholes can form slowly, making depressions in the landscape that appear as rolling hills and troughs. Both are caused by water eroding or dissolving the bedrock. This could happen over centuries, when rainfall mixes with plant material to form a weak carbonic acid and percolates into limestone, slowly dissolving it. Or, if that underlying rock is an evaporite—like salt or gypsum—it can happen fast, in months or even days. Especially if water flows through the rock. This might be from an underground river, which is what formed China’s Xiaozhai. Or from water leaking from human infrastructure, like a water main. In the U.S., sinkholes occur in all 50 states and cause 300 million dollars in damage a year. But they also form lakes and provide access to underground caverns. If you ever visit China, you can take the half-mile staircase down into Xiaozhai, where there are more than a thousand rare animal species. The river still flows beneath this massive depression, though, so be warned, the story may not yet be over.

Delicate T-Rex Smile

Remember the movie Jurassic Park? Or every time you’ve seen a reconstruction of a T. rex in a museum? Probably the image that sticks with you is the exposed daggers of fearsome teeth. Well, some Canadian grad students may have proven that image wrong. They discovered something rather elemental: T. rex teeth have very thin enamel. Even thinner than the enamel on mammal teeth. But early paleontologists had based their tyrannosaur reconstructions on crocodiles and alligators, which have exposed teeth. It turns out that the enamel on their teeth is very thick, enough to withstand constant drying out and wear from their environment. Teeth with thin enamel need to be protected and kept wet with saliva. And for that, you need lips. That’s why most lizards have them. The Komodo dragon, for instance. It’s one of the few living reptiles that has teeth like a therapod dinosaur. But you’ve probably never seen them, because they’re hidden behind the lizard’s scaly lips. A closer examination of their tooth and jaw structure, and those of other modern lizards, has indeed revealed that theropod mouth anatomy more closely resembled lizards than crocodiles. So T. rex probably had a face like a Komodo dragon. And like them, probably lacked the muscles to expose its teeth in a fearsome snarl. That toothy grin may sell movies. But in reality, T. rex had lizard lips.

Cities Are Sinking

We often hear that sea level is rising, but less often that cities are sinking—which may put them in more jeopardy. This may be caused by land rebounding from the last ice age, when it was covered by ice sheets. While some areas spring upward, others that were pushed upward around the periphery of ice sheets are slowly sinking. Or, Earth's surface may rise or fall when a fault slips, or tectonic plates subduct under or slide over other plates. Rivers move sediment to deltas where, over many years, they may accumulate so much weight they depress Earth’s crust. Normally, floods carry new sediment to replenish the sinking land, but human flood control measures, like levees, distort that. Our large cities, made of concrete and steel, can become so heavy that they start to sink. But the removal of fluids from the subsurface turns out to be the leading reason that land around cities subsides. Scientists surveyed 100 coastal cities and found that a third of them were sinking more than half an inch a year—mostly because of groundwater extraction. The fastest sinking city is the Indonesian capital of Jakarta; some neighborhoods are subsiding almost one foot per year. By comparison, global sea level is rising about an eighth of an inch a year. But the combination of sinking land and rising seas makes some coastal cities quite vulnerable.

Supersonic Jets

You may remember the Concorde, the passenger jet that rocketed from New York to London in just three hours. And you may wonder, why do we now settle for much slower airplanes? Speed is actually the reason. Achieving it required a great deal of maintenance and fuel, making the Concorde very expensive to operate. But a bigger reason was noise. The Concorde flew at twice the speed of sound, so it created a sonic boom. It was so loud, the FAA prohibited it from flying across land, relegating it to transoceanic flights. Few routes and high ticket prices meant the jet was unprofitable and thus taken out of service. A sonic boom happens when an object exceeds the speed of sound, about 700 miles an hour depending on air temperature. It pushes sound pressure waves in front of it so fast that they collide with each other, eventually fusing into a single shock wave that we hear as loud noise. Bullets and bullwhips move fast enough to make small sonic booms. Lightning heats the air so fast that it expands faster than the speed of sound, which we hear as thunder. New supersonic jets now in development are shaped so that the boom doesn’t reach the ground, creating a smaller effect that its designers call a “sonic thump.” More research in these areas could see a “boom” in supersonic air travel in the future.

Rain Shadow

Mountains can shape not only a region’s weather, but its climate. When wind blows warm moist air against a mountain range, the air is pushed upward by the mountain. As it rises, it cools. The moisture within it condenses into rain, which falls on the front side of the range, creating a lush landscape. But behind the range, the air—now empty of moisture—can leave the landscape extremely dry. The mountains have essentially blocked the passage of weather systems. This is called a rain shadow. These very dry areas occur frequently when oceans, or even very large lakes, meet coastal mountain ranges. The air charges with moisture over the body of water, dumps it on the coast and leaves the interior dry. We see rain shadows in California, where the Sierra Nevada catches the moisture from the Pacific and produces Death Valley to the east. And in China, on the Tibetan Plateau, where the Gobi Desert stretches out behind the Himalayas. And in South America, where the Atacama Desert has formed behind the Andes—it’s one of Earth’s driest environments. Even the Great Plains of the U.S., once called the Great American Desert, are kept sometimes dangerously dry by the Rocky Mountains, which form a barrier across the country. Rain shadows are a permanent climatic feature on the landscape—or at least, as permanent as the mountains and winds that create them.